[TowerTalk] One more dipole-balun question

[Jim Brown]

On 6/3/2013 9:45 AM, David Blake wrote:
> are we talking about a G5RV or a Windom or both?

My comments specifically address off-center fed antennas.

As I understand it, the G5RV is a symmetrical antenna, so any imbalance 
will be due to asymmetry of its surroundings -- trees, buildings, ground 
slope, the antenna sloping, etc.

I consider antennas like the G5RV a compromise, both because its pattern 
is different on every band, and because it's not easy to choke it. Many 
of us must make compromises, and this sort of antenna is one choice for 
hams with limited space.

One form of choke that would work for 80-10M is one wound with 12 turns 
of a pair THHN conductors on a #31 core, connected as a series section 
of transmission line.  Like the antenna itself, this choke is a 
compromise, because its own Zo (around 100 ohms) introduces a small 
impedance bump, which in turn changes the match a bit.  The bump is 
"small" because the length of the line is short (less than 3 ft) as a 
fraction of a wavelength, and because the Z of the antenna itself varies 
wildly with frequency.

This choke is NOT a good choice for an off-center fed antenna at power 
levels greater than about 100W, for the reasons noted in my earlier post.

73, Jim K9YC