[TowerTalk] R7000

[jimlux]

Joe Subich, W4TV wrote:
>> My test was with the capacitive hat (the short radial rods) 
>> at the lower end of the antenna about 12" off ground.
> 
> That's a rather rough test for the R7000 or any "no radial 
> vertical."  The radials are "hot" and couple rather heavily 
> to the soil.  Performance should improve significantly by 
> moving the base up to 15 feet or so (1/8 wave at the lowest 
> operating frequency). 
>   
> 
>
I don't know that the fractional wavelength is the right way to think 
about the spacing.  I'd say it's more a function of the physical size of 
the radiator.  (without actually doing any analysis)

My thinking runs along these lines..

If you think about a physically small dipole or loop, what you're 
concerned about is how much of the near field interacts with the (lossy) 
soil. The near field is where the energy is stored in the antenna system 
(compared to that energy radiated away). A physically smaller antenna 
will have a smaller near field (i.e. higher absolute field in a smaller 
volume)..


Compare two capacitors with equal capacitance, one with 1 meter square 
plates a meter apart, the other with 10 cm square plates 1 cm apart. 
Charged to the same voltage (i.e. same stored energy), in the big cap, 
the E field is 1/100th that of the small cap, but occupies 10,000 times 
the volume (1E6 cm^3 vs 100 cm^3).  I'm not sure, but I think that if 
you drew a boundary line around a certain amount of "contained energy", 
it would be smaller for the smaller cap (including all the fringing 
fields, etc.)

Same for an antenna. (maybe... I'll have to think about it)

In any case, I think the basic concept is more like an untuned radial 
field: radial length is comparable in length to the physical height of 
the radiator, rather than the electrical height.

Jim