Fellow RFI'ers:
Some months ago, I mentioned here that I had invested in the National
RF HFDF Vector Gun active HF loop antenna system to help me locate
HF-only RFI sources. At that time, I said I would provide reports on
the results I've gotten using that tool. Here we are in July and it's
time to bring you up to date.
The Executive Summary:
The National RF HFDF Vector Gun system works. I have had 4 HF-only RFI
cases to resolve in the intervening months since February 2011. Each
one has been successfully located using the Vector Gun. I recommend
this tool for those of you who need to locate HF-only RFI sources.
Learn about this package at
http://www.nationalrf.com/type_hfdf_vector.htm.
Detail:
When I first received the Vector Gun, I spent several hours learning
how to use it with the Radar Engineers Model 242, Icom R3 and other
compatible receivers I have. Radar Engineers (RE) product data can be
found at http://www.radarengineers.com/rfitvi.htm. At first, I used
known sources to gauge pointing accuracy. From 2 miles out, bearings
can vary off the mark but are usually in the right quad of the compass
rose. As I got closer, accuracy improved. When I was within a block or
two, accuracy on strong signals was very good. Poles producing strong
signals are easy to pinpoint. Weaker sources require more work. The
key to success is to keep the receiver from being overloaded. An
attenuator, either built-in or external to the receiver is very
important. The Vector Gun has a basic 2 position GAIN control: HI and
LO. Very strong sources can easily swamp the receiver making accurate
location impossible even when using the LO setting on the Vector Gun.
The Radar Engineers Model 242 has effective receiver sensitivity/gain
control. I had to use an external switch-type attenuator with the R3
on very strong sources. The key is to keep the signal strength
indicator at about mid-scale as much as possible.
After several hours of experimentation, I felt comfortable with the
Vector Gun. When the first HF-only RFI case came along, it was time
for a real workout. The customer was experiencing fairly strong HF
only RFI (to about 20 MHz). The Vector Gun took me about 1/4 mile away
and pointed to a pole which ultimately proved to have loose hardware
for a mounting bracket that was arcing nicely. When I was within a few
poles of the source, I could just barely make out the signal on the
Radar Engineers Model M330 320 MHz Yagi-equipped receiver. I could
have found this source by driving around all over the place and
eventually coming upon the pole. Since I get paid to do this sort of
thing, TIME IS MONEY!! Better to drive right toward it and find it
fast!
The next 2 cases were similar. The Vector Gun successfully pointing
out the offending source.
The most recent case involved an incredibly strong RFI signal that
could be heard easily from 1.0 MHz to about 20 MHz as far as 2 miles
away. I connected the 242 to the customer's antenna. This often
complicates matters because you will usually hear many sources. I had
to be sure I was hearing on the 242 what the customer was hearing in
his receiver. The 242 has an LCD scope display that allows you to
capture the "signature" of the signal you want to find. The
continuous, very strong source that was overwhelming all other sources
had a distinctive signature that was easily captured on the receiver's
scope and stored. Out to the yard with the Vector Gun and the 242. In
one 360 degree rotation, the strong source could be heard somewhat
weak but definitely in one direction. To the truck and off to a spot
in the general area about 1.5 miles away pointed out by the Vector
Gun. The source was much stronger with a definite bearing. In a total
of about 30 minutes I was standing under the pole that was producing
the RFI. The source could not be heard at all above about 25 MHz. This
is unusual for something so strong. The Radar Engineers M330 did not
detect this source at all. Nor did the RE Model 250 Ultrasonic
Detector. There were 3 lightning arrestors (LA) on the pole (1 for
each phase.) Only when the crew came out to work this job was I able
to find out which LA was bad. I could not have found this source so
quickly without the Vector Gun.
Interestingly, the RE M330 320 MHz receiver did sound off nicely 4
poles to the east where an inline switch's mounting hardware was loose
and arcing. This source was intermittent and had a completely
different signature. We fixed that problem when we replaced the bad
LA.
With the Vector Gun, I have been able to pinpoint 2 noisy switching
power supplies that produce RFI primarily in the 10 MHz range of the
spectrum. I was able to find them by walking around with a battery
operated shortwave receiver but I could find them more quickly with
the Vector Gun and the 242.
I found the best results were obtained when the receiver sensitivity
was adjusted so the signal strength indicator pointed mostly in the
middle of its range. As I got closer and closer to the source,
receiver sensitivity had to be reduced to keep the max indication at
the midpoint. Following this protocol deep nulls are possible. Deep
nulls mean accurate pinpointing.
One additional comment: Reflections can be a problem if you don't
understand how loop antennas work. Loop antennas point to the source
via deep nulls that are 180 degrees apart. If you are seeing multiple
nulls, you will have to relocate to a different spot, preferably one
not likely to produce reflections. With experience, you can learn to
separate reflections from the actual source but it takes time. Since
nulls are 180 degrees apart, multiple bearings must be taken in order
to see where the bearings cross. Drawing lines on a street map can
help. Practice using known signal sources to gain familiarity with the
behavior of the loop antenna system.
Bottom Line: The National RF HFDF Vector Gun works well for locating
sources that have an HF component. It is invaluable when the source
has only HF content. However, the receiver must have excellent control
over sensitivity or an external variable attenuator must be used to
prevent the receiver from being overloaded. This is VITAL to success.
Like any RFI location tool, practice to achieve mastery is important.
Feel free to ask any questions. I'll do my best to answer them.
Keep on locatin!!
73,
Frank N. Haas KB4T
Utility Interference Investigator
Florida
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