Topband: Low Dipoles

[donovanf at erols.com]

Its a fool's errand to use only antenna modelling software to evaluate 
the complexity of topband transmitting antenna performance. 


Why is that? Antenna models do not take into account polarization 
mismatch loss, a fact of life because the electron gyrofrequency 
is so close to 160 meters. See K9LA's excellent article: 


https://k9la.us/Polarization.pdf 


Antenna models also do not consider the lossy ground reflection 
in the Fresnel zone in front of the antenna. The Fresnel zone isn't a 
thin line along the ground in the direction of the distant receiving 
antenna, rather it has an elliptical shape with significant width. 
That's why HFTA analysis should always include ray traces at 
multiple azimuth angles in front of the antenna, unless the reflection 
zone has uniform terrain, such as several square miles of prairie or 
seawater. 


The classic reference on this topic is: 


https://nvlpubs.nist.gov/nistpubs/Legacy/TN/nbstechnicalnote139.pdf 


While the NBS publication primarily addresses horizontally polarized 
antennas, its general conclusions also apply to topband vertical 
antennas. 


More than half of the low angle radiation from a 1/4 wavelength 
160 meter vertical is formed in an approximately 500 foot wide 
zone in the direction of propagation from the transmitting antenna 
to the intended receiver. 


The far edge of the Fresnel Zone is several miles away for low angle 
radiation from a 1/4 wavelength 160 vertical, especially if the 
vertical is installed on a highly conductive surface such as sea water, 
salt marsh or wet marshy soil. 


The far edge of the Fresnel zone is much closer when a 160 meter 
1/4 wavelength vertical is installed over poorly conducting dry, 
chalky or sandy soil. 


A specific worked example: 


For a 10 degree elevation angle more than half of the radiation from 
a 160 meter 1/4 wave vertical is formed in an area approximately 
500 feet wide in the desired direction of propagation. The near 
edge of the reflection zone is approximately 3000 feet in front 
of the vertical and the far edge is about three miles from the 
the vertical 


Its obvious that an extensive radial system has no affect at all 
of the efficiency of the ground reflection in the Fresnel Zone. 
Radials affect only the efficiency of the antenna itself. 


73 
Frank 
W3LPL 





----- Original Message -----

From: "K4SAV" <RadioXX at charter.net> 
To: topband at contesting.com 
Sent: Friday, December 18, 2020 4:48:20 PM 
Subject: Re: Topband: Low Dipoles 

On 12/18/2020 10:18 AM, Mike Waters wrote: 
> A few words of wisdom about 160m antennas from W8JI, ON4UN and others... 
> 
> https://web.archive.org/web/20180815141931/http://w0btu.com/160_meters.html 

After you read W8JI's comments, put these two antennas on EZNEC. It 
will say the dipole smokes the vertical at all elevation angles. I 
trust W8JI's information because of his experience and his attention to 
detail when running tests like this. NEC is not telling you the whole 
story. 

Jerry, K4SAV 


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