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I0OJJ > ANTENN 01.07.10 23:42l 45 Lines 2126 Bytes #-5797 (0) @ WW
BID : 11344I0OJJ
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Subj: Re: language and antenna gain
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Richard Fry wrote:
> On Jul 1, 3:18 pm, Roy Lewallen <w...@eznec.com> wrote:
>> Guess I just don't understand how a resonant, helically-wound antenna is
>> "electrically short". Suppose you helically wound an eighth-wave
>> vertical in such a way that it was resonant. Its physical length is an
>> eighth wavelength. What would its "electrical length" be?
>
> Its overall height in free space wavelengths. This is the definition
> used by the FCC for the unloaded monopoles used in AM broadcasting
Oh, then the "electrical length" is the same as "physical length". Why
not just say "physical length" then?
> Do you expect your 1/8 WL high, self-resonant helical to have the same
> electrical length and feedpoint parameters as a self-resonant,
> straight radiator about 1/4 wave high in free space wavelengths?
You just said that the height of the antenna is the electrical length.
So no, I wouldn't expect two antennas of different heights to have the
same electrical length, using your definition.
> According to Kraus and other authors, your example above still has
> about the same radiation resistance as a 1/8 WL straight conductor --
> not that of a self-resonant, straight conductor about 1/4 WL high.
It'll be a little higher than a straight 1/8 wave conductor due to a bit
more favorable current distribution (see the article referenced in my
next paragraph for some actual measurements). But it'll surely not be as
high as that of a quarter wavelength conductor. Which of course is the
reason it's so important to minimize loss if you want good efficiency.
> An 1/8 WL high helical may be resonant, but it will not perform the
> same in a practical antenna system as a straight, self-resonant
> vertical whose physical height in free space wavelengths is about 1/4
> wave.
Depends. If the ground system is very good and you're willing to keep to
a narrow bandwidth without retuning, you wouldn't be able to tell any
practical difference between the two. For good experimental evidence see
"The W2FMI Ground-Mounted Short Vertical" by Jerry Sevick, W2FMI in
March 1973 QST.
Roy Lewallen, W7EL
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