Probable source of ~28.635 MHz interference

[John H. Klingelhoeffer]

Glen:
 
Your interfering signal on ~28.635 MHz is probably a whole lot less  sinister 
than being extraterrestrial!
 
One of the reasons you and MANY others hear this signal is because it is  
*exactly* the 8th harmonic of 3.579545 MHz, the TV color burst crystal  
frequency.  However, it does NOT necessarily have to come from a  TV.  In fact, I had 
an AT&T cordless phone/answering  machine several years ago that had an "S9+20" 
birdie on 50.11363 MHz that I  had to hunt down using a portable receiver and 
the breaker box.   50.11363 ended up being the 14th harmonic of 3.579545 MHz. 
 After I figured  out what it was coming from, I looked at other harmonics, 
and sure enough, it  was on 28.63636 MHz along with other frequencies.  That's 
also probably why  you don't hear much of any modulation on it.  They also 
normally drift a  bit in frequency due to house and unit temperature, and 
naturally the  multiplication of the birdie may move it around a kilohertz or so as 
you have  observed.
 
Many of the current plastic-cased, supposedly FCC compliant, unshielded  
consumer appliances and toys use a TV color burst crystal for their embedded  
processor clock.  They are very, very cheap in large quantities, only a  couple of 
cents each.  In the case of the AT&T phone / answering  machine above, it was 
mostly radiated from the line.  I took a large  ferrite toroid core, wrapped 
about 20 turns of the DC line cord through the core  snugged right up against 
the back of the machine, and it dropped the signal from  S9+20 to about S2 - 
still detectable, but much less bothersome.  I'm sure  if I'd wanted to go 
inside the machine and do some bypassing and shielding that  I could have made it 
disappear entirely, but I'm not that avid an operator on  6M.  
 
My suggestion would be to first eliminate appliances in your house  initially 
as the potential source of the signal.  With the gracious  cooperation of 
your family, pull all of the breakers for the other rooms of your  house except 
for the one that has your radio equipment, and of course, unplug  anything in 
their other than the radio and possibly the lights.  If the  birdie is still 
there, you are going to have to go hunting down the street, but  you can do it 
with a clean conscience.  
 
It's then a matter of walking around the neighborhood with a portable HF  
radio, hopefully with a BFO.  I have a little Sony unit that I use for  exactly 
that.  Figure out where the signal is strongest, and then determine  the 
relationship with that particular neighbor to discussing that they might  have a 
device that is interfering with radio communications.  Remember that  this birdie 
is every 3.579545 MHz, so in my case it was also on all kinds of  commercial 
and public service frequencies as well as 10 and 6 meters.  How  you handle 
the fix is a delicate subject that the ARRL treats pretty well in  their 
literature.  I won't go into it here.
 
I've been hunting down these things for many years, and there ought to be  an 
online database of devices known to emit stuff like this and how people "fix" 
 them, but I don't know of one.  Seems to me like a job for a national  
amateur radio organization, but I won't go there.  
 
Happy hunting and 73.  I'd be interested in hearing what device you  find is 
emitting this.  
 
John...
WB4LNM
"One of the Hams from Heath (VHF/UHF Senior Design Engineer  1979-1981)"
 
 
Glen Zook <gzook at YAHOO.COM> wrote:

>Last night I was  checking a local's SSB signal on the
>10 meter band.  After we got  through I started tuning
>around the band and ran across a steady  carrier
>located approximately at 28.635 MHz.  Although  there
>are numerous computer "spurs" around that frequency
>this  particular signal is very different from the
>"normal" computer  spurs.
>
>First of all, there is no modulation of any type  on
>the carrier, not even a trace of "hum".  There  are
>computer spurs on almost the same frequency but the
>signal  strength of those is such that an S-1 signal
>could be easily copied  through the spurs.  The signal
>is definitely vertically polarized.  When I switch
>from a vertical antenna to my 3-element yagi at  55
>feet above ground the carrier virtually disappears in
>signal  strength and the computer spurs take over the
>frequency!
>
>I  have checked with "locals" and they are hearing the
>same signal again  vertically polarized.  I first came
>across the signal about 11:00 PM  CST last night and it
>was as close to 28.635 MHz as I could read on all 3  of
>my Collins S-Line receivers and my Collins 75A4. 
>Since last  night the signal has drifted up in
>frequency almost 1 KHz and is now on  about 28.636 MHz.
>
>Since I haven't tuned around that part of the  10 meter
>band for months I have no idea as to how long the
>signal  has been on the air.  But, the signal level is
>running around S-8 on  receivers that have been
>calibrated for an S-9 reading of 50 microvolts.  
>Because of the fact that the signal level does not
>seem to vary  this source has to be local.
>
>I have heard from several people  around the country
>that are hearing signals on 28.635 MHz.  However,  all
>of those seem to be the "usual" computer "spurs" since
>they do  have some sort of modulation ("buzz", etc.).
>
>Anyway, I haven't  put my 10 meter mobile in the car
>and gone looking for the source of the  signal. 
>However, I am looking for suggestions as to just  what
>this might be.  Frankly, over the years I have run
>into  all sorts of signal sources.  But, none as
>"clean" as this  particular source.
>
>Glen,  K9STH
>
>=====



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