Projects

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Building a Tape Measure Yagi

In the September / October newsletter we discussed how to construct a transmitter that can be used  for radio direction-finding. But there is another piece of equipment you'll need to consider before heading out to the field for a transmitter hunt - an antenna! For reasons of simplicity, and because they are relatively inexpensive to make, we chose to build a "tape measure" antenna. Below, you'll find directions on how to build your own 2 meter direction-finding antenna.

Parts List

• 3  feet of ½ inch schedule 40 PVC pipe
• 2-½ inch schedule 40 cross connectors
• 1-½ inch schedule 40 T-connector      
• 1-½  inch schedule 50 caps
• 1  inexpensive tape measure (a 1 inch wide tape is recommended)
• 4  feet of coax cable with a connector on one end (UHF, BNC, SMA, etc.) and the other end prepared for soldering
• 6  #12 (11/16 inch to 1 1/4 inch) hose clamps 
• 5  inch piece of solid wire for hairpin tuning component (12 to 18 gauge will work)

Tools required

• Hacksaw or PVC pipe cutter
• Soldering iron or gun
• PVC cement
• Electronic solder
• Wire cutter
• Hot glue gun
• Tape measure
• Tin snips
• Black indelible marker
• Metal file
• Flat blade screw driver or 5/16 inch nut driver (to tighten hose clamps)

Step 1

Cut an 11 ½ inch, and two 7 inch pieces of PVC pipe. Before cutting the pipe,  measure and mark the length using a black indelible marker.


Step 2

Assemble the cut lengths of pipe as seen in the drawings to create the boom of the antenna.  Begin by connecting a T-connector  to one end of your 11 1/2 inch pipe, and a cross connector to the other. Then add one of the 7 inch pipes to the cross connector. Glue the remaining cross connector to the end of this pipe and connect the last pipe to this cross connector. Make sure that you align the cross connectors. The easiest way to do this is to place them on a flat surface and twist them to be flat with the surface. Finish this step by adding the cap to the open end of the last pipe.

Step 3

Cut four pieces from the inexpensive tape measure: 41 3/8 inch, 35 1/8 inch, and two lengths of 17 3/4 inches. Be careful not to cut yourself on the sharp corners or ends. Use your tin snips to carefully round off the sharp edges.

Step 4



Step 5

The next step is to take the 5 inch piece of wire and bend it into a "U" shape about 3/4 inch wide. Then tin the ends of the wire. This will be used to make the “hairpin match.” Scrape or file about 1/4 inch of paint off the back side of one of the ends of the two remaining pieces of tape. Tin the bare areas, then attach the hairpin match and coax wire with solder as shown in the diagram.

Step 6

Next attach this assembly to the boom using the two remaining hose clamps. The soldered joints are then covered with hot glue to seal and help waterproof it. For a balun,  4 or 5 turns of the coax feed wound round the boom should do the trick. Now your antenna is ready for use.





One of the most enjoyable projects that we tackled this summer was constructing a pair of transmitters for radio direction-finding. Of the two that we built, one is a modulated-CW beacon, like the ones used for International ARDF competitions, and the other is a "voice" beacon. While researching for this project, we discovered Bob Simmons' (WB6EYV) boards that could be used in their construction. We chose the MicroHunt and SqwakBox transmitters.

The MicroHunt transmitter produces about 50 milliwatts of FM-modulated CW on 2-meters on a board measuring 0.9 X 1.2 inches. It comes partially assembled, and the PIC micro (that generates the CW message an ID at regular crystal-controlled intervals) can be pre-programmed before shipping. What we find upon delivery is a tiny board with mounting points for power and an antenna.


The MicroHunt is intended for competitive transmitter hunts, where multiple transmitters operate sequentially on a single channel. For this reason, crystal timing is essential to prevent transmission overlaps. It can also be used as a DF "homing beacon" on free-flight balloons. It uses a PIC 12F675 micro for supervision as well as a strap programmable, crystal-controlled PLL synthesizer chip. Ground range, depending on antenna, is typically well over a mile.

Its sister, the SqwakBox, also produces 50 milliwatts of FM-modulated RF on 2-meters, but its assembled on a slightly larger board, measuring 0.9 X 2.4 inches, and includes a voice record/playback chip with 60-second recording capacity, and an integrated electret microphone to record a message.


The SqwakBox makes use of a PIC 16F84 micro for supervision, ISD2560 for voice record/playback and an ICS525 PLL CPU clock generator chip. Messages can be recorded by pressing a "record" button and speaking into the integrated microphone. Its mostly intended for recreational 2-meter transmitter hunts.

