Hardware Resources for RDF Home Builders

Sources for the Roanoke Doppler project

Complete construction plans for the Roanoke Doppler are in Transmitter Hunting---Radio Direction Finding Simplified (THRDFS). The following are sources of printed circuit boards for the display unit. Some may also offer critical parts, complete kits, and/or wired/tested units.

Disclaimer: The information above, and elsewhere on this page, is provided for the convenience of readers only. These suppliers are independent entities. They have no business association with Joe Moell or Tab/McGraw-Hill. A listing on this page does not constitute endorsement of products. Contact these suppliers directly for offers and prices.

Boards are no longer available from Ed Greany KB6DOL or Tom Lewis AB5CK.

Sources for RF attenuator projects

Resistive RF attenuators are best suited for mobile HF and VHF T-hunting with directional antennas such as loops, yagis and quads. Pete Ostapchuk N9SFX sells parts for resistive attenuators, including a 6-section enclosure machined from 6061-T6 aluminum bar stock and engraved with attenuation values. Also available from him are switches, resistors and connectors. Plans for his project are in QST Magazine  for May 1998. Send e-mail to n9sfx@aol.com for details and prices. The above disclaimer applies.

Another source of resistive RF attenuators is National RF, Incorporated (formerly Radio Engineers) of San Diego, CA.

For on-foot VHF/UHF "sniffing" and international-style foxhunting with a beam and handi-talkie or scanner, an "offset" or "active" attenuator is better because it eliminates the problem of signal leakage directly into the receiver's case. Plans for an active attenuator are at this Homing In site, click here.

Sources for fox-boxes

Hidden transmitter controllers, sometimes called "fox-boxes" are units that provide audio and timed key-up/key-down (push-to-talk) outputs. A fox-box turns an ordinary handi-talkie or mobile transceiver into a cycling hidden T. Here are four fox-box projects for home construction:

  1. The Un-Music Box by KØOV is an easy-to-build project using common easy-to-obtain CMOS ICs. No microprocessor programming is required. Provides on-off cycling or continuous transmissions. Callsign is hand-programmed using two switches; shift register memory has battery backup. Complete plans in the book TRANSMITTER HUNTING---Radio Direction Finding Simplified. You can remote control the Un-Music Box with a surplus pager receiver using plans in Homing In for December 1990.

  2. PicCon by Byon Garrabrant N6BG uses a Microchip PIC microcontroller and has a variety of programmed on/off modes and tone sequences for mobile T-hunts as well as IARU-standard MOE/MOI/MOS audio, timing and synchronization for international-style radio-orienteering. DTMF remote control and programming with your HT or external receiver. Delayed turn-on and turn-off timers. Callsign and programming info are held in EEPROM when power is removed. Reviewed in Homing In for March 1997. Complete kit is available from N6BG.

  3. TBOX by Ron Seese N6MBR uses an 80C51 microprocessor to provide three programmed tone sequences and/or a CW message. User-programmable on and off times in intervals from seconds to hours. Programmed via RS-232 serial port using commands similar to packet TNCs. Non-volatile memory (EEPROM) holds programming and message when powered down. Optional SmartWatch clock turns fox on automatically at programmed time. Plans for basic unit are in Homing In for October 1991. Assembly, object and binary files for the microcontroller program (10K total) are available at this Homing In site. Contact N6MBR regarding availability of kits and critical parts.

  4. Francois Tremblay VE2JX and Jacques Brodeur VE2EMM of Montreal have designed a new PIC-based foxbox for international-style hunts with MOE/MOI/MOS audio, synchronization of 2 to 7 foxes, delayed start of up to 3-1/2 hours in 1/2-hour increments, and automatic shutoff after four hours of operation. Callsign is held in non-volatile flash memory. Callsign changes and mode setup are easily done with dipswitches. This project is fully described in Homing In for April 1998.

Sources of PIN type RF diodes

For maximum receiving sensitivity and minimum problems with cross-modulation, good RF PIN diodes must be used in hard-switched dopplers such as the Roanoke Doppler. Breakdown voltage and power rating of the PIN diodes are not important in low-level receiving circuits. (You aren't going to transmit through the switcher, are you?) For low loss and good sensitivity, the diodes should have low RF resistance (Rs) at the expected forward DC bias current (7.5 mA for the wideband switcher). They should have low capacitance (Ct) at the reverse bias off-state voltage. Effective carrier lifetime and reverse recovery time (Trr) should be short. (PIN diodes with long carrier lifetime are ideal for current-controlled RF attenuators, but that's not what we want in this application.) Keep the diode leads very short to minimize inductance and make sure signal path lengths, including diodes, are equal for all elements in the array.

