Topband: End or Centre Feed?

[Roger Parsons]

There is a vague possibility that I may get the W4RNL
array up before the snow flies. This is a half wave
vertical parasitic array. I could practically feed the
middle driven element at the base or at the centre.
According to EZNEC, the 3:1 SWR bandwidth is about the
same with either feed method, and at the base the feed
impedance is roughly 3000 Ohms at resonance, with
major reactive components as I move away. At the
centre the feed impedance is roughly 8 Ohms with the
same situation as I move away. The gain and pattern
are the same in either case.

Seems to me that the pros and cons are:

End Feed:
Very high voltages, and potentially very high losses
if I get the matching network even a little wrong;
The possibility of providing a DC ground and a spark
gap;
When the practical antenna is not quite the same as
the model the matching network can be easily adjusted
- there is no feedline so no high SWR on the feedline;
Potentially more chance of rf in the shack although
the ground system is so extensive that I doubt this
will be a problem.

Centre Feed:
I could use a 1:9 (Z) balun at the feed point to give
a reasonable match to 75 Ohm hardline at resonance;
The balun would be seeing a reactive load off
resonance - not sure how it would like that;
The matching network would be at ground level so there
could be a high SWR on the hardline, but the matching
network would have quite an easy job;
In practice the antenna would not be resonant exactly
at the design frequency, so the match (at the antenna)
would quite likely never be perfect at any frequency
actually used;
I could raise the whole antenna an extra 20ft above
the ground which according to EZNEC gives me a
fraction of a dB more gain....;
DC ground could be at centre of element through a
balun, so presumably not so good as at the base.

Would be grateful for any thoughts.

73 Roger
VE3ZI


 


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