The first step regarding assembling these transmitters is to find an enclosure. Just about any electronics enclosure would work. We wanted something that was small, lightweight, and had a good surface to mount an antenna, and on/off switch. Checking several options online, we decided on plastic enclosures with removable tops, and room enough to mount the necessary 9-volt battery. Believe it or not, it was less expensive to purchase these from their manufacturer in Poland and ship them, than from the US distributor. The antennas we chose are Nagano 2 meter HT "flexi" ducks. We added an on/off switch so we didn't have to worry about disconnecting the battery when not in use. Besides the enclosure and switch, the only other parts required to assemble them were BNC chassis connectors for the antenna, some wire, and 9-volt battery connectors. 

For more info on the parts used in this project, please visit:
Doppler DF Instruments
Electronics Enclosures

The EL-Key is an important paddle in the history of amateur radio. This single lever design was the first commercially manufactured paddle offered to ham radio operators. They were made by R.E. Poucel (W2AYJ) and Sid Shore (K2FC) doing business as "Poucel Electronics," a division of Shore Mfg. Co., of Long Island NY, starting in 1959. Today they are very scarce, and sought after by collectors.

   

While searching for an EL-Key, we were looking for two important qualities - a key which had all the components intact, and one which still had the all-important name plate attached and undamaged. Everything else could be restored.

The first step in any restoration process is to decide what needs to be done. This one was in obvious need of new paint on the base, and the lever, upon initial inspection, didn't function as it should have - It was frozen and had no springing action. To begin, we would need to remove all of the components from the base. Before disassmbling anything, however, its a good idea to take pictures and to draw diagrams of how everything is attached and wired.

The main carriage comes off with just two bolts, allowing removal of the swing arm assembly. A flat head screw driver is the only tool that is required. The rest of the components are simple upright posts which act as electrical contacts, spring holders, or stops. They all remove with a single machine screw, threaded from the underside of the base.

   

Once everything is removed, the base can be sanded and prepped for paint. We used two different grits of sandpaper with a sanding block. The first was 220, followed by 320, making sure to smooth out all the chips and scratches. Then the base was cleaned with solvent and set aside for painting. Our base was painted by our good friends at Moody's Collision Center, in Scarborough, Maine. We chose Volvo paint code 019, which is a solid black, closely resembling the original color.


The rest of the components were cleaned and polished, and much of the stripped or worn hardware was disgarded and replaced. For the plastic paddles, which were yellowed and scratched, we used Mothers Plastic Polish, which removed almost all of the surface scratches and yellowing. Then we gave it a finishing coat of Zymol Concours Glaze. The metal components were polished with mag and aluminum polish, then finished with Zymol. We discovered that the problem with the side-to-side motion of the lever assembly was a missing spring, and a badly bent center bolt.  With these replaced, the left side motion was restored.

With all the components repaired and polished, and the base back from the body shop, we began the reassembly process. Consulting our photographs and wiring diagram, the process went smoothly. The wiring was straight forward, but required a few connectors to be soldered to the wires, and to each other, to match the original layout. On removal, some of these just fell apart. The finishing touch was to reapply the EL-Key nameplate, which was done with a two-part epoxy called J-B Weld.

   

 

We chose to build dipole antennas for our field activity because they are cheap to produce, very efficient, and easy to construct. But why are they effective? Let’s take a closer look, as we examine just what a dipole is...

A dipole is a balanced antenna. It consists of two haves, which extend in opposite directions from a feed point at the center. It is commonly called a “half-wave dipole” since the entire antenna is ½- wavelength long at the desired operating frequency.

Why do we start here? In its simplest form the dipole is very dependable. It’s easy to construct, tune, and to install, but it is also the building block for other, more complex antennas. Once you understand the techniques involved in making the dipole, they can be applied to any type of wire antenna including the “Full Sloper,” “Inverted V,” “End-Fed Zepp,” “Folded,” and “Trap” varieties of the dipole.

The first step is to determine the total length of the antenna. We do this (for frequencies up to 30 MHz), by using the following formula:

Length of half-wave antenna in feet  =     492 X 0.95 / f(MHz)    =     468 / f(MHz)
                                                               
A half-wave antenna for 14.200 MHz  =   468 / 14.200 = 32.96 ft.  Divide this number by 2 to get the length of each leg, and we have 16.48 ft. Keep in mind that it’s a good idea to cut the wire a few inches longer than the calculated length, since some of the resonating area of the wire will be lost when it is tied back onto the insulators.

After this is done, cut a length of feed line. We used what is commonly called “zip cord,” which is simply the electrical cord used on lamps or other appliances. Cut a length of about twenty-five feet, and strip the ends (about three inches on the end that will connect to the center insulator). Although there are many ways to connect the feed line, we chose to use a commercially available insulator made of plastic. This could also be made from PVC tubing, wood, or even a scrap of Lexan. The commercially made insulator has a fat round center with grooves in it, and holes drilled on the ends.  Secure the feed line to it by wrapping it tightly around the center grooves and securing it with a plastic tie, leaving enough of the stripped wire to loop around and eventually connect to each leg. 

Wireless Society of Southern Maine  25 Graham Road, Westbrook, ME 04092  
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