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
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