"Tam/WB2TT" wrote in message
...

"John Smith" wrote in message
...

"Tam/WB2TT" wrote in message
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"John Smith" wrote in message
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Okay, then, I will present data measured this day for this antenna:

http://www.sophisticatedsolutions.us...d%20Dipole.jpg

This is shown in "Antennas for All Applications" on page 820, figure
23-17 (a).

I built the antenna wholly from RG58. The center conductor of the right
half is not connected at either end. It is 14.375 inches wide and
averages a little less than .5 inches between the centers of the top
and bottom conductors. Where the coax is shown exiting the antenna, is
a female, flangeless, chassis mount, BNC connector so that I can
replace the antenna with a short.

My test set up is a VHF oscillator, a vector voltmeter, and a Narda
dual directional coupler. I use a 66 inch piece of RG58 from the output
of the directional coupler to go to the antenna. The short circuits I
use are the best I could make from BNC connectors. The 50 Ohm load I
used for comparative measurements is one of those used for network
terminators. Yes, I am aware they are not instrumentation quality, but
it's what I have.

For a given frequency, I replace the antenna with the short and adjust
the amplitude of the oscillator and the controls of the vector
voltmeter so that the reference channel (A) is 10 mV and the phase is
180 degrees. I record channel B's amplitude. I then remove the short
and connect the antenna. I then read and record channels A, channel B,
and the phase. From these data I calculate the impedance (per HP's AN
77-3, thanks to Wes Stewart).

The first item measured is the 50 Ohm terminator. I also measured it at
the conclusion of the tests to see if there were any differences and
there were none.

Here are the results computed from the data:

Freq (MHz) Impedance (Ohms)

410 46.4 + 6.0i (50 Ohm terminator)

380 9.7 - 12.5
390 3.5 - 5.7
400 5.1 + 1.3i
410 5.1 + 6.5i
415 4.0 + 10.0i
425 2.5 + 15.7i
........................................
John,
Discounting the absolute values, the numbers seem to make sense, except
for the 9.7. Might it have been 2.7? There seems to be resonance at
around 400. The impedance goes more reactive in both directions from
there, and the real part goes down monotonically, except for the 9.7

I looked at the picture, and it is not 100% obvious to me what gets
connected at the balanced point. Just for kicks, I am going to try that,
somewhere within the range of an MFJ269.

Tam/WB2TT


Hi, Tam -

I will try to repeat the test at that frequency.

By the balanced point, I assume you mean at the bottom center. It is a
female BNC connector, facing downward. A halfwave length of RG58 goes off
the left side and folds. The coax is soldered in normal fashion to the
connector. Another halfwave piece of RG58 goes off the right side and
folds. The center conductor of the right side piece is not connected on
either end. The shield of the right side coax is soldered to the shell of
the BNC and the two pices of coax is joined as shown in the figure.

If by the balanced point you meant at the top center of the figure, the
center conductor only of the left side coax is soldered to the shield
only of the right side coax.

If this description is not clear, let me know and I'll try again. I would
take a picture and make it available, but I'm afraid it would only
confuse due to lack of detail.

Thanks,
John

John, This turned out simpler than I thought. I did a quick and dirty test
with a folded dipole made up of two 3 foot pieces of RG58, and a feedline
1/2 WL at 160 MHz. Unfortunately, this puts me too close to the upper
limit of the MFJ, but you can see what happens. BTW, I meant the top for
the balanced point. Turns out it makes no difference whether the center
conductor of the left side is connected to the center conductor, or the
shield, of the right side.


My numbers:

F R X
145 30 89
150 10 60
155 ? 7? 14? This looks like 165
160 8 25
165 7 13
170.25 6 0
175 5 14

I would say that if anything, my numbers make less sense than yours. I
also got a reading of 1+j0 at about 65 MHz. Don't know what that means,
other than a dead short. I will hook it up as a regular folded dipole, and
see if I get anything like 300 Ohms. Probably tomorrow.

