Topband: Unun on FT-240-61

[Björn Mohr]

Thanks for your input Sinisa and George,

It is very useful information. I have to make a correction to my previous
posting though, the antenna measured around 16 ohm, not 25 that I initially
stated. I remember measuring this value at some time, but obviously I have
changed something since then.

I have today tried a bifilar winding of 11 turns and I got a good results
tapping about 1 turn from the centre tap.

Sinisa wrote:

4) With only two elevated radials quite strong common mode currents
   are possible, much worse than with a symmetric dipole. You'll
   need a very good common mode choke with common mode impedance
   in the kOhms range. Most "recipes" floating around will not provide that.

Would a RG213 coil made of 18 turns on a 11 cm plastic pipe be enough for
this you think? I have one that I could connect easily.

The whole antenna is based on the Spideream 18 meter pole and is top loaded
with a three-wire hat. My space is very limited, so the two elevated radials
are bent about 10 meters from the base of the antenna. Lots of compromises,
but the arrangement has worked out OK so far.


73 de Björn /SM0MDG