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

May 11, 2013

Abandoning Antenna Analyzers. Moving on to Antennas.

Filed under: ham radio — Cranky Sysadmin @ 7:01 pm

After learning a lot about antenna analyzers over the past few months, but not actually building a portable functional model, I settled on a very simple antenna analyzer based on the 74HC4046 pll chip. There are several designs that use this chip. I decided to combine the dual range vco from PE2ER’s design and the simple output from the tenna dipper. I thought this was novel, but of course someone has already done it and called it the deluxe tenna dipper. The schematic is so close to mine that I don’t care to post mine here. After all this work though, I found an MFJ-259b for sale at near-fest. It was a great deal, so I took a chance.

When I got it home, I cleaned some corroded battery contacts and popped in some batteries (drum roll please). Dead! Fortunately, the problem was easy to find. One of the battery wires was disconnected. After fixing that, it powered up and a few tests seem to indicate that I have a nice antenna analyzer. Now that I have an antenna analyzer, I started work on a small loop antenna There are lots of descriptions of various designs on the web. I’m a fan of AA5TB‘s site. He has some nice calculators and a complete, concise description of single turn small loops. For my friend in the nursing home, I settled on a multi-turn loop for 80 meters to go with the pixie II. A single turn loop made of copper pipe wouldn’t work in this situation as it’s too heavy and large and is likely to scare the residents. Below is a picture before I added the coupling loop.

IMG_0011At the top is a 5-25pf air variable with about 1 millimeter spacing between plates. I added a dowel to the shaft of the capacitor because my hand was affecting the tuning of the loop. The wire is 14 gauge solid copper from a roll of romax. The pipe is 1/2 inch pvc. Each arm is 17 inches. I spaced the turns about an inch apart. Fortune was with me. With the capacitor fully meshed, I can tune the antenna from 3.5 to 6 mhz. Most of that will be wasted on a pixie II of course. The capacitor can handle 5 watts at 3.6 mhz, but 10 watts causes arc-over.

I’ve not done any serious measurements of this antenna. On receive, it is easy to null out noise by turning the antenna. 80 meters was dead today, so it’s hard to tell how it performs. Once I prove that the antenna functions, I’ll probably build some different small loop designs.


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