Hi, Rick.
Those are interesting comments, so I modeled it up with EZNEC with
approximately resonant antennas to see what it looked like. Your
message says "inverted vee's" (plural), so I'm making the leap to assume
you had all three inverted vee's up at the same time and were able to
switch between them. Please correct me if I'm wrong ... and if I am
wrong you and everyone else will probably want to ignore that which
follows.
Here's what EZNEC says about inverted vee's individually at the various
heights:
30 ft ... max lobe straight up (90 degrees) of 6.4 dbi --- gain at 20
degrees (arbitrary mid angle) of 0.2 dbi
60 ft ... max lobe at 35 degrees of 5.8 dbi --- gain at 20 degrees of
4.0 dbi
90 ft ... max lobe at 23 degrees of 8.3 dbi --- gain at 20 degrees of
8.2 dbi
It gets more interesting when you look at the three antennas all
together on the same tower, but only one being fed.
30 ft antenna fed ... max lobe at 90 degrees of 7.6 dbi --- gain at 20
degrees of 2.7 dbi
60 ft antenna fed ... max lobe at 26 degrees of 5.5 dbi --- gain at 20
degrees of 5.1 dbi
90 ft antenna fed ... max lobe at 26 degrees of 6.4 dbi --- gain at 20
degrees of 6.4 dbi
None of this data should be taken too literally, of course, but the
model implies a lot of parasitic coupling between the three antennas
that affects the pattern even when only one of the antennas is being
fed. Individually, the signal level at 20 degrees varies by 8 db
depending upon whether the antenna is at 30 feet or 90 feet.
Collectively, the signal level of the stack of three antennas varies by
less than half that (3.7 db in this arbitrary case) no matter which of
the antennas is fed. In real life the difference across the stack might
be even less. If I had my choice, I'd prefer to have only the upper
antenna on the tower ... unless of course, as you say, someone wanted to
optimize the close-in performance. For longer DX, takeoff angles as low
as 10 degrees are useful and there the difference according to the model
jumps to 10 db.
It would be interesting to see someone hang an inverted vee from a pully
and rope and take signal strength readings at different heights. I
don't have my tower up yet at this new QTH, but if nobody has done so by
the time I get the tower up I'll promise to give it a try.
73,
Dave AB7E
Richard (Rick) Karlquist wrote:
Anecdotal results from anywhere are irrelevant ... that was my
point. I
don't trust software analyses implicitly, but I trust them more than
opinions that aren't backed by direct comparison of some sort (like
an A
vs B test of two antennas at the same height at the same time).
Yup ... well, close anyway. I used a fixed 2 element 40m wire yagi at
70 feet for a while. It worked great and I had a lot of fun with it.
It would have worked even better at 90 feet, and it would have
worked a
whole lot worse at 45 feet like the original message from NY6DX
discussed.
Dave AB7E
Interesting that you should mention A/B'ing. I did a lot of A/B'ing
of 40
meter
inverted vee's at 30, 60, and 90 ft. I thought the 90 ft one would
have a
substantial
advantage over the lower ones, but in actual operation they were
very hard
to tell apart. I listened to foreign broadcast stations and ham DX
stations
as
much as I could and looked for S-meter changes. On local stations
(<100
miles),
there was a substantial difference which agreed with conventional
wisdom of
the lower the better for locals. YMMV.
Rick N6RK
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