Stealth Antennas - Ideas & Thoughts
Warnings
BUT, first a few caveats. If metal file cabinets surround you, the results with these simple antennas may be somewhat limited. Metal picture frames are also a problem, as is a building with windows covered with metallic film and other RF absorbing materials. Some buildings are well shielded with metal mesh in the walls that will block RF signals. My office location is in the middle of a tilt-up pre-stressed concrete wall building with metal tie rods. In other words, a bunker!
There are places in my situation where even 800 MHz cell phones, 1900 MHz PCS phones, 2 meter and 440 MHz equipment cannot put a signal into the remote receiver sites at times, no matter what frequency I want to use. I even have trouble listening to a FM broadcast station with an ERP of 1.6 Megawatts from an antenna 1,600 feet high. The station is about 16 miles from our location. When people walk, around I notice there is a lot of QSB on the 440 repeater I use, despite the use of the beam described in last month’s issue. Two meters isn’t too bad though, as I am only about three miles from another repeater.
Use Low Power
As far as operating is concerned, low power must be used—just enough to hit the repeaters with a good signal. Otherwise, you will tear up all the Walkmans and other small portable entertainment things that people bring into their area. There are some other RFI problems that may arise, but that is not the subject of this discourse. So, assuming you want to put one of these antennas in your home or work place and understand the limitations you will face, we will proceed.
Number one, get the copper tape. I found my1/4 in. (.635 cm) tape at a hobby shop and got a huge roll for a minimal price. You could use wire, but unless it is large diameter wire, bandwidth will be less than with the tape. The tape will also allow the picture to lay flat against the frame whereas the wire may not, depending on the wire size. Also, for 2 meters, you will need a large picture and a non-metallic frame. Figure-1 shows a picture that will allow a 1/2-wavelength 2 meter dipole to be put on the back at a diagonal, with the feed point in the center.
Figure-2 illustrates a ground plane with a long radial running across the bottom of the picture. This type of installation needs to have a picture more like a banner with a minimum width of 39 inches (99.06 cm), and a height or 20 inches, (50.8 cm). Note that the tape is attached to the back of the picture—not the picture frame.
Figure-3 illustrates a ground plane with only one radial. This is my favorite as it has a slight amount of gain, and with the dimensions used, gives a nearly perfect match to the coax on both 2 meters and 440.
The dimensions for the antenna in Figure-3 are listed below
| Frequency (MHz) | Vertical Element | Horizontal Element |
|---|---|---|
| 146 | 19 inches, 48.26 cm | 21.25 inches, 53.97 cm |
| 446 | 29 inches, 15.97 cm | 6.75 inches, 17.14 cm |
To connect the coax to the antenna, coax with alligator clips will do well since it can easily be coiled up and stowed when not in use. If you can find the required adapter, RG-174 can be used with a BNC connector, as long as the length is not too long. Otherwise, the use of RG-58/U will have to do. Just keep the coax as short as possible.
To figure the lengths in feet, use the formula: 234/F (in MHz) and convert the resulting figures by using the conversion tables located elsewhere in this magazine.
I hope the antennas presented here have given you something to think about in solving your operatiing needs from work or home locations. There are other solutions, I am sure and an inventive mind can come up with numerous other variations of this method. Just keep the power low. Have fun!
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 25th April 2024