[RFI] RFI Direction Finding

Jim P jvpoll at dallas.net
Sat Feb 18 11:12:14 EST 2006

----- Original Message ----- 
From: "Ed -K0iL" <eedwards at tconl.com>
To: "'Jim P'" <jvpoll at dallas.net>; "'Paul Christensen'" <w9ac at arrl.net>;
<rfi at contesting.com>
Sent: Saturday, February 18, 2006 8:50 AM
Subject: RE: [RFI] RFI Direction Finding

> The FM broadcast wouldn't be affect on an FM radio since noise is
> "Amplitude" in nature.  FM rcvrs won't pick them up (usually, although
> rcvrs may).


Not these cheap ratio-detector equipped FM radios with only
4 transistor IF stages - which I used to check this phenom (these
radios aren't equipped with a full set of 'limiter' stages followed by
the usual Foster-Seely discriminator).

The IF strip is 'linear' up to the ratio detector, as the "ratio
detector"  (FM detector) stage doesn't require limiter stages
to preceed it, this is an advantage in producing cheap consumer
class gear. This allows a FAIR amount of AM component,
especially from impulse noise sources, to be heard (NOT to
mention the fact that some sources will COVER a weaker
station regardless if its FM or not).

Shoot - I used to hear aircraft traffic on this Sears radio
on account of the 'image' response to aircraft - the local
VOR where I grew up could be heard rather prominently
too, CW ID and all ...

> Most powerline noise sources, when you are in the neighborhood, do emit
> VHF high-band real nice and sometimes even lower UHF portion locally if

Only a partial "Bzzzzzzt! on this one.

An attempt on this basis to establish correlation of a particular source
and it's 'effect' that one has in his shack may fail given a rock-hard
assumption in this area.

Take for example, again, where the ham who took his Two Meter
Beam could not locate this source using VHF alone.

Maybe I should emphasize again that this source was about 3/4
mile away and was prominent in it's emissions in the AM band
and HF region.

> have a sensitive enough rcvr at those freqs.  I am not referring to ham
> bands here, just generic VHF (30-300Mhz) & UHF (300-3000).  At work we use
> somewhere around 250-300Mhz and usually have to start turning the gain

I pointed out a specific case where a general assumption did not
work; I just want to point out to people that there are no hard, fast
rules to this game, there are exceptions..

I have seen and worked with the power company and seen their
hand-held DF 'beam' that looks to be in the 300 MHz area. It was
quite effective at that time for the particular noise issue that I had
at that time.

Again, I want to stress the point that you MAY not have 1:1
correlation with any given noise source from HF into the VHF
or UHF range at some distance where the effect is experienced
on an HF band with a MUCH larger aperature antenna.

This is not to say, though in the case that you DO have correlation
that you should noy use it to your advantage in finding it,

If you do find correlation, you should use it, gaining the ability
to easily and accuratly pinpoint the source, using this facet to
one's distinct advantage.

*** TV Field Strength Meter DFing ***

Judging from the lack of comments, I also see that no one else
has apparently glommed onto the idea of using a wide-band
impulse receiver like a TV field strength meter to DF impulse/
power line noise sources (since this type noise generally follows
the law that noise power is proportional to BW); I have found this
to be particularly effective too, in DFing with little trouble distinct
noise sources that are 1/4 to 3/8 of a mile away that affect both
the low-band TV CHs and Six Meters. The nulls off the ends of
a dipole are rather sharp, as we should all know, yielding a
rather sharply-defined (compared to a TV log peridic) compass
bearing (with 180 degree ambiguity, of course).

And once in the proximity of said 'noise' source, if I can
observe correlation in characteristics (mainly by sound),
to the UHF area then DFing using a hand-held yagi will
isolate the house ot pole.

> when we get it down to the last couple of poles to figure out which one it
> is.  Then we switch to ultrasonic to confirm the results and determine
> hardware is the offender.

Ultrasonics is something I've not yet tried; I think I'd like to
try the 'corona observation technique', corona which yields
UV light spectra, as I have previosly brought to light/discussed
on the list.

And since I "don't climb poles" there is little need for me to isolate
to a specific piece of hardware. I'll leave that to THE professionals!


Jim P  -- WB5WPA  --

----- Original Message ----- 
From: "Ed -K0iL" <eedwards at tconl.com>
To: "'Jim P'" <jvpoll at dallas.net>; "'Paul Christensen'" <w9ac at arrl.net>;
<rfi at contesting.com>
Sent: Saturday, February 18, 2006 8:33 AM
Subject: RE: [RFI] RFI Direction Finding

> No, it does not make that assumption at all.  Not sure where you got that
> from.
> It makes the assumption that the noise travels FARTHER from the source on
> lower freqs than at higher freqs. You must be close to the source to get
> at VHF and closer still to pick it up on UHF.
> 73,
>  de ed -K0iL
> -----Original Message-----
> From: Jim P [mailto:jvpoll at dallas.net]
> ----- Original Message -----
> From: "Ed -K0iL" <eedwards at tconl.com>
> > The fact that you cannot rcv it at VHF indicates the source is some
> distance
> > away from your station.  It's either being conducted via lines to your
> This assumes that the source is flat from HF through VHF. This isn't
> always the case.
> > vicinity or it's just far enough away to be out-of-range on VHF but not
> > HF.

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