Chapter 2
Equipment for On-foot VHF Hunting
Introduction | Terms | On-foot VHF Hunt Equipment | Mobile VHF Hunt Equipment | Strong Signal Tactics | References

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Transmitter hunting is a great excuse for building your own equipment, and for experimenting with antennas and electronic projects. But even if the local fire authorities have confiscated your soldering iron, you can still come up with an effective set of transmitter hunting equipment. Below, you will find some examples of very effective, yet inexpensive, handheld radio direction finding tools you can build or buy.

Note: an asterisk* appears next to the heading of those tools that include their own receiver.

Beam, Attenuator, & Receiver

A basic on-foot transmitter hunting set-up consists of a beam antenna, an attenuator, and a receiver. No additional equipment is required to successfully participate in most recreational transmitter hunt events. Your choice of antenna, attenuator, and receiver will depend on your budget, and your preference to build or buy.

For an easy-to-build, low-cost, high-performance directional antenna, it is hard to beat the popular tape measure yagi. Attach an offset attenuator between the antenna and your receiver, and you've got the complete on-foot transmitter hunt set shown below. I have been unable to locate a source for offset attenuator kits, or assembled and tested offset attenuators. Does anyone smell a business opportunity?

 

Complete on-foot hunting setup
Tape measure yagiOffset attenuatorHandheld receiver
Buy a Handheld Beam

Would you prefer that someone else built the antenna? Here are several sources of pre-built antennas. Note: you'll still want an attenuator to use with any of these beams. 

Shoestring Antennas (of  Keyport, WA) offers a handheld 2-element quad antenna, that is light weight, and collapses into a small bundle of wire and fiberglass sticks that fits neatly into a nylon bag for storage. Best of all, it isn't a kit. It comes ready to use, with only some minor assembly required. Their 2-element quad, with one of the attenuators mentioned above, will turn your HT into a competitive transmitter hunting tool. Keep in mind, however, that quad antennas are prone to snagging on tree limbs and low branches. This quad is durable enough to survive the rough treatment, but extracting an antenna from an obstinate bush does tend to slow one down.

Arrow Antenna (of Cheyenne, WY) offers a couple of varieties of handheld 3-element yagis, including one that converts into a 4-element antenna. The yagi elements are made from aluminum arrow shafts. They should be sturdy, but not as forgiving as the tape measure yagi's elements.

WiMo Antenna Ltd. (of Germany) offers an HB9CV antenna that should be suitable for on-foot transmitter hunting. It can be dismantled, and stored in a small pouch when not in use. The HB9CV is a 2-element design in which both elements are fed off-center. The design is commonly used in Europe for transmitter hunting.

Ron Graham Electronics (of Australia) offers an HB9CV antenna designed especially for on-foot hunting. The model ANT1/144 antenna is optimized for front-to-back ratio. It is compact and light weight, and has flexible antenna elements that resist breakage. The antenna comes ready to use, and only minor assembly is required. 

A Complete Kit from Down Under*

This kit from Ron Graham Electronics (of  Australia) was popular at the Region 2 ARDF Championships held the Summer of 1999 in Portland, Oregon. It consists of two components that can be purchased separately: the model RX1 receiver, and the model ANT1/144 antenna. You can find out more about this kit at the web site above, or by sending e-mail to rongraham@magnet.com.au

The model RX1 receiver has adequate sensitivity for most on-foot hunts, and a built-in attenuator that eliminates the need for an external attenuator. The dual-conversion AM design works also for FM signals, using slope detection. When paired with the model ANT1/144 2-meter antenna (described above) the combination makes a complete, compact, self-contained transmitter hunting tool. RX1 price: $99 Australian + shipping, ANT1/144 price: $46 Australian + shipping (prices subject to change). Use a currency converter to see what those Australian dollars amount to in your favorite currency. Note: the receiver is not a suitable kit for a first-time builder, however, it might be possible to purchase one fully assembled and tested. Contact Ron Graham Electronics to inquire.

An Australian Sniffer*

In Australia, a receiver with a built-in attenuator is called a sniffer. In the recent past some enterprising Australian entrepreneurs have offered sniffer kits to fellow transmitter hunters, with reportedly excellent results. 

