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The Inverted L Antenna and NVIS

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In this article LodeRunner explains what an Inverted-L antenna is, how it works, and why you might strongly consider building one for use on the lower bands. While explaining its use with a high degree of technical information, he’s written it in a manner that’s easy to follow and digest. The diagram data is sourced from EZ-NEC, which a link to the software is provided in the sidebar.

The Inverted-L Antenna and NVIS

An “Inverted-L” antenna is basically a wire antenna, typically ¼ to ½ wavelength long on the band it is designed for. The Inverted-L antenna is a common antenna for the 160 meter and 80 meter amateur bands, where typical ¼ wave verticals are impractically tall for most amateurs.

In the Inverted-L configuration, the first portion of the wire rises vertically from the feedpoint, and at some height is bent roughly 90 degrees, and then extends horizontally to the unterminated end. The feedpoint is very close to ground level (typically not more than 3 feet above ground), and the antenna is worked against a Ground consisting of one or more ground-rods, and/or a counterpoise consisting of one or more radial wires – which may be buried, laid directly on the ground, or suspended above the ground at some low height.

Because the input impedance of a typical Inverted-L antenna is low, and the feedpoint is at or very close to ground level, where ground losses are substantial, it is very important to establish a good ground to work the antenna against.

Most of the amateur literature regarding the Inverted-L antenna is focused on optimizing performance of the Inverted-L antenna for “DX” operation – that is, high efficiency in radiating its energy in a pattern that is low in elevation (low Take-off Angle, or ToA) – typically below 30 degrees relative to the horizon – which maximizes the distance to which communications may be achieved. Effective NVIS communications, on the other hand, require an antenna which is optimized to produce a pattern where the majority of the radiated energy has a high ToA pattern – ideally between 60 and 90 degrees – to provide reliable communications from zero to several hundred miles.

Fig. 1: DX dipole pattern on 80 Meters

First, lets take a look at the difference between a good “DX Antenna” pattern vs. a good “NVIS Antenna” pattern –

Figure 1 is a diagram of a dipole optimized for DX communications; Figure 2 represents the exact same dipole, but the height has been lowered by approximately 1/3 wavelength to optimize the antenna pattern for NVIS communications.

Fig. 2: NVIS Dipole Pattern on 80 Meters

These diagrams make it easy to see the difference – first, the green line in each diagram shows the direction of maximum radiated energy – the Take-off Angle, which is commonly abbreviated “ToA”. For the DX dipole, the ToA is at 30 deg above the horizon. For the NVIS dipole the ToA is 90 deg, i.e. straight upward. In the above figures, it is the signal energy in the area between the two diagonal lines which produces NVIS propagation. The purple lines show the angles which are called the “Half Power” points. In simple terms, these are the ToAs where the signal strength drops to half of what it is at the best ToA of the antenna’s pattern. The angle between the two purple lines is called the “Beamwidth” of the primary lobe. The beamwidth of the NVIS dipole encompasses the entire NVIS “Sweet Spot” – which is roughly between 60 and 90 deg. ToA .

These pattern plots show us that NVIS and DX communications are essentially opposite objectives, so designs which optimize the antenna for DX will produce very poor performance for NVIS, and vice versa. Because I have not seen elsewhere any advice on optimizing an Inverted-L antenna for NVIS on the 160 and 80 meter bands, and because I believe that effective NVIS communications on these bands is essential for preparedness communications, I have prepared this article as a practical guide to achieving these objectives.

The objectives which will be satisfied are –

To meet these criteria, the antenna wire will need to be between 175 and 215 feet in length, and the horizontal section must have a height of 50 to 65 feet at each end. This means that the horizontal span of the antenna wire will be between 110 feet and 165 feet. A small amount of slope [ +/- 10 feet ] in the horizontal section is not a problem, and some sag in the middle of the horizontal wire unavoidable, but no part of the horizontal wire should be below 40 feet Above Ground Level (AGL) or above 65 feet AGL for optimized NVIS performance.

t’s important to note here that, even if antenna supports of greater than 65 feet are available, the antenna should not be hung at a height greater than 65 feet, unless the length is also increased (which will increase the challenge of matching the antenna to the feedline). If we hang a 200 foot wire at a height above 65 feet, then the Current Loop (point of maximum radiation) will be in the vertical section of the wire, and the NVIS radiation pattern will be substantially degraded (it will become a good DX antenna, but that’s not what we want in this case).

