So you wanna be a Guerrilla RTO-

Keypounder sent me this question and wanted to pose it to all of you.

There’s a lot more to being an effective RTO than knowing how to key a mic, or even calculating a simple equation for antenna length, and one of the very serious reasons to get a license now is to start building that base of knowledge before it’s go time and you’re up the creek. This realistic scenario should make you think and point out holes in your working knowledge, that is, if you don’t wanna be renditioned off into the night. If you’re new, it’s gonna shine a light down the path you need to travel.

Post your responses or RFIs in the comments section.


You are a spy for the World Wide Freedom Forces and you want to minimize the likelihood that your signals, containing critical information to the Resistance, transmitted from the capital of Slobovia to your spy base in one of the provinces about 300 miles away, can be intercepted by the Slobovian Security forces (SS) who have alarmingly effective direction finding equipment. If the Slobovian Security forces catch you, you will die a gruesome death, and so will all of your contacts in the Resistance.

Not only does Slobovia have an excellent SigInt section, but they have agreements with neighboring  countries to provide DF lines of position to Slobovia.  To counter this technology, you have been instructed to use the 1.8 mhz amateur band  and have been given an antenna and transceiver set for this frequency.  You intend to use Near Vertical Incidence Skywave communication during time periods when the D layer of the ionosphere is ionized by the Sun, to keep signals from traveling any farther than they have to go in order to reach your base 300 miles away. The D layer is found 35 to 50 miles above the Earth’s surface. Assume Slobovia is on the Equator. Assume that the D layer is instantly ionized by the Sun.

How many minutes before sunrise and after sunset is the D layer of the ionosphere fully illuminated by the Sun? (Assume only direct light from the Sun; neglect atmospheric scattering effects)
How many minutes before sunrise and after sunset do the Sun‘s rays just start to illuminate the D layer?
If there are thunderstorms ( which cause extremely loud noise and make communications impossible) 1500 miles to the west of you, and the ionized D layer is capable of damping that noise completely out, are you going to operate after sunrise or before  sunset?  What is the minimum time after or before; how much of a window do you have?

I’ll add – what modes may you utilize and what should be your PEP?

120 thoughts on “So you wanna be a Guerrilla RTO-

  1. W6VRF

    “I’ll add one more question- what modes should you utilize and what should be your PEP?”

    Why not run the lowest power and most reliable mode that will work?

    The longest DX contacts I’ve made were using JT65 with about 25 watts. In your scenario I’m not talking to Indonesia from Seattle, but 300 miles away – thus the NVIS. JT65 isn’t useful for much besides a CQ, a signal report and 73s. I suppose it might be possible to overload a simple protocol like JT65 to contain more details, but otherwise another digital mode would serve us better here.

    From my limited experience with day time operating, NVIS works best on 40 and 80 meters.
    I’ve never worked with a 160m specific antenna, but from listening know its a night time band.

    But you stated we should operate when the D layer is ionized. The D layer is effectively gone shortly after dark.

    So, theoretically I’d say an NVIS “inverted V” 160 meter wire with a digital mode some time after dawn would be the ticket. I need to research how soon after dawn this can be, and how soon after dark before the D layer disappears.

    This exercise is counter-intuitive for the DXer, as you want to PREVENT our TX from travelling far. I’d prefer to discuss something like this verbally, as I’m sure there are many clarifying points I’m not getting.

    Thanks for letting me think aloud here.

    1. You’re on the right track.

      Clandestine communications are different from normal amateur aims- but as you see, the foundation is laid.

      More info will be posted in a couple days, after this thought experiment fully manifests.

      1. Silence is golden, Morse is short and sweet, don’t forget your flags, smoke and drums. Be prepared to say nothing at all and remember, loose lips sink ships, so listen and learn. The life span of a Forward Observer is 15 seconds, so, shoot, move, communicate and move again. Walk when you shoot, never sleep after you send com’s, move, move, move. If you can’t move, become a hole in the air and live. PS, Don’t forget your pencil eraser. The Rooster.

    2. W6VRF-
      160 works nicely during the day for short hop NVIS- see post here:
      You get about 4x the D layer absorption on 160 that you do on 80, if I recall correctly.

      It is also a long enough wavelength that few orbital DF sats have the ability to provide a fix, and the accuracy is degraded in any case. Terrestrial stations will have more trouble still, as 160 propagates in odd ways, depending on your location with respect to the magnetic poles.

      I’ve used sideband and a hundred watts and done well in the immediate area.

      I’ve done some experiments on information transfer rates with various modes, and come up with some interesting information I will share on Sunday. Maybe at one of the upcoming Patcons we can have an NVIS demo, too.

