Topband: Efficiently Matching Low-Impedance Antennas to 50ohms

[Ron Majewski]

Hi All,

I'm looking for advice about how to efficiently match
my 160m inverted-L to 50 ohms.

Here's what's motivating this question:

My 160m inv-L is about 60ft tall with a 100ft long
horizontal section.  I have about 50 ground radials of
varying lengths spread under it.  My receiver noise
bridge shows that its impedance has a resistive
component that's around 20 ohms, and it has 
a small capacitive reactance.  Depending upon which
book you read, the inv-L should be around 15 ohms
resistive.  My noise bridge doesn't have sufficient
resolution when the impedances are that low, so I went
with the value of 20ohms.

To match this antenna to 50ohms, I installed an
MFJ-989C tuner at the feedpoint.  A match was readily
obtained, life was good, and I've been running this
way for a number of years.

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!

I borrowed a TenTec 238B tuner and replaced the MFJ. 
I found a match point to 50 ohms and then measured the
current on the antenna side again.  The news was
better -- only about 1.5dB of loss.

A homebrew tuner I have came in with about 1.8dB of
loss.

Field strength measurements I made with a coax loop in
the near field are in agreement with the ammeter
measurements.  That is, the field strength with the
TenTec tuner was about 2dB stronger than with the MFJ
tuner.

I can certainly get a TenTec 238, but I'm left
wondering if maybe there is another approach.  I
certainly don't want to heat the backyard all winter, 
especially if it means missing good DX.   :)

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.

I'd appreciate any advice that anyone can offer.

Thanks and 73,

Ron (W8RU).