If the total length of radiator on each band was a total of 1/4 wavelength,
it would be just like a fan dipole. People feed these with one feed line
easily and they can be configured to be very broad by having the fan
resonant at various places in the 160 and 80 meter bands. (and of course on
other bands as well - but this question is talking about Inverted-L's so I
comment only on the lower two bands)
I assume you're talking about a total of 1/4 wave of wire which wouldn't
normally require an antenna tuner as it is a low impedance current fed
For illustration speak about 160 meters and 80 meters.
A 1/4 wave of wire on 160 meters bent into an Inverted-L will be low
impedance, current fed antenna - a good match for 50 ohm coax having a 30
ohm resistive component and very low reactance. On 80 meters this wire has
a low resistance component but is very reactive.
On the second harmonic of 160 meters (80 meters) that beomes a high
impedance voltage fed antenna. If an additional 1/4 wave on 80 meters of
wire was used in parallel, that wire would be low impedance on 80 meters.
I've used a four wire two band antenna in this configuration - with it I was
able to cover the two sections of 160 meters I was interested in, and the
entire 80 meter band. 80 meters was the more difficult to adjust. I had to
make one wire resonant about 3550 kc/s and the other wire around 3875 kc/s
to cover the entire band with low SWR. I needed to cover the low DX end of
3.520 kc/s and I needed to get into the area CW net at 3658 kc/s and I
needed a good 75 meter antenna. I had to adjust the longer wire to a higher
frequency than I really wanted, and the shorter wire to a lower frequency I
really wanted to to get acceptable "whole band" coverage.
One thing I didn't appreciate is that the bandwidth of an antenna becomes
broader as the frequency gets higher. When you think in terms of wavelength
you immediately see this. Chart out a graph of frequency versus wavelength
for 80 meters and you wll see how a larger change in frequency on 75 meter
SSB results in less wavelength change that the same change in frequency on
80 meters. Somehow I didn't understand that wavelength had anything to do
with antenna length!
Duh, and I got a ham license? No wonders they call us amateurs.
David Ring, N1EA
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