      1. Jay Bree

        A rule of thumb here might be to observe what one hears on the AM broadcast band. Use their coverage as a guide….particularly the daytime only low power stn’s .

    3. black

      you would use an inverted v from a high spot, but your freq would be just below the LUF. so you would be shooting our signal, line of sight like 2 meter.

    4. I’ve made contact 400 miles using NVIS on 80M with 20mw psk-31 and worked 23 states in 23 days CW 5w on 40M with full size vertical loop up 50 feet. QRP works, but must have good antenna. 73’s W9USS Bob

  2. This was a question I gave to a young person I am helping with high school math.
    An exact answer requires some knowledge of trig, but an acceptably accurate approximate answer can be arrived at using simple geometry and basic math.

    If there is sufficient interest, I can provide questions similar to this on a regular basis.


  3. Chris

    Flat nvis antenna at near ground level a few minutes after Sun set or a few minutes before dawn. One of the digital modes at about a 1/2 watt or less if the radio can be setup that way. (I know just enough to get into trouble with the digital stuff). I would also try to be in a valley / ravine to help eliminate any line of sight detection.

    Looking forward to the answer.


    1. The ravine is good. There are digital modes and digital modes, but that idea is good.
      Low power is good, but what will you do if you do not get an acknowledgement? Retry? At the same power? How many times will you try before you abort? If you have to abort, will you attract attention being out and about before dawn?

  4. This is great- good way to learn.
    I’ll attempt to add to the discussion in morning- heading out to spread salt at our 14 commercial snow removal accounts in a couple minutes.
    Went from snow flurries to 4-6 inches on the roads in 3 hours or less early this morning.
    We get “lake effect” snow here in NE Ohio- lake Erie is open water, any W, N, or NW wind accross lake dumps snow from Cleveland east to Pa state line and accross Pa into NY.
    Every interstate was a mess and at basically a standstill this am-one guy told me it took him 43 minutes to go 1 mile on I-271 this am. On the plus side- when he got to work the lot was plowed and sidewalks shoveled and salted.

  5. I’m not sure on the best times, or the effects of thunderstorms 1500 miles away on MF transmissions but I’d say that the best mode would be CW in the 160 meter band, at 5 watts or less…Nobody really monitors the CW portion of the MF ham bands except other hams. The D-region typically attenuates radio signals, keeping them from penetrating to higher regions and reflecting so it will be acting as a signal attenuator. The D-region is tightly locked to the sun, starting to build rapidly with sunrise, peaking at local apparent noon and dissipating almost totally at sunset.

    For an antenna, if space is not an issue, a terminated rhombic (1-2 wavelengths per side) would be highly directional, minimizing intercept. Using NVIS may not be the best choice, since it creates a torroidal-shaped reception zone: On the other hand it is hard to DF.

    I can’t wait to see how wrong I am 🙂

    1. You’re not wrong at all, actually, you’re on the right path.

      Maybe I should have thrown in FRAGO #2…licenses within the AO have been cancelled due to the ongoing insurgency.

    2. Keypounder

      FlighterDoc- the first thing to remember is that for this type of communication, you very much want to RESTRICT the propagation of your signal. Ideally, your signal would be received at your desired location and nowhere else. Absent that, the attenuation provided by the D layer will act to muffle your signal at lower angles of radiation and reduce the probability of intercept. Gain is not always your friend.

      D layer attenuation peaks within a minute of being fully illuminated and dissipates just as fast; various experiments done during total eclipses confirm this. F layer ionization takes longer to build, lasts longer, and changes height during the day. ( hint)

      Interesting stuff so far; remember, folks, read the question carefully. The right answer will have numbers in it, and they should be the same for before sunrise and after sunset.
      (Another hint for the questions about thunderstorm noise.)

      Pythagoras is your friend.

      1. OK, I was trying to puzzle it out with the Pythagoras hint….I started calculating what angle I would want to launch the signal at to reflect off the F1 layer (at an average of 115 miles altitude or so), and bounce down to a target 300 miles away…so one side of a right triangle would be 300/2, the other side would be ~115 miles, and the hypotenuse would be 189 or so miles. But, the need for trig would imply that we would need to use the rule of sines to calculate the launch angle…..and while I play with NVIS more than most hams, I’ve never even HEARD of a way to adjust the takeoff angle beyond hoping it goes mostly…UP. If I did the math right, the takeoff angle should be around 53 degrees above horizontal…

        I have to admit it…this is VERY interesting to me. MORE!

