"John Smith" wrote in message
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"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