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Re: [RFI] MFJ-805 RFI detector - Any good?

To: <rfi@contesting.com>
Subject: Re: [RFI] MFJ-805 RFI detector - Any good?
From: "Frank Haas KB4T" <kb4t@arrl.net>
Reply-to: kb4t@arrl.net
Date: Thu, 10 Mar 2011 00:22:29 -0500
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
I want to add some comments to my earlier note in this thread. 

Earlier I mentioned that a 2 meter Yagi antenna makes a pretty good DFing
antenna for power line interference. I meant to include mention that there
are a pile of DIY (Do It Yourself) articles on the web and in books that
describe a dizzying variety of possible antenna designs. One of the best is
the Tape Measure Yagi. This type of antenna is a real favorite among the
Fox/Bunny Hunters in my area. They are simple and cheap to build. Check out
this web site for construction details: 


Tape measure Yagi's can take a beating and keep on leading.

When DFing any sort of signal, real transmitter or interference, it's
important to keep the signal strength of the signal you are chasing to the
minimum necessary to get sensible indications on whatever receiver you are
using. Most interference searching is done using a simple signal strength
meter. A pinned needle tells little. The RF Gain/Sensitivity control on the
receiver (if there is one) should be adjusted so the signal is mid to 3/4
scale. Not full scale. If the receiver does not have an RF Gain or
sensitivity control, you will likely need to use an external attenuator in
series between the antenna and the receiver. 

The objective is to keep the signal strength meter readings between midscale
and 3/4 scale. The goal is to avoid "swamping" or overloading the receiver.
This works very well for any type of signal location effort. 

Hunting high power transmitters usually requires specially designed
receivers with integrated sensitivity controls. As you get closer to the
antenna of a high power transmitter, the receiver is often able to hear the
signal even with max settings of an external attenuator. But this is the RFI
list, so...

When hams I work on the air find out what I do for a living invariably I end
up suggesting that DFing interference is a skill most hams will need as
poorly designed and poorly or unethically built electronic devices continue
to flood our cities. To be ready when the need is real, practice locating
weak signals you hear on 6 meters and 2 meters. Join the local Fox/Bunny
Hunting group and get comfy with the "tools of the trade."

If you are looking for a real challenge, contact your electric utility and
ask them how to report interference problems and inquire how they are
handled. It might be an eye-opening experience. Knowing ahead of time makes
resolution of real troubles go faster. If your utility has a good track
record for locating interference sources, don't hesitate to call them for
help. There simply isn't any good reason to suffer with a power line
interference problem any longer than necessary.  

I hope this rounds out my earlier recommendation. 


Frank N. Haas KB4T
Utility Interference Investigator

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