  6. I will throw in my .02 worth. From this chart for Eglin AFB, which was the closest I could find to the equator ( it looks like the best time would be about 40 minutes before sunrise (12:35Z) or 1 hour after sunset (0000Z) for 160m nvis. As far as what mode, I’m not sure. I would want the narrowest bandwidth, and as robust a mode as possible. I think I would take narrow bandwidth over robustness though. There are so many modes I wouldn’t even know where to start. As far as PEP, I’ve made voice contacts on 40m nvis using 10 watts at around 250 miles, with a random bush supported dipole, so I think no more than 2 watts for digital.

      1. As far as thunderstorms 1500 miles to the west of me. I think you will want to operate in the middle of the daylight illumination since the D layer will have had as much time as possible to absorb energy from the sun, and block out the noise from the T-storms. Since I would be in the middle of the area being lit (halfways between sunrise and sunset) I think the D layer would block my signal from going too far in any direction. But this also means, according to the chart I linked, that 160M NVIS is not very good. So maybe shortly after sunrise would be the best time, since the D layer will have charged everything to my east, dampening my signal from going that far, and the T storms will hopefully provide a noise screen on everything to my west. If my intended receiver is to my west, I would say wait until the shortly after sunrise at his location. So in the end, my first time choice would be middle of the day, to avoid begin DF’d, and my second choice would be shortly after sunrise.

  7. TT03

    I’ll give it a shot–
    How many minutes before sunrise and after sunset is the D layer of the ionosphere fully illuminated by the Sun? (Assume only direct light from the Sun; neglect atmospheric scattering effects)
    Approximately 8 minutes –6 degrees below horizon
    How many minutes before sunrise and after sunset do the Sun‘s rays just start to illuminate the D layer?
    30 minutes
    If there are thunderstorms ( which cause extremely loud noise and make communications impossible) 1500 miles to the west of you, and the ionized D layer is capable of damping that noise completely out, are you going to operate after sunrise or before sunset? What is the minimum time after or before; how much of a window do you have?
    After sunrise to maximize dampening coupled with thunderstorm expansion after sunrise with the heating by the sun. Its a good thing they are 1500 miles west. an hour on average for a window.

    I’ll add – what modes may you utilize and what should be your PEP?
    narrow bandwidth modes. CW is good, a digital mode can be encrypted with a one time pad and sent with Winlink or contestia, MT63, I would avoid image traffic if at all possible due to transmission time length.

    1. The amount of intelligence (data) to transmit would also have a factor in the mode to use I think….If it is very small (a few words +/-) CW would be ideal (assuming we can do that…maybe I should get my practice key out and see if it still works). If it’s a paragraph or more I’d encrypt/compress it and send it via digital….a narrow bandwidth mode with error correction…Olivia/Contestia 125Hz?

  8. Pingback: Brushbeater: So You Want To Be A Guerrilla RTO? | Western Rifle Shooters Association

    1. So after going back and re-reading the NVIS articles Keypounder put out earlier this year, he discusses using FSQ mode for NVIS. Here is a page dedicated to that on FLDIGI:
      And after looking at the results on that test you linked I think for a mode I would use either FSQ or QPSK-250. FSQ is 290 Hz wide, but I’m not sure how fast it is. I don’t know what the best balance of speed versus bandwidth is. I’m just thinking if I can get my message sent in 26 seconds that’s better than 3 minutes for something like PSK-31. This is all just my theory, I’m not any kind of expert on this so take it for what it cost. It will be very interesting to read Keypounder’s explanation.

  9. looking at my handy-dandy chart,, the opportunity to blot out the storm to the west is late evening when the window just opens up. else it looks like you’ve got a good shot from a couple hours after local sunset until a couple just before when it pinches off. as for modes, low-tech says run a key. a spot report at 5 wpm would be tiresome though. maybe a pre arranged set of code phrases would be just the thing this trip out.

    1. As for the math, there is one answer, although based upon guidelines.

      As for my question, there are multiple options provided they satisfy the logical constraints.

  10. dangero

    I’d probably go SSB voice using code words versus a digital mode or CW to transmit lots of information quickly to minimize key down time even if we’re using hard to DF NVIS. 20-30 watts should get the job done. I’ll be honest I don’t know the answer to the best operating time.
    Also just to be safe I would operate portable at least several miles from my base of operations if possible varying the distance and direction from it each time.

    1. dangero

      Oh and as an add I would instruct my listening party to construct a beverage on the ground antenna pointed in my direction to make sure they here me over the noise on the band.

      1. Paragraph 5 of the OPORD and standing SOI.

        A while back I posted a link to DanMorgan76’s instructions on building an SOI, because his version is short and sweet.

        I think I should probably re-post it.

