[Antennas] TA32 update-more puzzling than ever

2002年6月02日 23:14:40 -0500

John, one thought: standing within a three-foot jumper length of the
antenna to make the measurements sounds like it would put your body within
the antenna proper among the elements. That could be expected to alter the
tuning quite a bit. 
As far as the difference between the shack readings and the "direct"
readings, if the antenna looks like anything but the 50 ohm resistor your
analyzer is calibrated for, then the feedline acts as a transformer to
alter whatever the actual antenna impedance happens to be.
I do not have any ideas what could be wrong with your antenna, especially
since the problem seems to vary from time to time, but the two points above
could be involved.
Also, you didn't say which analyzer you were using. In addition to your
body, etc. detuning the antenna, the analyzer being so close to the antenna
may be picking up broadcast-band or other r-f signals which would affect it
on the roof but not at all or as much in the shack.
One thing I would try just to eliminate it as a possible cause would be to
temporarily run another coax from the antenna to the shack - just to see
what changes if anything. If the length is different, you will probably get
different impedance readings in the shack although if the coax is not very
lossy, the SWR readings should remain about the same.
Another aspect is whether or not you are using a balun to feed the antenna
and/or to isolate the analyzer case. Common-mode current on the outer coax
braid will affect your analyzer readings and will cause your SWR readings
to vary with position along the line. If that is happening, I would suggest
that you first of all put a 1:1 current balun at the feedpoint. If nothing
else, try winding up 8-10 turns of coax about 6-8 inches in diameter -
wound like a coil around a jug or something, not scramble-wound - and see
what difference that makes.
73/72/oo, George W5YR - the Yellow Rose of Texas 
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe 
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121 Kachina 505 DSP #91900556 Icom IC-765 #02437
John Geiger wrote:
>> Got some good feedback on things to check. Thanks to
> all those who responded. Here is the current
> situation. I took apart the traps to clean them (they
> were spotless inside) and to tighten the coil wires to
> the trap cover-both of these ideas had been suggested.
> Then got out the antenna analyzer and tried the
> antenna sitting on the ground-looked fine, just like
> yesterday. Put the antenna back up on the roof-in the
> dark even, and checked it with the antenna
> analyzer-looked great again. I am using a 3 foot
> jumper coax piece between the yagi and the analyzer.
> Unhooked this jumper and hooked up the coax to the
> shack. Went to the shack and tried out the yagi-fine
> SWR on 10, problems on 15 and 20 again. This was less
> than 5 minutes from when I checked it with the
> analyzer.
>> You may figure that the coax into the shack is
> bad-which wouldn't account for the fine SWR on 10, and
> the fact that it worked fine thursday with the MA5B.
> So, I take the analyzer back up, hook up the jumper,
> and now the SWR sucks on 15 meters-just like it did in
> the shack
>> Any ideas? Is running 100 watts thru it somehow
> messing it up on 15 meters-enough that the lower power
> of the analyzer still gets bad readings then? I
> thought that yesterday-but if I damaged it yesterday,
> how do you explain the fine SWR this evening after
> reinstalling it (before the shack test).
>> Any additional help would be appreciated!! It is
> things like this that sometimes make me think I need
> to take up a less stressful hobby, like bomb disposal.
>> 73s John NE0P