The antenna wire should not be closer to any non-conducting materials (such as tree branches) than 6 feet, and ideally the minimum separation from branches/support poles/building roofs/etc. should be 20 feet or more, in order to minimize the RF losses in the near-field of the antenna.

Separation from metallic objects should be at least as far from the wire as the antenna is above ground, i.e. 50 feet or more – this includes flag poles, utility poles (they have a ground conductor running down their entire length), metal roofing/siding of buildings, etc. This is particularly important because metallic materials in the near-field of the antenna may have substantial effects on its radiation pattern and efficiency.

DEFINITELY ENSURE THAT NO PART OF THE ANTENNA CAN COME IN CONTACT WITH ANY UTILITY LINE, EVEN IF ONE OF THE SUPPORT ROPES FAIL WHILE SUBJECT TO STRONG WINDS. YOUR LIFE, AND THE CONTINUING OPERABILITY OF YOUR EQUIPMENT DEPEND ON MEETING THIS REQUIREMENT.

First, let’s take a look at the typical (¼ wavelength) Inverted-L antenna’s pattern, and why it favors DX over NVIS communications –

Fig. 3: Elevation Pattern of a 1/4 Wavelength Inverted L Antenna

Looking at this elevation plot and the associated data, we see that the maximum gain occurs at 30 deg., and that this gain is roughly equivalent to a dipole above typical earth, i.e. ~3dBi (although this antenna, unlike the dipole, produces mostly vertically polarized energy).

We can see that the radiated energy in the NVIS sweet spot from 60 to 90 deg. is down 3dB to 5dB from the peak. Remember that -3dB = 50% power, a.k.a. The “Half Power Point”, so the NVIS signals are at least 50% weaker than those at the 30 deg ToA peak. This is obviously a DX antenna on 160 Meters, and would be very difficult to match on 80 Meters, so I won’t spend the time to go through the 80M plots.

Another important consideration with Inverted-L antenna is their losses. With a ¼ wavelength Inverted-L, the point of maximum radiation is right at the feedpoint, just above the earth. With typical soil conditions, a large portion of the signal radiated into the ground is absorbed, lost.

Fig. 4: Point of Maximum Radiation and Consequent Losses

In the above diagram of a ¼ wavelength Inverted-L antenna, the red circle indicates the area where the strongest field-strengths occur. Notice that essentially half of our radiated energy will interact closely with the ground. Depending on soil characteristics and the quality of the radial field under the antenna, somewhere between 30% and 70% of this energy will be lost.

Now, lets look at what happens when we change the antenna wire from ¼ wavelength to 3/8 wavelength, again at 1850Khz –

Fig. 5: Elevation Pattern of 3/8 Wavelength Inverted L Antenna

Pictured right is an Elevation Plot of an Inverted-L antenna which is 50 feet in height, and has a horizontal span of 150 feet – so the radiating element is 200 feet in length – this is roughly 3/8 wavelength at 1850Khz.

This pattern is much better for NVIS: the ToA for peak power is at 80 deg, and at no point in the 60 – 90 deg “NVIS Sweet Spot” is the signal strength down more than 1dB from that peak. You should also notice that the Max Gain is 5.69dBi. This is 2.6dB more gain than the ¼ wavelength Inverted-L, and the Beamwidth of this antenna includes the entire NVIS Sweet Spot. In round numbers, this antenna puts about four times as much RF energy into the NVIS Sweet Spot as the typical Inverted-L designed for DX work.

With regard to losses, we can see that moving the point of maximum radiation up into the horizontal wire has substantially reduced the portion of the radiating field which must interact closely with the earth. In Figure 6 below, the small blue circle indicates the (approximate) point where maximum radiation occurs on 160M. Increasing the height of the point of maximum radiation (in this case from 0 feet to 50 feet) reduces earth losses by approximately 1 to 3dB – again, depending on soil conditions and the quality of the antenna’s ground and/or counterpoise.

Fig. 6: Elevating the Point of Maximum Radiation

Another effect of moving the point of maximum radiation up into the horizontal section is this – the feedpoint has a much higher [real] resistance. Since the efficiency of an antenna is determined by the ratio of Rrad (radiation resistance) to the sum of Rrad + Rloss {Efficiency = Rrad / Rrad + Rloss}, we have two ways to improve the efficiency – increase Rrad and/or decrease Rloss. In the case of an Inverted-L antenna, it is much easier to increase Rrad than decrease Rloss,and increasing the efficiency of the antenna accounts for a good portion of the increased gain of the 3/8 wavelength versus the ¼ wavelength Inverted-L.

