Given the propagation conditions likely in the next few years, a
stacking strategy may be better if optimized for those conditions. For
tribanders, "optimal stacking" is where you want the best performance
and are willing to sacrifice elsewhere.
So my strategy would be to have the fixed US antennas at top and bottom
with the goal to maximize 20m performance and the EU antenna in the
middle. The arrival angles will be lower to/from US and higher to EU.
To maximize performance, getting the top antenna as high as possible
perhaps on a fixed mast, and the lower US fixed so that separation is
45' or greater would optimize for 20m.
As I understand HFTA it does not accurately predict performance at close
stacking distances, so I would advise using EZNEC to optimize the stack
patterns and checking for interactions. With the US direction downslope
you might make a judgement call about your effective tower height at
that azimuth.
And with tribanders a three high stack may not be worth the investment
and complexity. Another choice might be 2x 20m monobanders as a stack
and a fixed tribander on US for the times 15 or 10 (ha) open. Or just a
15m monobander.
The length of cables to a stack should be physically equal (same reel of
coax too) from the stackmatch since for a tribander you are covering an
octave of frequencies, the length only constrained by how much loss you
can live with. For monobanders, making the cables electrically the same
length at the antenna frequency would be the best method.
Grant KZ1W
On 7/17/2016 9:50 AM, Filipe Lopes wrote:
Hello guys,
I have read quite a few posts here but I still am not sure about the
distances I should use to stack my tribanders.
I am rebuilding my contest station CR6K in Portugal from scratch, meaning
all antenas are gone, new antennas are being constructed and also a new
self supporting cranck up tower 20m high is being built by my dad CT1CJJ.
For this new tower I want to make a stack of 3 x tribanders (10 15 20m) 11
elements each. I tried a few simulations with MMANA and HFTA and what I
found is that my stacking distances are best at 5.4m (+- 17 feet) from each
antenna, meaning I would have the 3 antennas at 11.4 m / 16.8 m / 22.2 m (
37 / 55 / 73 feet). I then read one post from someone who says that HFTA
has some sort of bug that considers maximums at < than 0.5 wl.
For info the top antenna will rotate when necessary and the other 2 will be
fixed to USA and my QTH slopes down a lot towards USA. Also this is mainlly
a USA tower because there is another tower dedicated to EU/ASIA.
So my questions are:
- Is a stack of 2 tribanders @ 60/90 feet enough?
- If I consider the 3 tribanders the top one might be a lot towards EU
and so the other 2 towards USA
- Is 9 m (30 feet) the optimum stacking distance for tribanders?
- Will it hurt having for example 30 feet stacking between the 2 bottom
antennas and 17 feet between the second one and top one?
- I know the coax lines between the antennas and the power splitter need
to be electrically the same but does it matter how long they are, for
example 3x 25m is ok?
All comments and suggestions are welcome.
Thank you so much.
73's Filipe Lopes
CT1ILT - CR6K
F4VPX - TM3M
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