Suitable PIN diodes include:

After I bought some Motorola PIN diodes from Debco Electronics, I mentioned their availability in Homing In for April 1995. A reader wrote to me after publication to say that Debco was astonished at the response. The order-taker said that they hadn't sold a PIN diode for over a year and then after the article came out they began selling like hotcakes. Who says hams don't build nowadays?

Other Sources of RF Parts

Hard-to-find RF parts such as air variable capacitors for tuning HF loop antennas can sometimes be found at dealers of "overstocked" or "industrial surplus" components. Here are three to try:

For more information about sealed RF "piston" trimmer capacitors and sources, see the Shrunken Quad page at this site.

Ferrite beads and cores for the dual-polarization quad (see Homing In for February 1997) and other RF projects are available by mail order from Amidon Associates. Locals can save shipping charges by picking up orders at 240 Briggs Avenue, Costa Mesa, California, during normal business hours. Call in advance to 800-898-1883 or 714-850-4660.

ByteMark Corporation, 7714 Trent Street Orlando, FL 32807 (407) 679-3184 also carries Amidon ferrite components and has technical information about Amidon ferrite and iron powder products on its Web site. In addition, ByteMark sells a Windows-based program for calculating the turns for coils on various Amidon cores.

The Philmore line of packaged electronic parts includes unusual items such as Amidon T50-6 cores (used in the ATX80 80-meter fox transmitter), International Rectifier HEXFETs, MPF-102 JFETs, etc. Unfortunately the Philmore Web site does not have a complete product list, but it does include a list of local distributors of these parts.

Crystals for 80-meter foxboxes are available from Brian Carling AF4K in Sanford, FL. He also sells some crystal-can oscillators that may be suitable for building offset attenuators.

Jerry Boyd WB8WFK provides this feedback on ON7YD's Fox Transmitter for 80 meters (ATX80), which was featured in 73 Magazine  for November 2000: "Motorola MC14001B Quad-NOR ICs from Radio Shack (part number 276-2401) didn't work. The oscillator would not start properly. I tried 6 different ones and they all acted the same. I put in an old Fairchild CD4001B Quad-NOR and now it works with no problem. I downloaded data sheets from the manufacturers' Web sites and the internal schematics are indeed different." Fairchild ICs are available from many suppliers, including Digi-Key. KØOV's six transmitters use Signetics (Philips) CD4001B's packaged by Jim-Pak and sold by local parts suppliers nationwide -- they all work fine.

Sources for building polar-plot bearing readouts

I have many requests for sources of unusual electromechanical parts used in two-dimensional bearing indicators such as the KK6CU motorized quad, the WB8WFK and KD6IFZ computer displays, and the NØMKJ Foxcopter. (See the Homing In Index page at this site for articles describing these RDF systems.)

For (relatively) inexpensive surplus sine-cosine potentiometers as used in the KK6CU motorized quad, try Servo Systems Company, 115 Main Road, PO Box 97, Montville, NJ 07045, (201) 335-1007, in USA (800) 922-1103. The last catalog I received from this company included sin-cos pots by Litton, Duncan, N.E.I., Samarius, and Gamewell. There were continuous turning and single-turn models with prices from US$18.50 to $69.50.

For a "Cadillac" system, check out these contactless sin-cos resolvers using internal magnets and Hall effect sensors, designed to be rotated at up to 10,000 RPM: Elweco Incorporated, PO Box 909, Painesville, OH 44077-0909, (440) 254-1716.

Small industrial rotary joints (slip rings) in the $30 price class are suitable for low-voltage uses. NØMKJ used one of these for the two-meter coax feed in his Foxcopter. They are available from McMaster-Carr Supply Company, PO Box 54960, Los Angeles, CA 90054-0960, (310) 692-5911.

Another promising source of rotary joints is Mercotac, Incorporated, 6195 Corte del Cedro #100, Carlsbad, CA 92009, (619) 431-7723. Unfortunately the Web site does not list prices. If you use one of these products in your RDF system, please let me know how it works and what it costs.

Inexpensive electronic compasses with digital output (Vector 2X series) are manufactured by Precision Navigation Incorporated, 1235 Pear Avenue, Suite 111, Mountain View, CA 94043 (415) 962-8777. They are sold by Jameco Electronic Components, 1355 Shoreway Road, Belmont, CA 94002, (415) 592-8097, in USA (800) 831-4242. They are also sold by JDR Microdevices, 1850 South 10th Street, San Jose, CA 95112, (408) 494-1400, in USA (800) 538-5000.

From Peter PB2RDF comes this suggestion for a source of obsolete and out-of-stock electronic components for replacement or for older designs: 1-Source Electronic Components, 591-B Bicycle Path, Port Jefferson Station, NY 11776 (800) 966-8826 or (631) 642-2479.

Also see these Homing In site pages for specific project parts information

If you discover a good source of unusual parts for RDF homebrewers, please e-mail me.

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This page updated 5 May 2008