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ZL1ANM > HELP 04.04.07 01:33l 71 Lines 2738 Bytes #999 (0) @ WW
BID : ZL1ANM68
Read: GUEST
Subj: Re: Info, 1/2 Wave End Fed
Path: DB0FHN<DB0MRW<OK0PPL<DB0RES<ON0AR<ZL2BAU<ZL1AB
Sent: 070403/2324Z @:ZL1AB.#06.NZL.OC #:58516 [AUCKLAND] FBB7.00i $:ZL1ANM68
From: ZL1ANM@ZL1AB.#06.NZL.OC
To : HELP@WW
>> |<---------------1/2 wave---------------->|<-----1/4 wave----->|
>>
>> C B F A
>> _______________________________________________________________
>> _____________________|
>>
>> F
>>
>>
>>
>From: VK2TV@VK2TV.#MNC.NSW.AUS.OC
>
>Actually, if you take the two-wire feed portion of the J-pole away at 90ø
>to the 1/2 wavelength section, the antenna can, and does, find horizontal
>application.
>
>Such a feed method for an end-fed dipole is to be found in the literature
>as the Zeppelin or Zepp feed.
The Zepp was originally a half-wave horizontal radiator, end-fed by
a quarter-wave transmission line of presumably 600-ohms, and was
developed for the Zeppelin airships of the 20's and 30's. With time,
the term "Zepp" came to mean a resonant (any multiple of 1/4 wave)
length of horizontal antenna, end-fed by two-wire transmission line.
However, the horizontal J-pole that I described differs from the
Zepp in two respects:
The quarter-wave matching stub AB is not part of a transmission
line (the term "transmission line" implies that the line's
dimensions are chosen to be some particular impedance, such as
600 ohms or 300 ohms). The stub AB is uncritical in respect of its
spacing and wire diameters. Its only critical dimension is its
length, 1/4 wave, so it's not a true transmission line, nor is it
fed as such.
The second point of difference is that the end at A is closed by a
short-circuit and the feed point is tapped along AB at the point
marked FF in order to match the desired transmission line, which
may be 600, 300, 75, 50 ohms, or any other similar impedance.
>The term is applied nowadays to any resonant antenna fed at the end by a
>two-wire transmission line although it originally applied to end-feeding a
>quarter wave antenna.
No, it applied originally to feeding a half-wave suspended antenna
on the airships.
>Keep in mind that the shorted, two wire feed (a stub), with the main
>transmission line being "tapped" up the stub is nothing more than a
>matching network of inductance and capacitance, the same thing that can be
>achieved with discrete components ... provided you can calculate them.
>Perhaps empirical methods are more practical!
>
>Cheers ... Ray
True, but inductors introduce significant losses (a real problem for
QRP operation), while capacitors unless adequately rated (and expensive)
are prone to break down under mismatch conditions, especially if
exposed to the weather.
73 de Neil ZL1ANM
T4 1.5à24
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