      2. Dangero:
        I hope your insurance is paid up, and that you are fast on your feet!

        Beverages work best for lower angle vertically polarized incoming signals. I use mine for low band DXing and listening to AM broadcast and lower HF shortwave broadcasts, up to perhaps 10 mHz. NVIS signals are high angle and often horizontally polarized. If the incoming NVIS signal is horizontally polarized, there is a 20 db loss for cross polarization, and at least another 10 to as much as 30 db loss for the overhead null. See ‘Low Band DXing’ (Chapter 7?) for a better understanding of Beverage directionality. At any rate, a 50 db loss would mean that your listening party would not hear your 30 watt SSB signal, but the Slobovian security probably would.

        What WOULD be an awesome NVIS antenna for a fixed location and band would be a Jamaica. I’ve run some models and they offer significant advantages. This is two dipoles side by side about 10 to 25 feet up, fed in phase and about 1/2 wave apart. For 80 meters, this means 125′ apart, and for 160 about 250′ apart. Set in a steep sided valley with the ends pointing at the valley walls, this would be virtually ideal for a fixed location. It is NOT, however, a low profile antenna, to NC Scout’s point.

    2. 1. NVIS is not hard to DF.
      2. SSB voice is far less than ideal, due to no guarantee that your intended receiver can hear you or get a 100% copy.
      3. Voice is bad for a few reasons, namely the bandwidth involved, the intelligence value of voice pattern analysis, and the time it takes to get a message out.
      4. 20-30w is a VERY BAD idea, especially when attempting to restrict your propagation.
      5. While not co-locating your TOC and your clandestine transmitting site is very important, a few miles may not always be practical or necessary. But that’s situationally dependent.

      1. dangero

        PSK31 takes FOREVER to transmit even 2 lines of information so voice by comparison is much quicker but it does require more power. Also voice analysis is not any more of a threat that CW. Read up on WWII intercepts, the people (typically women) listening could tell who specifically by name was transmitting even Morse code by their keying, they were that intimately familiar with the senders.

      2. Voice analysis IS a big deal these days.

        Similar to what you pointed out with CW, except that with pattern analysis done via a computer, some analyst doesn’t have to listen for days on end. Its instantaneous.

        SSB Phone is out of the question.

      3. Dangero:

        PSK31 is slow, but so are most other narrow band modes.

        Keep in mind that computer CW has exactly the ‘fist’ it is programmed to have, and I would expect any competent resistance organization would either have a program that allows changes letter by letter, and changes them for each transmission, or simply uses the standard defaults for everything.

        I recognize different fists for the people I work regularly on CW that use manual keying, but more people use keyers or computers than use straight keys or bugs.

    3. Digital Modes, dude.

      You can cram more information in a shorter transmit time.

      Phone is absolutely, totally out of the question regarding the hypothetical scenario outlined above; as is < QRP PEP. FEC will allow you to make like War and be a Low Rider.

      Remember, the consequences of fucking up are you getting a JDAM enema from the Slobovians as apprised to just not getting a QDL Card.


      1. No, it’d look more like a group on the ground hunting you with an AOR 8200 and a man-portable Adcock array, because you’ve obviously set a pattern. Now when you don’t set a pattern…they don’t know what to hunt.

        A JDAM is expensive, overly destructive, and has no intelligence value. A warm body captured gives up (eventually) a whole network.

      2. dangero

        Sounds like if they are that good and you are that concerned then you shouldn’t be transmitting at all. Sounds like we need couriers running messages in person.

      3. Borders closed. No one in or out. Last attempt at a courier resulted in a replay of the opening scene from LeCarre’s “The Spy Who Came In From The Cold”.

      4. Actually on that note, something Bracken brought up in his third “Enemies” novel with Boone…

        .45 is subsonic and can be made very usefully quiet.

        Anyway, I’m getting myself off track. Another topic for another day. 🙂

      5. Hey, what is the matter with a fedora? *I* wear a fedora, from time to time. (the last time I did, somebody called me “Rebbi.”) A very practical and stylish addition to my attire, I might add. Waterproof, warm, keeps the rain off your glasses and provides some protection against cuts and bumps. And you can hide some good stuff in a fedora, too. Money, weapons, 20 ga wire antennas, even small radios. Next time you walk past a guy wearing a fedora, don’t assume all that it covers is his scalp……;-)


      6. Nothing wrong with a J frame Smith, any vintage. Beats rocks or knives. I *like* wheelguns. What did the best pistol shot ever in history use? Revolvers. Forget the folded paper, though. That will get you killed…….

        but I digress.

