[Elecraft] FW: K2 Frequency Shift On 80 Metres

[Guy Olinger K2AV]

A dipole can have a center impedance anywhere between 50 and 90 ohms,
depending on height above ground.  Off-center feed impedances are
subject to wild variation depending on height.  A guaranteed 300 ohm
feed at a certain offset is simply a myth.

Further, with off-center feed the issues with the feedline shield are
exacerbated, and switching short end to shield and long end to shield
connections make the antenna behave quite differently.  This MIGHT
improve your feedline current problem, just depending on the
particulars.

If that were not enough, baluns which can behave with 50 ohm feed Z
and near resistive impedance become ineffective to the point of
invisible when confronted with largely reactive impedances in the 500
ohm plus range.

To be blunt, your setup is practically guaranteed to have high current
on the feedline.  You would never get me to bet real money that it did
NOT.  Sucker bet.

If you want to have a "hanging" network off center feed to multiband,
use a stepdown transformer THAT HAS NO CONNECTION between the high and
low side (isolation transformer) and use balanced feedline to the
ground. Use a second isolation transformer to go over to coax.
Unfortunately, nobody makes those, and they have to be fabricated at
home.

73, Guy.

On Tue, May 3, 2011 at 1:26 PM, Ray Spreadbury <ray2.s at btinternet.com> wrote:

> The antenna is currently fed with RG213 cable and there is a 4:1 balun at
> the feed point so as the impedance at the dipole is supposed to be about 300
> ohms I thought it would be a reasonable match for the 50ohms cable?
>
> Anyway the antenna was OK until yesterday, when I raised it a bit, but
> obviously changed something to now cause the rf in the shack.