A question from MT-01 over at Lower Valley Assembly:
I have a neighbor who was a special forces engineer sergeant in Vietnam. I helped him get his amateur license. Sometimes we talk about radio comms in Vietnam. He tells one story where the comm sergeant put the end of his antenna into a stream in order to make a contact. Is that feasible? I keep wondering if he misunderstood what he saw, and they guy was grounding something instead of using it as an antenna. If it didn’t ground the signal, I guess that I could see it being a really long wire antenna, but if it worked at all it’d just work as a NVIS I would think. I’ve tried googling the idea, but haven’t found anything. Thought I’d run it by you.
The antenna was most likely a variation of the Longwire, using the water as a reflector. It can work on any band, from HF to UHF, depending on what application you need and cut to the appropriate length. What’s nice is that it’s a somewhat directional antenna as is, but when you take the cold end and make it vertical with the hot end sloping towards the ground, you’ve got an even stronger directional antenna. Add a terminating resistor to the mix and now you have a completely directional antenna pointing in the direction of your sloping wire.
You may not be able to tell from the photo, but the same antenna that’s pictured in the diagram is also right in front of you. And that’s using clear coated, somewhat shiny speaker wire. One leg is running into a small stream at the bottom and the other strung across to the opposite bank. This is a VHF antenna, with the hot wire cut one wavelength for the VHF frequency I’m using (936/frequency= length in feet) coupled to a terminating resistor on the opposite end sloping into the ground on the other side.
The antenna itself is not for NVIS propagation at all- it does the opposite. The radiation pattern is for a strong groundwave pulled in one direction; think of a flashlight pointed in a direction and that’s very close to the radiating pattern.
On VHF this type of antenna is very easy to construct on the fly and including a terminating resistor pulls the current on the wire towards the end along with your signal. The cold end is running to the water and grounded out with an aluminum tent stake. The water itself serves as the reflector along with the cold leg of the antenna being vertical.
I built this antenna entirely on the fly out of my ruck on site just for the photos to demonstrate what can be done- the same way we do it in class. Is it perfect? Nope. Does it work? Well, the signal report on the repeater I checked into that’s quite a long ways away says so- and this transmitter spot is located at the base of two hills. That’s not always the perfect gauge of effectiveness, but it is one nonetheless. Improvements to make include soldering and sealing the crimps with shrink tubing for robustness and soldering the resistor in place. Immediate improvements would include electrical tape on all the potential points of failure.
There’s nothing quite like having the skills to build your own gear on the go and as needed. As any professional soldier knows, Small Unit skills go far, far beyond simply being a trigger puller who can flank a target. Communications is the third leg of tactical proficiency and is by far the least understood. We cover improvised antennas and much much more in the RTO Course, and I’ve got a few coming up- 14-15 July and 11-12 August. Email me at [email protected] for more details.
We’ll see you out there.