Topband: [RFI] Powerline noise question

[John Kaufmann]

The WD8DSB mini-flag truly is a fantastic RFI-hunting tool.  When I saw the original article in QST, I recognized immediately that this was a great "invention" by WD8DSB.  Like many of you, I deal with quite a bit of man-made RFI on 160m, so I bought one of the very first ones sold by DX Engineering.  Of course, it's not hard to build your own from the QST article.  The antenna is very broadband and works from the medium wave bands up to at least 50 MHz.  Having a unidirectional pattern with a sharp null off the back is a huge advantage over conventional bidirectional DF loops.  You can narrow down direction of signal arrival unambiguously to within a few degrees.
As W3LPL says, it is a very low gain antenna that does need a very good preamp behind it.  You need a preamp with not only a lot of gain but also a very good noise figure because ultimately the sensitivity for hunting weak signals is limited by noise.  I have the companion preamp that DX Engineering sells for this antenna.  Its noise figure is about 4 to 5 dB, which is quite good for a reasonably priced, battery-powered preamp that you can carry around with you.  By way of comparison, I also have a homebrew high gain (35 dB) preamp, made with MMIC's, with a noise figure of 2 dB, which is very close to the best you can do for HF.  However, most of the time I use the DX Engineering preamp because it is good enough for all but the weakest signals.
Bottom line is that the WD8DSB mini-flag and a portable receiver are all you need for tracking down noise sources.  As one more auxiliary tool, I sometimes use the tinySA Ultra (an improved version of the original tinySA hand-portable spectrum analyzer) to look at the spectral signature of a suspected source in the field to confirm it's the same source that I'm seeing at my station.  See tinysa.org/wiki/ for more information.
Disclaimers:  I have no affiliation or commercial interest in DX Engineering or the tinySA product.
73, John W1FV


-----Original Message-----
From: Frank W3LPL <donovanf at starpower.net>
To: Richard <richard at karlquist.com>
Cc: rfi <rfi at contesting.com>; PVRC <pvrc at mailman.qth.net>; topband <topband at contesting.com>
Sent: Fri, Dec 30, 2022 8:09 pm
Subject: Re: Topband: [RFI] Powerline noise question

Rick,

You'll be very pleased with the performance of the WB8DSB RFI hunting
flag antenna.  

Don't forget to need lots of preamp gain. The WB8DSB flag is about 60 dB
down from a dipole on 160 meters. 40 dB of preamp gain is needed until
you're very close to the RFI source.

73
Frank
W3LPL

----- Original Message -----
From: "Richard" <richard at karlquist.com>
To: "Frank W3LPL" <donovanf at starpower.net>, "rfi" <rfi at contesting.com>, "PVRC" <pvrc at mailman.qth.net>, "topband" <topband at contesting.com>
Sent: Friday, December 30, 2022 8:25:47 PM
Subject: Re: [RFI] Powerline noise question

Thanks Frank.  You got me unstuck.
I have ordered the DX Engineering Kits and the PL330 receiver.

73
Rick N6RK