In one of their older books, ARRL featured a helical which had approximately
1/2 wavelength of wire on the form. This article said the antenna had a low
impedance so the project featured an L-network tuner to step 50 ohms down to
the lower Z of the antenna. Why the matching network was needed I don't
know... many mobile helical and homemade helical antenna don't seem to need
the impedance stepdown, and if they do, why not make a simple broadband
ferrite RF transformer with taps?
The 1/2 wavelength of wire in some helical designs does not give you the
performance of a 1/2 wave vertical, it's just that 1/2 wavelength and other
lengths give an impedance close to 50 ohms. The antenna will perform like
what it is, a short 1/4 wave vertical. The tradeoffs for short physical
length are a little less efficiency and a narrower SWR bandwidth.... you're
probably talking 15 Khz on 160 meters, 30 khz on 80 meters.
In a basic experiment, I've tried winding a helical for a low band (40 or 80)
and then feeding it with a simple L-network tuner (impedance stepup type) to
get it to work on all higher bands. I don't think this worked too well,
perhaps the helical winding was acting as an RF choke on some higher
frequencies? Has anyone tried this?
I've noticed that some helicals may have two resonant points, the fundamental
and approximately a third harmonic.
I've only briefly tried tapping a helical. This idea is very similar to the
"Outbacker" antenna, a helical mobile whip with taps for Ham and (optional)
marine bands. The user shorts the unused winding with a jumper and banana
plug, fine tuning is done by a steel rod on the top of the antenna.
For those building an 80 or 160 meter helical, it might be worthwhile to
experiment with taps on the windings for low, mid and high portions of the
lower bands. You probably won't need multiple taps for the higher bands.
I've found that helicals, like all short verticals need a good RF ground. If
you build a helical over one RF ground system and then move it to a different
RF ground you may have to change the taps on the winding.
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