Well no response to this so far so I'll give my $0.02 and see if it sparks
some interest.
It appears from recent threads here on towertalk that there is a great
interest in triband stacks. That makes sense as they provide wonderful bang
for the buck and single tower solutions. I've spent a lot of time diddling
with models and such trying to beat a single tower triband stack with
monobanders on a single tower and can't figure out how to do it. I'm sure
coutless others have done the same analysis for themselves and wonder if
they've come to the "correct" conclusions.
First, why stack at all? Simple, you need antennas at different heights to
cover different angles so you work more marginal guys. No surprise there. I
think stacking tribanders for forward gain is a total waste of time. You
might realize a db of gain on a band or two, but you won't get perfect
patterns or optimal gain so don't spend forever worrying about it. If you
have to have band opening performance you need to think about proper monoband
stacks and the proper New England location to erect them.
So what about different tribander arrays? I asked forthoughts on C3 (or C3E)
vs the C31XR since many people here like Force12 antennas. I think if we're
talking tribandes we should also consider the Skyhawk. I'm excluding older
antennas like the TH6 and Kt34XA and also the X9 as it still uses traps.
I'll throw out the following arguement for a three stack of C3's with the
following logic:
(1) It's clear that the C3 works very well and provides a fairly cheap way to
get three antennas up covering three bands. See N7ML or N5CQ for exceptional
implementation arrays using these antennas. (2) I think 25G would be
sufficient for a C3 tristack, others might disagree. 25G is available
locally for $25-30 a section. (3) While modelling 10L 100 foot boom 20M yagis
we often forget about how difficult erecting real antennas can be. Model big
stacks all day and then go pay someone to erect it for you. The C3 is light
and at least 4 of 7 elements are within easy reach from the tower. An
individual could reasonably expect to erect a C3 stack with minimal rigging.
To me this is a huge plus. Dean Straw often tells stories of building his
TH6's on the tower. I don't think anyone wants to do C31XRs that way. (4)
Rotation. Big rotors are expensive. A C3 does not require anything special
or costly to rotate. A Ham4 or T2X should easily rotate two C3s together on
one sidemount and a small rotor up top turns the third (5) For SO2R you can
use the WX0B box and run the lower antenna for mults and still have a two
stack going on teh run rig. (6) Where the C3 falls short is in pattern and
gain. 2L antennas don't have great F/R but the modelled gain is reasonably
close to 3L for the boom lengths we're talking about here. I undestand the
F/R to be worse when these antennas are stacked. (7) Small tribanders are in
demand. If you build a three stack and want to move up to monobanders later
you'll have no trouble selling off C3s at a decent price. Minor point perhaps
, but we all dream of bigger and better.
Next step up. I think folks are missing a step in triband stacks by not
discussing the Skyhawk. It's similiar to the F12 overlayed monoband design
and looks like a *great* antenna. It also has plenty of room for
sidemounting on a swinging gate mount. The F12 C19XR is similar but has
different element spacing which looks to preclude this type of side mounting.
When I thought about using 45G tower a tristack of Skyhawks looked very hard
to beat. Big advantage here is you have some F/R so if you need that look
hard at this possibility!!
C31XR stacks. Sounds great, but these are BIG antennas and I'm not sure I'd
be comfortable putting three of these on 45G. Maybe it'd be fine, but maybe
not. I'm sure we'll see some of these go up and we'll see if they stay up!
A two stack might make more sense unless you have 65G tower.
Finally, for a C31XR tristack you are looking at $3,000 for the antennas, a
beefy tower, and some big rotators. I figure the system cost to be around
$12,000 - $13,000 if one is paying retail. For that kind of cash I'd be
looking at how to get two towers up so I could do proper monoband stacks:
20/40 and 10/15!!!!
Synopsis: What started all this was Dink's question about a single tower
contest station. I think most of us assumed all band coverage. Since there
is no single answer I think the debate will continue interesting. For me I
think the conclusion is the following: (1) Get your upper antenna to at
least 90-100 feet so you'll do well on 20M. (2) For contesting three smaller
antennas will be better than two big ones since your goal isn't to open bands
but to be capable once the band is open. (3) Since any triband stack is a
compromise the best value lies in the smaller antennas when one includes
function, overall cost, ease of erection and maintenance.
As a total aside - if I were still on the west coast I might focus on being
loud on a single band rather than OK on 10-20M. A 90 foot tower makes a
great support for a 6x6 15M array that will play right up there with the best
stations. Single band15M is fun from the west and when you can work both
Europe and Asia it's a total blast. You can add 20 and 10M with smaller
antennas - perhaps 3L 20M up top and 4L 10M at 60 feet or so. Alternatively
a second short tower with a tribander gives you more flexibility.
Cheers,
Mark
KI7WX
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