[TenTec] Antenna is up, but the coupler doesn't like 160 meters

[Jim Brown]

On 10/11/2013 9:36 PM, Mike Bryce wrote:
> I have a 160 meter doublet feed with 600 ohm line. 130ish feet on each side.

Please define "doublet."  Do you mean a single wire dipole, or do you 
mean a folded dipole?  How high is the antenna?  How long is the feedline?

A single wire dipole has a feedpoint Z in the range of 40-80 ohms, 
depending on height and the nature of your ground. A "folded" dipole 
tends to look more like 300 ohms if it is high enough. Neither of these 
is a good match to 600 ohm line, so the transmission line will transform 
the antenna Z to some value that depends on the antenna Z and the length 
of the line.

You say "1:1 balun."  What is it, really?  Is it a common mode choke?  
How is it made? Is it a string of ferrite beads, or something better?  
Most things called 1:1 "baluns" are a string of beads wound onto 50 ohm 
coax.

If I had a single wire 1/2 wave 160M dipole (and I did until about a 
year ago), I would feed it with 75 ohm coax if it was higher than about 
100 ft (mine was), and 50 ohm coax if it was lower.  If I had a folded 
dipole, I would feed it with 300 ohm line, put a 4:1 transformer near 
the shack, and run either 50 ohm or 75 ohm coax to the rig.  These 
solutions make the antenna a fairly good match to the line, so the 
transmission line doesn't transform the antenna to some wild value that 
the tuner can't match.

The only good reason I can think of for a folded dipole is slightly 
greater bandwidth, and we can get the same benefit from using spaced 
parallel conductors from the common feedpoint, which keeps the feedpoint 
Z in the 50-75 ohm range, which means we can (and should) use coax to 
feed it with a serious ferrite common mode choke at the feedpoint (up in 
the air).

73, Jim K9YC