[TowerTalk] Radial question

[jimlux]

On 2/29/16 3:01 PM, Richard (Rick) Karlquist wrote:
>
>
> On 2/29/2016 2:29 PM, jimlux wrote:
>>
>> So, if you had a 5-10 meter tall radiator, being used on 80m or 160m,
>> there's not a lot to be gained by going longer than 15-20 meters for the
>> radials.
>>
>
> This often heard concept makes intuitive sense.  Kind of
> like how the roots of a tree should be of a length related
> to the tree height or width.
>
> However, it simply isn't true.  Actually, the opposite
> is true.  Jerry Sevick, W2FMI, wrote  an article for QST
> a long time ago that famously had a photo of a 40 meter
> vertical that was something like 7 feet high, with a top
> hat, but worked over a huge radial field.  The thing
> that was funny about the photo was that the antenna was
> made of a clothes dryer with metal arms that served as
> the top hat and the pole was the radiator.  The concept
> was that the less you have in the air, the more you need
> on the ground.  Jerry did seem to know what he was talking
> about.
>

I'll agree you need to have a LOT more on the ground "close to the 
radiator", but did Jerry ever try his 7 foot "Hills Hoist" antenna on a 
radial field that was, say, 30 feet in diameter (1/8 lambda radials in 
free space, radials twice the height of the radiator), but still much 
shorter than quarter or 0.4 lambda.


I'm basing my statements on work with tesla coils - there's not much to 
be gained by making the counterpoise under the coil (a very conductive 
sheet: chicken wire or solid aluminum foil or similar) have a radius 
much greater than the height of the top load above it (e.g. a 4 foot 
high tesla coil works about the same with a 8 foot diameter and 12 foot 
diameter counterpoise, but not so well with a 6 foot diameter counterpoise).

A single short monopole might be very different than one with a capacity 
hat: the current profile on the radiator will look like a ramp with the 
straight monopole and more uniform with the capacity hat.

For the tesla coil world, you can analyze it pretty much as an 
electrostatic system: everything is a tiny fraction of the wavelength 
(operating frequencies in 100s of kHz range).

The antenna case is not quite so simple, but for very short radiators, 
maybe it is.



> The "capture area" of a short vertical is nearly the same
> as that of a 1/4 wave vertical, no matter how small it is
> (at least in the absence of losses).  Thus there is no
> justification for shortchanging it on radials.

I'm not sure that's been actually modeled: I've got a copy of Nec4, so 
maybe it's time to try it.