[ARC5] QST articles.

[Kenneth G. Gordon]

On 14 Dec 2008 at 15:37, Ian Wilson wrote:

> Not if you want any decent power output. You need to transform the
> real part of the antenna down to a few ohm to match the optimum load
> that the ARC-5 TX wants to see.

Correct. Furthermore, by doing as Bob suggests, and as 
many other hams did, you also completely defeat any 
harmonic suppression built into the rig.

> The best way to do this on 40m I have found is to use a parallel
> capacitor of around 1200pF.

Yes. That WILL work.

However, the very BEST method, if you wish to feed a 50 
ohm load (like a modern antenna coupler) is to use a three-
element "Tee" matching section.

The left arm of the Tee being part of the normal ARC-5 
loading coil, the middle  (vertical) arm being a capacitor to 
ground from the antenna connector, and the remaining 
(right) arm of the Tee being another capacitor in series with 
the antenna connector and the antenna.

Values can be easily calculated using Zin of 5 ohms, and 
Zout of 50 ohms, or a 1:10 transformation.

I will post the necessary formulae asap.

As I remember what I recently read concerning this, on 80 
meters, the inducance end required about 7 turns on the 
normal ARC-5 loading coil, a 680 PFD cap to ground from 
the antenna connector, and a 365 PFD cap to the antenna.

But don't hold me to those values. My memory isn't that 
good any more. (Obviously!)

I only mention those to illustrate the range of values 
expected.

Then you set the link to about 1/2 open.

Power output was somewhat over 30 watts at 450 VDC on 
the plates of the 1625s.

Ian's method will work fairly well, and is, in my opinion, a 
version of the Tee matching network, the series 
capacitance of the antenna comprising the third leg.

HIs method would probably work best with an end-fed wire 
antenna, or an antenna that exhibited more than 50 ohms 
impedance.

Ken Gordon W7EKB