      7. Going back more to the meat of the matter, I also like tube rigs and tube amps. Overload a solid state rig and it’s toast. Overload a tube and it will still work. There is a reason the Russians use tubes in their avionics………… Older tech is not necessarily bad or even obsolete.

        How long have knives been around? Since the stone age?

  11. Topbander

    I have fifty years experience on 160. I use a full wave loop that shoots the signal straight up on 160 but on 40 meters it is gangbusters to Europe at night. A storm within 1500 miles really makes it unusable so in the summer it is pretty much useless. In winter I have a 1200 mile + range on 160. DF is pretty hard to do on 160 but I have seen old timers do it on 80 meters with a vertical mobile antenna and a two foot wide aluminum wash pan. I say for secret operation, use a dipole suspended at seven foot high in the woods, run a hundred watts for a couple minutes to make sure the msg gets through clearly them MOVE the whole operation quickly before the chopper gets there.

      1. Chris S

        Using an PRD-13, I could DF San Francisco and Oakland area radio stations (all below 160 m) from Gilroy with ease – because of groundwave. That is 60 miles, but then, they were 50 kW stations with antennas optimized for groundwave. Groundwave signal strength falls off as R^(-4). Do the math, it would be easy to DF 10W of groundwave at a couple of miles. Night time might be harder because of interference from distant stations – but I wouldn’t count on it. BTW, every horizontal dipole over ground has vertical radiation off the ends – you can check this with NEC type antenna modeling software. That’s what will radiate your groundwave.

      2. It’s simple to DF groundwave if they’re looking for it, but 160 can be an interesting animal.

        As per the vertical radiation off the ends, you’re absolutely right, hence why I’ve understated inverted vees and terminating resistors. Even still, getting caught is absolutely in the realm of possiblities.

      3. Indeed, Chris is correct, groundwave can be reliably DFd for some distance, depending on frequency, ground characteristics, and power level. There are a number of DF techniques that could be useful; using computerized time difference of arrival is probably the most common, but the Allies were DFing U Boats in less than 5 seconds using Adcock arrays during WW2, and that technique is still useful today, and works on skywave signals. The higher the incoming angle of the signal, the more difficult it is to DF it or even for a vertically polarized array, as many of the DF antennas are, to hear it at all.

        Chris is also correct that dipoles emit vertically polarized RF- that was mentioned in the NVIS articles as one reason that valleys and terrain discontinuities were useful to restrict ground wave propagation.

        Bad ground for ground wave propagation is also a built up area with lots of above and underground utilities, high rise steel structures or cast in place concrete structures, etc. Like a capital city. One could easily place a very low profile NVIS antenna along the top of a board fence, with each end pointing at a nice tall building. Hard to DF a signal with multiple reflections, especially one that is weak and short duration.

    1. Dear Topbander:

      I just worked a new country this week about 4500 miles away from my fixed location on 160. The interesting thing is I worked Argentina on 160 during the summer, and routinely work 160 DX year round. At night. My 160 transmit antenna is an inverted vee with the apex up about 60′. The 160 meter full wave loop is a good NVIS antenna, and has usefullness on other bands too, as you noted

      The trick for using 160 for *short range* comms is doing it during the day, which cuts the noise way down, AND reduces the probability of intercept.

      NC Scout’s question about mode, and your comment in response for the need for speed are spot on.


  12. Chris S

    I don’t have a lot of experience on the Top Band, or any from the equator. I know that distant AM stations disappear within 30 minutes of sunrise, and come back 30 minutes after sunset (sometimes). So I only want to transmit in the daytime window. But – even with NVIS antennas, you will have some ground wave, and if the SS RDF teams are close, you are screwed. (You’re in the capitol, after all.) The neighboring country DF sites aren’t going to be a problem – they aren’t accurate enough to pick out a site in a city. It’s the nearby ones I would worry about. I would look for burst communications at random times and frequencies if I have the technology.

  13. The Lost Okie

    Man, reading this makes me realize I am SOOO far behind it’s depressing. I got my General ticket and got 2M mobile, 2M HT, SWL RX and an Icom R6 but no HF TX capability yet. Herr General did not teach me if I can run digital modes at low power which I think is the answer here.

    I’m running (6) coax out of the radio room for future antennas next week. Currently saving for a 857-type radio. Then I got to figure out how to run a digital mode through a QRP radio like that one. THANK YOU THANK YOU for all that you do here. I have learned SO much.

    1. You’re not as far behind as you think! This is a tough and thought provoking question, hence why it was asked 🙂

      I thank you for reading.

