Ham Radio Forum

by **skywave** » 22 Jun 2006, 13:56

Attics can provide a good way to hide antennas, but coupling to and from power and phone lines can be serious problems in some cases. On the receiving side attic antennas can pick up lots of power line interference. When transmitting coupling into power lines can cause lots of interference to radios and stereo equipment, and coupling into phone lines can cause telephone interference. Even so, attics may be the best solution where the installation of external antenna is prohibited.

by **yolkman** » 24 Jun 2006, 14:17

I think attics are more suitable for VHF than for HF antennas. I have a 20 meter half-wave dipole in my attic and a ceiling light in one of the bedrooms lights up to about half its normal brilliance when I transmit. The power that lights that lamp obviously isn't radiated and it is likely that a lot of other power is being lost in other lamps that heat only slightly.

Because of that, I wouldn't install an HF antenna in an attic again. Of course, a half-wave dipole hanging a few inches above a roof top wouldn't be much better.

by **skywave** » 26 Jun 2006, 15:37

Another twist to MstSage's metal flower box antenna suggestion is to cut a thin half-wave long horizontal slot in the side of metal flower box and then attach a feed-line somewhere across the slot to form a slot antenna (also known as a current sheet radiator). The impedance across the center of a narrow half-wave slot antenna in a large flat sheet of metal tends to be around 500 ohms, but the impedance drops toward zero moving toward either end, so almost any feed-line impedance can be matched simply by positioning the point of feed-line attachment along the slot. The feed point impedance also depends on the width of the slot and the proximity of the antenna to other nearby conductors. In a flower box application, the rear metal panel of the flower box would serve as a plain reflector that would significantly increase the forward gain, but that would also change the feed impedance. However, an impedance match could be obtained experimentally by simply moving the point of feed line attachment along the slot to find the point that provides the lowest SWR.

Slot antennas haven't been used much by amateurs, but a horizontal slot antenna in a large metal sheet (large compared to a half-wave) has radiation pattern and efficiency characteristics identical to a vertical half-wave wire dipole and a vertical slot antenna has radiation pattern and efficiency characteristics identical to a horizontal half-wave wire dipole, so slots provide interesting stealth antenna possibilities (for example, a slot in the slide of a metal garage wall, metal storage shed wall, solid metal fence wall, or other such things).

by **kc4ybe** » 26 Jun 2006, 15:50

Years ago in Denver Colorado the "cable" television was provided by microwave from transmitters on the nearby mountains and people would buy a high gain antenna from the provider. As people are, enterprising folks started making their own antennas copying the design of the television provider. This business would then send teams of people out to verify every antenna on housetops throughout the area was indeed one of their customers and not a thief of their service.
Even more enterprising people would construct fake fibreglas chimneys that attached to the housetop and hide their illegal antennas! Such a method would work best for VHF/UHF work and has the advantage of placing the antennas up high.

by **Matt** » 27 Jun 2006, 16:42

That's a great idea. The same idea could be applied to stealth antennas in fake air conditioning equipment.

by **Roberto Velasco** » 28 Jun 2006, 16:32

I am amazed at the range of stealth antenna ideas suggested. They give me a lot to think about. Thanks for taking time to answer.

Roberto

by **blindsnake** » 30 Jun 2006, 02:39

Stealth antenna possibilities haven't been exhausted yet. For example, no one has suggested using a buried antenna. Various military organizations around the world have done a lot of experimenting with underground antennas. It is true that buried antennas are not very efficient compared to above-ground radiators, but they work, and they work better than most hams probably would guess without trying one. High frequency sky-wave energy penetrates many feet below ground level. The depth of penetration for a given amount of attenuation decreases with increasing frequency, but there is significant penetration even at the upper end of the HF spectrum. The reverse also is true in that transmitting antennas buried below ground level radiate energy above ground.

by **wGEric** » 03 Jul 2006, 13:20

If an antenna is going to be used only for receiving, various types of active antennas are good stealth antenna possibilities. Basically, any kind of small antenna with a built-in RF amplifier is an active antenna. A thin metal whip or well-insulated vertical wire as short as 18-inches works amazingly well against a large metal ground surface throughout the HF band, for example. An untuned, amplified loop antenna as small as 18-inches in diameter also works well without a ground plane throughout the HF spectrum with proper design.

by **Ken** » 06 Jul 2006, 17:22

Tree antennas shouldn't be overlooked as stealth antennas. A vertical wire running vertically up the trunk of a tall tree is nearly invisible. Resistive losses in the nearby tree trunk smooth antenna reactance variations with frequency and greatly increase the antenna's bandwidth. If used with buried ground radials tree verticals function reasonable well transmitting despite losses in the tree trunk and they make excellent wideband receiving antennas.

Ken

by **WA7OCR** » 09 Jul 2006, 14:34

["skywave"]Another twist to MstSage's metal flower box antenna suggestion is to cut a thin half-wave long horizontal slot in the side of metal flower box and then attach a feed-line somewhere across the slot to form a slot antenna (also known as a current sheet radiator). The impedance across the center of a narrow half-wave slot antenna in a large flat sheet of metal tends to be around 500 ohms, but the impedance drops toward zero moving toward either end, so almost any feed-line impedance can be matched simply by positioning the point of feed-line attachment along the slot. The feed point impedance also depends on the width of the slot and the proximity of the antenna to other nearby conductors. In a flower box application, the rear metal panel of the flower box would serve as a plain reflector that would significantly increase the forward gain, but that would also change the feed impedance. However, an impedance match could be obtained experimentally by simply moving the point of feed line attachment along the slot to find the point that provides the lowest SWR.

Slot antennas haven't been used much by amateurs, but a horizontal slot antenna in a large metal sheet (large compared to a half-wave) has radiation pattern and efficiency characteristics identical to a vertical half-wave wire dipole and a vertical slot antenna has radiation pattern and efficiency characteristics identical to a horizontal half-wave wire dipole, so slots provide interesting stealth antenna possibilities (for example, a slot in the slide of a metal garage wall, metal storage shed wall, solid metal fence wall, or other such things).

Thanks for the info on this Skywave. Can you pls elaborate on the calculation for the width of the slot and how/where on the slot to attach the feed line, i.e. on the inside edge of the cut?

Ham Radio Forum

Stealth Antenna Suggestions

Attic Antennas

HF Antenna Coupling to Power Wiring

Stealth Slot Antennas

Stealth Antennas in Fake Air Conditioning Equipment

Thanks for all Your Stealth Antenna Ideas

Buried Antennas

Stealth Active Antennas

Vertical Tree Antennas

Re: Stealth Slot Antennas

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