You overlooked one other complicating factor, feedline length. The
length of the feedline on the unused antenna determines if the unused
antenna looks resonant or not.
Considering only two of the antennas for the moment, if the upper
antenna is unused and has a multiple of a half wavelength of feedline
attached and the source end open, the lower antenna would look much the
same as any other inverted vee at the same height. If the top feedline
happened to be an odd multiple of 1/4 wavelength and the source end
open, nearly all the radiation from the lower antenna goes straight up.
Other feedline lengths cause all sorts of radiation pattern changes, and
everything reverses if the unused end is shorted instead of open.
Experimental A/B testing of closely mounted antennas with uncontrolled
parameters of feedline length and source impedance can produce some
really weird results.
On the other hand, using controlled feedline lengths and switching
source impedance can produce different patterns which may be useful.
More useful patterns can be obtained by feeding two or more antennas.
You can get some gain, or for the ultimate NVIS antenna (if you really
want that) try feeding two of the antennas 180 degrees out of phase.
Jerry, K4SAV
David Gilbert wrote:
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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