2

Many suggestions on the Internet for a "starter" HF long wire, suggest running a wire in the attic. My (South Australian) roof has a 30 degree pitch, and what little space remains contains air conditioner ducting. Not to mention the steel roof on the carport...

Can I use that aircon ducting to my advantage, as a counterpoise wire? I thought the general idea of running a wire at height was to get away from ground: that is just not going to happen here.
If I can't take advantage of the earthed ducting, how far above it should I try to get?

Alan Campbell
  • 246
  • 1
  • 7

2 Answers2

3

If you run a wire up in your attic, all other conductors such as ducting, house wiring, gutters, and so on are part of your antenna system. You want to avoid near-field capacitive coupling as your losses will definitely go up in that case. Therefore, a wire antenna close to other conductors that run parallel to the antenna conductor is not good.

I am a big fan of numerical modeling of antennas. I model every single wire antenna I have ever built in the last 10 years using NEC2 (National Electromagnetics Code). Actually, I bought the license for NEC4 and use that a lot but NEC2 is free and can easily be run on PC Windows, Mac OS X, or any Linux or Unix platform (assuming you build it yourself, source is available).

With NEC2, you can actually model the antenna wire as well as other near by conductors whether they be wire or metal ducting (typically modeled as a mesh). Doing it up right means spending time creating the model but you can learn a lot about an antenna.

What happens when you have nearby conductive surfaces?

  1. If they are close, you can have greater losses.
  2. Your matching impedance can be changed (important for resonant antennas).
  3. Your propagation pattern will very likely be changed in creating nulls and higher gain lobes where you least expect them.

Items 2 and 3 are determined when you model with NEC2 and NEC4. I mean, that is the principle reasons I do modeling is to calculate the matching impedance and to determine the location of nulls and higher gain lobes and take off angle and so on.

To get started with NEC2 you can acquire the free version of EZNEC software available from ARRL. Or, you can possibly get a 30-day trial from the EZNEC vendor W7EL (at http://www.eznec.com). Or, you can go to http://nec2.org or merely google NEC2.

K7PEH
  • 2,998
  • 11
  • 14
  • The free ARRL version of EZNEC is only available (I think) with the DVD that comes with purchase of the ARRL Antenna Book and possibly the ARRL Handbook. – K7PEH Nov 18 '14 at 01:47
1

You could just use the ducting in lieu of the wire. Anything conductive makes an antenna to some degree. For example, gutters. I wouldn't suggest putting more than 10W into your ducting unless you want to find out how your air handler deals with RF current, but the same advice would apply to a wire running in close proximity to your ductwork also.

If you do run a separate wire, the ducting is going to be a "counterpoise" whether you deliberately make it so or not. Unless you deliberately install a more effective counterpoise (like a ground plane, or the other half of a dipole) and make sure that antenna is properly balanced, then the counterpoise is going to be whatever conductive things are nearby, like the feedline, the earth, your electrical wiring, and your ductwork.

Community
  • 1
Phil Frost - W8II
  • 51,701
  • 6
  • 88
  • 212
  • You may want to investigate whether your ducts are directly connected to the air handler chassis, which may be directly connected to the handler electronics (by chassis ground). – Ken - Enough about Monica Nov 17 '14 at 22:36