A good match?
Ampro stick antennas are great, but sometimes, a little extra is needed.
As in personal relationships, getting a good match with antennas is critical to success. But when it comes to portable ‘stick’ antennas, reaching that ‘sweet spot’ doesn’t always go according to plan.
For many years now, I’ve been using Ampro ‘stick’ antennas to great effect whilst operating along the North Wales coast. Helically wound, all but the higher HF and 6m versions have varying degrees of closely-wound, very fine wire centre or near-top loading to achieve resonance. An adjustable stainless steel whip allows for adjustment of the matching.
In fairness, matching for these simple antennas, which in my case means screwing them onto a 3-base magnetic mount, is very good. Whilst an antenna analyser will typically reveal non-ideal complex impedance, the match is always good enough to persuade a transceiver’s SWR bridge that things are fine. The worst I ever get with my sticks is about 1.2:1 with the 14MHz version; everything else reaches 1:1.
Or, it was until I got the 7MHz stick this week. The first chance I had to adjust the stick was at the side of a dark, flooded country road. It wasn’t a very good location, but I was busy with other things at the time as well. The match was pretty good for a first stab with such a short antenna in relation to wavelength - about 1.6:1.
The following evening, which was a lovely star-filled night, I set off to the beach with the car, to see if I could improve matters - and get some real QSOs!
As I might have expected, the vastly improved ground conditions around the vehicle - seawater-saturated sand - led to the resonant frequency lowering by about 30kHz, to slightly outside the 7MHz band. A quick shortening of the antenna improved matters, but only to 1.5:1 at the FT8 frequency.
Although most might simply now use an ATU to correct matters, I’m never a fan of this approach. I’m happy to use one when the mismatch is slight, but not at 1.5:1 or above. That’s just failing to think about and trying to correct what is leading to the mismatch in the first place.
Do remember that, at 7MHz, stick antennas like this are inevitably quite narrow-banded; you won’t get a similarly good match in the SSB regions of the band without adjusting something. The longer the wavelength, the narrower-banded they get.
By now, my car is a full mobile shack, with every tool, roll of PVC tape, record book and - yes - a ready-made shunt coil, I am ever likely to need. So it was simply a case of rummaging around in the back seat to find, first the coil, then the two battery crocodile clips used to attach the coil to the magmount system.
After the coil was installed - which was a ‘guesstimate’ size, based on other coils I’ve made for matching full-sized quarter-wave verticals - the matching was a perfect 1:1. The coil, incidentally, can of course be stretched or squeezed for fine matching, if needed.
A very small loss is likely in the use of a shunt coil, but this is of no real consequence when operating from such a spectacular radio location as a beach, where the environmental gain, and at angles down to much less than 1 degree, is typically in the 10-14dB range. It’s not infrequently found to be over 20dB.
So, with a perfect match and a good location, what did I get? Well, with 1W WSPR at 7MHz, I was getting to two stations in Australia, where no other British station was getting to at all at that time. The third VK station I was getting to was only seeing one other British station. The FT8 signals, of course at a higher, 15W output for this fairly wide signal, were getting to much the same parts of the world.
So there’s little doubt that, when it comes to a good match, a shunt coil is a good friend, and the results I obtained at 7MHz are a testament to that!