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
David Jordan wrote:
>Hi Folks,
>
>
>
>Looking for comments regarding questions about the Pan Cake dipole antenna:
>
>
>
>What is the theory behind the antenna configuration?
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>Has anyone attempted to model the antenna?
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>
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>What is the expected input impedance at resonance?
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>Is there a difference between actual and advertised SWR 2:1 bandwidth?
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>Has anyone confirmed SWR bandwidth claimed versus actual?
>
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>Is there a defined radiation pattern or is it just a BLOB pattern?
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>73,
>
>Dave
>
>Wa3gin
>
>
>
>
>
>
>
>_______________________________________________
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>
>
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>
>
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