The closer you get to the transmitter
you're hunting, the stronger the signal gets. The strengthening signal
is a good sign, but it can also make the end game a special challenge.
Strong signals are a problem because receivers are only able to recognize
changes over a limited signal strength range. Once a signal's strength
exceeds a receiver's dynamic range the receiver no longer responds to signal
level changes. The receiver will indicate maximum signal strength, and
will be useless for showing changes in strength as you proceed with the
An attenuator is the ideal tool for knocking
a signal down to size. The two types of attenuator most commonly used for
recreational transmitter hunting are described below.
Instead of directly attenuating the incoming
signal, an offset attenuator works by adjusting the strength of an offset
signal. When using an offset attenuator, your receiver needs
to be tuned to the offset frequency, instead of the frequency of the signal
you are hunting. This can take some getting used to! It will make your
life simpler to have the offset frequency already programmed into one of
your receiver's memory locations before you start the hunt.
Offset attenuators are inexpensive, and
easy to build. The fact that your receiver is tuned to a different frequency
from that of the signal you're hunting means that your receiver will not
be as affected by a strong hunt-frequency signal entering through the receiver's
plastic case. Designs for an offset
attenuator (or offset
attenuator #2, or offset
attenuator #3) are readily available. Unfortunately, I have been unable
to locate a source of offset attenuator kits, or assembled and tested offset
One disadvantage of offset attenuators
is that they require a battery to make them work, so you've got to remember
to turn them on... and off! Another drawback to most offset attenuator
designs is that their attenuation level is not calibrated; so you can never
be certain just how much attenuation you're really using. Also, since most
designs use a potentiometer for adjusting attenuation level, it is difficult
to return precisely to a previous attenuation setting. And because most
offset attenuator designs include no filtering between the antenna and
the mixer, the attenuator is potentially a very prodigious intermod producer.
This can be a real problem when an offset attenuator is used in an area
with strong out-of-band signals, such as neighborhoods near broadcast or
Note: You should take care to avoid transmitting
through an offset attenuator. Doing so could possibly destroy the attenuator.
If it doesn't destroy the attenuator, it will almost certainly result in
strong out-of-band signals being generated by the mixer in the attenuator.
When it is necessary to reduce the strength
of a received signal, a passive resistor-network attenuator can be used.
Most attenuators of this type are operated using a series of slide switches.
By configuring the switches appropriately, any attenuation level
between 1 dB and 80 dB can be selected, in 1-dB steps.
For much of the hunt, a resistor-network
attenuator will work quite well. Unlike most offset attenuators, resistor-network
attenuators provide accurate, and very repeatable, attenuation settings.
Also, most passive attenuators will not generate intermod signals that
might interfere with your hunting efforts. Designs for a resistor-network
attenuator can be found in references ,
and . Arrow
Antenna (of Cheyenne, WY) offers an affordable attenuator
kit. Radio Shack sells a fully-assembled
For transmitter hunting, passive attenuators
have several drawbacks. The most serious drawback results from the tendency
of most receivers to pick up strong signals directly through their plastic
cases, pinning the S-meter, regardless how well the attenuator reduces
the signal arriving at the antenna jack. It is possible to address this
problem by installing additional shielding (e.g., aluminum
foil) to the case of your receiver, and using double-shielded feedline
between the receiver and your antenna. Using a receiver spur (see below)
in conjunction with a passive attenuator to further attenuate a strong
signal can often get the job done. But you might find it is simpler to
overcome the leaky case problem by using one of the offset
attenuators mentioned above.
Note: You should take care to avoid transmitting
through a resistor-network attenuator. Doing so could destroy the attenuator
if it is not designed to handle the transmitter's output power level.
What do you do if you have no attenuator,
or when the battery on your offset attenuator dies during the hunt? You're
not out of luck. Here are some attenuator substitutes.
a Receiver Spur
If your receiver uses an IF frequency
of 455 kHz, then try tuning 910 kHz above or below the transmitter's frequency.
At one of those frequencies you should be able to weakly hear the fox's
signal. Try it out ahead of time, using a nearby transmitter, to see which
frequency you should tune to, and the amount of attenuation to expect.
It's like having an attenuator built into your receiver, albeit an attenuator
with only one attenuation setting. Your receiver may have other useful
spurs, providing different levels of attenuation. Consult reference 
for more ideas.
Tune Off Frequency
Try tuning your receiver 5 kHz or more
above or below the transmitter's frequency. This effectively uses the skirts
of the IF passband to attenuate the signal and keep you in the hunt. Since
you are no longer tuned atop the signal's carrier, you'll probably notice
that any modulation of the signal will cause the signal strength to appear
to jump around. So this is a less-than-ideal substitute for a "real" attenuator.
