Richard Fry wrote:
On Jul 1, 3:18 pm, Roy Lewallen 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