MGFoster wrote:
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Has anybody had any experience using a helical antenna for
transmissions. I put a small helical antenna (65' 14# wire, 172 turns
on 1-3/8" dowel) on my antenna tuner: on 40 meters (7MHz) the SWR was
about 8:1; on 15 meters (21MHz) the SWR was 1.6:1). I thought the 40M
would tune better than the 15M because 65' is about 40M's 1/2 wl.

My antenna tuner manual suggested a balun to tune in any band that
doesn't initially tune-up. I'm thinking the ubiquitous 4:1 balun.

Suggestions, comments?

Tnx,

Only a couple of comments.

1. Winding a half wavelength of wire doesn't make an antenna which acts
like a half wavelength antenna. Turn-to-turn coupling makes the total
wire length a lot less important than other factors like turn diameter
and spacing.
2. My experience with using helical antennas is very limited, so other
folks can much better tell you what combinations might provide you with
a good match. But a helical antenna that's physically quite a bit
shorter than a half wavelength will have a low radiation resistance. So
if you get a good match without a tuner, it means that there's enough
loss to raise the feedpoint resistance to 50 ohms. Another good
indication of loss is the bandwidth. If it has low loss, the bandwidth
will be very narrow; a wide bandwidth means more loss. Most people,
though, run at least several times the power needed to communicate, so
trading some efficiency for bandwidth, easy matching, and compact size
might be a worthwhile exchange in a lot of cases.

If the loss is low, the feedpoint resistance at resonance would be
considerably less than 50 ohms, so a 4:1 balun, with the high impedance
side toward the tuner, might make things easier for the tuner.

Roy Lewallen, W7EL