Was this claimed to be a 50 Ohm antenna?

Tam/WB2TT


Hey, Tam!

This is great information! According to the dimensions, it should be
resonant at 173 MHz. (Close enough... I subtracted 1 inch on each side for
the loop). Your 6 Ohms is close enough to my 5 Ohms to satisfy me that I'm
not getting useless measurements.

I wouldn't worry about the 65 MHz reading, as that's so far away from
resonance that it is probably acting like a shorted half wave transmission
line or some other kind of crazy network.

No, there was no statement given about the antenna impedance. That's why I
started this thread. I was interested in the antenna because:

1. It is at DC ground.
2. It is a half wave, giving a nice omnidirectional pattern if mounted
vertically.
3. No balun is needed as it is inherent.

However, it appears to be not worthwhile after all, owing to its low
feedpoint impedance.

I really appreciate you taking the time to perform your experiment. I'm now
convinced that the antenna is next to worthless.

Thanks,
John
(KD5YI)

John,
I had breakfast with some friends this morning. One of them has a PHD in EE,
specializing in antenna design. He thinks the antenna should work, but
suggested changing the spacing between the upper and lower conductors. He
also told me to look for multiple resonances. I just now tried that, with
the spacing essentially 0. What I got was a new resonance at 165 MHz, with
Z=18 + j0. . Interestingly, I now seem to have hit parallel resonance, like
you did, and R goes down either side of 165. In fact, accross 100 -150 MHz
the Z is 0 -jX. When I was messing around with spacing, at one time I got
202 + j0 at 129 MHz; but that is way off calculated frequency. I probably
should not be doing this inside, as it is not entirely repeatable.

I also tried a longer piece of coax (RG8X, Vp=.75) that gave me a 5 foot
folded dipole. That should be resonant at about 93 MHz. I clearly got
multiple resonances:

F R X
92.5 3 j3
100 11 j44
110 80 j181
120 4 j30
127 4 j1
160 53 j14 SWR=1.3
170 11 j0

The feedline was also 5 feet, since I did not know what a wavelengt was
going to be. This looks like it wants to be a 160 MHz antenna, instead of
93. Lastly, I tried the 5 foot antennawith a 2 inch feedline.Resonance was
at 156 MHz. If I get a chance, I will try it outdoors tomorrow.

Tam/WB2TT

"Tam/WB2TT" wrote in message
...
John,
I had breakfast with some friends this morning. One of them has a PHD in
EE, specializing in antenna design. He thinks the antenna should work, but
suggested changing the spacing between the upper and lower conductors. He
also told me to look for multiple resonances. I just now tried that, with
the spacing essentially 0. What I got was a new resonance at 165 MHz, with
Z=18 + j0. . Interestingly, I now seem to have hit parallel resonance,
like you did, and R goes down either side of 165. In fact, accross
100 -150 MHz the Z is 0 -jX. When I was messing around with spacing, at
one time I got 202 + j0 at 129 MHz; but that is way off calculated
frequency. I probably should not be doing this inside, as it is not
entirely repeatable.

I also tried a longer piece of coax (RG8X, Vp=.75) that gave me a 5 foot
folded dipole. That should be resonant at about 93 MHz. I clearly got
multiple resonances:

F R X
92.5 3 j3
100 11 j44
110 80 j181
120 4 j30
127 4 j1
160 53 j14 SWR=1.3
170 11 j0

The feedline was also 5 feet, since I did not know what a wavelengt was
going to be. This looks like it wants to be a 160 MHz antenna, instead of
93. Lastly, I tried the 5 foot antennawith a 2 inch feedline.Resonance was
at 156 MHz. If I get a chance, I will try it outdoors tomorrow.

Tam/WB2TT


Thanks for the hard work, Tam. I'm not sure I know what to make of all this,
but it appears that the antenna is not what I thought it would be.

Thanks again.

John