At 09:04 AM 5/16/98 EDT, K7LXC wrote:
> I presented the tribander comparison info on Friday at K3LR's Antenna
>Forum to an SRO audience. It seemed to be well received and of course some
>people had nits to pick about one thing or another. My comment to them for
>them to conduct some testing and present THEIR results. We're actually hoping
>that some people will pick up the challenge. There were quite a few
>TowerTalkians present and I was happy to meet as many as I could.
Aloha, just back on island last night from mainland, and earlier at Dayton.
Yes, I sat on the floor up near the front during Steve's presentation;
and was
quite convinced. However, a great friend, Larry, KM6IU, also heard the talk
and is still wondering what to do with his big Pro 67 which covers 40 up to 10
meters, and with which he has 315 countries confirmed since 1993 when he
put it up in the hills at his LosAltos, CA QTH; he also worked the 3B7 on
40 meter SSB with it just before leaving for Dayton!!? Have told him his
antenna does not really work at all, and he should return all the QSL's,
as his signal was obviously not legitimate with such a poor Mosley
antenna. Also my TA-34 XL, also a four element Mosley tribander on
21 foot boom, and the 187 or so countries I have collected with it, is
very suspect, I suspect! For each of us, most of these are on 20 SSB, but
my DXCC+ is mixed, as I like CW also; Larry is not fond of CW!! However,
he certainly spends far more time than I chasing after DX, thus the 315
over roughly the same time period.
OK, now that our bias is established, let me ask some questions.
Steve stated that his test range antennas were about a mile separated,
on 55 foot towers, and at the "same height, within 35 feet or so", whatever
that exactly means. In any case, a yagi on 20 meters is going to have its
maximum radiation lobe, the first one formed at something like a 30
degree launch angle, and that I presume is the lobe in mind when
talking about an antenna's gain, isn't it?
Well, lets say the angle is at 30 degrees. This gives us, on Steve's range,
a 1, 2, 3 triangle; where the "2" is the 5280 feet for the one mile
separation between radiating antenna and the test "probe" antenna.
The "1" is the side of the triangle opposite the 30 angle from the radiating
antenna at which, lets say our test antenna is radiating it's maximum
energy, out at the one mile range is up above the ground by 2640 feet!
That seems a bit above the height of the test probe yagi and dipole used
by Steve and the others in their tests!
A more elegant approach to determining the height of the peak of the first
lobe radiated at a given range is given in the last several editions of the
ARRL Antenna Book; mine, the 17th edition, covers all of the info about
antenna test ranges and the heights needed for the test probe antennas
at pages 27-14 and the following. They also point out why a dipole is not
a good probe antenna, as Steve and the others used as a "control" probe.
Or, just use YO to find the angle of the max gain lobe for a given tower
mounting height and wavelength to be radiated and measured.
I have no doubt that Steve and the group did measure energy, but have
no idea of how it got to the probe antennas; was it the induction field from
the transmitting antenna only a mile away, ground wave or ? We do doubt
seriously that what was measured represents what the tested antennas were
radiating up at their maximum radiating angles for the height in wavelengths
at which they were mounted. In fact YO shows typically negative gain at 1/2
degree
up in a yagi's radiation pattern.
Also, Steve's data for 20, 15, and 10 meter bands, are all with the
antenna
physical heights unchanged. How can they possibly represent the way the
tested antennas actually perform at DX distances; there is a 2:1 difference
in wavelength height between the 20 and 10 meter tests as reported! That
certainly impacts the radiation angle for the two bands with fixed antenna
heights.
Of course these same points about the needed height to the probe antenna
are why the ARRL has for years rejected antenna manufacturers antenna
range test gain claims!
We are in no position to repeat the tests as challenged as to do so, our
way, would require a helicopter from which the probe antenna could
be properly suspended well below. Per the ARRL material, the correct
probe antenna at the correct height ought to be, probably, a two
element yagi whose axis of max gain is pointed directly down the "beam"
at the axis of the main lobe of interest coming up from the yagi under
test. Also GPS instrumentation, in addition to everything else, would
be required to assure accuracy of location and test antenna height.
Of course, the probe yagi must be for the wavelength being measured
and radiated from the antenna under test.
As Steve and the others tested with another triband yagi at the
one mile range, no telling what it's gain was at height and
angle with respect to the that of the antenna being tested, rather
like beams forming a crossed sword pattern in the sky?? Wonder how
they coupled?
Well, the above pretty much, at least suggests our questions about
the reported tests. We believe the ARRL's new policy of allowing
computer model results along with all applicable model data to be,
at present, the proper way to compare antennas: at least until comparative
tests are run at "reasonable" distances (certainly much more than one
mile and induction field problems), and at the proper altitudes to really
measure the main lobes with the proper probe antennas.
Now, I greatly respect Steve; I wish I too owned Force 12 and not a
Mosley antenna, but I bought it before ever hearing of Force 12;
and there is probably a very good reason why Steve and the others
can defend what they did. I, and others, await anxiously the explanations
as to why what is outlined above, and the ARRL's long standing policy
about such HF range tests are now to be set aside as irrelevant.
73 and much Aloha,
Jim, KH7M
On the Garden Island of Kauai, (and very glad to be back here!)
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