Topband: Propagation improves from VK6 into Europe
lee at k7tjr.com
lee at k7tjr.com
Fri Nov 16 13:44:34 EST 2018
Well said Frank,
When I was first experimenting with the Hi-Z all active 8 element array through modelling I placed a simple 125 foot vertical in the model and found serious interaction at 800 feet separation and some pattern perturbation at 1000 feet. However, we routinely can recommend nesting a 160 meter and 80 meter version of 8 element high impedance receiving arrays (200 and 100 foot diameter) as we have also not seen serious mutual impedance affects. RDF or directivity with any close physically long pseudo element like power wires or resonant element comes with a price on top band for sure. My conclusion at that time was that a single antenna with greater than 13.5 dB RDF was not likely to work at most ham radio sites due to interaction with something. However, phasing two lesser-compromised high RDF antennas is working well providing modelled and anecdotally observed RDF of 15 dB (Where is the government budget when we need it). Fortunately even with some pattern or RDF compromising interference a good receiving antenna remains much better than most simple transmitting antennas for low noise receiving.
Lee K7TJR OR
Corollary: You can't have too many antennas except when they interfere with each other which they often do on Topband where a wavelength is approximately 500 feet...
Even on 20 meters, the pattern of a Yagi is noticeably degraded when it points through another 20 meter Yagi or tribander 500 feet distant or more.
Fortunately most Topband receiving antennas don't interfere with each other -- except at extremely close spacing -- because their mutual impedance is negligible. But transmitting antennas and other antenna-like radiators such as power lines -- even 1000 feet away or more -- can significantly degrade the directivity of Topband receiving antennas.
More information about the Topband