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Re: [RFI] National RF HFDF Vector Gun now in hand

To: "Frank N. Haas KB4T" <kb4t@arrl.net>, rfi@contesting.com
Subject: Re: [RFI] National RF HFDF Vector Gun now in hand
From: "Dale Svetanoff" <svetanoff@earthlink.net>
Reply-to: svetanoff@earthlink.net
Date: Wed, 9 Mar 2011 17:11:38 -0600
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>

Thank you VERY much for the detailed report on the antennas.  This is the
sort of report info that many of us will find useful.  I look forward to
your future updates.  In particular, I think many of us would like to get
an idea as to what distance from the source you were when the antenna
actually directed you toward it.  (Yes, I know range will vary depending
upon source characteristics, but even "ball park" values are handy to know

73, Dale
Sr. EMC Engineer

> [Original Message]
> From: Frank  N. Haas KB4T <kb4t@arrl.net>
> To: <rfi@contesting.com>
> Date: 3/9/2011 4:29:49
> Subject: [RFI] National RF HFDF Vector Gun now in hand
> 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
> 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
> 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
> 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
> my package. A couple of weeks passed and the package arrived on March
> Back in January, after I had placed my order for a new system, I
> an auction on eBay for a pre-2008 version of the HFDF being offered by a
> 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
> investigations. 
> 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,
> 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,
> a map or one's eyes,  one must determine where lines drawn along the
> bearings of the nulls actually cross. The source will be near where the
> lines cross. 
> 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
> 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
> when the signal levels are relatively low. The receiver must have some
> of variable RF gain control or an external attenuator can be used to
> 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
> use. 
> 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
> cover specific frequency ranges. The box to which the antennas attach has
> pistol grip that allows the antenna to be pointed and controlled easily.
> In my experience, faulty transformers have been the most challenging
> 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
> 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.
> 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
> how loops work and to practice simple DFing. The new package will remain
> 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
> pleased. 
> 73, 
> Frank N. Haas KB4T
> Utility Interference Investigator
> Florida
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