That's pretty much been my impression of this antenna. It generally
gets rave reviews on eHam.net (doesn't everything?), but all you have to
do is look at it to see a few obvious things:
1. Three feet of linear element is going to have a VERY low radiation
resistance.
2. The spiral ends equate to lots of inductance and lots of distributed
capacitance, none of which radiates effectively.
3. Direct coax feed without a balun or shield choke is a dead give-away
for dependency upon feedline radiation.
It's merely an expensive tank circuit at the end of a radiating
feedline. It would be interesting to see someone who owns one replace
it with a traditional coil and capacitor and compare the performance, or
as you say simply test it with and without a good common mode choke.
Dave AB7E
K4SAV wrote:
> I recently got some real dimensions and rebuilt my model for this
> antenna. This antenna is supposed to cover 40 meters thru 10 meters by
> selecting taps on the coils.
>
> Radiation resistance of the antenna on 40 meters without the feedline is
> about 0.23 ohms. Feedpoint impedance on 40 meters in free space without
> a feedline or any kind of matching network is about 3.2 ohms.
>
> When you attach the feedline things change a lot. With the proper taps
> on the coils and with the right length feedline you can get a very broad
> low SWR curve across the band (40 meters). With the antenna at 20 feet,
> the gain is about -2.4 dBi with a take-off angle of 30 degrees. The
> pattern is all vertically polarized (all feedline radiation). Notice
> that this antenna has no common mode choke and is fed directly with
> coax. When adding a good choke to remove the feedline radiation, the
> gain goes to -9.5 dBi with a take-off angle of 90 degrees. Some of this
> gain reduction is due to feedline loss because removing the feedline
> radiation also increases the SWR. In this case since I only had 40 feet
> of RG8X the coax loss was only 1.7 dB. If the feedline had been 100 ft,
> the gain would have gone to -12 dBi or 4.2 dB of coax loss.
>
> I expected to see the antenna do a little radiating on the higher bands,
> but some unusual things happened. Depending on feedline length and the
> taps you select, you can direct a lot of current down the feedline or
> not. In some cases with a lot of current down the feedline the take-off
> angle was very high and low angle gain was low. In other cases with
> different feedline lengths, the gain at low angles was reasonable (-2 to
> -4 dBi). You can't determine which condition you have by looking at the
> SWR. Playing with various combinations on 15 meters, the best gain I
> ever got was -2 dBi and the worst was about -11 dBi.
>
> Summary: On 40 meters, the antenna acts as a variable unbalanced load
> that injects large common mode currents onto the feedline. The feedline
> is the major radiator. The thing on the top called the "antenna" is
> used to fine tune the resonant frequency and SWR. On the higher bands,
> performance will be very unpredictable and can vary widely with each
> installation. In some cases it may approach the performance of a
> no-radial vertical.
>
> Or so says EZNEC.
>
> Jerry, K4SAV
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