Topband: Efficiently Matching Low-Impedance Antennas to 50ohms

[Paul Elliot]

>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. 

I have a similar antenna, though probably not quite as long as yours (I have
never measured the length).  I have been under the impression, from reading
the literature, that the feedpoint had an inductive component--I have never
measured it.  Using a variable capacitor in series with the antenna at the
feedpoint, by adjusting the length of the antenna and the value of the
series capacitor, I obtained, after several iterations, an SWR of 1:1 at
1827 kHz.  I estimate the value of the series capacitance as approximately
170 pf.  Bandwidth for an SWR of less than 1.5:1 is about 35 kHz.  Since the
capacitor never gets warm under load, I don't think that I am losing much
power there.  People with more knowledge and skill in modeling than I have
can  probably tell from the numbers above whether my ground system is any
good--but it is the best that I can do on a small city lot. 

The use of an antenna longer than one quarter of a wavelength in conjunction
with a series capacitor has been around a long time (W1BB?).

73 Paul W5DM