Dak Pek - Radio Direction Finder

In late 1967, the Electronics Maintenance Shop at Nha Trang was called upon to develop a prototype system of 'VC Intrusion Alarm' radios. The system would consist of small, tamperproof, easy-to-operate radio beacon transmitters and a receiver equipped with a direction-finding antenna. The purpose of the system was to silently alert the nearby Special Forces camp of any VC activity in local villages. The receiving system would be set up in the SF camp, and a transmitter given to each village chief, along with instructions as to its use.

The transmitters were fabricated by cannibalizing a standard HT-1 transceiver (Hallicrafters): removing the case and receiver section, adding a tone generator section, and potting the entire transmitter unit in a rugged rubbery potting compound. The final appearance of the unit was a solid block of rubber with an antenna and one wire hanging out. The wire had a small pin plug on it and functioned as the On-Off switch. To activate the transmitter, one had to merely insert the plug into a small receptacle on the unit and extend the telescoping antenna. I am not sure how the batteries were implemented on this unit. On the normal HT-1, 8 D-cells were used. On this unit, a much smaller battery arrangement was used. This is probably the only existing picture of the unit, shown next to a field-strength meter and a pack of cigarettes for size comparison.

The receiver consisted of a standard TR-20 transceiver (Hallicrafters) equipped with a custom-made Adcock-type direction-finding antenna. To design the antenna, I went directly to the ARRL Antenna Book to research the best type of directional antenna to use in this situation. WO1 Leo F. Butcher (Signal Maintenance Officer) offered much good advice, as he was an experienced amateur radio operator. The Adcock type was selected because of its very sharp null point and apparent ease of construction.

The antenna was built from sections of aluminum antenna mast, hardwood axe handles for insulators, HT-1 replacement antennas for the active elements, and a custom-wound coupling transformer. The boom was guyed to the mast using the nylon guy lines from the mast kit. It was fairly simple to calculate the element spacing, plus the elements were already made for the HT-1 frequencies. We taped the joints (black bands) to keep the elements from telescoping together by themselves. The transformer was mostly guesswork. The big question was 'Would it work?'.

The rig was set up and tested. It worked almost flawlessly the first time! It had very even reception over most of the antenna rotation, with a very distinct and sharp 'null' or dead-spot when pointed directly at the transmitting station. We were all surprised and pleased that it worked so well.

Next came field deployment and testing. The Special Forces camp A-242 at Dak Pek was selected for the first test in early December, 1967.

WO1 Butcher and I arrived at Dak Pek with the radio system and plenty of spare parts and tools. We put up a shelf in a corner of the team house and cut a hole in the tin roof to pass the antenna mast and cable. We powered it up and briefly tested it.

All we could do then was wait for something to happen.

While we were waiting, WO1 Butcher and I 'adapted' the team's KWM-2A rig (Collins) and put up a special dipole antenna to allow operation on a HAM (amateur radio) band. He then operated (clandestine) as a maritime mobile station somewhere in the South China Sea (even though we were many miles inland) and placed several phone patches back home to the states for members of this rather isolated team.

I also had the opportunity to do some photo work while there. Seems as though someone had ordered a complete small darkroom equipment setup and lots of supplies, then left the camp (unknown circumstances). When I arrived, it was piled in a corner because nobody there knew how to use it. I volunteered to set it up and teach them what it was for. The wash-house was the choice for the darkroom. I shot and developed many rolls of film for them and myself. Unfortunately, one night when I had several rolls of film hanging up to dry, we had a rather severe dust storm, coating all the negatives with grit. Not realizing what would happen, I attempted to clean them, resulting in terrible scratches. That is the explanation for the bad scratches in most of my black-and-white Dak Pek photos, including this one.

Then it happened! One night, the VC Alarm went off! We went to the receiver, and rotated the antenna to locate which village was sending the signal. A response team was quickly dispatched, only to find a party going on. Apparently, the chief got drunk and wondered 'what will happen if I stick this plug thingy into this little hole (on the transmitter)?'. Well, he found out and we proved that the system worked as designed. I don't know if our people got to partake in the party.

All together, WO1 Butcher and I spent about 21 days at Dak Pek. I don't know what became of this project. If there was any after-action report written, I would certainly like to find a copy.