[ARC5] ARC-5 antenna match

[Kenneth G. Gordon]

On 29 Jul 2013 at 10:21, D C _Mac_ Macdonald wrote:

> A photo of the schematic with component values at a 
> clickable URL link would be of great help to those of us who 
> have trouble visuallizing things!

OK. I can do that. It will be somewhere on my website when I get it finished.

Here is the main URL for that:

http://www.w7ekb.com/

It is quite simple: just a coil in parallel with a capacitor, sized for the frequency range of 
interest. As I said earlier: I used a roller coil out of an ARC-5 transmitter and paralleled that 
with a two or three section BC variable cap.

Ground one end of the coil (doesn't matter which end). The coax shield (if any coax is used) 
goes here too.

Two taps to the coil near the ground end. 

One tap nearer ground for the ARC-5 transmitter, say at two turns from ground. 

The second tap "higher" up (towards the ungrounded end) for the antenna, say at 5 turns 
from ground.

The reason for connecting it this way is that USUALLY the transmitter exhibits LOWER 
impedance than the antenna, and ARC-5s always do, as far as I know.

The higher impedance goes closer to the ungrounded end.

First, tune the coil-capacitor to resonance at the frequency of interest. 

If you have placed the taps as I have stated above, then when you hit resonance, there will 
be some indication on the transmitter plate current: either a dip or a peak. But whatever it is, 
it will be quite plain. If you have a way to see power output, like a simple field-strength meter 
sitting near the antenna, that will also peak, and the tuning will be quite sharp.

Once you have peaked resonance, leave that adjustment alone and don't touch it again.

Then adjust the taps a little to get maximum power output, but adjust the antenna tap first 
rather than the transmitter tap.

MOST of the time, if you have placed your taps "judiciously" and the coil-capacitor is "about 
right", you will not gain much by adjusting the position of the taps, unless your antenna 
exhibits significantly higher than 50 ohms impedance, like if it is a short end-fed wire.

If it is an end-fed wire, depending on the length vs frequency, you will have to move the 
antenna tap closer to the ungrounded end of the coil-capacitor, i.e., toward the higher-
impedance end.

What it amounts to is you do "something", see what effect it has, then do "something" else, 
and keep doing this, until you get maximum output.

Oh. Two more things: 1) set the ARC-5 antenna coupling control to no more than 1/2 open, 
and 2) set the roller coil to about 1/3 open.

After you have the external coil-capacitor and taps set about right, you can "tweak" the output 
with the two controls in the ARC-5 transmitter. Most of the time there will be very little 
advantage to adjusting those. In any case, DO NOT use maximum coupling. If you have to 
use maximum coupling to get peak power output, then one or the other or both taps are not 
yet set at their optimum point.

Believe it or not, all of the above can be done in less than 15 minutes the first time you do it, 
and in only a few seconds after you have gotten used to how it is supposed to work.

Another advantage to doing it this way is that the external parallel-tuned circuit is a pretty 
good harmonic filter, as if any was needed.

Ken W7EKB