Discussion:
DOUBLE RESONANCE IN DIPOLE...THE CAUSE?????
(too old to reply)
Dr. Slick
2004-11-09 01:13:32 UTC
Permalink
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
Richard Clark
2004-11-09 01:23:43 UTC
Permalink
Post by Dr. Slick
guess what the problem was.
Why do you think it was a problem?

73's
Richard Clark, KB7QHC
Dr. Slick
2004-11-10 19:36:02 UTC
Permalink
Post by Richard Clark
Post by Dr. Slick
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.
Richard Clark
2004-11-10 19:40:16 UTC
Permalink
Post by Dr. Slick
Post by Richard Clark
Post by Dr. Slick
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
Dr. Slick
2004-11-11 04:16:40 UTC
Permalink
Post by Richard Clark
Post by Dr. Slick
Post by Richard Clark
Post by Dr. Slick
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.
Richard Clark
2004-11-11 04:39:43 UTC
Permalink
Post by Dr. Slick
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
2004-11-11 20:11:26 UTC
Permalink
Post by Richard Clark
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."
Double dips (or even triple and more) certainly
at harmonics of the fundamental, certainly.

88.1 and 92 aren't exactly harmonically related!
Post by Richard Clark
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).
A broadband antenna usually doesn't have as good a
match as a dedicated antenna. 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.
Post by Richard Clark
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.
Again, didn't need a choke for this one.

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

Anyone else?


S.
Richard Clark
2004-11-11 21:00:17 UTC
Permalink
Post by Dr. Slick
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.
Post by Dr. Slick
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.
Post by Dr. Slick
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.
Post by Dr. Slick
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.
Post by Dr. Slick
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
Dr. Slick
2004-11-12 14:07:50 UTC
Permalink
Post by Richard Clark
Post by Dr. Slick
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.
I'll bet you money that a well designed, dedicated, tunable
antenna can achieve a better SWR than any discone.

There are broadband folded dipoles for VHF,
but you don't get something for nothing! They may
be adequate across the band, but nothing beats
a dedicated antenna.
Post by Richard Clark
Post by Dr. Slick
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
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.
Neither. We have varied the coax length, certainly not
keeping to multiples of 1/2 wavelength, with results
remaining the same. We just don't need it with our
design.
Post by Richard Clark
Post by Dr. Slick
Someone infered the first problem, not high
above off the ground.
So what was the second thing i did?


S.
Cecil Moore
2004-11-12 15:30:48 UTC
Permalink
Post by Dr. Slick
There are broadband folded dipoles for VHF,
but you don't get something for nothing! They may
be adequate across the band, but nothing beats
a dedicated antenna.
Here's something for virtually nothing. Take a
commercial FM folded dipole, vary the feedline length,
and you have a tunable folded dipole peaking at your
frequency of choice. The tuning can be done by mounting
two telescoping antenna sections side-by-side.
--
73, Cecil, W5DXP
Richard Clark
2004-11-12 15:37:13 UTC
Permalink
Post by Dr. Slick
I'll bet you money that a well designed, dedicated, tunable
antenna can achieve a better SWR than any discone.
I don't want your money (nor your ganja). Time and tide won't wait
for you to offer an example either. :-)

73's
Richard Clark, KB7QHC
Dr. Slick
2004-11-12 20:16:27 UTC
Permalink
Post by Richard Clark
Post by Dr. Slick
I'll bet you money that a well designed, dedicated, tunable
antenna can achieve a better SWR than any discone.
I don't want your money (nor your ganja). Time and tide won't wait
for you to offer an example either. :-)
Don't take my word for it...go measure
and compare for yourself...


S.
Richard Clark
2004-11-12 20:37:28 UTC
Permalink
Post by Dr. Slick
Don't take my word for it
Never entered my mind to do that.
Post by Dr. Slick
...go measure and compare for yourself...
Been there, done that. Time and tide are not still for the
expectation of ganja-man's example. :-)

73's
Richard Clark, KB7QHC
Dr. Slick
2004-11-13 23:36:02 UTC
Permalink
Post by Richard Clark
Post by Dr. Slick
Don't take my word for it
Never entered my mind to do that.
Post by Dr. Slick
...go measure and compare for yourself...
Been there, done that. Time and tide are not still for the
expectation of ganja-man's example. :-)
Like i said, i'm willing to bet money.

