I'm trying to make a body whip antenna for some friends of mine running at 60 Mhz at 10 W power. We are in the military, and we are trying to make an antenna that can be routed through our gear to prevent a massive bulky antenna from hitting us in the face. The downside of this, is the antenna is relatively close to the ground at shoulder height, so it wont have the same range as an antenna that extends about 1-2 feet over the shoulder. Usually I would use a RG58 cable as an antenna base material, but its flimsy and if left with no support will just fold onto itself. I was hoping Nitinol wire might be good to use because of the shape retention attributes that will keep it up in the air. Anyone with any experience with Nitinol wire have any advice?
A very general rambling answer that might help a bit. Body-worn and low-profile antennas have a long and unhappy history - they generally don't work very well. The main factors are that:
- the body is not a great conductor, so it absorbs some of the radio power
- the person and equipment is usually in the way of the radiation, so it is directed more in some directions than in others
- the configuration is not stable, so the resonant frequency and other properties of the antenna are always changing. Either this requires re-tuning (example automatic tuned antenna) for different situations, or it requires a low performance broadband antenna that doesn't change performance much when it's moved around.
So I'd always recommend getting the antenna as high up as possible. At 60 MHz, you should ideally have 3-4 feet of whip antenna, shortening that by half will cost you maybe half the range.
Antennas are always first of all a mechanical challenge. I've spent years working on ways to make them flexible and impact resistant, but stand up straight. They can either be flexible over the whole length, usually a fairly thin stainless-steel whip or a "tape measure" antenna; or they can have some sort of base-mounted spring or break mechanism that allows them to fold or hinge at the base. I've made both for manpack radios - springs store a lot of energy so the thing tends to whip back and forth and hit you on the helmet when going through a doorway. Break mechanisms (also seen on some road island chevron/reflector poles) have cables and springs; they're heavier but can respond better.
For the whip itself - we'd always buy some pre-existing metal whip, actual tape measures, or a fibreglass tube, not wanting to get into the metallurgy ourselves. There are a lot of SS whips on the market, you can choose a diameter and taper to suit you. You could also use something made of plastic, wood, carbon fibre etc, with the antenna wire taped to it.
The Nitinol itself doesn't seem to offer any big mechanical advantage, unless you plan to somehow re-heat it. If you find it does - by all means use it. A small caution though - it might be much less conductive than steel or copper, and further reduce the transmitted power. It's hard to find good data but it seems to be about 100x less conductive than copper, which puts it close to generic stainless steel, so probably not a big deal.
Last thought - we are in an amazing new age where anyone can own a network analyser - for less than \$100! So if you want to experiment, get a NanoVNA, almost any one will work for 60 MHz, and a handful of cables and connectors from amazon etc, and start trying things. You can use regular stainless steel wire first for electrical tests. Look for a clear dip in the VSWR near your frequency, that moves around when you walk close to a wall or metal object. Once the antenna is somewhat tuned for your frequency, you can set up simple tests with a \$20 SDR and a laptop, to compare the transmitting performance of various different antennas. [but ask a new question about that]