So how does this 50ft/150ft antenna perform on 80 meters? Lets take a look –

Fig. 7: 80M Elevation Plot, in line with horizontal wire, e.g. North-South

I’ve included both elevation plots (Fig. 7 & Fig. 8) because I want to make obvious that the orientation of the horizontal wire is very relevant to the coverage provided by the antenna on 80M. So if you desire broader coverage East-to-West, then the antenna wire should be hung North-South, just as you would for a dipole.

Notice in Figure 8 that there is a vertically polarized radiation pattern from the antenna (in red) which will provide some East-West coverage beyond NVIS range when band conditions are favorable.

Fig. 8: 80M Elevation Plot, perpendicular to horizontal wire, e.g. East-West

All of the preceding assumes that the antenna is worked against a “good” ground. At a very minimum, this antenna requires three radials of at least 100 feet in length, and the efficiency and pattern will be much better when the antenna is worked against a set of 6 or more radials (ideally) of at least 150 feet in length each.

Using the minimum of 3 radials, the layout should look something like Figure 9, where the red line indicates the antenna wire, and the black lines represent the ground radials. The center radial is directly below the antenna wire, and the two outer wires are “fanned out” to run parallel to the center wire at a distance of 15~25 feet to either side. If the three radials can be extended to the same length as the horizontal section of the antenna wire, this will improve the antenna’s efficiency and VSWR at best match.

Fig. 9

A better arrangement of ground radials is shown in Figure 10, above. The three radials going to the right of the antenna feed point are the most important, and the optimal length for these is ~150 feet each. The radial going to the left of the feedpoint is least essential, and can be bent, shortened, or omitted if space constraints require. The outer half of the radials going vertically in Figure 10 can be bent to suit the available space without substantial effect on the efficiency or pattern of the antenna; if these radials are shortened, this may affect the efficiency and/or matching requirements, but not to such a degree as to make the design of the antenna matching unit described below unworkable.

