Topband: Comments on High Performance RX Antennas for a Small Lot (Webinar)

donovanf at donovanf at
Fri Mar 4 12:03:37 EST 2016

Hi Tim, 

While RDF is helpful, nothing substitutes for devoting the effort 
to analyze the detailed antenna pattern. RDF is especially useful 
in quiet rural areas with very few homes and power lines and 
within several miles of our antenna, but its insufficient for most 
of us. 

Very few of us live in an extremely quiet local RFI environment 
any more, especially now that just one RFI generating electrical 
or electronic device in a single home within a mile (or more) can 
suddenly ruin our previously quiet RFI environment. Life was 
much easier when we only had to worry about power line RFI. 

RFI caused by a high efficiency heating and air conditioning system 
in a home a mile from my QTH caused me to install the 8-circle 
W8JI receiving arrays at W3LPL to provide a much narrower main 
beam than I could achieve with Beverage antennas. The 8-circle 
often provides a tremendous improvement compared to my 580 foot 
Beverages which I still have and use. I don't have adequate space 
for longer Beverages or arrays of phased Beverages. 

Most of us care more about narrowing the beamwidth of the main 
beam at elevation angles below about 30 degrees while also 
inimizing RFI arriving at all azimuths outside the main beam at 
low angles. RDF doesn't do that for us, it optimizes over the entire 
hemisphere, often at the expense of better RFI rejection at low 

Better RFI rejection usually results in a different optimization than 
RDF alone can provide. RDF provides a good starting point, but it 
doesn't provide the complete answer for most of us. 


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

From: "Tim Shoppa" <tshoppa at> 
To: "Richard (Rick) Karlquist" <richard at> 
Cc: Topband at 
Sent: Friday, March 4, 2016 2:59:34 PM 
Subject: Re: Topband: Comments on High Performance RX Antennas for a Small Lot (Webinar) 

The RDF seems to be the best we have at the moment, for taking a 
3-dimensional pattern and turning it into a single number. Of course the 
details of the 3-dimensional pattern are lost. 

In addition to the quantitative RDF or S/N numbers, the qualitative change 
in pattern as you move up the RDF is remarkable. We go from 

* no directivity 
* a null in back with not much differentiation between forward and side 
* increasing side rejection 
* near-complete side rejection 
* increasing rejection of directions near forward but not quite forward 

The 8-circle is mind-blowing. 

Tim N3QE 

On Fri, Mar 4, 2016 at 12:37 AM, Richard (Rick) Karlquist < 
richard at> wrote: 

> In this webinar, it was asserted (without explanation) that 
> for every 1 dB increase in RDF, you get 1.5 to 2.0 dB 
> improvement in S/N ratio. I've never heard that before 
> and don't even see how it makes sense. Actually, I don't 
> even know how you can make generalizations like that 
> unless you are describing a theoretical QTH with uniform 
> isotropic noise. I'd like to believe this is true. 
> Can someone educate me as to why I should believe this? 
> Rick N6RK 
> _________________ 
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