But your stubborness and inability
to receive new information has kept you
ignorant this long, so why should you
ever change? Stay the way you are
and always have been...


Slick
Richard Clark
2004-11-14 00:05:35 UTC
Permalink
But your stubborness and inability to receive new information
What new information? Time and tide washes over empty claims. :-)

Quit tokin' on your spliff and name a simple "marijuana garden variety
dipole" antenna that outperforms a broadband
Discone
Biconical Dipole
Rhombic
Log Periodic Dipole

73's
Richard Clark, KB7QHC
Dr. Slick
2004-11-14 12:10:05 UTC
Permalink
Post by Richard Clark
But your stubborness and inability to receive new information
What new information? Time and tide washes over empty claims. :-)
So the time and tide will wash over your
empty brain too!
Post by Richard Clark
Quit tokin' on your spliff and name a simple "marijuana garden variety
dipole" antenna that outperforms a broadband
Discone
Biconical Dipole
Rhombic
Log Periodic Dipole
Don't need to. "Jack of all trades and master of
none" applies to broadband amplifiers AND antennas as
well.


S.
Richard Clark
2004-11-14 17:10:27 UTC
Permalink
Post by Dr. Slick
Don't need to. "Jack of all trades and master of
none"
How apropos.

Are you still enjoying the legs (stumps actually) on this Lee? Time
and tide wasn't waiting for a response to your posts either. Perhaps
you might attract an answer if you put it in the format of 20
questions:
Is it bigger than a breadbox? :-)

73's
Richard Clark, KB7QHC
Richard Harrison
2004-11-12 16:29:51 UTC
Permalink
Dr. Slick wrote:
"They (terminated folded dipoles) may be adequate across the band, but
nothing beats a dedicated antenna."

A rhombic will clean your clock.

Best regards, Richard Harrison, KB5WZI
Cecil Moore
2004-11-09 03:08:40 UTC
Permalink
Dr. Slick wrote:
.> I'm certain that one of two things (or a
Post by Dr. Slick
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
Bill Turner
2004-11-09 15:09:40 UTC
Permalink
Post by Dr. Slick
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
Fractenna
2004-11-09 16:08:48 UTC
Permalink
Post by Bill Turner
Post by Dr. Slick
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
Dr. Slick
2004-11-10 19:42:21 UTC
Permalink
Post by Fractenna
Post by Bill Turner
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.
Post by Fractenna
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
Richard Clark
2004-11-10 20:02:03 UTC
Permalink
I didn't need a choke with this one = the other factor
Dr. Slick
2004-11-11 04:17:36 UTC
Permalink
I didn't need a choke with this one = the other factor
Incorrect. I simply didn't need it for this dipole.

Anyone else?


S.
Cecil Moore
2004-11-10 21:47:48 UTC
Permalink
Post by Dr. Slick
A properly tuned and positioned dipole will
be resonant at only one frequency. Double-dips
are a bad sign, and the return loss suffers.
If something is wrong with a dip on 40m and a dip on 15m,
someone should warn all the hams who are using their 40m
dipoles on 15m. :-)
--
73, Cecil, W5DXP
Cecil Moore
2004-11-10 22:05:27 UTC
Permalink
Post by Cecil Moore
Post by Dr. Slick
A properly tuned and positioned dipole will
be resonant at only one frequency. Double-dips
are a bad sign, and the return loss suffers.
If something is wrong with a dip on 40m and a dip on 15m,
someone should warn all the hams who are using their 40m
dipoles on 15m. :-)
Forgot to add that dipoles are resonant near all odd
half-wavelengths, i.e. 0.5WL, 1.5WL, 2.5WL, 3.5WL, ...
They are also antiresonant (purely resistive) on all
integral wavelengths, 1.0, 2.0, 3.0, ...

