Here is a quick and dirty guide to making a pretty decent receiving antenna.
- A length of tape measure cut to slightly more than 1 full wavelength
- A PVC T-joiner
- A TV Matching Transformer (or 4:1 receiving balun, typically $5)
- Two hose clamps, zip ties also work.
Step 2: To begin we center the tape measure segment on the T-joiner, longer lengths may require some tape to keep it in place. Put the hose clamps into position and tighten them slightly, we still need to fit the ends back in but the less time you need to secure it in the end is better. From there we fold both ends back on themselves. I’ve been told conflicting things as to whether to measure the entire length or just the straight section to determine final frequency, since we are using a large conductor and a wide-bandwidth design it doesn’t necessarily matter as the antenna will compensate for this. I typically leave a space of 0.5-1″ between the ends of my conductors, I’ve gone with 1″ here.
Step 4: From here solder both ends of the matching transformer to the exposed metal, since this is a symmetrical balanced antenna it doesn’t matter which there is no need to ensure one conductor goes to a specific point.
A few things to consider that may or may not improve your antenna:
- Weather proofing, these are very durable in the elements if you take the time to weather proof the solder joint, I use liquid electrical tape for this task.
- Hose Clamps vs Zip Ties, for smaller wavelengths zip ties are the way to go due to the increased relative size of the metallic hose clamps. But for larger wavelengths I’d suggest hose clamps for mechanical stability and for their robustness in the elements.
This antenna was made with the end of a tape measure I had already cannibalized for a few different projects such as the first image of my Marine AIS receive antenna. On the plus side the dimensions work for this to be on the high end of the 70cm band, which worked out rather well.
So why a folded dipole? Well since we are dealing with ships on an ocean we need more gain towards the horizon, preferably with a uniform receiving pattern around the vertical axis. A simple dipole can do this, but a stacked array of dipoles would work better. Since space can be, and is, an issue we turn to other types. For this application since it is receive only, 75 ohm RG6 Quad Shield works wonders. We can then use our previously mentioned 4:1 transformer to convert the nominal 300 ohm impedance of a folded dipole to that of 75 ohm coax straight to the receiver. So not only do we have a resonant antenna with gain towards the horizon, it is also well-matched to deliver all the RF energy it receives to the receiver.