Topband: Efficiently Matching Low-Impedance Antennas to 50ohms

Mauri i4jmy at iol.it
Thu Sep 22 11:07:18 EDT 2005


> I got curious (always dangerous) about how much power
> was actually making it through the tuner, so I used an
> RF ammeter to measure this.  On the input side, the
> Bird wattmeter and  the RF ammeter were within 5w of
> each other, so that gave me some confidence about the
> approach.  I moved the RF ammeter to the antenna side
> of the tuner and got a real surprise.  The
> measurements indicated that there was about 3dB of
> loss in the tuner.  Wow!

Assuming the ammeter is based on a thermocouple, your measurement has sense, 
even if the impedance is other than 50 Ohms and reactive. The same 
doesn'apply to bird wattmeter, which doesn't measure power directly and 
whose accuracy is affected by loads different from 50 Ohms, and reactive.

> One thought I had was to get a 2:1 or 4:1 UN-UN from
> Amidon and step up the impedance to a value that's
> easier for the transmatch to handle.

A coreless step up transformer, a grounded coil of proper size, with 2 taps, 
will work fine, expecially considering you have a capacitive reactance.

Have a good time & Dx,

73,
Mauri I4JMY 




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