  14. Carlos.....Danger

    I’d use a book code or one-time pad, encrypt my message, use a simple steg program to embed it in a picture of the glorious leader of Slobovia and email it to the Secretary of State of Slobovia at her private email address . Within hours base could hack said server by Phishing and have the message.

    But then, I’m lazy.

  15. Coyote Hunter

    Mr Keypounder,

    Thank you for your illuminating articles on NVIS. They are by far the best explanation of the topic that I have ever read. I learned even more by considering your questions and working the numbers. I believe these figures are accurate…

    1) Roughly 30 minutes. If you want to keep the entire 600 mile circle under the D-layer umbrella, you should subtract off 18 minutes to account for the NVIS edges that are roughly 300 miles away.
    2) Almost 6 additional minutes.
    3) Evening operation will keep T-storm interference from the west under the D-layer umbrella.
    4) A T-storm 1500 miles away will cross into or out of the D-layer umbrella in approximately 1-1/2 hours.

  16. As far as a digital mode is concerned, after going back and re-reading Keypounder’s articles on NVIS, he mentions FSQ. I did some research this morning and I’m pretty sure that is the way to go. Some articles to explain it:
    So it is 290 Hz wide, at 60 WPM max speed, although it sounds like it is more reliable at 45 WPM. Is that too wide? I just wrote a simple SALUTE report in word and it was about 25 words total using abbreviated codewords with the vowels and spaces removed. I also did the same thing for a list I had made, and it came to about 75 words for a 9 step list. So if I was going to send both of those at 45 WPM it would take about 2 minutes to get it all sent. Is that too slow for that wide of a bandwidth?

      1. Badger

        Since I’m the guy out in the wind, you’ve already issued me the antenna to use in your initial scenario, thanks HQ (it’d better damn work). And BTW thanks for issuing me my decked-out KX3, into which I can key, using CW into one of the buffers) my already encrypted SITREP. Then xmit FSK-D/PSK or some such low-overhead flavor in a burst, like 60wpm. Xmit exposure is extremely short. My govt in exile specified a freq in that band just offset from an existing beacon; how ’bout that? Am leaning toward just before sunset as the DE is west of me, but still working on a location – since I don’t think there are ravines in the Slobovian capital but might be man-made equivalents – as well as the other “clock-sensitive” stuff. Interesting puzzle.

    1. Assume that they have you on a listening schedule- X frequency at such and such a time, and that the receive equipment is top-notch- K3S or equivalent, with an excellent receive antenna array, phased Jamaica’s or equivalent.

      Lots of talk about what sort of comms- I would expect that there would be a code-word vocabulary (“John” = ‘Have arrived safely with all equipment’; “Fish” = ‘have made contact with local resistance group E’; etc. and that there would be a dedicated one-time pad for encryption for things not covered in the book. You can transmit a LOT of information very quickly with this sort of code, just a couple of words can be two sentences or more.

      A lot depends on whether you are trying to conceal all hint of any transmission or whether you want to conceal the existence of a resistance cell communicating from the capital.

  17. ZPO

    We’re now into the 2nd decade of the 21st century. Sending any significant volume of information via HF in a hostile DF environment is a losing proposition on many levels. Likely it was decided upon by a retired military signal type who came of age in the cold war or a ham who is way out of his depth. HF stations in sanctuary sending out one-way broadcasts to be received by assets in denied areas still makes sense. Those agents transmitting back via HF seldom does.

    Lets break it down…

    1) The mission statement gives our location as the capital city of Slobovia. This is likely a very built-up urban environment.

    2) Resistance HQ is using a simple dipole so we’re limited in our ability to shift our frequency to be very close to a powerful broadcast station that can be placed in an antenna null at HQ.

    3) Absent things like loading coils, an antenna for 160M is going to be at least 130ft long for a quarter wave or 260ft long for a 1/2 wave dipole. Getting it up at least 1/10 wavelength is a height of a bit over 50ft above ground.

    4) Since our HQ is also in Slobovia, any ARQ modes are a no-go since we don’t want HQ to expose their position by transmitting. A digital mode with FEC that can get the message through in under 10 minutes (preferably 5) would be my choice.

    5) PEP – Enough to get the message through on the chosen mode plus 10-15% margin.

    My plan:

    Location of transmission – I’m going to take some poetic license and assume that there are disused industrial areas in and around the capital of Slobovia. Perhaps caused by crippling sanctions or general crappiness of central economic planning. I’m also going to assume that a simple 1.8MHz transmitter and the gear to modulate my encoded message are expendable items. The transmitter will be placed in an abandoned building (hopefully surrounded by other abandoned buildings) with a very visible antenna on the roof. It can be setup the night before or days before. Especially it the content can be loaded remotely.