Using a Smith Chart, if one plots the feedpoint impedances
of a dipole VS frequency and connects the dots, that locus
of points will describe a (very rough) spiral. (Traversing
once around that rough spiral from resonance to resonance
is one full wavelength, not 1/2 wavelength.)
--
73, Cecil, W5DXP
Irv Finkleman
2004-11-11 02:23:59 UTC
Permalink
Post by Cecil Moore
Post by Cecil Moore
Post by Dr. Slick
A properly tuned and positioned dipole will
be resonant at only one frequency. Double-dips
are a bad sign, and the return loss suffers.
If something is wrong with a dip on 40m and a dip on 15m,
someone should warn all the hams who are using their 40m
dipoles on 15m. :-)
Forgot to add that dipoles are resonant near all odd
half-wavelengths, i.e. 0.5WL, 1.5WL, 2.5WL, 3.5WL, ...
They are also antiresonant (purely resistive) on all
integral wavelengths, 1.0, 2.0, 3.0, ...
Using a Smith Chart, if one plots the feedpoint impedances
of a dipole VS frequency and connects the dots, that locus
of points will describe a (very rough) spiral. (Traversing
once around that rough spiral from resonance to resonance
is one full wavelength, not 1/2 wavelength.)
--
73, Cecil, W5DXP
Explaining an antenna concept so simple to Dr. Slick after
the statement that he made seems to me akin to reading Shakespeare
to a cow.

Irv VE6BP
--
--------------------------------------
Diagnosed Type II Diabetes March 5 2001
Beating it with diet and exercise!
297/215/210 (to be revised lower)
58"/43"(!)/44" (already lower too!)
--------------------------------------
Visit my HomePage at http://members.shaw.ca/finkirv/index.html
Visit my Baby Sofia website at http://members.shaw.ca/finkirv4/index.htm
Visit my OLDTIMERS website at http://members.shaw.ca/finkirv5/index.htm
--------------------
Irv Finkleman,
Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP
Calgary, Alberta, Canada
Cecil Moore
2004-11-11 03:42:38 UTC
Permalink
Post by Irv Finkleman
Explaining an antenna concept so simple to Dr. Slick after
the statement that he made seems to me akin to reading Shakespeare
to a cow.
Awwwwhhhhhh Irv, everyone has a senior moment now and then.
--
73, Cecil, W5DXP
Irv Finkleman
2004-11-11 04:56:45 UTC
Permalink
Post by Cecil Moore
Post by Irv Finkleman
Explaining an antenna concept so simple to Dr. Slick after
the statement that he made seems to me akin to reading Shakespeare
to a cow.
Awwwwhhhhhh Irv, everyone has a senior moment now and then.
--
73, Cecil, W5DXP
The devil made me do it. Aside from senior moments I also have
second childhood flashbacks!

Irv
--
--------------------------------------
Diagnosed Type II Diabetes March 5 2001
Beating it with diet and exercise!
297/215/210 (to be revised lower)
58"/43"(!)/44" (already lower too!)
--------------------------------------
Visit my HomePage at http://members.shaw.ca/finkirv/index.html
Visit my Baby Sofia website at http://members.shaw.ca/finkirv4/index.htm
Visit my OLDTIMERS website at http://members.shaw.ca/finkirv5/index.htm
--------------------
Irv Finkleman,
Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP
Calgary, Alberta, Canada
Richard Harrison
2004-11-11 05:33:17 UTC
Permalink
Cecil, W5DXP wrote:
"Awwwwhhhhhh Irv, everyone has a senior moment now and then."

That`s my experience.

Arnold B. Bailey in "TV and Other Receiving Antennas" is more thorough
than most. Bailey writes on page 291:
"We have already seen that such conditions (efficient operation) can
exist for many resonant lengths of the rod. The rod if divided into two
sections and connected to a load at its center, will exhibit resonance
when the total length of the rod is any multiple of one half-wave.
----only two resonant lengths will give a simple directivity pattern ---
if the rod operates at its fourth resonant frequency, no signal is
picked up from directions broadside to the antenna in contrast to
operation at its third, second, or first resonance. Only at first
resonance is the directivity pattern as indicated by Figs. 6-20 and
6-21.

At all other resonances above the first, the pattern is going through a
progressive change which will later be more explicitly shown in a
quantitative manner."

On page 348, Bailey gives radiation resistances and drivepoint
resistances for dipole resonances 1 through 10, and shows their current
distribution patterns.

