Your antenna is resonant a little BELOW 7.000 MHz.

Is that what you expect?

W1MCE

Jack Twilley wrote:

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The weekend was beautiful, so I was able to do some outside work.

I measured the antenna's impedance over 40m, then shortened the 40m
legs by two feet each. I then measured the antenna's impedance again,
and came up with numbers more closely resembling what I'd expect:

Frequency Resistance Reactance Impedance
7.00 14 6 15.2
7.05 18 15 23.4
7.10 18 21 27.7
7.15 21 20 29.0
7.20 24 27 36.1
7.25 28 36 45.6
7.30 28 43 51.3

These measurements were done using a slightly different method. This
time, I set the noise bridge for R=50 X=0 with the power off, then
tuned the drive on my receiver for maximum noise before powering on
the noise bridge and finding the null. The noise bridge manual is not
as detailed as I would like, and it's not clear what the "proper"
method is, but this produces reasonable results.

The impedance is well within a 2:1 match throughout the voice segment,
and a 3:1 match across the entire band.

In other news, I happened to tune across 20m for the last five minutes
of the Virginia QSO Party, and I made two contacts (NC4S and N4NW).
I feel better about the performance of my antenna on that band, but I
really hope that it's because the tuning somehow helped, and not that
contesters are the only ones willing to dig into the noise.

So it seems like I'm on the right track. I am going to try
lengthening the antenna by six inches and doing another round of
testing before moving on to the second set of legs, unless someone
- From the newsgroup pipes up with a correction to my methodology.

Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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"Dave" == Dave Shrader writes:

Dave Your antenna is resonant a little BELOW 7.000 MHz. Is that what
Dave you expect?

Actually, my goal is for the antenna to be resonant at 7.150. If it's
resonant too low, then I need to shorten the wires even more, right?

Dave W1MCE

Jack.
(higher frequency is shorter wavelength)
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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On Mon, 22 Mar 2004 17:10:02 -0800, Jack Twilley
wrote:
Actually, my goal is for the antenna to be resonant at 7.150. If it's
resonant too low, then I need to shorten the wires even more, right?

Hi Jack,

Resonance is not all that it is cracked up to be. Be careful of
seeking something that yields little return on your effort.

For instance, the numbers you've offered show a very low Z for that
anticipated goal. This means you will still need to transform that
value to reduce SWR, which in its own pursuit is an equally low yield
return.

You will need a tuner any way you look at it, and the higher R values
at the higher frequencies are probably more due in part to the loss of
nearby ground, rather than the miracle of a neighboring resonance.

My guess is that you have already obtained an optimal situation = it
don't get any better than this even with all the trimming you may
accomplish. Worse, you could double its height and contacts may never
notice the improvement in "efficiency." Basically all your work is to
achieve bragging rights, and you could cheat and start bragging right
now to no one's challenge.

73's
Richard Clark, KB7QHC

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"Richard" == Richard Clark writes:

Jack Actually, my goal is for the antenna to be resonant at 7.150.
Jack If it's resonant too low, then I need to shorten the wires even
Jack more, right?

Richard Hi Jack,

Richard Resonance is not all that it is cracked up to be. Be careful
Richard of seeking something that yields little return on your
Richard effort.

This is a very good point.

Richard For instance, the numbers you've offered show a very low Z
Richard for that anticipated goal. This means you will still need to
Richard transform that value to reduce SWR, which in its own pursuit
Richard is an equally low yield return.

Yes, but pursuing a match close to 1:1 and an impedance close to 50
ohms seems to be a reasonable pursuit, if I understand correctly.

Richard You will need a tuner any way you look at it, and the higher
Richard R values at the higher frequencies are probably more due in
Richard part to the loss of nearby ground, rather than the miracle of
Richard a neighboring resonance.

Possibly. I hope a accurate description of my antenna's geometry and
surrounding structures will assist in resolving this question.

Richard My guess is that you have already obtained an optimal
Richard situation = it don't get any better than this even with all
Richard the trimming you may accomplish. Worse, you could double its
Richard height and contacts may never notice the improvement in
Richard "efficiency." Basically all your work is to achieve bragging
Richard rights, and you could cheat and start bragging right now to
Richard no one's challenge.

That's a reasonable guess. However, I think I've got a bit more
experimenting to do before I can reach that conclusion. I disagree
with the assertion that "bragging rights" is the goal here, though. I
don't see any point in bragging -- there's nothing terribly unusual or
special about what I'm doing. I'm not going after DXCC with a
magnetic loop the size of a ream of paper, or WAS from a radio in an
Altoids can. I'm just trying to have the best antenna my environment
and budget can support.

Richard 73's Richard Clark, KB7QHC

Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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On Tue, 23 Mar 2004 00:29:43 -0800, Jack Twilley
wrote:
Yes, but pursuing a match close to 1:1 and an impedance close to 50
ohms seems to be a reasonable pursuit, if I understand correctly.

Hi Jack,

This is an aspiration on par with monks seeking nirvana. The two
illustrate it is the path, not the destination that is meaningful.

