Back in January I solicited recommendations for DFing antennas that work
well down to 3 MHz. There simply isn't much to choose from. I discovered
through Internet searching that NationalRF.com offers an amplified loop
antenna system that works down to 8 MHz stock.
I called National RF and had a nice chat with one of the engineers. At that
time he told me that each HFDF is custom built as there isn't much demand. I
placed my order. In an act of full disclosure I was told that it would
likely take several weeks for my order to ship because of some personal
matters they had to deal with out in CA. I told them to take their time. I
was not in a rush and will gladly wait.
I also asked if they had considered an antenna for the system that would
work down to 3 MHz. I was told that a prototype had been designed and built
but little had been done to refine the antenna since there was little
demand. I thanked them for their honesty and full disclosure and paid for my
order via PayPal.
In late February, I received a call advising that my package was ready to
ship. I asked again about the 3 MHz capable antenna and was told that they
would gladly build one for me if I understood that it was not fully tested
and proven. I agreed and asked them to include the additional capability in
my package. A couple of weeks passed and the package arrived on March 8th.
Back in January, after I had placed my order for a new system, I discovered
an auction on eBay for a pre-2008 version of the HFDF being offered by a ham
in CA. I won that auction. I've had the older version system for about a
month and have used it several times during some of my interference
After only several sessions with the National RF HFDF, I have become
comfortable with its use. When faced with an interfering signal that can
only be heard at the customer's home on HF frequencies no higher than 8 or 9
MHz, the HFDF has successfully pointed me in the right direction, generally
indicating the direction to the distant source. I say distant because, in
most cases that I work as an investigator for an electric utility, an
interference source copyable to only 8 or 9 MHz is usually some distance
from the affected party's location.
Keep in mind that the HFDF is a LOOP antenna system. That means two nulls
are produced. To figure out which direction one must actually follow, at
least one (and preferably 2) additional bearings must be taken. Then, using
a map or one's eyes, one must determine where lines drawn along the compass
bearings of the nulls actually cross. The source will be near where the
Distance to the source can be roughly gauged by the manner in which the
lines cross. When two bearings are taken and the nulls are at similar
angles, the source is a long way off. If the two bearings produce nearly
reciprocal angles, the source should be nearby. This type of data is easily
acquired from the HFDF.
The HFDF is simple to use. The most important point to remember is to use
the least amount of gain possible at the receiver. Accuracy is much improved
when the signal levels are relatively low. The receiver must have some sort
of variable RF gain control or an external attenuator can be used to adjust
the input to the receiver as needed. The 4 page instruction sheet that
accompanies the HFDF clearly explains this important part of the system's
You supply the receiver, the coax that connects the HFDF to the receiver
and, if needed, external attenuator. The HFDF uses one of 3 or 4 antennas to
cover specific frequency ranges. The box to which the antennas attach has a
pistol grip that allows the antenna to be pointed and controlled easily.
In my experience, faulty transformers have been the most challenging sources
to pinpoint because they typically produce interference that can only be
heard (even up close) only up to about 8 to 10 MHz. Some faulty transformers
can produce RFI detectable to 350 MHz but most that I run into here in
Florida can only be heard up to about 10 MHz.
It's hard to carry a 10 MHz Yagi around. A few years ago I paid $120 for a
G4TPH magnetic loop antenna system at the Dayton Hamvention. The mag loop
works very well but it's not convenient to use for my type of searching. The
HFDF Loop antenna system makes the job much, much easier.
The eBay package I bought will be loaned to area hams to allow them to learn
how loops work and to practice simple DFing. The new package will remain in
my professional "toolkit" where I expect it will make some of the cases I
investigate easier to resolve.
As I get more experience with this system, I'll report again, probably in
another 3 months, to share the results of my experience. Thus far, I am
Frank N. Haas KB4T
Utility Interference Investigator
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