[TowerTalk] HyGain TH-2 problem

[Brian Alsop]

You would think this simple antenna should give one no problems.

Wrong.

Maybe some expert here can help solve this problem.

Unfortunately I can mount it only up 22' on a chimney mount.  Currently 
fed with 61' RG8X and a 6 turn 6" diameter RG213 choke at where the 
feedpoint spreads in Y fashion to the element halves.

It works with one problem.   The 15M SWR is 2.7 at the low end of the CW 
band.  The settings were made for CW.  The resonant point in the HyGain 
manual for this "CW" setting is 21.175 MHz.  That's what I see. The 
curve has not got the Hygain bandwidth.  The lower frequency slope is 
quite a a bit larger.  The above resonance slope doesn't depart too much 
from HyGain's.  This larger slope is what is causing the high SWR at the 
low end of the CW band.

I'm using the K3 SWR readings.  They are not that different from what an 
AUTEK RF-1 gives.

The elements are like this:

--------Y-----T10---X---T15-----Z----   Half driven element.

Where X,Y,Z are dimensions.
T10 is the 10M trap
T15 is the 15M trap.

I wanted to try and move the resonance on 15M down the band about 85 KHz 
to bring the CW SWR under 2:1. I determined that that meant lengthening 
X by 3/8" on the driven element half and 0.5" on the reflector half 
element.  This was determined by extrapolation of the slopes of the 
length curves given in the manual.

"This height at 15M is 1/2 wavelength so it should be high enough to 
avoid too close to ground effects.  Obviously it is about 6' over 
standard shingled roof.

It was expected that the bandwidth is less than the HyGain manual gives 
for 70' up."

I made these length changes and NOTHING changed.  The resonant point was 
identical!

I've ruled out backwards 10M traps and driven element 15M traps are 
indeed on the DE.  20/10M are fine.  Turning the beam 90 degrees doesn't 
change anything.

So what magic dimension does one change to bring the resonant frequency 
down on 15M?

And why 15M and not the other two bands.

"USE A TUNER" is not a solution.

Brian/K3KO

(Previously had a TH-2 at 80' on a HAZER arrangement.  Tweaking it was 
straightforward by numerous up/downs to 40')


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