There is a very clear point to be made that unless you elevate the
dipole, any 50 Ohm characteristic you observe will more ground loss
than radiation resistance. This is reasonable only in the sense of
ease of tuning and the reductio ad absurdum is bliss can be found in a
dummy load. Clipping the ends of the current structure do not lead to
increased benefit unless you shave your advantage in tenths of a dB.
To put that in perspective, unless you are a teenager, you and your
contacts couldn't possible hear any change below 1dB variation. To
force that perspective further, propagation variation through one QSO
easily varies by that much or more (unless we are talking line of
sight FM with full quieting).

73's
Richard Clark, KB7QHC

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"Richard" == Richard Clark writes:

Richard On Tue, 23 Mar 2004 00:29:43 -0800, Jack Twilley
Richard wrote:
Yes, but pursuing a match close to 1:1 and an impedance close to 50
ohms seems to be a reasonable pursuit, if I understand correctly.

Richard Hi Jack,

Richard This is an aspiration on par with monks seeking nirvana. The
Richard two illustrate it is the path, not the destination that is
Richard meaningful.

Richard There is a very clear point to be made that unless you
Richard elevate the dipole, any 50 Ohm characteristic you observe
Richard will more ground loss than radiation resistance. This is
Richard reasonable only in the sense of ease of tuning and the
Richard reductio ad absurdum is bliss can be found in a dummy load.

Oh, I see your point here. What you're saying is that my dipole is so
far from the ideal that reaching 50 ohms won't mean anywhere near the
same thing that it does to the ideal case. That begs the question,
then -- if not 50 ohms, what is the best value for my particular
configuration?

Richard Clipping the ends of the current structure do not lead to
Richard increased benefit unless you shave your advantage in tenths
Richard of a dB. To put that in perspective, unless you are a
Richard teenager, you and your contacts couldn't possible hear any
Richard change below 1dB variation. To force that perspective
Richard further, propagation variation through one QSO easily varies
Richard by that much or more (unless we are talking line of sight FM
Richard with full quieting).

I've listened to QSOs where they've ebbed down to the noise floor and
flooded back several times over the course of a minute, so I
understand what you're saying.

Richard 73's Richard Clark, KB7QHC

I am really interested in knowing what sort of target values would
represent an optimal configuration, if that's at all possible.

Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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On Tue, 23 Mar 2004 13:34:38 -0800, Jack Twilley
wrote:
Oh, I see your point here. What you're saying is that my dipole is so
far from the ideal that reaching 50 ohms won't mean anywhere near the
same thing that it does to the ideal case. That begs the question,
then -- if not 50 ohms, what is the best value for my particular
configuration?

I am really interested in knowing what sort of target values would
represent an optimal configuration, if that's at all possible.

Jack.

Hi Jack,

For the height you are at, you are probably already at the optimal
solution. If you could optimize further, at this height, it would
barely eke out 1dB difference. Ground dominates your design.

This is a heresy with dipole aficionados, but building a ground screen
will help toward the quickest, easiest 1dB return. You don't have to
do anything but shallow bury copper for the length of dipole + 20% and
maybe a quarter wavelength wide, beneath it. It need not attach to
anything to be beneficial. A grid of wires, 1M on a side is a good
first step.

You will undoubtedly note a tuning change, use a tuner to adjust (you
need it anyway). Some may distract you pointing out that it simply
sends more signal up. True, but as a screen, it removes loss, and
allows more signal out toward the horizon too.

73's
Richard Clark, KB7QHC

"Jack Twilley" wrote in message
...
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"Richard" == Richard Clark writes:

Richard On Tue, 23 Mar 2004 00:29:43 -0800, Jack Twilley
Richard wrote:
Yes, but pursuing a match close to 1:1 and an impedance close to 50
ohms seems to be a reasonable pursuit, if I understand correctly.

Richard Hi Jack,

Richard This is an aspiration on par with monks seeking nirvana. The
Richard two illustrate it is the path, not the destination that is
Richard meaningful.

Richard There is a very clear point to be made that unless you
Richard elevate the dipole, any 50 Ohm characteristic you observe
Richard will more ground loss than radiation resistance. This is
Richard reasonable only in the sense of ease of tuning and the
Richard reductio ad absurdum is bliss can be found in a dummy load.

Oh, I see your point here. What you're saying is that my dipole is so
far from the ideal that reaching 50 ohms won't mean anywhere near the
same thing that it does to the ideal case. That begs the question,
then -- if not 50 ohms, what is the best value for my particular
configuration?

.................................................. ...........................
....................
Jack,
You may have a misconception about impedance here. Making the antenna
resonant is not going to make it 50 Ohms. Extrapolating your numbers, it
looks like resonance would be at around 6.9 MHz, and give you an impedance
of something like 12 + j0. Might be a good match to a 1:4 (not 4:1) balun,
but you would have to see what happens on the other bands.

Tam/WB2TT

YEP!!

Jack Twilley wrote:



Actually, my goal is for the antenna to be resonant at 7.150. If it's
resonant too low, then I need to shorten the wires even more, right?