[Moon-Net] 2M antenna matching Data
n1dpm at verizon.net
Mon Nov 7 21:55:19 CET 2016
I sent this to Kurt before but since someone else asked I'll post this here....
If the antennas are physically identical (element length, position, t match shorting bar position , t match wire length and spacing from the DE including the part past the shorting bars) they should measure very nearly identically. I suspect your analyzer may be being "thrown off" a bit by using 2 different cables between the antennas and the analyzer. I would suggest that you first find a 50 ohm load that measures good on the analyzer (50 ohms with no reactance) or as close as you can get. Next using this load evaluate all of your interconnect cables / phasing lines for match only by inserting the cable under test between the analyzer and the load you just measured. Pick out the cable with the best characteristics (the one that shows the least disturbance to the direct measurement of the load. Use just that cable now to measure each antenna. Be sure to dress the cable down the boom and away from the antenna in EXACTLY the same manner each time. This now removes the uncertainty of different cables and allows you to measure the antennas. They again should measure nearly identically. The only better ways to do this are to either be able to calibrate out the test cable like can be done with higher end network analyzers, or to make a purpose built test cable that is an even multiple of a 1/2 wavelength at the measurement frequency so it will repeat the antenna's impedance at the analyzer.
As far as the antennas being high in frequency....This is true. It is because of 2 reasons...
1. When the antenna gets wet the resonant frequency will drop. It's better to have it a bit high and when raining still work OK than to get it right on the money dry and have it become useless wet. This can be a BIG issue at 432 and higher especially.
2. When antennas are stacked the resonant frequency of the array tends to decrease a little because of coupling between the antennas as the optimum stacking distance has the antenna apertures just overlapping a bit, so one will effect the other.
Hope this helps.
On 11/07/16, Curt Nixon<radio.ku8l at gmail.com> wrote:
Sorry about the data formatting...tried to fix it with this resend.
Good Morning All:
So, I am at the edge of my personal experience database here! I had a
brand new K1FO built by Directive Systems and I built a second one to
match. When I adjusted the Driven element Tee, all I had was an SWR
analyzer....and not a fancy one.
So trying to figure out what is wrong with this system, I now have a
simple vector impedance analyzer--I have collected sweeps on most of the
components now but am out of my depth at interperting the results.
Hopefully the group can enlighten me.
Here is the data taken from each antenna mounted on the cross boom using
the jumpers as they are installed except dropping the ends to the
analyzer well below the antennas.
Antenna data taken with Times T100 and CzCube software utility
Value / Antenna ID
F 144.700* 144.775*
ZL 52.12+J6.70 51.25+J4.11
RL -23.35 -27.54
G 0.02 +J0.07 0.01+J0.04
angle 70.8 70.7
* Data at F= low point in SWR/RL Value
With jumpers arranged as they will be in use
F 144.1 144.1
ZL 54.34+J9.22 56.43+J5.57
RL -20.26 -22.05
G 0.05+J0.08 0.01+J0.05
angle 59.7 37.9
Clearly they are not the same...and both indicate they are tuned high.
But I still have some questions on what I should be aiming for as far as
Return Loss and how important are the other numbers. I did not think to
do both of them using the same jumper---but will do that when I start to
I did measure the divider alone with 50 ohm terminations on each end and
it shows 50.02 ohms and RL of -31dB but was not in the data above.
I also looked at the jumpers disconnected from the antennas and see some
differences between the ones I built and the ones that came from an
older 4x array I purchased that was running well when taken down. But
for the above data both were similar in RL and length using the R+Jx
So..how close should the two yagis be? What values should I be using to
determine if they are adjusted properly?
Thanks a bunch---and I would be happy to take this discussion off the
board unless it may help others going thru the same thing.
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