RadioBanter - View Single Post - DOUBLE RESONANCE IN DIPOLE...THE CAUSE?????

November 11th 04, 09:00 PM

On 11 Nov 2004 12:11:26 -0800, (Dr. Slick) wrote:

Double dips (or even triple and more) certainly
at harmonics of the fundamental, certainly.

88.1 and 92 aren't exactly harmonically related!

Never said they were, and until these recent posts, we all had to
guess. And besides, not all antennas are harmonically resonant.

A broadband antenna usually doesn't have as good a
match as a dedicated antenna.

You have too little exposure to the world of antennas to paint them
with that broad brush. A simple example is a discone antenna which is
a variant of the biconical antenna, which is the genesis of ALL
dipoles. Either the discone or the biconical display a very wide
bandwidth (octaves) and are eminently matchable by definition.

This is why when i
had two dips, the min. SWR was NOT as good as when i
had only one resonant (not incuding harmonics) freq.

You have yet to disclose what SWRs were present to make this a
problem.

Again, didn't need a choke for this one.

You have yet to show that it is not your problem, much less the lack
of need (which is a strict requirement for tuning). "Not needing" it
can be accomplished through one of two means:
Luck,
or
a hardwired solution (the customer, like with CB whips, cannot
vary the length without causing a major shift in dynamics).
Either way, the two are probably the same solution, an even halfwave
multiple length line. The longer the line, the more tenuous the
solution.

Someone infered the first problem, not high
above off the ground.

Which will broaden the response (lower the SWR) if too close.
Proximity to ground will shift resonance too, but not add resonances.

Your problem was feedline related. You simply (and without taking
note of it) changed that along with the melange of other activity and
pushed the "problem" up/down the spectrum. The addition of the other
dip is harmonically related to a structural issue that has a physical
dimension related to the wavelength and velocity factor. There is no
other way to accomplish this with "a garden variety dipole" except
with a short transmission line to a SWR tester (I will bet your test
cable was a generic 3 foot coax). Any coax line that is as long as an
odd multiple quarter wavelength (1/4, 3/4, 5/4...) of the unusual dip
will do the same thing if it is not snubbed at the drivepoint.

73's
Richard Clark, KB7QHC