[TowerTalk] 80 meter antenna choice

[jimlux]

On 7/9/20 9:48 AM, Richard (Rick) Karlquist wrote:
> 
> 
> On 7/9/2020 9:07 AM, jimlux wrote:
>>
>> Then, there's the "power absorbed in the soil" issue - an elevated 
>> vertical dipole, or a limited number of above ground horizontal 
>> radials both have the potential problem that the near field of the 
>> antenna is "in the soil" and absorbing power.  Highly conductive or 
>> highly wet soil both have a good (high) reflection coefficient, so 
>> less of the field winds up being absorbed.
>>
> 
> I have heard this kind of theory before.  But I did A/B testing of a 1/4
> wave ground mounted vertical with 32 1/4 wave radials vs a 1/2
> wave ground mounted vertical (driven from the bottom) with no
> radials or counterpoise (except the coax shield running away.
> This was on 20 meters.  As far as I could determine from listening
> to signals on the air, the antennas were equal, over my high
> conductivity ground.  The drive impedance of the 1/2 wave
> was around 900 ohms; a base mounting matching network
> was employed.

Yeah, I think that the "soil loss" aspect is probably over estimated in 
most cases.  Of course, you did have 32 radials, as opposed to say, 4.

was the base mounted matching network "grounded" in any sense (other 
than from the coax shield capacitively coupling).

So in this case, the monopole was at ground level, and the dipole's 
center was 5 meters up - so the "radiation center" was slighly higher.

That might have an effect on the null at zero elevation.


For a horizontal dipole, laying on the ground (or very close to it), the 
power that goes into the ground vs the air is larger by a factor of 
epsilon^1.5 (see the paper from Rutledge and Muha)
So for epsilon=13, it's a factor of 46 (that is, the power radiated into 
the air is 17 dB *less* than the power into the ground)
I would think that this is similar to the case with the radials - that 
is, they're not contributing as much to the radiation as the currents in 
the dirt.



*D. Rutledge and M. Muha, "Imaging antenna arrays," in IEEE Transactions 
on Antennas and Propagation, vol. 30, no. 4, pp. 535-540, July 1982, 
doi: 10.1109/TAP.1982.1142856.

Rutledge's results were analytical, but they match NEC4.2 models for 
wires of various lengths and heights over soil with epsilon=3.

> 
> 73
> Rick N6RK