I want to thank everyone who wrote me both publicly on this list and
privately with suggestions and comments on this subject.
I have been reviewing literature for about a dozen different loop style
antenna systems. Units reviewed range from the $99 original Q-Loop to the
$6400 Cubic Communications Military HF DF Antenna Model HH1307a.
I have chosen to evaluate the ($240) NATIONAL RF Model HFDF system. I
believe it will fit nicely into my preferred search methodology. The package
is well suited to foot searches and should work nicely with all of my
receivers. I hope to have one in hand in 2 weeks or so. (Each unit sold is
built to order so it takes time to get them.) Once I've had some time to
evaluate its performance, I'll share my comments with this list.
Interestingly, no one commented about the loop antenna products I mentioned
in my original post. Nor did anyone comment on the 80 Meter ARDF equipment
that is popular in Europe or the U.S. Most of the input I received focused
on products that are no longer in production. I can only conclude that few
subscribers to this list do much HF DFing of RFI. With all the new sources
affecting the HF spectrum these days, many of which are mistaken for power
line interference, I would think that good HF DFing tools are going to be
much more in demand.
A recent case I worked where the customer's receiver was affected by
significant power line interference coming from a distant (about 1 mile)
source has proven to me that being able to quickly get good bearings at HF
frequencies can speed the search.
The customer was affected up to about 8 MHz only. The RFI was very strong
all around the customer's neighborhood but could only be heard up to about 8
MHz. This particular customer lives on the shoreline of the Intracoastal
Waterway. It was easy to find 4 RFI sources near the customer's home that
produced RFI copyable to VHF frequencies. But none of these sources were the
real offender. Nearly a mile to the WSW, across the Waterway, was a
substation where the real offender was situated.
With 4 sources eliminated and the basic problem still raging, it was clear
that the 8 MHz source was not nearby. Only when I started using my G4TPH
loop antenna system was I able to draw lines on a map that pointed directly
at the substation. The source at the substation was strong enough to be
heard 3.53 miles away on 8 MHz. The higher frequency components (30 to 350
MHz) could only be heard within a 1/4 mile of the substation.
Despite the fact that the substation (located on the opposite shoreline of
the same Waterway) has 2 submarine cables that come ashore 2 blocks south of
the customer's home, radiation, not conduction, was the primary medium for
this RFI. It is possible, I suppose, that overhead conductors at the
substation could be acting as antennas. Perhaps the submarine cables were
also acting as antennas. However, my HF DFing clearly pointed to the
substation itself as the source of the lower frequency components of this
This particular case proved that I need a more efficient method of DFing
lower frequency sources. Had I been able to drive directly across the
waterway instead of having to drive either 2 miles north or 4 miles south to
cross the waterway slowed the search's ultimate progression across the
waterway. Every case teaches at least one lesson.
The number of cases involving only low frequency or HF only sources has been
increasing over the last 2 years. New technology televisions are among the
varied offenders. The increasing trend of this threat is a clear sign that
more efficient methods of tracking lower frequency sources will be a
requirement for RFI Location success.
Frank N. Haas KB4T
Utility Interference Investigator
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