Topband: Dual Band Inv L - 2 Points to clarify

[i4jmy@iol.it]

Hi Brian,

if my memory doesn't fail, once I analyzed something very similar for a 
friend, the feed point impedance of your antenna should be in the range 
of 40 Ohm resistive on 80m (3.5 MHz) and 6 -j400 on 160m (1.8), this 
assuming your ground losses are only a few ohms.
In my opinion the 6 ohms are already in a critical area to be managed 
and to have a decent performance or you minimize line losses on top 
band or you lenghten the radiator to rise its feed point impedance.
As W8IJ already explained, feeding an unbalanced antenna with a 
balanced line causes a number of extra troubles that add to inherent 
line losses, expecially in the case where the feed point antenna 
impedance is rather low.
Having a real good ground one may try choking the balanced line (you 
can try to wind it over some ferrite stick, like on PA filament chokes) 
but if I were you I'll will use coax, lenghten the 80m radiator until 
it's close to an equivalent 120° and I'll use some matching system at 
the antenna base.

73,
Mauri I4JMY

 


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