    Method of transmission – Using a FEC based digital mode that will get the message out in 5-10 minutes. Message will repeat several times (at least twice)

    Securing the transmission location – Enough explosives and scrap metal to deal with the Slobovian Security Forces that respond to the DF hits. Inside the building. Rigged for tamper of the transmitting gear with backups of tripwires and command det. If I’m feeling artistic, likely assembly areas outside will also be rigged with enough AP demo to take out anyone responding to the initial blast.

    Summary: Rather than trying for LPI, I’m going to assume the message(s) will be intercepted, DFed, and there will be a security response. The problem statement describes a security regime that tortures prisoners for information. Thus there isn’t an ethical dilemma with killing and/or maiming as many of the responding SSF as possible. The next time their DF network indicates a transmission in progress, they will respond much more cautiously and likely with EOD support. That takes more time to get rolling.

    There are many other and more modern ways to move information across 300 miles than HF radio. Even more so if it is a large quantity of information. Even a simple non-broadcasting wifi dropbox on an OTR truck can be part of a ratline moving info. Lots of other more esoteric possibilities are there too.

    1. ZPO:

      your objections are noted, and to some extent, valid. Keep in mind, this question was created as a word problem in mathematics for a high school student.

      There is no doubt that there would be significant issues in the real world if a guerilla force went up against a first-class government using HF comms. However, there are limits to what governments can afford to do, and the fact that most covert traffic these days is by encrypted digital spread spectrum VHF and UHF satellites means that the institutional knowledge of HF DF is fading or gone. The technical capability may still exist, but the institutional knowledge base is fading. The US has decommissioned many of their land-based HF DF stations, for example.

      How many people know how to design, build and operate a steam engine these days?


      1. HM 01 comes to mind.

        Clandestine HF is very much alive and well- I went through an entire course learning about it, paid for by y;alls’ tax dollars.

      2. Cuban Numbers Station. It’s a hybrid voice/digital mode that can be heard very often.

        It emanates from Radio Havana’s transmitter…even though they deny it. 🙂

      3. ZPO

        I concur that national capabilities for HFDF are lagging in most areas. I might have even worked at some of those now closed land-based DF stations back in the 90s. 🙂

        My intent was to show the options available to get the message through and turning the risk of being DFed and located into a probable tactical win.

  18. Badger

    ZPO makes one specific point I must concur with (not in terms of this exercise) but in terms of tradecraft. The list of very quickly rolled up operatives dropped into German-occupied territory 70 yrs ago were rolled up often after the first transmission or betrayed before they ever got their set fired up. We should suppose that someone can do it a bit better nowadays.

  19. couldn’t take it any longer – here’s the math. the 50 mile up d-layer gets hit when its 631 miles away from you. at an earth rotational speed of 17.3 miles a minute, that gives you 36 and a half minutes before sunrise or after sunset where you’re at, neglecting refraction. in practice this will be different and you’ll need to plan on a longer window, just to make sure, because d-layer start up and fading aren’t symmetric even under ideal conditions.

    your radio choice will be determined by who’s funding your partisans, or if they have to just make it up as they go along. if fedex delivers to slobovia, then buy something top of the line from lnr precision. but keep it simple. might be your grandma has to make the trip and send the message, just because nobody would suspect your granny.

  20. Jay Bree

    I think some of the conditions are in more detail than of practical use, while others are ignored.

    – practical concerns- Lugging around computer gear and interfacing it to radios is a significant layer of complexity that may or may not be needed, given that we don’t know how big the content of the traffic. If it’s big, then lugging around a computer and interfacing it MIGHT be of use, otherwise it’s orders of magnitude more complexity on both ends.

    – If it was me, I’d do this at dawn to avoid QRN from late afternoon storms, throw up a random wire ~100′ long in the trees, lay down some counterpoise wires and send the traffic re: CW.

    Power? 300 miles @ grey line on 160.. 50 watts to start…

    1. No, they haven’t been.

      It was a math question to start.

      Lugging a computer is not exactly required, either.

      Third, you’re in a city. A 100′ random wire is inefficient on 160- and 50w will get you killed.

  21. I’ve made NVIS contacts using 2 1/2 watts sideband on 80 meters. 4x the attenuation on 160 means I should be able to make a contact using 10W SSB. (I’ve done it with 5 watts….) Not that I would recommend that under these conditions, but I bring it up so that we can get some idea of the power requirement. Equivalent power is about 1/50 of that using digital modes, say about 1/4 to 1/2 watt output should be plenty.

