The Cranky Sysadmin A world of technology, fun, and ignorant rants.

May 27, 2013

A Field Day Antenna for 80 meters (I hope) pt.1

Filed under: ham radio — Cranky Sysadmin @ 7:20 pm

I spent this weekend starting work on an 80 meter antenna. The design is pretty pedestrian, but I’m hoping for acceptable performance. If I’m lucky, I’ll be able to get the antenna onto 160 meters with reduced performance.

This is going to be a short vertical (about 28 feet) with capacitive top loading and a loading coil about half way up the mast. I should do some modeling, but I’m going to wing it for my first real antenna project and hope for the best. Surely that’s a path to guaranteed success. What could possibly go wrong? I will be measuring along the way, so hopefully I’ll be able to set a good course. Even if it’s a failure, I’ll probably learn a thing or two.

This is a portable Field Day antenna. Its not meant to be a permanent installation, and it will probably collapse in the first big wind. An ice storm will also probably take it down. I also expect that multiple installations and tear-downs will probably wear out the crummy construction quickly. With all that said, here are some details and pictures.

My “tower” is a Jacklite 31 foot telescopic fiberglass pole. It seems pretty sturdy. My capacitance hat is simply 8 12 gauge wire radials which are two feet long. I connected the ends of the radials with similar 12 gauge wire to form a 4 foot diameter circle.

 

Finished capacitance hat

Finished capacitance hat

The center hub is a short section of 1 inch PVC pipe which I drilled eight holes through around the outside to shove the wires through. I stripped about 3/8ths inch of insulation from the end of each wire and bent them to conform to the curve of the pipe. I pushed each through and soldered the end of each wire to the butt of the next wire where it comes through the pvc. Below isĀ  a picture of the finished product:

IMG_0022I’m depending on the solder and compression to keep the wires in place. This is probably not a mechanical masterpiece for the ages. In fact, I’ll be lucky if it makes it through Field Day. After I completed this hub, I soldered the tip of a 14 gauge stranded wire to the inside and inserted a copper ring so that the whole thing won’t move around when mounted on the pole. Below is a picture of this.

IMG_0021The stranded wire is going to be the radiating element. I stood up the pole in a light breeze as a test of the structure. It all seems to work pretty well.

Ugly antenna on a pretty day.

Ugly antenna on a pretty day.

I have the support base for a bird feeder which I slid the bottom of the pole over so it would stay up. The wind loading in a light breeze isn’t too bad, but anything more would knock it over.

Next steps for this antenna are a ground system, the feed system, and the loading coil. The ground system will be 8 25 foot radials attached to a ring at the bottom of the antenna. The feed system will be a piece of sheet aluminum with some holes drilled for the connector, ground, and the radial system. Before I build the loading coil, I will need use an antenna analyzer to find where the antenna is resonant. I’ll also need some matching transformer to match the antenna to 50 ohms.

The work left is about a day if all goes well. I’ve ordered the parts I need and hopefully, it will all show up before next weekend.

 

 

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