Best regards, Richard Harrison, KB5WZI
Dr. Slick
2004-11-11 08:43:26 UTC
Permalink
Post by Irv Finkleman
Post by Dr. Slick
A properly tuned and positioned dipole will
be resonant at only one frequency. Double-dips
are a bad sign, and the return loss suffers.
Explaining an antenna concept so simple to Dr. Slick after
the statement that he made seems to me akin to reading Shakespeare
to a cow.
Irv VE6BP
What if the resonant frequencies are 88.1 and about 92.0 MHz?

Not exactly harmonically related, are they! Think about it.

I think you need to change your diapers, Irv.



Slick
Cecil Moore
2004-11-11 20:33:27 UTC
Permalink
Post by Dr. Slick
What if the resonant frequencies are 88.1 and about 92.0 MHz?
Log Periodic?
--
73, Cecil, W5DXP
Dr. Slick
2004-11-12 13:59:04 UTC
Permalink
Post by Cecil Moore
Post by Dr. Slick
What if the resonant frequencies are 88.1 and about 92.0 MHz?
Log Periodic?
Or perhaps a discone of some sorts, sure.

But we are talking about a simple dipole here.

S.
Mark
2004-11-12 15:22:45 UTC
Permalink
Post by Dr. Slick
Post by Cecil Moore
Post by Dr. Slick
What if the resonant frequencies are 88.1 and about 92.0 MHz?
Log Periodic?
Or perhaps a discone of some sorts, sure.
But we are talking about a simple dipole here.
All this sounds like the characteristics of a dummy load......
Dr. Slick
2004-11-12 20:18:54 UTC
Permalink
Post by Mark
Post by Dr. Slick
Or perhaps a discone of some sorts, sure.
But we are talking about a simple dipole here.
All this sounds like the characteristics of a dummy load......
Who you calling dummy, dummy? :)

the ideal dummy load would have a perfect match across
the entire band.


S.
Dr. Slick
2004-11-11 04:14:06 UTC
Permalink
Post by Cecil Moore
Post by Cecil Moore
Post by Dr. Slick
A properly tuned and positioned dipole will
be resonant at only one frequency. Double-dips
are a bad sign, and the return loss suffers.
If something is wrong with a dip on 40m and a dip on 15m,
someone should warn all the hams who are using their 40m
dipoles on 15m. :-)
Forgot to add that dipoles are resonant near all odd
half-wavelengths, i.e. 0.5WL, 1.5WL, 2.5WL, 3.5WL, ...
They are also antiresonant (purely resistive) on all
integral wavelengths, 1.0, 2.0, 3.0, ...
Using a Smith Chart, if one plots the feedpoint impedances
of a dipole VS frequency and connects the dots, that locus
of points will describe a (very rough) spiral. (Traversing
once around that rough spiral from resonance to resonance
is one full wavelength, not 1/2 wavelength.)
Granted, i'll give you the harmonics, but a double-dip
at for example, 88.1 and 93.7 MHz would indicate a BIG
problem!

:)


Slick
Richard Harrison
2004-11-11 06:00:37 UTC
Permalink
Dr. Slick wrote:
"I`ll give you the harmonics, but a double-dip at for example, 88.1 and
93.7 MHz would indicate a BIG problem!"

Does the dipper dip on one of these frequencies in te absence of an
antenna?

Is the receiver connected to the antenna, and can you move one of the
dips with the receiver tuning?

Does augmenting the antenna cable move one or both dips?

Does relocating the antenna change the dips?

There seem to be two coupled resonant circuits.

Best regards, Richard Harrison, KB5WZI
Dr. Slick
2004-11-11 20:20:02 UTC
Permalink
Post by Richard Harrison
"I`ll give you the harmonics, but a double-dip at for example, 88.1 and
93.7 MHz would indicate a BIG problem!"
Does the dipper dip on one of these frequencies in te absence of an
antenna?
???? The MFJ-259 is obviously infinity to one when you
have an open....
Post by Richard Harrison
Is the receiver connected to the antenna, and can you move one of the
dips with the receiver tuning?
not using a receiver.
Post by Richard Harrison
Does augmenting the antenna cable move one or both dips?
Didn't try this. Didn't have to change this
to fix the problem.
Post by Richard Harrison
Does relocating the antenna change the dips?
Again, one of the two things i did to fix this
problem was to increase the height above the ground
from 3.5 feet to about 6.5 feet.

Can you guess what the other factor was?

Hint: no inductive choke was needed, and the radial
lengths remained the same.


