Topband: Inverted L SWR Jumps ???

[Tom W8JI]

> 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