A complete, assembled, sniffer is available from Bryan Ackerly (VK3YNG). The VK3YNG sniffer comes fully built and tested. Just add batteries and antenna. Take a look at the user's manual here: http://www.users.bigpond.net.au/vk3yng/foxhunt/2m_sniffer/manual.htm. As of this writing (Dec 2002) this unit is available for purchase, but don't wait, they tend to sell out quickly. Contact Bryan Ackerly for payment and shipping details.

A Korean Competitor*

Agayon Amateur Radio Research Group (of South Korea) has developed the AGAYON model AR-2e, a competition-grade ARDF receiver/antenna combination. Clearly, a lot of effort has gone into this design. I have no experience with this device, and have not heard any reports on its performance. I have repeatedly requested price information, and clarification on whether this is a kit or a fully-assembled unit. So far I have never received an answer to my inquiries. Please contact me if you have any information on this device.

The Venerable Little L-Per*

L-Tronics (of Santa Barbara, CA) has been selling its VHF and UHF radio direction finding receivers for search and rescue since the 1970's. These units are still available today. They are not often used by the recreational transmitter hunting community for several reasons. Price is one obstacle: a new model LH-20 covering the frequency range of 136-150 MHz costs $475 (price subject to change). Also, the Little L-Per is rock bound: a crystal is required for each hunt frequency. But if you have access to an L-Per, or have the opportunity to purchase one at a bargain price, it might be worth making the investment in some crystals for the usual transmitter hunt frequencies. Consult reference [3] for a much more thorough treatment of the Little L-Per.

Handy Finders

The term "handy finder" is often applied to various time difference of arrival (TDOA) radio direction finders, such as the HANDI-Finder® (reference [4]). TDOA RDF units consist of two identical antennas, mounted to a rigid support, and spaced less than 1/2 wavelength apart. An electronic switching mechanism alternately connects one antenna, and then the other, to an attached FM receiver, at a rate of several hundred times per second. The FM receiver detects the induced phase shift in a received signal, and outputs an audible tone. When both antennas are equidistant from the signal source, there is no induced phase shift, and a null in the audio tone is observed. In use, a handy finder's support is rotated until a null is observed, indicating that the transmitter lies along a line perpendicular to the plane of the antennas.

Handy finders are simple and inexpensive to build. They also do not suffer from overload in high signal strength conditions. 

Unfortunately, handy finders have several drawbacks. One serious drawback is the 180-degree ambiguity of bearing measurements. Until one resolves the ambiguity, by moving a distance along a path tangential to the signal wavefront, it is not possible to determine whether one's direction of movement is toward, or away from, the transmitter being hunted. Frustration and fatigue set in quickly when a significant amount of time and effort is wasted moving sideways to the direction of the transmitter. 

As serious as the directional ambiguity, is the handy finder's susceptibility to multipath signals. A raspy sounding audio tone, with multiple nulls as the antennas are rotated, is a classic indication of a reflected signal interfering with a TDOA unit. An experienced handy finder operator will recognize the telltale tone, and will attempt to find a better location for bearing measurements. Under the same signal conditions, a hunter using a simple beam antenna might notice the peak in signal strength in the direction of the reflected signal, but would also observe the stronger peak in the direction of the transmitter, and would know immediately in which direction to head. The handy finder hunter is at a serious disadvantage.

The limitations of TDOA devices restricts their usefulness to hunting within a hundred meters or so of a transmitter, where a few steps to the side results in a noticeable change in bearing (and body fade can also be used for removing bearing ambiguity).  At such close range a line-of-sight signal is likely to overwhelm most reflections, helping neutralize the handy finder's multipath vulnerability.

TDOA units that include narrow audio filters, and circuits that determine left-or-right signal direction, are a step above the simple handy finder. If you're thinking about building a TDOA DF tool, consider this design by Joe Leggio. Its synchronized sampling of the receiver audio, and superior antenna switching design, should make it perform more dependably than simpler designs.

The Bottom Line

Every tool for transmitter hunting has its advantages and disadvantages. Do a little research and choose what is best for you based on your budget, time, and skills. In the end, it is those who know their equipment that fare the best. So, whatever you use, practice with it ahead of time. Use tape, velcro, or cable ties to hold things together. Remember: you'll be walking around with this equipment, and you may have a map, or other tools to carry as well.

Be sure to check Joe Moell's RDF Equipment Ideas for Radio Orienteering page, and visit the Links Page for links to sites containing many more equipment ideas and sources for plans and parts. 

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

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