Restricted Space Antennas


'What was that about restrictive covenants'

80 Meter Antennas in an Antenna Restricted Community

So, what about 80 meters? This question presented quite a dilemma. What could be done in such a small space? Maybe a large magnetic loop laid on the floor of the attic. Probably could be done but the bandwidth would be pretty small with constant retuning. There is insufficient space for vertical mounting of the loop and horizontal mounting would suffer from low height above ground. What about spending the money for an Isotron? I pondered all manner of antennas until I hit on a possibility - a short vertical!!!! But in the attic? Not totally!

I ran a small single wire from the soffit directly down the side of the building. This is cheating a bit but the condo police haven't objected so far. And of course the antenna is easily removed if they ever do. This 30 ft vertical wire is the primary radiating element.

80 meter short vertical diagram

Now, do I inductively load the antenna on the ground or use a capacitive top hat in the attic? Reading through the ARRL Antenna Book it is clear that capacitive top hat loading is more efficient. I modeled both alternatives and just as noted in the book the radiation resistance went from 6 ohms inductively loading to 14 ohms with a top hat. And up to 20 ohms if I move the feed point to my deck. With the prospective of "ohms" of ground loss the higher the radiation resistance the better.

So what does the top hat look like? I modeled several configurations and found that a simple perimeter wire running around the attic provides more than enough capacitance to resonate the 30 ft wire at 3.5 Mhz. Interestingly, a 3meter by 4 meter top hat is normally dismissed as mechanically infeasible but this application it is the perfect solution. But I'm going to use the building to "support" the top hat.

What about radials? Of course, they must be buried and limited to be unobtrusive. I use 4 (ground is pretty hard) twisted pair wires 40 ft long. The ends connected and the second wire cut at 20 ft giving approximately 60 ft long radials. The wires were installed during some quick after dark sod-cutting operations.