Slick
Richard Clark
2004-11-11 21:03:50 UTC
Permalink
Post by Dr. Slick
Garden variety dipole.
Hint: no inductive choke was needed, and the radial
lengths remained the same.
What do you grow in your garden?
Dr. Slick
2004-11-12 14:14:26 UTC
Permalink
Post by Richard Clark
What do you grow in your garden?
Loading Image...


S.
Tom Bruhns
2004-11-11 19:55:02 UTC
Permalink
Post by Dr. Slick
Granted, i'll give you the harmonics, but a double-dip
at for example, 88.1 and 93.7 MHz would indicate a BIG
problem!
So are you measuring _just_ the dipole, or the dipole through a
feedline? Do you have any idea at all how many coupled resonators you
are measuring? See, e.g., King, Mimno and Wing, to educate yourself
about antenna systems seen as a set of coupled resonators. Disabuse
yourself of the notion that you actually understand what your
measurements mean at this point.
Dr. Slick
2004-11-12 04:50:55 UTC
Permalink
Post by Tom Bruhns
Post by Dr. Slick
Granted, i'll give you the harmonics, but a double-dip
at for example, 88.1 and 93.7 MHz would indicate a BIG
problem!
So are you measuring _just_ the dipole, or the dipole through a
feedline?
Through about 20 feet of RG-58.


Do you have any idea at all how many coupled resonators you
Post by Tom Bruhns
are measuring? See, e.g., King, Mimno and Wing, to educate yourself
about antenna systems seen as a set of coupled resonators. Disabuse
yourself of the notion that you actually understand what your
measurements mean at this point.
I understand it quite well... two non-harmonically
related resonances in a very narrow band (88-108 Mhz) is
a bad sign if you didn't design you antenna for broadband.


S.
Nilla Wafer
2004-11-12 05:49:11 UTC
Permalink
Post by Dr. Slick
What do you think was the other factor?
Slick
Were the two halves of the dipole of equal length?
Richard Harrison
2004-11-12 16:14:52 UTC
Permalink
Nilla Wafer wrote:
"Were the two halves of the dipole of equal length?"

The adjacent close-spaced wire ends have much mutual resistance and in
most cases there os a considerable conductance between them too. In
these cases, they resonate as a unit. But, imbalance enhaces radiation
from the feedline which can produce a separate resonance.

Best regards, Richard Harrison, KB5WZI
Richard Clark
2004-11-12 17:00:28 UTC
Permalink
Post by Richard Harrison
But, imbalance enhaces radiation
from the feedline which can produce a separate resonance.
Hi Richard.

However, out stellar designer has managed to negate that through a
"marijuana garden variety dipole" with radials. ;-)

I wonder who would be more interested. The DEA or the PTO?

73's
Richard Clark, KB7QHC
Cecil Moore
2004-11-12 17:34:39 UTC
Permalink
Post by Richard Clark
However, out stellar designer has managed to negate that through a
"marijuana garden variety dipole" with radials. ;-)
Radials? I missed that. How are the radials coupled to
the dipole?
--
73, Cecil, W5DXP
Richard Clark
2004-11-12 17:38:39 UTC
Permalink
Post by Cecil Moore
Radials? I missed that.
Hint: no inductive choke was needed, and the radial
lengths remained the same.
Problem: Unconventional usage leads to unconventional claims; shortly
followed by unconventional physics offered as proof in terms of
unconventional usage - familiar?
'Doc
2004-11-13 15:20:29 UTC
Permalink
Dr. Slick wrote:
"(1)A properly tuned and positioned dipole will
be resonant at only one frequency.
(2)Double-dips are a bad sign, and the return loss suffers."

