Archive for August, 2009

Automatic Coupler

This week I have mostly been planning and designing antennas. As you know I have no HF capabilities in the condo as for one I don’t have an HF radio and more importantly don’t have the antennas needed.

I am a bit spoiled with access to the club station but feel usage of that is much better suited to contesting and cluster spotted DX hunting. For casual local/regional QSOs a small set-up at home would be much better.

So how to get on the air you say? There are many compact antenna systems that may adequately work from my condo balcony. You have shortened coil-loaded verticals like the the Buddistick or the SuperAntenna MP1. You also have shortened coil loaded dipoles like the Buddistick or SuperAntenna YP2. At the other end of the spectrum you also have compact High-Q small loops like the MFJ-1788.

All of these antennas are suitable and would work to some degree from the balcony. However I feel paying cash up front for one kind of antenna system would limit experimentation and the fun involved in building home-brew antennas. I need something that would grow with the station. Hopefully sometime in the future I may find myself with a backyard and plenty of room for antennas, a high-Q small loop or shortened dipole would be kind of pointless then, no? I’d end up having to invest more money into an antenna system then.

I am starting to look at automatic antenna couplers like the Icom AH-4 or any of the SGC models. They would allow my HF antenna to grow if the need arose. So this week I began modelling some small wire loops in EZNEC. I have chosen wire loops as this removes the need for radials and maximises the volume of space I have on the balcony.

A 2 turn loop approximately 8ft x 8ft brings feed-point impedance within the range of most feed-point mounted automatic couplers. Most of these couplers can handle in the range of 100:1 SWR, if 1 is 50 ohms, then a range of 50 to 5000 ohms. In fact the loop I have modelled presents very favourable feed-point impedances between 40m and 12m. Anything less than 1:50 SWR is favourable, this loop presents most bands below 30:1 SWR or 1500 ohm at the feed point. Placing the coupler at the feed-point as opposed to using a tuner at the radio reduces losses in the coax due to high impedances.

Radiation patterns will be an issue and I will need to experiment more in ENZEC with feed-point positioning. But here in lies the joy of a automatic coupler. I can experiment with any kind of antenna and feed it into the coupler and it should tune it to within 3:1 SWR, hopefully!

There is nothing stopping me from designing short coil-loaded verticals, helically wound verticals, dipoles or loops of various kinds and using them all with this coupler. Think of these couplers as more of a toolkit item than a specific antenna purchase.

Purchasing a compromise antenna like the above mentioned models, means just that, a compromise antenna built specifically for either restricted or portable use. Though initially a costlier item an automatic antenna coupler is a purchase that should keep on giving long after I’ve moved out of my current QTH.