Rotate Your Antenna
You can attenuate the signal you're hunting
by orienting your antenna so that its angle of polarization
is not aligned with the signal's polarization. For instance: if
you hold a yagi antenna so that its elements are aligned vertically, the
antenna will be less efficient for hunting a signal that is horizontally
polarized. The yagi antenna will pick up the signal more weakly when it
is misaligned in that manner, which is just what you want.
Tune to a Transmitter Spur
Transmitters often generate in-band signals
that are offset from the main transmit frequency. But it is difficult to
know where those transmitter spurs are, unless you happen to be very familiar
with the transmitter you're hunting. For instance: if you know that you're
hunting an Alinco model DJ-S11, try tuning 4 MHz above the hunt frequency
when the signal on the hunt frequency becomes too strong. The DJ-S11 has
a 4-MHz oscillator internally that is used to run its microprocessor. The
4-MHz signal produced by the microprocessor oscillator mixes with the transmitted
signal to produce a weak spur at multiples of 4 MHz above and below the
Note: the previous four suggestions
will help knock the signal strength down so that you can continue to use
your receiver and VHF beam antenna to hunt with. The remaining suggestions
will require that you reconfigure your hunting equipment and hunting style.
Remove the Antenna
Do you think you might be right on top
of the transmitter, but you're not sure? Try this test: disconnect the
antenna from your receiver. Do you still hear the signal? If yes, you're
getting close. If the signal is still full scale with the antenna disconnected,
you may be very close! But be careful. Most handy talkies today
have plastic cases, and may pick up the signal without an antenna when
you are still pretty far away from the transmitter.
Try Body Fading
Body fade (or body shielding) is a technique
that can be used when you're stuck without better hunting equipment. The
technique involves holding the receiver close to your abdomen, and then
rotating your body slowly while observing the signal strength indicated
by the receiver. When you observe minimum signal strength, the direction
to the transmitter is most likely behind you. This technique works best
when you can ensure that the receiver is picking up only line-of-sight
signals from the transmitter. Most reflected signals will not be strong
enough for your receiver to pick them up without an antenna. Which means
you will be most successful with the body fade technique when you are close
enough to the transmitter that you can perform body fade with no antenna
connected to the receiver.
Is the signal too strong even with no
antenna attached to the receiver? Try this: wrap some aluminum foil around
your receiver. (CAREFUL: Place some electrical tape over any exposed metal
charging pads on your receiver. You don't want to short out the battery!)
The aluminum foil should help shield the receiver, and knock the signal
strength down again (still no antenna attached). Instead of wrapping your
receiver with foil, you can put foil around a cardboard box or tube, and
then place the receiver inside the foil-covered container. In either case
you'll need to cut a hole in the foil (and box) so that you can see the
signal strength indicator. Once you've done that, try the "body
fade" technique, this time with the foil shielding in place and no
Tune to a Harmonic
If you have a receiver capable of tuning
to a multiple of the transmitter's frequency, try listening to the second
or third harmonic. (That's two times, or three times the transmitter's
frequency.) If you can hear one of the harmonics, you're getting close.
Once you can hear a harmonic, you can switch over to an antenna designed
for the harmonic frequency. For instance, a 440-MHz
beam works nicely for tracking the third harmonic of a 146-MHz transmitter.
A UHF beam is roughly one-third the size and weight of a beam for the 2-meter
band... but it still might be more extra bulk than you want to carry around.
Hunt an Oscillator
If you have a sensitive receiver that
is capable of receiving at the frequency of an oscillator inside the transmitter,
you can hunt a signal that leaks from within the transmitter. This technique
requires that you be very knowledgable about the transmitter you're hunting.
It also requires a portable HF band receiver. A signal leaking from within
a transmitter tends to be very weak, so you probably won't be able to receive
it until you are within a few meters of the transmitter. But this technique
can be very helpful for tracking down a cleverly disguised transmitter,
especially since it permits you to keep hunting even when the transmitter
is off the air!
Try a Field Strength Indicator
If you're very close to the transmitter,
you don't need a receiver at all. An amplified
field strength indicator has enough sensitivity when you're within
100 meters or so of a hidden transmitter. A standard
field strength meter also works very well, but you'll have to be that
much closer to the transmitter before you'll begin picking up the signal
on the meter. If possible, connect the field strength indicator to your
beam antenna. You'll be amazed how well this works. But beware: most field
strength indicators are not very selective, and will not let you distinguish
between the signal you're hunting, and a strong signal operating on a different
The Bottom Line
It is a cruel irony that that closer you
get to the transmitter you're seeking, the tougher it is to find! But there
are many techniques that can help you overcome the strong-signal challenge.
Try to find several methods that work for you, and be prepared to switch
to the alternative on those occasions when the usual technique just isn't