Chapter 4
Strong Signal Tactics
Introduction | Terms | On-foot VHF Hunt Equipment | Mobile VHF Hunt Equipment | Strong Signal Tactics | References

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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 hunt.

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.

Offset Attenuators

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 attenuators.

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 paging towers.

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.

Passive Attenuators

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 [1], [2], and [3]. Arrow Antenna (of Cheyenne, WY) offers an affordable attenuator kit. Radio Shack sells a fully-assembled attenuator.

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. 

Attenuator Alternatives

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.

Tune to 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 [5] 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 transmit frequency.

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. 

Try Reynold's Wrap
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 antenna attached.

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 band!

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 working. 

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Introduction | Terms | On-foot VHF Hunt Equipment | Mobile VHF Hunt Equipment | Strong Signal Tactics | References

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