80 Meter Pizza Pan Antenna
By Chuck Hines, K6QKL
This is a resonant, half-wave, vertical antenna. It takes up little space in the back yard, was designed for operation on a single frequency 80 meter PSK net, and is reasonably inexpensive to construct. This one has been in operation for sixteen months in the St. Louis area. It has worked into NY, NJ, KS, OK, TX, NV, LA, AL, GA, MI, CO, MN, OH, AR, and PA driven by a Warbler running 2.2 watts. The world's greatest antenna? No way. But it works, is small and can be quickly broken down and hauled to another location.
|K6QKL's homebrew Pizza Pan 80 Meter Antenna. Click on the photo to view a larger image.|
Most wire antennas used in the amateur service are not resonant. We cut our wire by formula to get it useful across a frequency spectra of interest, string the wire up, then adjust by using some kind of matching network to minimize SWR at the transmitter. The Warbler doesn't move around much more than a kilohertz. You can observe the utility of resonance at audio frequencies if you have a piano around the house. Hold down the far right pedal of a piano with your foot, then sing a note aloud near the strings. After you stop singing, you will hear your voice being echoed back to you from the piano strings. At resonance a piano string produces energy at a particular frequency when struck. The string also selectively absorbs energy of that same frequency. Same kind of resonance behavior happens at radio frequencies. We just can't hear it.
With the exception of the length of wire remaining after trimming, none of these dimensions are critical. Feel free to use different materials, form diameters, size wire, etc. Started with a ten foot long, fir wood full round pole 1 ¼ inches in diameter. Hardware store item. Normally used to make clothes closet poles for hanging up stuff on hangers. Since it was going to be out in the weather in all seasons, gave that pole three good coats of polyurethane marine varnish - including both ends of the pole. Calculated the number of feet in a half wave at the frequency of interest and added a couple feet out of caution. Measured and cut my length of twelve gauge multi-strand insulated wire. Leaving room for connections at the top of the pole, simultaneously wrapped the wire and a roll of mason's cord, of roughly the same diameter as the wire, around the pole in bifilar fashion. Idea was to use the string between each wire turn to keep the turns equidistant and the capacitance between turns reasonably uniform. The wire/string wrap of the half wave length took up about fifty inches along the pole. Temporarily secured the ends of the wrap with tape and varnished the wrap to waterproof it and hold it in place.
Bought a steel pizza pan, to use as the antenna's top capacitor, for about $3 at Target. Wandered around the plumbing section in the hardware store and found end caps for plastic pipe with 1 ¼ inch inner diameter. Drilled a hole in the pizza pan large enough for the base of the plastic end cap to pass through. Then gave the pizza pan three coats of red paint. Pizza pans are steel and will rust quickly outdoors if not corrosion protected. Drilled a small hole in the plastic end cap and secured a two foot length of metal rod through the hole in the end cap with nuts, lock washers, and a solder lug. Bored out some space inside the top of the fir rod so everything inside the end cap could fit when the cap was in place, drilled a small hole from the outside of the fir rod into that space so the wire from the top of the antenna could be connected to the metal rod. There is no electrical connection to the pizza pan. Neither metal rod nor wire contact the pizza pan. The pizza pan is held in place with a plastic ring and glued using the kind of glue plumbers use on plastic pipe joints. Soldered a NE-2 bulb at the top of the metal rod. One of the NE-2 wires is connected. The other NE-2 wire just points out into space. The NE-2 illuminates just fine when the antenna is driven by the Warbler. Verify continuity from BNC center conductor to top tip of metal rod.
Used a three foot piece of plastic pipe as a mount for the antenna. Dug a small hole, filled with hole with a small bag of patching concrete, added water, leveled the pipe so it was vertical, then let the concrete set up. Sawed a vertical slit or kerf in the top of the pipe mount, inserted the base of the fir rod, then held it all together using three stainless steel compression fittings - metal bands which tighten with the turn of a screw, normally used on connections under your sink.
Attached a foot of RG-58/U with BNC. The BNC center conductor connects to the bottom wire of the wrap. The BNC outer conductor is strapped to a six foot copper ground rod pounded into the earth.
The critical part: bringing that half wave coil of wire wrapped on the pole into resonance at your frequency of interest with no matching network. Really just two steps. First, realize that no matter how ridiculous your SWR may be, the frequency of lowest SWR is the point at which the antenna is currently resonant. Trim. Check again the frequency at which SWR is lowest, make notes to yourself about how much the resonant frequency changes when you remove X-inches of wire. Patiently trim and check until the lowest SWR is right on the frequency where you want it. If you trim too much, don't worry. You can always splice on another foot or two of wire, wrap it around the pole, secure the end with tape, and work to trim to resonance. Last step is to build and insert a matching network, at the intersection of the antenna and your feed line, to minimize your SWR at the transmitter. Mine required about a seven micro Henry coil in series with the BNC center conductor and the base of the antenna. Matching coil was wound on a nylon toilet paper roller with the internal spring removed and small holes drilled at both ends of the roller to drain condensation. SWR is 1.1 to 1 in the immediate vicinity of the resonant frequency. This is not at all a broadband antenna. A few KHz either side of resonance the SWR begins to rise steeply.
The cotton masonry cord used in the bifilar winding was a bad idea. The cotton wicked moisture from the air and changed the resonant properties of the antenna. So I had to unwind all that cotton cord. I did all my adjustments for resonance and feed line matching using an Autek Model VA-1.
October 9, 2003