Good morning.
Regarding the 20/40 above the 10/15...
The problem might be that the 15 is below the 20 and that the 20 mtr elements
are either acting as poorly tuned reflectors, or performing some kind of
"shading/shielding" on the higher frequency antenna.
Whenever building or planning a project, I always remind myself that elements
can couple efficiently out to at least 1/4 wavlength, so every item within that
range is "suspect" and considered in the design. For 15 mtrs, this is close to
12'.
I have done two major projects in regards to stacking Yagis related to this
subject.
One was using an "inverted stack", with the 20 on top, then 15, then 10. The
memory is a little fuzzy, but the spacing was at least 8' between each antenna.
I ran this set-up for just over a year, operating several contests, besides
casual operating (every day). I also made careful notes on how it performed
relative to other locals who were not changing anything. I then changed the
antennas with the 10 on top, then 15, then 20 on the bottom. The improvement on
10 and 15 was very impressive. As far as we could tell, 20 was essentially the
same. The computer model does not necessarily indicate that the "inverted
stack"
(20 on top) would result in this situation. At least my long term testing
showed
that it did and I haven't put up another like it since.
The second involved having a large 10 mtr at the top (8 elements), then a 15
mtr
(6el on a 36' boom), then a 6el 20 on a 44' boom. Spacing was just over 8'
between 10/15 and 9' between 20/15. I installed relays in all the 20 and 15 mtr
elements so that they could be opened individually to "remove" the element(s).
The trusty computer model indicated that 10 would be losing close to 3dB and 15
over 2, besides having a poor F/B, all the while the VSWR curves were
acceptable
and nothing out of the ordinary. Sure enough, after testing this for about a
year using a spectrum analyzer (to see small variations), the true life
condition tracked the computer model. If you were checking 10 mtrs, as each
element on 15 was opened, you could visually see the increase (showed right at
3dB). The same for 15 (abt 2.5dB), as the 20 mtr elements were opened, besides
the 15's F/B returning.
So, the situation might simply be the first case (inverted stack) that the 20
is
above the 15, even with the 14' spacing. I know that we always want the lower
frequency antennas higher, but perhaps that is not always the best choice.
Maybe others have done similar testing.
Have a great day & 73,
Tom Schiller, N6BT
President, Force 12, Inc.
Amateur Antennas, Commercial Towers & Antennas
P.O. Box 1349 Paso Robles, CA 93447
Phone: 805.227.1680 FAX 805.227.1684
Web Site: http://www.QTH.com/force12
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