(1) - Nope, just not true.
(2) - Not true either. (see #1)

With the two 'dips' not being harmonically related, it
appears that there is some 'other' reason, the feed line
for one, or something near the antenna (a person for instance?).
'Doc
Cecil Moore
2004-11-13 16:56:49 UTC
Permalink
Post by 'Doc
"(1)A properly tuned and positioned dipole will
be resonant at only one frequency.
(2)Double-dips are a bad sign, and the return loss suffers."
(1) - Nope, just not true.
(2) - Not true either. (see #1)
With the two 'dips' not being harmonically related, it
appears that there is some 'other' reason, the feed line
for one, or something near the antenna (a person for instance?).
'Doc
He said something about radials. Radials not attached to the
antenna are like parasitic elements. Two shorter radials will
resonate at a higher frequency than two longer radials.
--
73, Cecil, W5DXP
Dr. Slick
2004-11-14 22:20:24 UTC
Permalink
Post by 'Doc
"(1)A properly tuned and positioned dipole will
be resonant at only one frequency.
(2)Double-dips are a bad sign, and the return loss suffers."
(1) - Nope, just not true.
(2) - Not true either. (see #1)
What i meant to say was, a properly tuned
simple dipole will be resonant at only one
frequency and it's harmonics.

My double dips were not harmonically related.
Post by 'Doc
With the two 'dips' not being harmonically related, it
appears that there is some 'other' reason, the feed line
for one, or something near the antenna (a person for instance?).
'Doc
raising the height of the dipole above the ground
from 3.5 to 6.5 feet was one of the things i did to
fix this problem.

The other one no one has guessed yet...


S.
Tam/WB2TT
2004-11-15 04:26:26 UTC
Permalink
Post by Dr. Slick
Post by 'Doc
"(1)A properly tuned and positioned dipole will
be resonant at only one frequency.
(2)Double-dips are a bad sign, and the return loss suffers."
(1) - Nope, just not true.
(2) - Not true either. (see #1)
What i meant to say was, a properly tuned
simple dipole will be resonant at only one
frequency and it's harmonics.
My double dips were not harmonically related.
Post by 'Doc
With the two 'dips' not being harmonically related, it
appears that there is some 'other' reason, the feed line
for one, or something near the antenna (a person for instance?).
'Doc
raising the height of the dipole above the ground
from 3.5 to 6.5 feet was one of the things i did to
fix this problem.
The other one no one has guessed yet...
S.
This sounds like 20 questions. You are not telling us what you have. One
would not expect somebody to measure an antenna at a height of 3 feet. A
dipole would have an impedance of around 75 Ohms, which would give an SWR
into 50 Ohms of around 1.5:1 Are you using 50 Ohm coax and a 50 Ohm meter?
Is there a gamma match, or such, to adjust? Is it a dipole or a folded
dipole? Is there a balun? Did you buy the antenna or build it (I suspect the
intent would be different)? Since you are using an SWR meter, are you
putting enough power into it to get out of the nonlinear range?

Tam/WB2TT
Richard Harrison
2004-11-15 15:44:36 UTC
Permalink
Tam, WB2TT wrote:
"One would not expect somebody to measure an antenna at a height of 3
feet."

For reception of nearby broadcast stations a wire at 3 feet above the
earth is probably fine but the wire would have a low radiation
resistance.

The radiation resistance of a 1/2-wave dipole versus its height above
ground in wavelengths is probably a familiar graph for many. It starts
at zero ohms at zero height and rises in an almost straight inclined
line to about 99 ohms at nearly 5/8-wavelength. Then, with increased
height, the radiation resistance oscillates as a damped wave around 73
ohms.

Best regards, Richard Harrison, KB5WZI
Dr. Slick
2004-11-15 20:57:25 UTC
Permalink
Post by Tam/WB2TT
Post by Dr. Slick
raising the height of the dipole above the ground
from 3.5 to 6.5 feet was one of the things i did to
fix this problem.
The other one no one has guessed yet...
S.
This sounds like 20 questions. You are not telling us what you have. One
would not expect somebody to measure an antenna at a height of 3 feet. A
dipole would have an impedance of around 75 Ohms, which would give an SWR
into 50 Ohms of around 1.5:1 Are you using 50 Ohm coax and a 50 Ohm meter?
Yes, all 50 Ohms.
Post by Tam/WB2TT
Is there a gamma match, or such, to adjust?
No. But hint: a physical parameter of the
dipole was adjusted.



Is it a dipole or a folded
Post by Tam/WB2TT
dipole? Is there a balun? Did you buy the antenna or build it (I suspect the
intent would be different)? Since you are using an SWR meter, are you
putting enough power into it to get out of the nonlinear range?
MFJ-259. Home-made dipole using aluminum tubing.

There are only so many physical parameters of a
dipole, folks! Just list them and you should hit it!


