[TowerTalk] Impedance at the Feedpoint

[Jim Brown]

On 11/6/2010 9:53 AM, Bill Gillenwater wrote:
> If you have a feedpoint impedance of 32 ohms on an inverted "L" for 1.8 mhz, can anyone venture a guess as to what the impedance would be of the same antenna at 3.5 mhz ??

VERY high -- if it's 32 ohms on 160M, it's close to a quarter wave, 
which would make it close to a half wave long wire on 80M.  WAG about 
10K ohms.

A good compromise is to make it a bit shorter (or longer) on 160, which 
makes the Z not nearly so high on 80 (because the Z rises VERY fast as 
the antenna hits a half wave).  How much is "a bit short or long?"  I'd 
say 10-20%. Several years ago, I had a Tee vertical that was close to 
resonant on 160, fed by about 30 ft of RG8.  I could load and run full 
power to it through my Ten Tec 229B on 80 CW, but not on 75M -- it was 
too close to a half wave there.

73, Jim K9YC