[FoxHunt] DF 27 MHz.

[Dale Hunt, WB6BYU]

I think the loop is your best bet - if well built it should have
sharp nulls for taking bearings (nulls are much more accurate
than peaks in an antenna pattern.)  There are a number of
possible designs, using wire strung on spreaders, stiff copper
tubing, coax cable, etc.  Hunting on 10m was particularly popular
in the 50's and 60's, so there are a number of articles of
that vintage available.

One problem when you get close like that is that the radiation
may be conducted along wiring, etc.  I've tried sniffing around
buildings on both VHF and 80m, and bearings are rarely reliable.
On VHF/UHF reflections are the biggest problem, but on HF it is
coupling between the DF antenna and the power wiring.  The
wiring acts as an antenna and the DF loop couples to it:  I've
noticed this on 80m where a mobile loop antenna nearly always
reads relative to overhead power lines rather than the actual
direction to the signal source.  You can try it around the 
outside of the building, but when you get inside you may just
have to use the loop as a hand-held sniffer to see where the
signal is strongest.

One of the older designs was the "Snoop Loop", which included
a single-transistor amplifier and an indicator (a light bulb
was better than a meter for night hunting.)  What does the
modulation sound like when you put your radio on AM?  We've
used simple crystal detectors for ELT signals and they give
excellent audio indication of signal strength.  If there is
adequate audio in AM mode you can try a diode detector and
a pair of high-impedance headphones, or add a small
amplifier of some sort, or a small meter.  You'll want a
gain control as well for when you get close.  That gives
you a simple hand-held receiver that will give bearings
on very strong signals.  You can run it along the outside
of a wall and find the location of an antenna behind it,
(though likely the signal strength will be a better indicator
at such close ranges than the actual loop pattern.)


Harmonic sniffing as Bernie suggested is also a good
approach if your receiver covers the higher frequencies.
I use a 6-element 732 MHz yagi to hun the 6th harmonic
of 121.5 and the 5th harmonic of 2m with very good
results.   A 3-element yagi for 243 MHz requires only
a short length of PVC pipe and 8' of #12 or #14 solid
copper house wire.  I happen to have a number of beam
designs for DF on 216 MHz (wildlife tags, etc) and 
243 MHz (ELT beacons).  You can get pretty good gain
and pattern in a 4' yagi.  Of course such antennas
can be built for any frequency - calculate the
harmonics of the signal and try listening on them the
next time you are close to the signal, then build a
beam for the highest harmonic that you can hear.
In fact, I need to build a 216 MHz antenna in the next
week or so for the local program that uses wildlife
tags to track Alzheimer's patients - shouldn't take
more than an hour at most to build.  (The hardest
part is deciding which of the designs to build!)
Anyway, if you want some simple designs that are
easy and inexpensive to build, I'll be happy 
to help.

    Good luck!

          - Dale   WB6BYU