Phil:
There are good reasons ultrasonic detectors are not spoken of much. They are
only useful up close and sound off only in a small number of cases. They
tend to be costly to buy, can be costly to build and are only marginally
useful.
Ultrasonic detectors DO NOT hear all RFI sources. I use the Radar Engineers
ultrasonic detector on every pole I pinpoint. Only about 40% of these scans
produce something useful. That's 4 out of 10 and, frankly, not a high enough
percentage to warrant building or buying such a detector unless you do RFI
searches for a living.
Every RFI source I've ever found that produced a reaction in the ultrasonic
detector was first heard at HF or VHF on a regular AM capable receiver.
Remember, DFing the RF signal gets you to the pole.
The purpose of the ultrasonic detector is to pinpoint what device/part/point
on the pole is the actual source. This implies that the detector is precise
enough to do this with accuracy measured in inches. I wonder how many
homebrew ultrasonic detectors are this precise?? My multi-Kilobuck Radar
Engineers ultrasonic detector is very precise and even it can be off
somewhat.
Unfortunately, all sources are not detectable with an ultrasonic detector.
Sources inside any sort of enclosure like an insulator or transformer may
radiate RF nicely but the sound they make can't be heard. Ultrasonic
detectors are "line of sight" devices. This means the source and the
detector must have an unobstructed path between them or the detector remains
silent. It's entirely possible that an unobstructed path cannot be created
when pointing up from the ground.
Keep in mind that it is not absolutely necessary to pinpoint the actual
device producing the RFI. Finding the pole is usually good enough. In all
honesty, I don't want to be that precise. Instead, I want the crew to
examine everything and work the pole from the top to the neutral. I want all
the hardware tightened, all the staples hammered in, all the bond wires
dressed, all connections examined and repaired if necessary. If I say,
"Tighten the bottom nut on pole top pin support bracket." The crew will do
ONLY that and move on to the next job!!!!!! Since the crew is there, I want
them to do everything possible to keep the pole from being a problem again
for a year or few. (Pole top pin is the insulator at the top of the pole.)
Even if I know precisely what part is causing the problem I often don't say
so just to be sure the crew works the pole the way I want.
I think it's more important to find a way to work with the crew or insure a
knowledgeable person works with the crew to insure the work done is
successful at eliminating the RFI. Attitudes about having the customer
around while work is being done vary widely from utility to utility. Since I
write up the work orders, I accompany the crew on every interference
resolution. The crews appreciate knowing that their work was successful.
Ultrasonic detectors are nice to have but not essential. Roughly half the
time, they will point out the likely source. If you enjoy building projects,
making such a detector for yourself might be fun and possibly useful. In my
view, the ultrasonic detector is not an essential tool for the amateur RFI
searcher.
73,
Frank N. Haas KB4T
Utility Interference Investigator
>>> Original Message <<<
Date: Mon, 28 Mar 2011 16:14:33 +0000
From: Phil Duff <na4m@arrl.net>
Subject: Re: [RFI] FW: Topband: Exciting noise sources (Tim Duffy K3LR)
To: rfi@contesting.com
On 3/28/2011 15:52, Frank Haas KB4T wrote:
> The majority of real power line interference sources can be heard at VHF
and
> low UHF frequencies as you approach the source (within a 1/4 mile
> typically.)
What about ultrasonic receivers/systems such as the one described by
W1TRC in April 2006 QST to zero in on arcing on a pole?
I've not seen much if any mention of the use of ultrasonic receivers
since the QST article was published.
73 Phil NA4M
--
-. .- ....- -- -. .- ....- -- -. .- ....- --
Phil Duff
Georgetown, Texas
http://priceless.apduff.com
http://stockphoto.apduff.com
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