[FoxHunt] DF 27 MHz.

[Mark Harrison]

Hi Kurt,

Could it be a 27MHz paging system?
It's just the sort of thing a fire department would have (or had 30
years ago and has been forgotten about!).
They often use several hundreds of watts, and may even use multiple
transmitter sites  - for instance, one transmitter at the station for
good coverage inside buildings, and more on surrounding mountains for
wider mobile coverage.  Multiple transmitters will of course make
tracking all the more interesting!

We used small shielded loop antennas with great success for tracking
CB in the old days.
They are simple to make out of a short length of RG8 coax.  Look up
designs in the ARRL Antenna Handbook under "RDF", 28MHz Shielded loop,
or "Snoopy loop".  From memory we didn't worry about complicated
matching networks.  The length was fairly critical, with the cable
capacitance and the gap in the shielding being chosen for resonance.
Unfortunately these magic lengths has been lost in time....
In any event, having a lossy antenna actually helps a lot when close
to the source and won't affect the directional pattern much at all.  

good luck,
Mark VK3BYY

> -----Original Message-----
> From: KD7JYK [mailto:kd7jyk at earthlink.net] 
> Sent: Wednesday, 18 July 2007 3:05 PM
> To: Radio Direction Finding
> Subject: [FoxHunt] DF 27 MHz.
> 
> What I am hearing is a telemetry signal that repeats every 
> eight seconds on
> 27.255 MHz.  The transmission is roughly 2.5 seconds length.  
> I listen to it
> in USB because it is pleasant to my ears and it is easier to 
> home in one,
> but it is a very steady signal in FM.
> 
> I first heard the signal November 2006.  I suspected it was 
> coming from the
> east and drove fourty six miles to Fallon, NV where I 
> actually found what I
> had heard.  I spent about two hours circling in on the source 
> switching in
> about 117dB of my available 131dB.  I found that I could only hear
the
> signal near one building or in the parking lot behind it and 
> only on the
> same side of the street, of any street around the building.  
> I could drive
> the opposite direction, only 20' further away and not hear 
> the signal.  The
> building is a fire department.
> 
> One day while driving west along Hwy 50, towards Carson Ciy, 
> NV, I heard the
> signal again, this time I zeroed in on a building in Mound 
> House.  This
> building was only 15 miles from my house, most likely the 
> signal I heard
> originally came from Mound House.  I managed to use enough 
> attenuation that
> I only heard the signal in the area in front of the building 
> just between
> the building and the highway.  Moving twenty or thirty feet 
> in any direciton
> caused the signal to drop off.  I went inside and talked to 
> the owner who
> was very interested, he even told me about his alarm system, 
> fire reporting
> system and showed me various vehicles on the property with 
> antennas, no
> luck.  Interestingly, there was a fire department about 1/4 
> miles behind
> this building, but the signal rapidly dropped off as I got 
> near the fire
> department.  Even more interestingly, there is a 1/4 wave CB 
> antenna on the
> fire department.  Just what I was looking for.
> 
> I spoke to the Battalion Chief, he was no help.  I spoke to 
> three or four
> members of the department, one who even showed me his 400W 
> linear connected
> to a CB in his truck, depending on who I spoke with, the 
> antenna was either
> disconnected or connected to a scanner.  I was unable to see 
> the radio end
> of the co-ax.
> 
> Thinking this may be a Fire Department related signal, I 
> spoke to a woman at
> the front desk who was very interested in what I was hearing 
> and gave me the
> locations of fire departments within a 50 or so mile radius.  
> In my checking
> around, the signal does not appear to be associated with fire 
> departments.
> I have heard four other signals in three counties.
> 
> I hear one signal near a vacant K-Mart building in Carson 
> City.  I hear two
> near Dayton, NV and one in Carson City near Baskin Robbins Ice
Cream.
> 
> With most signals, I can zero in on an area, one even out in 
> the open and
> not see anything!
> 
> The equipment I am using is an Icom IC-706 MKII, a Kay 101dB 
> attenuator and
> a 30dB fixed attenuator.  My antenna is a 24" CB whip 
> mag-mounted in the
> center of a Toyota 4Runner.
> 
> I have switched in about 117dB at which point even one or two 
> dB more will
> cut out the signal completely.  Disconnecting the co-ax, even with
no
> attenuation, I receive nothing, even at the focal point of 
> the signal, the
> radio is tight, well grounded and surrounded by metal.
> 
> I am not concerned with array size, I can set up tripods, 
> masts, whatever
> out in the field and take bearings.  What about something 
> like TDOA with a
> display to show when the phase is EXACT so I can sight 90º 
> and dead center
> of the array and hopefully see only one building?
> 
> For anyone interested in what I am hearing, I have a 
> recording, about 1 MB
> in size, I will be more than happy to send it.
> 
> Kurt
> 
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