Nuts and Volts - August 2015

My crutches are sold by Walgreens (Item 276734) for around $46. However, I have found that crutches must be surplus for a lot of other people. Since starting this project, I have seen several sets at flea markets for around $2.

That’s definitely the place to buy this antenna hardware.

I found the bottom (foot) of each crutch was almost the perfect size for a slip-fit inside 3/4 inch ASTM D 2241 PVC pipe. The feet had already been drilled with 1/8 inch holes. Reaming the holes just a little allowed #8-32 screws to pass through.

The holes turned out to be perfect for securing the crutches to the pipe, but the 3/4 inch thin wall PVC looked a little light for supporting the weight of extended crutches. A 1-1/2 inch schedule 40 PVC tee and two 3/4- 1-1/2 reducing bushings appeared to be just the right size for reinforcing the lighter 3/4 inch PVC. Check out Figure

2 and Figure 3.

Home Depot had all the PVC parts and the hardware in stock for less than $11. Total cost of my project would have been around $12 if I had bought my crutches at the flea market.

Initially, I bought #8-32 x 3 inch machine screws long enough to pass all the way through the tee, fasten the crutches, and terminate the RG- 58 coax. As I was drilling the PVC, I noticed #8-32 nuts would fit in the space between the crutch foot and the 3/4 inch PVC, so I decided to drill holes in only one side of the tee and the short 3/4 inch sleeve. I believe this design provides better electrical connections and also increases structural rigidity.

I built the antenna (Figure 4) in just over two hours after I had collected all the materials. The only tools I used were a hacksaw, screwdriver, drill bit, and pair of long nose pliers.

When the completed antenna was up in the air on top of a short joint of 1-1/2 inch schedule 40 rigid conduit, the dipole needed to be shortened to 98 inches for an SWR of 1.1 from 50 to 51 MHz (Figure 5).

I’m certain this novel antenna would never survive high winds or ice loads, but it was not built with permanent installation in mind. I think the best uses for these crutches are teaching antenna resonance, getting attention at hamfests, and starting conversations about emergency communications at public venues promoting ham radio. NV

Robert Fischer WB8BEL is a physicist and electrical engineer. He specializes in process instrumentation and maintenance reliability. He can be reached at BobFischer@Fischer Technical.com.