To: <towertalk@contesting.com>
> Date: Tue, 26 May 1998 12:42:46 -1000
> From: Jim Reid <jreid@aloha.net>
Hi Jim,
You ask some good questions below. The questions touch on a lot of
questionable conclusions published in amateur and professional text
lately. I have no idea what Steve did in the test, so I'll comment in
general terms.
> 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.
Good question! What does that mean?
> 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?
Absolutely. Measuring absolute gain outside of the main lobe
maxima can be very misleading, or it can be OK. It's a lot like
measuring feedpoint impedance and concluding efficiency changed in a
known way due to a measured feedpoint impedance change.
> 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!
Good point. To accurately measure gain, the test antenna should be in
the main lobe.
With a short (H<1/2 wl) vertical, the main lobe is along the ground.
The ground would attenuate both signals equally so the comparison
would be fair, and the path along the ground would be the angle of
maximum radiation (ignoring ground attenuation that is equal for any
two verticals that are focused along the ground).
Where this wouldn't work, is comparing two verticals that are
dissimilar in vertical angle, for example a 1/4 wl vertical compared
to a 1 wl vertical with a NULL along the ground.
Now let's consider the case of a yagi. If the test was done with
EXACTLY the same height of antenna under test and if the
antennas were not greatly focused at different angles due to
construction or tuning, the test method would work pretty well. I
don't believe this is the case, however, from what I have heard.
We should NEVER depend on a dipole to yagi gain comparison
outside the main lobe because multiplication of "ground image" and
free-space antenna patterns are different when a dipole is compared
to a yagi. The antenna's individual free space patterns are
dramatically different, and the dipole has much greater FS directly
below the antenna and is more affected by earth below the antenna.
> 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.
I haven't read much of the Handbook, but there are cases where use of
the most directional antenna available for a probe is desirable, so
the probe is not influenced greatly by reflections from things around
and below the probe. Even the earth below the probe influences the
gain of the probe.
(This is why we can not measure the true arrival angle or polarity of
an incoming wave with an antenna near earth, yet our books accept
data taken by that flawed method!)
But if we place the probe in EXACTLY the same spot and switch between
reference antennas having about the same directional pattern, the
test is "pretty" valid. This is even true with a dipole reference in
non-cluttered (no multipath) test ranges. It works because gain and
pattern variation of the probe occurring with height changes remains
fixed. Even if the gain is off, it is off the same amount for both
antennas being tested!
> 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 doubt that we know enough about the earth around the antenna to
accurately model the earth, but it should be closer than a guess.
> 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 ?
Unlikely to be induction fields at that distance. Impossible to be
ground wave with a horizontally polarized antenna. But let me toss
out one more critical comment.
It is best to avoid measuring a horizontally polarized antenna along
the ground, especially at any distance. Besides the reasons above,
polarity errors (such as those caused by feedline radiation) will
greatly skew the results. Any response to vertical polarity in the
system at either end will greatly influence FS readings, because the
attenuation along the ground is vastly different for vertical and
horizontal signals.
If I wanted to test various yagis for gain or efficiency (I've
done that for "hire") I'd install a large ground screen and place the
yagis in vertical configuration.
Let me give an example of an obvious error caused by using horizontal
polarity in a test at a large groundwave distance. A friend of mine
measured a ladder line fed dipole by using a reference antenna a few
miles away. When he changed matching systems, he picked up
five "S" units!!! Calculations show if he was losing 15-30 dB
(an S unit is generally three to six dB on most meters, and usually
nearer the three dB end) the matching system would have melted from
heat if it was the source of efficiency loss.
He merely changed the level of radiation from the feedline, and the
vertically polarized signal greatly increased with the new matching
system. He actually had HURT efficiency by causing greater feedline
unbalance, but measured an increase in FS because of the flawed test
method!!!
73, Tom W8JI
w8ji.tom@MCIONE.com
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