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Need FM antenna with very limited/directional signal

 
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Need FM antenna with very limited/directional signal
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SomeOtherGuy
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Joined: 05 Sep 2006
Posts: 2

Post Need FM antenna with very limited/directional signal Reply with quote
I'm relatively new to this, and would greatly appreciate if someone could point me in the right direction:

We have an application where there will be as many as a dozen cars parked in adjacent parking spaces and we need to broadcast a different FM signal to each vehicle. There are enough unused frequencies at the site to use a different frequency for each car, if necessary. We'll probably be using one PCI MAX 2006+ card (350mW) for each car stall.

The question that I have is this: What type of antenna could we use to broadcast as narrow a signal as possible with as little bleed outside the target car stall as possible? (In other words, it would be nice if someone parked two or three spaces away from the target stall wouldn't be able to pick up the signal.) Each vehicle will be positioned under a canopy, so the antenna could be positioned in front of, to the side of, or directly above the vehicle. Does anyone know of any off-the-shelf antenna that would be suitable for this application? Any other advice?

Thanks in advance!!!
Tue Sep 05, 2006 8:48 pm View user's profile Send private message
Norwalk Electronics
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Joined: 01 May 2006
Posts: 7
Location: Connecticut USA

Post Limited directional antenna Reply with quote
I think one of the simplest solutions would be to hang a quad loop in the front of each car stall. The vertically polarized full wave loop exhibits bi-directional characteristics that radiate on the front and back side of the loop but has deep nulls on either side. It's basically a 10 foot loop of wire shaped like a square. You break the loop in the middle of either the left or right side and feed each side of the loop wire with coax (a balun would be useful here if we were looking for higher power range).

Because of the limited range you seek, you will need to keep the power level very low, probably around 10 mw. or less. Unfortunately I think your goal of reaching one stall but not being able to hear the broadcast from an adjacent stall is not practical on the FM broadcast band. The FM band is not high enough in frequency where you can accurately beam a signal over this narrow of a target. Furthermore, I'd be careful not to reduce the power level to the point where you can only hear it in one stall. This is because the signal may be so low at this point that variations in some FM receivers may cause them not be able to hear the desired signal.

If it is important to to reach only one stall with each transmitter, you may still have another option. Since it is not possible to beam this narrow of a signal below microwave frequencies, you may want to consider RF shielding between the walls of each car stall to block the unwanted signal. Depending on the application you may be able to line the walls between stalls with aluminum foil. Some 4x8 sheets of Styrofoam insulation have foil on one side. The combination of low power, directional antenna, and shielding between stalls may give you the results you seek.
Wed Sep 06, 2006 2:27 am View user's profile Send private message Visit poster's website
SomeOtherGuy
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Joined: 05 Sep 2006
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Post Reply with quote
Thanks -- That's just what I needed! With your suggestion and little research I think my first take will be to mount an elongated quad loop to the 12' ceiling of the canopy directly over each vehicle. It's okay if there's a little bleed between stalls, but I really want to avoid (as much as possible) any interference with the FM receivers in other vehicles as well as the surrounding properties. Between a quad loop pointing straight down and a very low wattage signal, I hope to be able to achieve this!
Wed Sep 06, 2006 2:24 pm View user's profile Send private message
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