Re: grounding and counterpoise - Zepp

[Mike Morrow-3]

Ron wrote:

>... the most famous "aircraft" antenna is the Zeppelin antenna...
>balanced feeders from the transmitter to the END of a 1/2 wave
>antenna. At the antenna, one feeder connected to the end of the
>radiator and the other feeder simply stopped at an insulator...

>Most Ham "Zepps" today are not like the original Zeppelin antenna

The common VHF/UHF "J-pole" antenna is the most commonly encountered Zepp
antenna used by hams today.  The J-pole has every characteristic of the HF
wire version, except that it is usually stiff and vertical, and the coax
section is usually attached mid-section in the matching section.

Moxon's "HF Antennas For All Locations" book, pages 47 to 49, has some
interesting and convincing observations on the Zepp which conclude that it
should NOT work either theoretically or practically as it is typically
depicted, were it not for the effect of stray capacitances to ground and
between conductors in the matching section.

73,
Mike / KK5F

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[Ron D'Eau Claire-2]

Mike, KK5F wrote:
Moxon's "HF Antennas For All Locations" book, pages 47 to 49, has some
interesting and convincing observations on the Zepp which conclude that it
should NOT work either theoretically or practically as it is typically
depicted, were it not for the effect of stray capacitances to ground and
between conductors in the matching section.

------------------------------
Moxon makes a number of odd statements about what people believe is true
that can be confusing. Reading his book carefully, I came to the conclusion
they are the result of his not distinguishing between "Ham lore" and
engineering data. For example, when speaking of an HF antenna he says that
"antenna current used to be regarded as a good indication of antenna
efficiency..." He uses that to indicate the confusion over how a 1/4 wave
section of transmission line works, as on the classic Zeppelin antenna.

More current means more efficiency was correct when antennas were worked
against ground and were less than 1/4 wave long, such as when all Ham
operations were around 200 meters, but I've never heard anyone knowledgeable
say that more current meant greater efficiency at the frequencies where the
Zeppelin antenna was used. It can be true under certain very specific
conditions, but that text is the first time I ever saw someone claim that it
might be generally true at HF.

Perhaps Moxon is speaking to some misconceptions that British hams held that
never made it across the Atlantic <G>. He never explains who thought that
antenna current was regarded as an indication of efficiency.

The Zeppelin feeder arrangement is simply a 1/4 wave transmission line,
such as is used in a great many antenna designs.

The J-pole is a good example as you point out, although that also includes a
matching stub to work with 50 ohm coax.  

The degree of balance in the Zeppelin antenna is not perfect, but it's good.
There is leakage at the insulator at the un-connected end of the feeder that
allows some current to flow and, of course, the impedance at the end of the
1/2 wave wire is not infinite, so some current flows there too. The exact
amounts and their phase relationship is not perfect, but perfect balance is
never achieved.


Ron AC7AC


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