[TowerTalk] 40m Moxon over or under 4-el Steppir?

[Tony Brock-Fisher]

I have had a 2 element 40m beam and a tribander on the same mast/tower 
for decades. At first the 40m beam was a 40-2CD, later a W6NL Moxon. In 
early days the tribander was a KT34-XA or KT36-XA, but for the last year 
it has been a Skyhawk. Very early on I had the 40m beam at the top of 
the mast and the tribander under it at the top plate. This seemed to 
make sense with the windloading of the 40m beam being less than the XA. 
It was also 'conventional wisdom'.

But then I rethought things. I analyzed the antennas with HFTA (Thanks, 
Dean)! By swapping the two antennas, the tribander picked up around 1 db 
on all three bands from the added height, while the 40m beam lost less 
than 1 db from being lower. My decision was based on +1db on three bands 
being much more valuable than -.7db on one band. So from then on I have 
had the tribander at the top and the 40m beam underneath it. Performance 
has been excellent. Changing from the 40-2CD to the W6NL Moxon was a 
night and day, slap yourself in the forehead, why didn't I do this 
sooner, improvement. But that's another thread...

In making the swap, I also made another change. Originally I had 68' of 
Rohn 45 with a long mast sticking out of the top plate. When I reversed 
the antennas, I added another section of Rohn 45 so I now have 78'. The 
tribander now goes on a short mast at the top plate and the 40m beam 
went on a swinging gate mount, now a ring rotor, below it. The top guys 
are still at 66', so there is about 1 section of tower above the top 
guys. This configuration has been much easier to maintain than a long mast.

Over the years there have been interactions. Most notably the KT36-XA 
boom is resonant on 40m, and caused SWR on the 40m beam to go above 2:1 
depending on relative alignment. This was cured by insulating the 
reflector from the boom on the KT36-XA. This is not a problem with the 
KT34-XA as its elements are insulated from the boom, but that's yet 
another thread...

Bottom line: I suggest you analyze your choices with HFTA.

-Tony, K1KP