Copyright © 1999-2011 by Harold Melton KV5R. All Rights Reserved.
Understanding Shortwave Antennas: Page 12
Simple Indoor Shortwave Antennas
Last but not least, let us examine simple wire antennas which may be designed and used indoors, for little or no cost. Such are need by renters and apartment dwellers, and are always better than the way-too-short telescopic whip antenna.
The Indoor Longwire
The simplest antenna which will drastically increase the amount of signal power (compared to the telescopic whip) is merely a much longer monopole — i.e., 40 to 80 feet of fine wire, thumbtacked to the ceiling and connected to the radio.
- Go to Radio Shack and a get a little roll of fine enameled magnet wire, and if your radio has an antenna jack, get the male plug to fit it (usually a 1/8th″ “earphone” plug). If not, get a small “alligator” clip. Stop by Wal-Mart and get some thumb tacks.
- Pick a layout through your house that will allow you to string up the longest wire along the ceiling, in a fairly straight line. For example, start at the far corner of the living room, go down the hall, to the far corner of the bedroom. String up the wire on the ceiling using thumb tacks, dipping down under doorways where necessary. Bring it down the wall at one end. Attach the plug for the radio. Scrape the enamel coating off of the wire, then solder it to the center pin of the plug. If your radio has no antenna jack, simply solder a small alligator clip to the end of the wire and clip it to the telescopic antenna.
You will be amazed when comparing a signal on the telescopic whip and the fine ceiling wire. When connecting the wire, the signal meter will jump way up. You will be able to pick up weaker signals, and will have less problems with fading. It will also pick up a lot more static and noise from electrical devices in your house…
How much better is the full-sized, outdoor, multiwire dipole antenna? On the strong signals, no better. But on the weakest ones, considerably better. If you regularly seek out weak signals, try to get a multi-wire up outdoors — and considering purchasing a nice antenna tuner and an active audio filter. Mainly, a large outdoor antenna will have a much better singal-to-noise ratio. It will pull in more radio signal than electrical interference.
Indoor Electrical Noise
Almost all indoor antennas (including those expensive “active” ones) will pull in lots of electrical noise, requiring you to go around and turn off TVs, VCRs, computers, dimmers, florescent lights, and ceiling fans — before your favorite shortwave radio program comes on. Even worse, the electrical interference may come from street lights or even neighbors — sources you cannot control. This is why you should put up as big an antenna as possible, to get the induced signal power over the noise level.
I miss Radio Shack!
Yes, the dipole should be 1/4-wave on each side. Total length 468/f(MHz) = feet.
The 984 and 492 are “free-space” wavelength (and half-wave). It’s a little shorter in wire, hence the 468 for half-wave wire dipoles.
You should cut your dipole for the lowest frequency, not a mid-point.
I do not recommend wrapping it around PVC (a helical antenna works but not as well as the full length). If you don’t have the space, bending the ends of the wires into L-shapes will be better than winding a helical.
I’m new to shortwave listening and would like to make a dipole for 4.75 to 10.0 MGz and have several newbie questions.
1) Page 7 says dipoles should be 1/2 wavelength so I assume that would be 1/4 wavelength for each side?
2) Page 5 says wavelength in feet=984/freq in MGz and page 8 says 1/2 wavelength in feet=468/freq in MGz. Half of 984 is 492 so why is 468 used in the halfwave formula and what formula should I use for each side of my dipole?
3) Page 8 says to design your antenna for the lowest frequency you will listen to so should I use 4.75 or would 7.420 which is the midpoint of the range of frequencies I want be better?
4) I would like to keep this as small as possible so can I wrap the wire for each side of the dipole around a piece of (ie 3/4″) pvc?
Thanks for a great web site!