Doty Antenna Update

Well, it’s basically all finished (except for putting some tools away), and I must unfortunately report that the remaining work I did today accomplished only a modest improvement in the antenna’s performance. There does appear to be less influence on reception from indoor noise sources, but it also appears that the source that makes the noise floor below about 11 MHz so high is elsewhere in the neighborhood. That said, the improvement at 11 MHz and above is nothing short of astounding.

I do have two notes to add to the instructions John Doty wrote nearly two decades ago:

  1. Weatherproof metal project enclosures (“miniboxes”) are not so easy to find locally, and many of the mail-order sources for them have onerous minimum orders. My solution was to purchase a metal, outdoor-grade electrical box, spare plug (they all tend to come with three holes, and plugs for two), and cover plate and use that for an enclosure. Because one of the holes is on top of the box, I used a liberal amount of Coax-Seal inside the box to help ensure moisture cannot enter. I put all connections on the bottom of the box to minimize the chance of water ingress.
  2. The 300:75 ohm TV matching transformer I took apart had a ridiculously small ferrite core in it. Even if I used the recommended 40-guage enameled wire (which is annoying to use because it breaks so easily), I see no way I could ever get a total of 40 windings around it. I ordered a ferrite toroid (Core F-50, Ferrite Mix 61) from Palomar Engineers (they have no minimum order and shipped my order very promptly) and used that. Instead of 40-guage wire, I just bought the set of 3 spools of magnet wire that Radio Shack sells, and used the medium size for the 10-loop winding and the smallest size for the 30-loop one.

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