At least one ground rod should be installed at the feedpoint of the antenna as a lightening ground, and a suitable surge protector/lightening arrestor device installed at the feedpoint.

~~~

Since construction of the 3/8 wavelength Inverted-L antenna was thoroughly covered in a previous post by JohnnyMac, I’ll move on to describe how to obtain a simple and high efficiency match from this antenna to your feedline.

On 160 Meters, a simple series capacitance will suffice to resonate the antenna on 160 Meters. Depending upon the details of wire length, height, and soil conditions, a total capacitance of 80pF to 180pF will resonate the antenna on any desired frequency within the 160 Meter band. The VSWR curve will look like this –

At the bottom of the 160M band, a capacitance of 145pF (0-j600 ohms) gives us this:

While not perfect, this is a very workable match for the auto-tuners built into many modern transceivers, as well as external tuners such as produced by LDG and other companies.

In the top half of the band, 102pF (0-j800 ohms) obtains a nearly perfect match to a 50 ohm coaxial feeder all by itself:

Looking at possible matching solutions on 80 Meters – again, a simple capacitance of 560pF (0-j75 ohms) gives us a very usable mid-band VSWR plot as follows –

Towards the bottom of 80M, no capacitance at all is required for an excellent match. This is the point at which the antenna wire is ¾ wavelength long. This VSWR plot is with a direct connection between the feedline and the antenna wire –

What this tells us is that, to obtain a good match at the bottom of 80M with a 200 foot (50’v/150’h) radiating element, an inductive element (coil) will be needed at the feedpoint to match the antenna, which would complicate our matching arrangements – but should be within the ‘reach’ of most commercial matching units.

However, if we increase the length of the wire a bit, we can shift all the needed tuning reactances into the negative (capacitive) range across both bands, and keep our matching system simple and efficient for both 160M and 80M.

If we extend the wire another 10 feet horizontally (50’v/160’h) then 93pF (0-j900 ohms) obtains a near-perfect match at 1900Khz.

And 121pF (0-j725 ohms) will give us a very workable match at the bottom of 160M –

Adding 10 feet to the horizontal section shifts our 80M matching points as follows —

At 3600Khz, 590pF (0-j75 ohms) obtains a near-perfect match which looks like this:

Obtaining a 1:1 VSWR match at 3500Khz would require over 1000pF of capacitance (0-j40 ohms) so the lowest frequency with a ~2:1 or better VSWR match using this method will be approximately 3550Khz. This limitation only applies if you are building the simple capacitive matching unit described below – with a commercial tuner a 1:1 VSWR can be achieved anywhere in the 80 Meter band.

In the middle of the 80M band, 236pF (0-j180 ohms) obtains this very good VSWR curve –

And at the top of 80M , 115pF (0-j350 ohms) obtains a very workable match, also –

With all this data, we can confidently conclude that an Inverted-L antenna of 200~215 feet in total wire length, with a vertical portion 50 to 65 feet in height, can be effectively matched to a 50 Ohm coaxial feeder on 160M with nothing more than an adjustable capacitance in series between the feedline and the beginning of the antenna wire. Assuming we can install the antenna with a total length of 210~215 feet, then a series capacitance is all that is required to resonate the antenna across most of the 80 Meter band, also.

If your primary interest is towards the bottom of both bands (CW and Data segment) then a total radiator length of 210~215 feet is optimal (depending on soil conditions); if your interests incline more towards the middle/upper portions of the band for ‘phone operation, then 200~205 feet will suffice.

Assuming that you are able to place a good antenna tuner right at the base of this antenna, then no series capacitors (outside the tuner itself) should be required to obtain a good match across most of both bands, and you are ready to get on the air.

If you wish to “roll your own” tuner, either as a stand-alone, or to bring the VSWR to a workable value right at the feedpoint (and then finish matching with an auto-tuner built into your rig, or located away from the feedpoint of the antenna) then read on.

Assuming a 210 foot total wire length (50’v / 160’h) over a good counterpoise for all of the below calculations, then:

At 1900Khz we simply adjust the series capacitor for the best match, which will be very near the calculated value of 93pF(some value between 90 and 100pF will obtain the best match), and a variable capacitor covering the range of 85 to 125pF will provide a match of better than 1.5:1 across the entire 160M band. If this is not the case, then the length of the antenna wire should be adjusted to achieve the best match at 1900Khz with 90~95pF of capacitance. This will have to be determined experimentally once the antenna wire is installed in it’s final location. If you can get a “perfect” 1.0:1 VSWR at 1900Khz with a capacitor setting very near 93pF, then the antenna is optimized for 160M, and you are set to operate across all of 160 meters – only the variable capacitor will need adjustment.

To obtain a 1:1 match at the bottom of 80M (~3600Khz) only the series capacitance is required, and this will be very close to 590pF. Since this is a larger value than commonly available in a transmitter-rated variable capacitor, a combination of a fixed capacitor (e.g. 250pF, 400pF, or 500pF) and the variable cap in parallel can easily be used.

To cover all of 160M, and the majority of 80M the wiring of the “match box” looks like this –

Fig. 10: Simple Tuner for 160 and 80 Meter Inverted L of 210~215 Feet total length

For a transmitter of up to 150 watts, suitable components are as follows:

SW1 is a DPDT manual switch or relay rated for 250VAC/5Amps or more. If using a relay, the coil voltage can be whatever is available and convenient – relays with a 12VDC coil are common, and can be powered from the same power supply as your transceiver. 16Ga. or 18Ga. “Zip Cord”, or “Thermostat Wire” are inexpensive, and can be used to carry the switching power from the control position to the matching unit at the feedpoint of the antenna. The entire match-box assembly should be built in a good weatherproof enclosure.

If you plan to operate at power levels >150 and <=1000 watts, then the voltage ratings of the capacitors should be higher – 2000 Volts for up to 1000 watts of transmitter power, and SW1 must be rated for at least at least 1000VAC/10Amps per set of contacts. Otherwise, flashover of the relay and/or capcitors will occur and your transmitter may be damaged.

Other configurations of antenna wire and/or matching unit are certainly able to produce a good NVIS pattern on 160 and 80 Meters, but for all possible configurations the key elements for optimized NVIS performance of the antenna are:

  1. The horizontal portion of the wire should be no more than 65 feet AGL to maintain an NVIS radiation pattern on both 160 and 80 Meters;
  2. The Current Loop (where the majority of signal is radiated from) must be in the horizontal section of the wire on both bands, and
  3. You must work the Inverted-L antenna against a good ground or efficiency will be reduced and matching will be more difficult to achieve

I hope this article has given you all the info you need to put a strong NVIS signal on the air. I will answer questions posed in the comments section.

73, LodeRunner

~~~References and Annex Materials~~~

EZ-NEC file data for 210ft antenna:

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