Slick
Tam/WB2TT
2004-11-16 00:27:20 UTC
Permalink
Post by Dr. Slick
Post by Tam/WB2TT
Post by Dr. Slick
raising the height of the dipole above the ground
from 3.5 to 6.5 feet was one of the things i did to
fix this problem.
The other one no one has guessed yet...
S.
This sounds like 20 questions. You are not telling us what you have. One
would not expect somebody to measure an antenna at a height of 3 feet. A
dipole would have an impedance of around 75 Ohms, which would give an SWR
into 50 Ohms of around 1.5:1 Are you using 50 Ohm coax and a 50 Ohm meter?
Yes, all 50 Ohms.
Post by Tam/WB2TT
Is there a gamma match, or such, to adjust?
No. But hint: a physical parameter of the
dipole was adjusted.
Is it a dipole or a folded
Post by Tam/WB2TT
dipole? Is there a balun? Did you buy the antenna or build it (I suspect the
intent would be different)? Since you are using an SWR meter, are you
putting enough power into it to get out of the nonlinear range?
MFJ-259. Home-made dipole using aluminum tubing.
There are only so many physical parameters of a
dipole, folks! Just list them and you should hit it!
Slick
Having no adjustments, and presumably no 62 Ohm coax, you must have bent it
into a V to get 50 Ohms. If you put a parasitic element on it, it is not a
dipole.

Tam/WB2TT
Dr. Slick
2004-11-16 23:08:03 UTC
Permalink
Post by Tam/WB2TT
Post by Dr. Slick
MFJ-259. Home-made dipole using aluminum tubing.
There are only so many physical parameters of a
dipole, folks! Just list them and you should hit it!
Slick
Having no adjustments, and presumably no 62 Ohm coax, you must have bent it
into a V to get 50 Ohms. If you put a parasitic element on it, it is not a
dipole.
I didn't have to do that. It's a straight regular dipole.

No parasitic either.


s.
Reg Edwards
2004-11-17 03:02:43 UTC
Permalink
Post by Tam/WB2TT
Having no adjustments, and presumably no 62 Ohm coax, you must have bent
it > > into a V to get 50 Ohms.

================================

Having no record of who said what, or what the discussion was about, someone
may be interested in little program INV_VEE which, amongst other things,
demonstrates how the feedpoint impedance of an inverted-V dipole varies with
the enclosed angle from 0 to 180 degrees for any height above ground.

Download INV_VEE from website below.
----
...........................................................
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
...........................................................

Tam/WB2TT
2004-11-09 16:43:55 UTC
Permalink
Post by Dr. Slick
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
What happened to the total bandwidth after you tuned it?

Tam/WB2TT
Steve Nosko
2004-11-09 16:49:31 UTC
Permalink
Post by Dr. Slick
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
At what two frequencies? What construction, feed type...? What method
of measurement and equipment type/numbers?

First, let's determine if there even is a problem with the antenna...
Post by Dr. Slick
I did some modifications, and the double
resonance was was gone, plus the swr was down
to less than 1.1:1
What mods?

73,
--
Steve N, K,9;d, c. i My email has no u's.
Post by Dr. Slick
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
Hal Rosser
2004-11-10 05:40:55 UTC
Permalink
Post by Dr. Slick
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
but i wanted to
see if any of you seasoned antenna boys could
guess what the problem was.
twarnt no problem - if the dipole was 'clean' the double-dip may be due to
how close you were to it, the coax length, a nearby object, - your presence
near the antenna, the way you held your mouth while taking the measurements,
the measurement methods themselves, a passing cloaked Klingon bird of prey,
an ionized whif of exhaled air, or something none of us have ever seen.
(many causes - most just accept it is as - in Walter Cronkite's words
"That's the way it is.")


---
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Lee Hopper
2004-11-12 21:34:16 UTC
Permalink
Post by Dr. Slick
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.
Dear Dr -

This is just a WAG, but were you supporting the dipole on an aluminum
ladder at first? Then substituted a non-metalic support?

Thanks for the interesting topic - I see it has long legs!

Lee H, NB7F
antenna boy...
Lee Hopper
2004-11-14 04:11:52 UTC
Permalink
Along those same lines: did you swap a metal mast for a fiberglass one?

NB7F
antenna boy
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