[GreenKeys] OT: transmitter (fox) hunt stuff

[Jim Walls]

 It's been a LONG time since I was doing that regularly, but was quite 
active a few decades ago.  Almost all of my T-Hunting was with directional 
antennas.  The first dopler based systems came out while I was active and I 
did build one of the kits.  It worked OK with strong signals, but not at 
all for really weak signals.  I started out using a 4 element yagi on a 
mast that was held out the passenger's window.  by running the mast through 
the armrest on the door, it was easy to keep in position.  Later I put a 
hole through the roof between the seats.  On the floor there was a wooden 
block bolted in place with a hole that the bottom of the mast dropped into. 
That worked far better because it was easily usable by either the driver or 
passenger, and did not require holding it while in motion.  There was one 
of the Los Angeles area hunts that was well known for VERY weak signals at 
the common starting point.  For that hunt my team used a 16 element yagi 
with a GasFET preamp at the antenna feeding into an all-mode radio.  Even 
with the yagi and preamp, we would often switch to SSB for the initial 
readings.  Going to SSB would allow hearing a far weaker signal than with 
the radio in FM.
  
 BTW, one key for using a directional antenna and attenuator is that you 
MUST use well shielded cable between the attenuator and the radio and the 
radio MUST be well shielded.  Good quality double shielded cable should be 
considered a minimum.  A radio with adjustable RF gain also helps (another 
advantage to using the all-mode radio).
  
 Although I said I have not done any amateur T-Hunting in a long time, I 
recently did one at work.  Professionally I run a UHF public safety trunked 
system.  I got called in because of a lot of problems with inbound signals 
getting interfered with.  After some investigation I ended up with a 
spectrum analyzer connected to a spare port on on a receive multicoupler at 
one of the repeater sites (it's a voting and simulcast system), looking at 
the frequency ranges where our receivers were.  I saw that there was a 
constant signal on every one of our input frequencies in one of the 
frequency ranges that we use.  Almost immediately I realized that there was 
a channelized BDA that was self oscillating.  I spent the next several 
hours narrowing it down to about a 1 block area.  Got the FCC headed in for 
an assist (yes, they will respond on a Saturday night when a public safety 
system is being affected).  By the time the FCC engineer arrived, the 
building had been identified.  It was a brand new building that had not yet 
been occupied and there was no on-site security.  Hardest part was trying 
to find a responsible party with keys to open it up so the BDA could be 
turned off.  Almost had the fire department come out to open it - the hard 
way...  
  
 Jim - K6CCC
 Hidden Transmitter Enterprises
  

  
  
  

----------------------------------------
 From: "Gil Smith" <gil at baudot.net>
Sent: Wednesday, September 2, 2015 11:52 PM
To: "aaa-greenkeys" <greenkeys at mailman.qth.net>
Subject: [GreenKeys] OT: transmitter (fox) hunt stuff   
  Hey folks:
  
 Just a bit off-topic, fyi:
  
 Courtesy of Byon at byonics.com, we have a local Arizona transmitter-hunt 
(fox hunt) every month or so.   My son, Alec, K7EZX, and I have only had 
modest results finding transmitters (though the subsequent lunch has always 
been great).  Distance bearings were a pain due to frequent stops to get 
out and hook up the tape-measure yagi.  When we finally got close to the 
fox, the signal strength was generally overwhelming, and we often gave up.
  
 To help with the distance issue, I am taking a cue from a fellow hunter 
who mounted a yagi through his car's sunroof -- I just did the same thing.  
I got three tv rabbit-ear antennas on ebay, made one into the driven 
element (with coax and hairpin match), and the other two became 
reflector/director (with simple shorts).  Mounted them on a pvc boom, 
crimped/cut them to proper length, with a little blob of hot-melt glue on 
the ends for safety.  They are very light, worked well in road tests up to 
50mph, and they fold down nicely for storage.  Some pvc pipe pokes through 
a lovely piece of 1/4" cherry plywood secured in the sunroof opening.  My 
son will man the antenna as we drive.  We will use a switched attenuator 
with the roof yagi.
  
 For close-up stuff with the tape-measure yagi, which has always been our 
final location problem, I have now built an active attenuator kit, and 
hopefully we can finally shorten the time-consuming last segments of our 
t-hunts.
  
 Anyone else on the list into t-hunts?
  
 gil
  
  
 gil smith, AF7EZ
greenkeys moderator
gil at baudot.net
  


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