[TowerTalk] 80m Vertical

[Tom Rauch]

> >I would absolutely series feed the antenna with a large number of 
> >radials on each element. Either buried, on the ground, or in the air
> >radials are fine as long as you have at least 30 on each vertical.
> >
> >My 160 meter "four-square" (using 115/230 degree phase shift) has 
> >60 shallow buried radials, and it actually measures over 6 dB up 
> >from a 200 foot tower with 100 200 foot radials.
> 
> I presume that these field strength readings are made on the ground at a
> number of wavelengths from the array.  How is the relevance of these
> measurements affected by their being made at ground level -- presumably a
> fractional degree of elevation?  Isn't this the same problem that people
> (including me) had with the K7LXC/N0AX tribander tests?   

Ground wave measurements of verticals are NOT influenced at a 
distance by polarization "filtering".

Imagine a path with many dB more attenuation to horizontal 
signals than vertical. If you want to compare two verticals, distance 
is not a factor once you are out of the near field. 

Both vertical antenna systems, providing they are close to the 
ground, try to have maximum radiation along the ground and have 
similar elevation patterns. Any ground loss affects both equally at 
any distance because patterns are similar, broad, and in a 
polarization that favors least attenuation along the path. 

(Don't go by the Eznec patterns, they are calculated at an extreme 
distance over flat earth where attenuation has brought ground level 
signals to zero. The "real field" is not attenuated near as much at 
reasonable distances at ground level. I wouldn't measure my 
verticals 100 miles away, so the zero degree FS won't be zero.)

On 160 meters, there is absolutely no chance I have scattering 
from power lines or other reradiators. There is nothing metal within 
five miles of here more than 40 feet tall, and nothing in the direction 
I measured at all except pasture land with no buildings. 

This is not the case when the path attenuates the antenna 
polarization much more than a "non-radiated" polarization. Even at 
my location, I would be worried about power lines and other large 
horizontal conductors if they were anywhere near as close to the 
antennas as the measurement point. In this case, if the distance is 
not significantly shorter to the measurement point than it is to the 
"range pollution", even a small amount of reradiation from anything 
in the path (or at either end) can add vertical components. Vertical 
components are not attenuated nearly as much with distance as 
the horizontal components. The longer the path, the greater the 
error.

Try this test. Put a dipole up several wavelengths from a vertical 
and rotate it while watching the signal. You'll see the FS nulls 
when the dipole is broadside to the vertical! You can actually have  
more signal near or off the ends of the dipoles than off broadside. 
This can also be true with horizontal antennas at each end, when 
the distance is large.


73, Tom W8JI
w8ji@contesting.com

--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com