Thru-the-Earth Radio-Location

The performance is modest, but a pair can be constructed in a weekend, and with the two-way capability they are great fun.

    This radio, named the Basic-1, uses a single dual op-amp IC to perform both receive and transmit (beacon) functions in one unit. The operating frequency is 1750 Hz. Transmit can be either CW (Morse Code) or a beacon pulsing 5 times/second. The loop antennas are 19 inches (48cm) diameter. A single 9V radio battery will power a radio for days, even in transmit mode. A pair of Basic-1 radios can communicate straight down through 200 feet (60 meters) of rock in ideal conditions. Accurate Radiolocation (which requires a strong signal for sharp signal nulls) is limited to about 67 feet (20 meters) depth.
    Another version of this radio, the Basic-2, uses a second dual op-amp and a heavier loop to improve receiver performance and greatly increase the transmitted signal. The Basic-2 costs significantly more to build because of the high cost of copper wire, and has half the battery life, but increases range to 300 feet (90 meters) for communicating and 100 feet (30 meters) depth for radiolocating. The signal drops off as the cube of range (doubling distance drops the signal to 1/8), which makes long ranges difficult with simple equipment.

Basic-2 Radiolocator with loop formed from PVC electrical conduit

I have assembled several radios and loop antennas that have been range-tested on the surface. During testing in an open field I detected noise from power line harmonics that I eventually traced to a buried wire that I was able to radiolocate and follow for hundreds of feet. The abandoned wire was not connected to AC power at either end. Apparently, magnetic fields and/or earth currents from distant power lines induced current in the wire. Later, I found a second abandoned wire in an adjacent field! This is another good use for a sensitive broadband audio frequency receiver. One successful cave radiolocation test was done with a Basic-1 in less than ideal conditions.
I have put together a few kits with most of the required parts including PC board, case, Telex 610 mono 600 Ohm headphones, wire, controls, connectors, knob, and electronic parts, which are 100% thru-hole. Not included is the battery, the bubble levels needed for accurate locating, the loop form, the loop wire, and the optional piezo sounder (a Radio Shack item). Links to documentation for assembly and use are found below. I am looking for feedback to make improvements in the kits and documentation. It would not be difficult for an experienced person to assemble a radio from junk box parts even without the PC board, but some test equipment might be necessary for frequency alignment. I am selling at my cost as a service to the small Cave Radio Community. If there is enough interest, quantity purchases should bring the prices down.
The following costs exclude postage. Contact me for a quote at the email address on the bottom of this page. I will take a personal or bank check, or PayPal. PayPal will be 5% extra if you are paying by credit card rather than withdrawing from a PayPal account. Note that you do not need to have a PayPal account to send money. I will send you the name of my PayPal account.

A single Basic-1 radio kit is $34.00. You must purchase your own loop wire (~$20.00-$25.00) plus 2 bubble levels (~$4.00) bringing the total cost to ~$58.00 not counting the loop frame, optional piezo sounder, or battery.

Note that you you need two Basic-1 radios and loops to do radiolocating, bringing the total to ~$116.00.
A single Basic-2 radio kit is $35.00. You must purchase your own loop wire(~$35.00-$45.00) plus 2 bubble levels (~$4.00) bringing the total cost to ~$74.00 not counting the loop frame, optional piezo sounder, or battery.

Two Basic-2 outfits will cost ~$148.00.
For those with a well stocked junk box, I will sell the PC boards at my cost of $4.25. These are professional epoxy double-sided boards with plated-thru holes.
Note that the two parts kits are not interchangeable because the values of the loop tuning capacitors are different. The loop wire gauges are also different. However, the same PC board is used in both kits.

NOTE: 7/4/12 For people who have already purchased kits or built radios. I have found a minor change that will improve beacon operation of the Basic-2 at high temperatures. Especially if just the piezo speaker is used without headphones, the units may stop "beeping" and go into continuous ON mode at a temperature of 80-90 degrees F. The cure is to add a third 1N914 diode, in the same polarity, in series with diodes D5 and D6. THe schematics and parts lists have been updated, and I will include the diode in future kits. Email me if you can not obtain a diode locally and I will send you one. The Basic-1 kits are not affected.

A complete history and technical description of the Basic-1 and Basic-2 radios, including specs, schematics, and parts lists are found in The Simplest Radiolocator.
Complete assembly instructions for the kits are found in Constructing the Basic-1 and Basic-2 Radiolocation/communicators from kits.
See Locating Ground Zero to understand the shape of the magnetic fields and how to use them to easily locate the point directly over a beacon placed in a cave. Practice, practice, practice, before attempting a cave, even inside a 2-story house or barn! I am always available to answer any questions.


Searching for Ground Zero with the prototype Receiver and 18 inch loop	Raising the loop 25 feet above the ground for an extreme ratiometric depth measurement