-=> On 10-15-97  10:08, Roy Witt said to Ivy Iverson,<=-
-=>"About High Gain Antennas for 2M...,"<=-
-=> 14 Oct 97 15:23, Ivy Iverson wrote to Roy Witt:
 
 RW> Hello Ivy.
 
Hi, Roy;
 
 RW>> antenna  is stronger off the front vs skewed to one side, as in
 RW>> a gamma matched system.
 II> That will give a good idea of the pattern, but not for finding
 II> the gain relative to a dipole.  I suppose I could set up a
 II> dipole and compare the readings, though... a little difficult
 II> for me to read my current J-pole, though, because it's up on the
 II> roof and I can't get to it.   :-{
 
 RW> I knew your were going to say that.  Put your dipole up there too,
 RW> then take  your readings.
 
Actually, that's a good idea, but it won't work!  (Won't be accurate).
Why not?  Simple. (2 reasons, actually).
 
1) Interaction between the two antennas.
 
2) The way an antenna gets gain over a dipole is by "compressing the
doughnut."  In other words, it takes power from places, (up and down),
where it's useless, and concentrates it toward the horizon where it does
the most good.  (This is the general definition for a gain omni, for a
beam it's similar).  So putting a dipole up on the roof won't give
accurate results because more of the signal from the gain antenna would go
right over my head on the ground.  Even putting both at ground level won't
help much because a 2M dipole is about 3 Ft. long, while a 6 dB Co-linear
5/8 J-pole is about 10 Ft. tall, so the closer you are the more signal you
will "miss," unless your receiving antenna is the same as the larger
antenna under test.  Of course, the farther you are away from the
transmitting antenna the less this error will affect the test results.
 
The way they test TV receiving antennas is on a special test range.
They have a transmitting antenna, fed by a controlled and calibrated
source, in a "pit" in the ground, (so as not to interfere with other
TV's).  The antenna under test is mounted on a tall mast, well away from
any other metal or other signal sources.  The mast is tilted in such a way
that the antenna under test is pointed directly at the transmitting
antenna.  Sorry, but I don't have the facilities to do this kind of
testing on a new antenna.  For me, the proof of the pudding is in the
eating, and when I can work a DX station on 2M with 5 watts that someone
a few miles away can't hear with his 5-element beam and 50 Watts, (he's
in a bit of a hole, though), and when I can reliably work repeaters that
are over 50 miles away with the same set-up, I figure it's doing a great
job.  (The fellow that I out-heard and out-talked in a very marginal
QSO, asked me for the plans for the J-pole!).  Meanwhile, I'm planning a
4-section version which should get close to 12 dB... around 40W ERP from
a HT!  That should put me up with the folks with a 50-Watt mobile and a
ground plane for a home rig.  Of course, about 18-20 Ft. over the roof
DOES make a good lightning rod!   >KA-BOOM!<  Ever see a HT smoke?  :-<
 
73 DE KB9QPM
   Ivy
 
 
 
... Double your dipoles. Double your flux.
 
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