Topband: Unun on FT-240-61

[Sinisa Hristov]

Björn Mohr wrote:


> I am about to make an unun for my top loaded topband vertical using a
> FT-240-61 core. I need a 2:1 ratio as the antenna has around 25 ohms feed
> point impedance using two elevated radials.


1) With FT240-61:  preferably at least 8 turns of a trifilar "cable";
   connect all 3 wires in series (keeping connections short and close
   to the core); connect 50 Ohms to the "top" and the "bottom" leads;
   connect 25 Ohms to the "next to top" and to the "bottom" leads.

2) Without a ferrite core:  make a 4.35 uH inductance and connect it
   directly to SO239 (female) connector; connect vertical wire and
   radials to the connector; tune top wires for resonance (DON'T tune
   by changing the inductance).

3) Even a direct connection to 50 Ohms coax will do no harm,
   SWR of 2 at such a low frequency will not introduce any
   significant loss.

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.


73,

Sinisa  YT1NT, VE3EA