[ARC5] ARC-5 antenna match

[Kenneth G. Gordon]

On 28 Jul 2013 at 19:49, neilb at ihug.co.nz wrote:

> An antenna tuner can match a 50 ohm transmitter to a wide range
> of antenna impedances.
> I got to wondering whether one could be connected backwards to
> match the unknown output impedance of an ARC-5 transmitter to
> a 50 ohm antenna.

Of course it can. I used this method many years ago to build what one could, 
I suppose, call a very simple antenna coupler.

My reasoning at the time was that a parallel-tuned circuit with one end 
grounded is essentially a wide-range impedance auto-transformer, with the 
high-impedance end being at the "top", i.e., the end opposite "ground".

Therefore, one should be able to find a point on the coil, somewhere, that 
matched fairly closely any impedance between "high" and "zero".

Tapping the transmitter in at some point and the antenna in at another point 
would then result in a proper "match" to both.

In practice this worked very well, although it took some experimentation to 
get the best results.

Using a coil with as high "Q" (read, "lowest possible ohmic resistance") as 
possible, in combination with a fairly high value of tuning capacitor of a good 
design would result in an "antenna coupler" of fairly wide range, covering 
something like a 2:1 frequency range, at least.

My favorite such unit used a roller coil from a defunct Command transmitter, 
and an old, but very well-built, BC tuning capacitor. Taps would always be 
made between the "roller" and the ground end, the "roller" determining the 
basic frequency range over which the "coupler" would be used.

I also sometimes used a simple two-turn link around the "ground end" of the 
roller coil in lieu of a tap for the antenna IFF I was using a 50 ohm antenna. 
Otherwise, a second tap was used for the antenna.

YMMV.

Ken W7EKB