Hi,

I recently tuned up a VHF dipole, and i got
a double-dip, double resonance for the swr, and
also the minimum swr was around 1.3:1

I did some modifications, and the double
resonance was was gone, plus the swr was down
to less than 1.1:1

I'm certain that one of two things (or a
combo of both) did the trick, but i wanted to
see if any of you seasoned antenna boys could
guess what the problem was.

Thanks for your input!


Slick

On 8 Nov 2004 17:13:32 -0800, (Dr. Slick) wrote:

guess what the problem was.

Why do you think it was a problem?

73's
Richard Clark, KB7QHC

Richard Clark wrote in message . ..
On 8 Nov 2004 17:13:32 -0800, (Dr. Slick) wrote:

guess what the problem was.

Why do you think it was a problem?


A double dip is a bad sign, and your
return loss is not optimum.

S.

On 10 Nov 2004 11:36:02 -0800, (Dr. Slick) wrote:
Richard Clark wrote in message . ..
On 8 Nov 2004 17:13:32 -0800, (Dr. Slick) wrote:
guess what the problem was.
Why do you think it was a problem?
A double dip is a bad sign, and your
return loss is not optimum.

What makes a double dip a bad sign? Most antennas have many.
What does any dip have to do with non-optimal return loss? By
definition a dip in SWR indicates better return loss.

73's
Richard Clark, KB7QHC

Richard Clark wrote in message . ..
On 10 Nov 2004 11:36:02 -0800, (Dr. Slick) wrote:
Richard Clark wrote in message . ..
On 8 Nov 2004 17:13:32 -0800, (Dr. Slick) wrote:
guess what the problem was.
Why do you think it was a problem?
A double dip is a bad sign, and your
return loss is not optimum.

What makes a double dip a bad sign? Most antennas have many.
What does any dip have to do with non-optimal return loss? By
definition a dip in SWR indicates better return loss.


A double dip in a very narrow band, like 88-108 MHz
for example, is a real indication of something wrong.
It has always meant that the maximum return loss suffers.

S.

On 10 Nov 2004 20:16:40 -0800, (Dr. Slick) wrote:

A double dip in a very narrow band, like 88-108 MHz
for example, is a real indication of something wrong.
It has always meant that the maximum return loss suffers.

Well, for openers, and by your own description of a "garden variety
dipole," you don't have the prospects of anything but a narrower band
than 88-108 MHz (even if you opt to match directly to 73 Ohms). A
"garden variety dipole" centered in this band will only match 73 Ohms
from 89.5-99 MHz - it will only match 50 Ohms from 90.5-97 MHz.

As for "double dipping" none of your posts to date have any facts to
test your complaint. If we are to assume these two dips occur within
the same band, that is actually to your benefit as it could only
enlarge the matching prospects. However, your paucity of details
leaves this as speculation on both sides. If the two dips occur
within and without the band, then you have offered nothing to
distinguish this from the natural order of things. Simply put, ALL
dipoles have many dips throughout the spectrum. In this regard there
is nothing special about your "double resonance."

As for the disparaging comment of "maximum return loss suffers," that
too is in conflict with expectation. There is nothing inherently
sufferable about having more than your share of "dips." Additional
resonances does not detract from any other resonance's capacity to
perform within its region of match. A second resonance doesn't
necessarily rob another and it could be argued that it is actually a
boon if you wish to enlarge the bandwidth of an antenna (which by your
only specification of 88-108 would be a positive feature).

Now, as to HOW you could achieve TWO SWR dips within the FM broadcast
band with a "garden variety dipole," then that is revealed by your
comments about not needing (and by inference not having) your
driveline choked. Simply put, it sounds distinctly like your
transmission line length (combined with velocity factor) added a
resonant circuit in parallel with the dipole to offer this second dip.
You munged things around with the antenna, but changed lines and the
second dip went away (as a function of a different line length, or its
becoming balanced or choked). You would have to have stumbled onto an
unique antenna design to have forced these two dips into this FM band
and this is negated by your own description of a "garden variety
dipole." On the other hand, transmission line common modality is as
common as rain in Seattle.

73's
Richard Clark, KB7QHC

Dr. Slick wrote:
.. I'm certain that one of two things (or a
combo of both) did the trick, but i wanted to
see if any of you seasoned antenna boys could
guess what the problem was.

If it has two elements (sure, verticals can have two elements)
then a difference in resonance between the two elements could
easily have been the cause.
--
73, Cecil, W5DXP

I'm certain that one of two things (or a
combo of both) did the trick, but i wanted to
see if any of you seasoned antenna boys could
guess what the problem was.

_________________________________________________ ________

There is no problem. Dipoles and other antennas always show multiple
resonances. Just take an SWR analyzer and sweep a wide frequency range.
You'll find resonances all over the place.

--
Bill W6WRT


Dipoles are essentially harmonic devices, so they experience harmonic
resonances.

If you find resonances that are not harmonic on a dipole, then there are
loading objects that are producing them.

What exactly is your dipole? What is in proximity to it? How do you know that
the coax is choked properly?

73,
Chip N1IR

(Fractenna) wrote in message ...

There is no problem. Dipoles and other antennas always show multiple
resonances. Just take an SWR analyzer and sweep a wide frequency range.
You'll find resonances all over the place.

Bill W6WRT



A properly tuned and positioned dipole will
be resonant at only one frequency. Double-dips
are a bad sign, and the return loss suffers.




Dipoles are essentially harmonic devices, so they experience harmonic
resonances.

If you find resonances that are not harmonic on a dipole, then there are
loading objects that are producing them.

What exactly is your dipole? What is in proximity to it? How do you know that
the coax is choked properly?


Garden variety dipole.

But you have hit on one of the factors, i believe, which
is that initially, i was only about 4 feet off the ground.
Supposedly, dipoles need to be significantly over a 1/4 wavelength
above the ground. The second time, i had it at 6.5 feet or so.

I didn't need a choke with this one.

I'm using an MFJ-259.

What do you think was the other factor?


Slick

On 10 Nov 2004 11:42:21 -0800, (Dr. Slick) wrote:

I didn't need a choke with this one = the other factor