There are a number of approaches for this. The first
problem is that, as the antenna size decreases, so does
the received signal pickup. If you are looking in a close
area where the signal is fairly strong, this probably
isn't a problem. But it is hard to build a small antenna
that will be sensitive enough to use at a long distance.
Some combination of the following may work for you:
SMALL ANTENNAS
For a yagi, you can shrink the elements 50% or so and
still have it usable, but you can't shrink the boom much.
The shortened elements tend to have very narrow bandwidth,
however, so need careful tuning to maintain a pattern.
A 2-element yagi can be made very short (4" to 6" in
the 150 MHz range) but the tuning is even sharper. You'll
find some good information on shortened yagis here:
http://www.cebik.com/fdim/fdim1.htmlWhere signals are strong a loop antenna would be a good
choice, such as this:
http://www.arrowantennas.com/fhl.htmlThe pattern will be bidirectional, so you will have to
triangulate to locate the transmitter. I'm sure you
could build your own - I've seen a couple VHF loop antenna
designs on the internet, and the basic design is simple.
HARMONIC SNIFFFING
Most transmitters also radiate some signal on harmonics,
even if it is down 60dB or more from the fundamental.
At close range it may be easier to sniff the harmonics
because antennas are smaller and can have a much sharper
pattern. For example, I use 730 MHz beams for hunting
both 2m (5th harmonic) and 121.5 MHz (6th harmonic).
A 6-element yagi is a bit over a foot long and 8" wide
and has a very sharp pattern compared to a 3-element
yagi for the fundamental. I can only hear some of my
2m transmitters when I'm within 100 feet, but it works
fine for sniffing them out of the bushes. The 2nd and
4th harmonics of 450 MHz fall near the cell phone bands
around 900 and 1800 MHz, and you may be able to find
commercial beams designed for those frequencies. I've
had good luck scaling these designs for harmonic sniffing:
http://www.clarc.org/Articles/uhf.htmor you can use W4RNL's quad calculator formulas here:
http://www.cebik.com/quad/2mq.htmlOf course, this requires that you have a receiver that
covers the harmonic frequency.
TDOA (Time Difference of Arrival) DF SETS
These units require a relatively simple antenna and can
give good direcitonal resolution. They can also get
horribly confused with reflections, so use them with care.
Basically they consist of a pair of dipole elements mounted
perhaps a quarter wavelength apart with a switching
system that connects them alternately to the receiver.
The output of the receiver goes to a phase detector that
indicates whether to turn the antenna left or right.
There are a number of constructional projects on the web -
here is one example:
http://home.att.net/~jleggio/projects/rdf/tdoa1.htmYou can use shortened antennas with these as long as you
carefully match them.
CAMOUFLAGE
What would someone reasonably be walking around carrying
that could hide an antenna? A 2m quad can fit in a box
on top of a car - perhaps a large pizza sign? Political
or protest signs can have yagi elements inserted into the
corregated plastic that is normally used these days. A
tennis racquet could hold a loop or a small yagi woven
into the strings. Especially for harmonic sniffing the
item doesn't have to be large: a plastic briefcase or
binder will hold a yagi, a dummy telephoto lens could
hide a quad. Anything rectangular could enclose a corner
reflector antenna built into one side. You just have
to think of some object that doesn't look like an antenna.
I actually built a TDOA set once for 2m with a pair of
whips mounted on the sides of a set of headphones. The
whips had balls mounted on the ends so they looked like
"dealy-bobbers" (though I must admit that I was a bit
too old to look like I would wear such a thing.) I had
the TDOA circuitry connected so the receiver audio would
appear in my left ear if I had to turn the antenna towards
the left, etc. Two problems: first, reflections are a
problem with TDOA units. Second, trying to walk down a
sidewalk while taking a bearing on a station to the side
caused me not to pay attention to where I was going.