Hello Rich, I’m not sure if this will be inline with the thread you’ve
created, a new thread or somewhere entirely separate... sorry if not inline
where I intended it to be.
I've had what you're planning on doing - a 3 stack of C31's. See the picture
on QRZ.com call sign search (K9RX). It was taken down in April of 2012 after
having been up for 6 1/2 years (we moved).
I can tell you that first off there are few times that a separate antenna
will be better than the stack. On 10 meters it would happen occasionally, 15
less often and 20 probably less than a couple times all told! I had antennas
at 41, 81, 121. I felt this was the best overall spacing. If I had it to do
over again I'd probably reduce it to 37' or so as it really is too far apart
on 10 and for a relatively short boom 20 if you model it you can find that
37' is a slight increase in gain over 40' (with a slight reduction in FB).
So using single or dual stacks just isn't something you'll end up doing much
of. There's too much coupling into the non-connected antennas and thus
little change in overall performance. Once - on 10 meters, the lowest
antenna was 20 db stronger than the top ... but the stack was just a bit
down from the stack.
And indeed - the overall heights matter just as much. My low antenna at 41'
was really too low for 20 ... and it is beginning to be too high on 10...
that stated I'd probably go with that one at 45' and then 37' spacing from
there putting the top antenna at 119'. Note however - its really not that
critical - just stack them 35' - 40' apart and around these heights...
I sold mine - I hope to be in South Carolina soon (this year - purchased 11
acres in Piedmont - just have to move and build a house)... and this time
I'm going with 2 stacks of 20m 5el and 15m 6 el on a rotating tower... the
pattern and gain is better overall.
The C31 stack played quite nicely on 20... and on 15... but I never really
felt it was doing what I'd expect on 10. I had it with a Force12 240N 2 el
40 meter at 130' and their Magnum 80 meter rotary dipole at 136'. And the
SWR on 15 was never great - I was one of those that had the 'notorious' 15
SWR problems that I could never get quite right.
As an aside I'd often ask people to guess which of the antennas/systems
worked best overall... most would say the 3 stack of C31's... but the best
antenna on that tower was the dipole on 80 at 136'... it was awesome. Why?
Because it was competing against mostly wire dipoles at 50' - 60'.
Gary, K9RX
I've been following the mail on this subject. I'm going to start my new
tower this summer. The base and anchors are already in. It's going to be a
137' rotating 55g tower with three F12 C31XR's stacked. I've been using HFTA
to come up with the best compromise heights (compromise as I'm trying to get
the best results on all 3 bands). Then it dawned on me. Should I be worried
about stacking results or individual heights. Isn't the height of an antenna
for a specific band at a certain time of day more of an advantage than the
combined stacking?, and of course it's still a compromise since we're
talking about tribanders. Don't get me wrong, I'm still going to try to come
up with the best stacking distances for my location, but it seems to me that
the most important feature of a stack is having 3 different heights to
choose from, isn't it? I sure wish the project was finished, it would have
come in handy this past weekend. Rich - N5ZC
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