    Antenna selection has already been made; that was not a question. (if I were running things, I’d have a 160 meter antenna cut for the bottom of the band using 18 gage coated copperweld wire and a nice light balun.) I would plan on using a digital mode on a small tablet, or even a smart phone, to run the rig although again, this wasn’t part of the question.

    The point of this question was primarily simply to challenge the reader’s mathematical ability; the secondary point was to raise awareness of the importance of understanding the dynamics of the ionosphere and how they can affect HF operation, especially NVIS.

    I will be following up this question with others designed to elicit thought on various other aspects of radio operation.

  22. Jay

    Qty of traffic is important to know.

    If its just “QRV” then there are many options. Shoot and scoot w cw.

    If it’s sheets of text then things get more tricky. JT can be done with very little ERP but it takes time.. Id consider fast cw at low erp as the best practical solution.

    Sorry for tablet text…. I hate virtual keyboards. 🙂

    1. The math is no less important.

      Vital, in fact.

      Commo windows are real, purposefully constructed, and critical to clandestine communications.

  23. Jay Bree

    As a degreed EE, I fully understand the need for math. However in a practical sense, I’m not going to be doing trig problems in the field if I have 2 min to decide what mode/freq/power to use. I suspect I’ll be tired, dirty, hungry cold and behind schedule, and concerned about being shot.

    I’ll add in that propagation is rarely as predictable as the trig problem predicts. The calculations will get you close, but there’s huge variance in practical application.

    The 300 mile situation is an interesting one. It’s too close for traditional band choices and too far for reliable VHF without a lot of hardware. 160 would be the worst choice given antenna geometries. A very low dipole on 40 would work better and be more practical to implement since we’re only talking 66 ft of wire and QRP would work under the conditions.

    Balancing the math (which is implicit in the “practical experience” solution) , the practicality of implementing something quickly out of a backpack( fed by batteries?) I’d go NVIS dipole, CW on 40 with a few watts from a non-synthesized xcvr. If I needed to be secure I’d use one time pads.

    Of course that’s not the question, but it’s an interesting communications issue.

    1. You don’t “have 2 minutes to decide.”

      Prior proper planning.

      An EE degree does not correlate to understanding trade craft and clandestine communications.


      1. Badger

        “…trade craft and clandestine communications.” Quite sure this’ll be discussed in due time, so I’ll agree that – for purposes of the exercise – the other stuff (including some of mine) is white noise. Other aspects will be the onus on the distant-end to always be listening, choosing time of day (ergo freq & other parameters) are going to be driven to a large extent by the one in the NPE situation trying not to be killed (or worse) but that’s less the math problem – thanks for the exercise & hosting this discussion. Worthy stuff.

      2. Some of it has been discussed a while back.

        Look up the sections I wrote on report formats and commo windows.

        They will be brought up again however.

    2. Jay Bree:

      >As a degreed EE, I fully understand the need for math. However in a practical >sense, I’m not going to be doing trig problems in the field if I have 2 min to decide >what mode/freq/power to use. I suspect I’ll be tired, dirty, hungry cold and behind >schedule, and concerned about being shot.

      no time limit was stated. As an EE, I’m sure you can formulate an answer, and I look forward to reading it and your explanation of how you got there. The meat of the question is not whether or not you know trig, it is about being able to think through a problem. There are at least two ways to solve this, one without trig, but that is not the point. The point of training and teaching is to learn and practice ahead of time, so that when you are ‘tired, dirty, hungry, cold, and behind schedule …’ you don’t have to think about this sort of thing, you already know it.

      This is analogous to knowing your NPOA, and the mechanics of loading and firing a rifle. (Then again, I know some doctors who think that if TSHTF and they need to shoot something, that they’ll just unwrap their boxed up AR-15 and shoot whatever they need to.)

      >The 300 mile situation is an interesting one. It’s too close for traditional band >choices and too far for reliable VHF without a lot of hardware. 160 would be the >worst choice given antenna geometries. A very low dipole on 40 would work better >and be more practical to implement since we’re only talking 66 ft of wire and QRP >would work under the conditions.

      ‘Antenna geometry’ has nothing to do with the case. The function of this question is to help people understand NVIS operation and how it is affected by the ionosphere. NVIS is ALL ABOUT filling the gap between ‘traditional band choices’ and ‘reliable VHF.’ On that subject, it is entirely likely, as we have seen repeatedly this year, that when there are few or no sunspots and the solar flux drops, that NVIS may not be possible during the day on 40 meters. Further, the fact that D layer absorption on 40 is about 1/16 or less the level of absorption on 160 is NOT A GOOD THING for clandestine comms.

      You may want to reread the three articles that NC Scout posted earlier this year on NVIS operation.


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