Topband: Low vertical and high inv-L

[Ford Peterson]

Uli wrote:

> Questions:
> Is there anything that could be done to improve the GP towards
> North-West with the existing feedpoint?
> Any chance that the inv-L is better into NW because the GP acts as
> reflector? (Detuning the GP doesn't show any difference)
> If the feedpoint is elevated to the same level as the inv-L (like
> grounded tower feed system by N4KG), would there be an improvement
> towards NW?
> Can elevated radials compensate for irregularities in the ground below them?
> Has anyone a similar problem, and how was it solved?
> 
> Thank you very much in advance.
> 73s Uli, LA0CX

Uli,

In general, the hat wire on an inverted L will skew the pattern by a couple of dB.  The pattern tends to favor the direction opposite the hat direction.  So if the hat is running NW, the pattern will be skewed to favor the SE.  

Likewise, radials will also skew the pattern when mounted on one side of the tower only.  Not by much--maybe a couple of dB.  The pattern will generally favor the direction of the radials.  So your 45-270 degree pattern of radials will generally cause a peak response in the same general direction.  With so many radials down (you don't indicate the ground quality), my guess is that you would not expect more than a couple of dB of skew.

Could it be that you have a combination of effects that cause the 10dB skew?  The second resonant antenna so close may also effect things a bit.  Can you fabricate a quick way to detune the unused tower remotely?  Perhaps an assistant can help you with an A-B test by manually detuning / tuning the tower while watching the feedpoint impedance on the other antenna.  Any variation in feedpoint impedance would 'suggest' tight coupling, and therefore would have an effect on pattern as well.  It would take careful modeling of the system to identify what effect that might be.

My guess is that you have up to three contributing factors.  Isolating each one sounds like a long weekend project--and there may not be a 'fix'.

Good Luck.

Ford-N0FP
ford at cmgate.com