Topband: Clark SCAM 21 Mast on 160

[Chris G3SVL]

At 17:17 11/01/2009, Denis Jackson wrote:
I have an opportunity to acquire an ex-military pneumatic mast which 
pumps up to 21m high.  I am thinking about how to deploy it on 160

Hi Denis,

Welcome to 160m.

I have a 17m version of your mast -  mine is actually a Racal, but 
the concept is similar.  The first thing to note is that the 
pneumatic seals are neoprene and thus non-conducting, as you 
suspected.  However, all is not lost - here's what I have done in the past.

To use it as a vertical I run a 4-wire cage up the outside of the 
mast - if you spread the four wires out at the base to an 18 inch 
diameter they will stay away from the mast and thus avoid flapping 
against it in the wind and making it sound like a sailing boat 
marina!  I put a 10m flexiwhip on the top of mine and taped a wire to 
that so that I had a 27m vertical. Pay attention to the radial mat 
and it will perform well.

To use as an inverted L, I found the flexiwhip didn't like being 
pulled over at an angle and so I just ran a more solid 3m pole out 
the top and brought the end of the L back to another support. it 
really depends on if you want some high angle as to whether you chose 
the vertical or the inv L.

To match the vertical I used a base loading coil made of 8mm copper 
pipe (from B & Q) and found a taping that got me a good SWR. To match 
the L, I made it 170ft total length and matched it with a series capacitor.

As for the 11KV lines you should obviously ensure that there is no 
way that the vertical will EVER touch them, under any 
circumstances!  But that raises the question of your rx antenna - the 
vertical will be noisy the majority of the time and you should be 
considering a K9AY, beverages, tuned loops etc. to allow you to hear 
stuff.  Of course when it's damp the HV lines may give you hash no 
matter what you are using - there's little you can do about that.

And finally, if you don't have ON4UN's 'Low band DXing' book, you 
should get a copy!

Hope this helps

73 Chris, G3SVL