Tom:
MOST antennas are in a "neither" world of being neither perfectly
balanced nor perfectly unbalanced.
How about an inverted L longer than 1/4 wave but optimized with series
capacitor? Any closer to perfect unbalanced?
Bob VE7BS
Hi Bob,
Take the case of a 1/4 wave groundplane with four perfectly resonant
radials. There is enough common point voltage to excite the supporting mast
and coax with so much current the pattern is changed and SWR changes a quite
noticeable amount with mast or cable length. When designing and testing a
commercial low-band VHF groundplane I found SWR could be made to go out of
acceptable range from common mode. I never would have thought so, but it
did.
The point I want to make is **perfect** unbalance is as rare as perfect
balance. This does not mean we need perfection, but we should be aware that
a "not balanced" feedpoint does not mean the antenna is perfectly
unbalanced. Most systems are in a world of being neither perfectly balanced
nor perfectly unbalanced, and live someplace between the ideal condition of
perfect balance or unbalance in a shade of gray.
Certainly the less current driving the ground, the lower ground system
common point voltage will be for a given ground system.
I don't mean to alarm people into thinking they need extreme measures (they
almost never do with more than a several resonant radials), but in a case of
high common mode noise or higher radial voltages (from sparse or truncated
grounds) some additional isolation or feedline planning can be good.
Isolation may not be necessary, and certainly we have gone overboard in some
cases, but isolation is almost never harmful.
I would not worry on ground mounted verticals with 15-20 or more reasonable
length radials, unless the installation had high levels of common mode
noise. I would worry with smaller systems, and the smaller the ground system
the more I would worry.
73 Tom
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Topband reflector - topband@contesting.com
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