[TowerTalk] Shunt fed resonance changing.

[Jim Brown]

On 12/3/2018 12:26 PM, James Wolf wrote:
> My problem is that the resonance on 160 meters changes about 50 kHz.  This
> usually seems to happen when using amplifier power.

How are you determining resonance?  Do you mean minimum SWR? How are you 
measuring it? Are you using the same SWR indicator for low and high 
power levels? Or the rig when running barefoot and the amp when it's in 
use?

How are you feeding the tower?  Through fixed capacitors?  Many high 
voltage capacitors have high temperature coefficients, and many HV caps 
have significant loss.  If you're using fixed caps anywhere in the RF 
path, I suggest that you TX at power for a while, then feel them and see 
how warm they are.

Capacitors have a characteristic called Equivalent Series Resistance 
(ESR), which represents the loss in their dielectric at radio 
frequencies. Caps used in the signal path need low ESR; many HV caps are 
used simply for bypassing and don't carry much RF current. Caps made for 
this purpose often have high ESR, but they're a lot cheaper.

> do around the bottom of the tower, or the feed point to create the resonance
> change.  However, I can at times move guy wires and see the resonance
> change.

Guy wires that can move can make the tower unsafe -- they work by 
putting triangulated tension at the tower guy points!  Are your guy 
wires broken up with egg insulators to minimize interaction with your 
antenna(s)?  Could banging on guy wires be causing a flaky mechanical 
connection to go intermittent?

73, Jim K9YC