[TowerTalk] 16om vertical and the number of radials

[Lux, Jim]

On 12/3/22 12:39 PM, Leeson wrote:
> My thought on this is that the very shallow angle grazing reflection 
> from the dielectric discontinuity at the distant ground surface is 
> pretty much 100%. The ground permittivity and conductivity variability 
> would have a bigger effect for higher radiation angles.

Only for Horizontal Polarization.

For vertical pol, it's a lot more influenced by the soil properties, 
more to the point, the phase angle of the reflection varies with soil 
properties and angle, a lot more than for H-pol. H-pol has a reflection 
phase of pretty close to zero degrees regardless of the angle of 
incidence.  (this is why HFTA only does horizontal pol - it's easier to 
calculate)

The ARRL Antenna Book has some graphs that show that at 1.8 MHz, the 
pseudo brewster angle (where the reflection coefficient magnitude is 
lowest) changes from about 1-2 degrees for the proverbial salt marsh to 
30 degrees for "extremely poor", with "average" coming in around 8 degrees.





>
> For fire protection and layout reasons, I have only two elevated 
> radials on my full-size quarter-wave vertical, and it seems 
> competitive enough. Because the SWR is low without any matching, I 
> assume there's some ground loss, but my 12° sloping foreground seems 
> to make up for it. From modeling, I expect a little gain to the 
> northeast from the 140' tower reflector behind it.
>
> We used two full-size half-wave verticals above a sloping foreground 
> in our HC8 station, with very good results. One was always a little 
> better, but it was hard to predict which one by direction alone. 
> "Person with one watch knows what time it is; with two, never quite 
> sure."
>
> Dave
>
> On 12/3/22 10:21 AM, Brian Beezley wrote:
>> Paul, W9AC: "Isn't the measured probe result only useful for 
>> near-field system efficiency analysis? By near-field I mean to obtain 
>> system efficiency within a wavelength or so of a vertical radiator.
>>
>> But for skywave propagation field strength, don't we also need to 
>> know more about the ground conductivity much further out to more than 
>> 1km on 160m?"
>>
>>
>> Paul, I don't know how distant ground affects 160m specifically. But 
>> if you're on a hilltop, at low elevation angles your signal may 
>> reflect or diffract from ground miles away. Its permittivity and 
>> conductivity might be quite different than that of the ground beneath 
>> your antenna.
>>
>> I think the main usefulness of knowing your ground characteristics is 
>> to predict antenna efficiency. That could easily determine your 
>> choice between a horizontal and vertical antenna on 80m or 40m.
>>
>> If you have some idea of the ground quality far away, you could 
>> create a separate model with that value to study low-angle effects. 
>> You could take a drive and go measure distant ground. But you'd 
>> probably have to take a number of measurements to satisfy yourself 
>> that you had a representative sample. My ground probe calculator 
>> includes a utility that will average probe measurements. I had in 
>> mind making multiple measurements near the antenna, but you could use 
>> it to create an average of far-away ground for a low-angle model.
>>
>> Brian
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