The National RF HF DF products do not cover the spectrum between 2 and 8
MHz. I spoke with the engineer at National RF about the 80 meter band. He
told me that they had started working on a loop for 80M but they never
finished it. Other projects demanded their time and the 80M loop was set
aside. They told me that there simply wasn't enough demand to justify
investing the development time. He also cautioned me that the 80M loop would
likely end up being rather tallish.
I asked how they thought the 8 MHz loop would work at 4 MHz. They told me
that the nulls would likely be skewed and not as deep. The vast majority of
my "HF only" cases were copyable to at least 7 MHz. In the majority of those
cases, as I got closer I could hear the RFI at least one more MHz higher.
Those cases which are no higher than 4 MHz will just have to be worked with
the G4TPH magloop. For all the other "HF only" cases, the National RF Model
HFDF should do nicely and be a great deal easier with which to work.
In any case, I elected to proceed with my order for the National RF Model
HFDF active antenna device and all 4 available loops. The loops cover 8 to
50 MHz. If the system works very well, I might commission an additional loop
that covers 80 meters as well.
As to popular 80M handheld DF units popular with ARDF competitors, there are
some commercial unit available in Europe but none in the U.S. that I know
of. I don't know the names of any of the European units and, frankly, I
haven't tried to find out.
One person who contacted me told me about Wellbrook, a UK firm that makes
nice active loop antenna systems. (http://www.wellbrook.uk.com/antennashop/)
The model I would like to try costs nearly $400 including shipping. While
I'd be willing to spend the money, the form factor doesn't look like it
would suit my need to be easily mobile and on foot.
I've been told that many ARDF competitors build their own handheld 80M DF
units. There isn't much published info about current designs. In fact, I
have only found one elderly article about a homebrew unit. Parts for that
design have become obsolete and unavailable. I'm not interested in building
anything anyway. Perhaps others on this list know the brand names of popular
commercial European 80M handheld DF units used by ARDF competitors?
Frank N. Haas KB4T
Utility Interference Investigator
RFI mailing list