Engineering query: Part 15 AM antennas on metal apartment buildings?

[Black_Shire]

Hello All,

It looks like I will soon be moving to an all-metal, eight-story apartment building. It has crank-out casement windows. I have an idea for a Part 15 AM antenna installation there, and I welcome any comments or suggestions regarding it.

My thought is to use a Talking House ( www.talkinghouse.com and www.actradio.com ) transmitter with the vendor-supplied outdoor Antenna Tuning Unit (ATU) and an 8.5' whip antenna installed just outside a window. I would make a narrow "shim" window adapter just wide enough (5/8" or so) to keep the window from pinching the RG-6 coax to the ATU when closed, while still sealing the cold air and rain outside.

I would not ground the ATU to the building wall as per the Part 15 AM rules. The ATU would be mounted to a flat wood or metal bracket affixed inside the window, and the ATU/whip antenna would be tilted either up or down 45 degrees to emit some vertically-polarized radiation. The frequency will be either 1610 or 1700 kHz.

The building's bare, unpainted external metal wall should provide a low-loss environment for the antenna's near field. (The metal building wall should serve in this capacity even better if the ATU/whip antenna were mounted at 90 degrees to the wall, but the antenna would emit all horizontally-polarized radiation as it would be parallel to the horizon and the ground.)

Depending on the floor I get an apartment on, the antenna could be anywhere from 10' to 90' above ground level. The pattern will not be omni-directional due to the "shadowing" effect of the building (unless I could shunt-feed the entire building... <GRIN>), but that cannot be helped.

Many thanks in advance to anyone who can help.


-- Black Shire

 

[Neil E.]

Black Shire,

This is a situation where you may just have to try it and see what happens.

Have you thought about the possible consequences if the antenna gets away from you and falls several stories? Maybe an indoor antenna is worth considering.

Neil

 

[Black_Shire]

Black Shire,

This is a situation where you may just have to try it and see what happens.

Have you thought about the possible consequences if the antenna gets away from you and falls several stories? Maybe an indoor antenna is worth considering.

Neil

I've already had a few panic attacks just thinking about that--anything I manipulate outside the window (including tools) will be secured by kevlar cord. I have also been thinking about a planar-wound magnetic loop antenna like this one ( http://web.telia.com/~u85920178/antennas/frameant.htm ) inside the window, but its Part 15 legality is doubtful at best. If the 1.2 meter wide loop and its tuning capacitor also served as the transmitter's LC output tank circuit (a kaliotron oscillator, I believe it's called), it just might pass legal muster.

As it turns out, the TalkingSign and Talking House transmitters are designed in just this way. Each one has an internal LC output tank circuit that is directly coupled to the 3 meter indoor wire antenna. When the 3 meter antenna/external antenna switch is set to "external," the internal LC output tank circuit is disconnected. The outdoor ATU that is used with an 8.5' CB whip antenna with both transmitters is actually a remote LC output tank circuit that is connected to the transmitter's circuit board via a length of coaxial cable.


-- Black Shire

 

[Neil E.]

Black Shire,

Just some brainstorming going on here. Maybe you could use the 1/4" window foil that alarm companies use and form a nice multiturn loop antenna on one of your casement windows and tune it with a variable cap at the feed point. At the turn of a crank, you could steer it, and your guests will think it is art.

During my dorm days, I dropped a 10 foot length of about #28 wire out the window for my part 15 AM tx. At least if it fell, it wouldn't hurt anyone. This was not a resonant antenna and I can forsee with a coil loaded wire that it would detune everytime the wind blows.

In the dorm, and I presume in your proposed apartment, the listening audience was very concentrated so I didn't need a mile range. A wire antenna taped to the walls worked OK. I had about a 50 foot range in all directions which included a lot of people. Our rooms were about 10 X 12 so the density was high. I hope your apt. is bigger.

Neil

 

[Black_Shire]

Black Shire,

Just some brainstorming going on here. Maybe you could use the 1/4" window foil that alarm companies use and form a nice multiturn loop antenna on one of your casement windows and tune it with a variable cap at the feed point. At the turn of a crank, you could steer it, and your guests will think it is art.

During my dorm days, I dropped a 10 foot length of about #28 wire out the window for my part 15 AM tx. At least if it fell, it wouldn't hurt anyone. This was not a resonant antenna and I can forsee with a coil loaded wire that it would detune everytime the wind blows.

In the dorm, and I presume in your proposed apartment, the listening audience was very concentrated so I didn't need a mile range. A wire antenna taped to the walls worked OK. I had about a 50 foot range in all directions which included a lot of people. Our rooms were about 10 X 12 so the density was high. I hope your apt. is bigger.

Neil

I thank you very much for those suggestions. At the low Part 15 AM power level, directly feeding the loop (connecting it to the transmitter's external antenna and ground terminals, with the tuning capacitor connected across them parallel to the loop) would probably be better than using a coupling loop inductively coupled to the main loop.

Another thought I had was to capacitively feed a 3 meter monopole antenna *through* the window in the same manner as the through-glass "stick-on" 2 meter Ham radio automobile antennas. A sheet of self-adhesive copper foil could be affixed to each side of the window glass, and a thin self-supporting 3 meter wire (or a thin fiberglass rod carrying a wire) would be physically and electrically affixed to the copper foil sheet on the outside of the window. I would coat the outdoor copper sheet (as well as the indoor one) with epoxy to prevent oxidation, and I'd also use non-conductive top guy supports to keep the monopole from whipping in the wind and to prevent it from falling--these guys would be fastened to the window frame. Feeding the antenna through the window capacitor would incur some loss, but probably not as much as at the VHF 2 meter frequencies. What size would be optimum for such a capacitor I couldn't even guess, though (I know that it would electrically shorten the already short antenna, requiring a bigger loading coil).

I'm going for broadcasting to the neighborhood outside the building. Since the building is all metal (the frame as well as its external panels), I would probably have to use a Carrier Current AM system to get any significant range indoors.


-- Black Shire