Barrett wrote:
I have two beams I want to install on the same mast. They are, Cushcraft
MA5B and an 9 element crossed Tonna.

What is the optimal distance that these antennas should be spaced?

What is the minimum distance apart can they be mounted before they have a
negative effect on each other?

Does it matter what beam is mounted on the top?

Does it make any difference how close the beams are mounted to the rotator?

This is the first time that I have put any beams up, so I thought I better
get some good advice first.

This page will show you how to think about it:
http://www.ifwtech.co.uk/g3sek/stacking/stacking2.htm

You'll see that most of your questions have no definite answers. Very
close stacking will cause some degradation in performance - especially
for the 2m antenna [1] - but how much degradation depends on the
specific antennas, and can only be determined by modelling.

The "optimum" stacking distance will also depend on your priorities
between HF and VHF, and on how much degradation in performance you are
willing to accept.

There are also mechanical considerations. It might be better to mount
the HF minibeam at the very top of the stub mast [2], to put it a larger
number of wavelengths above ground; but that will create a lot more
bending force than mounting the HF antenna below the VHF one. Again,
this is very much your decision.

With winter coming on, I'd suggest that you go for strength over
performance, and mount the HF antenna very low on the stub mast. The 2m
yagi could then be about 1 metre above it. This may not be completely
"optimum", but it stands a better chance of still being there next
spring.




[1] Whichever way up you install the two beams, the mast will affect the
performance of the vertically polarized part of the 2m crossed yagi,
because it's in the same plane as the vertical elements. Even if you
mount the crossed yagi at the top of a fibreglass mast, the coax
feedlines will have much the same effect. However, if you are only using
vertical polarization for semi-local working with good signal/noise
margins, you may decide to accept the reduced performance. Most people
do, or aren't even aware of it.

[2] Since you are posting from the UK to an international newsgroup,
we'd better be aware of a US/UK language difference. In the USA, the
"mast" usually means only the tube above the rotator, and people may be
assuming that of course you'll have a lattice tower :-) In the UK, the
word "mast" is often applied to the whole structure (even to a lattice
tower) and we usually call the rotating part a "stub mast".



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek