Directional Antenna Questions

[FMROB]

Hello all,

I have a situation where I need to install a primary receive site (could be used as back up transmitter) in the 5th floor of a seminary. Problem is they dont want the equipment outside and there are no flat roofs. This building is ont he outskirts of thier district, so it doesn't have to be complete omni directional and I have a large open single pane window in the coopola overlooking the coverage area. There is a cell site with panel antenna mounted behind almost all of the windows except the one we need.

My question, what type of antenna would you use. They are a 471 channel, and like I said it will be primary RX site, but could need to function as a back up TX site.
Looking for best performance for the application?

Im thinking corner reflector or yagi. Could I use like a 3db omni fiberglass behind the window?

Thanks, Rob

[Bill_G]

What is th noise floor like in T band before you go investing in something that might not work?

[Will]

Corner reflector or flat panel sector antenna. You only want to look outside the window, not behind it....

[d119]

Corner reflector or flat panel sector antenna. You only want to look outside the window, not behind it....

Agreed.

[tvsjr]

If you do use it as a TX site, you need to consider RF exposure as well, especially if the fifth floor is occupied. A corner reflector will also help with this...

[FMROB]

Thanks all for the replies,

I haven't had a chance to measure the noise floor as of yet, this is just is pre planning stage, however I think it is only cell and VHF low band at the site right now.

I was thinking corner reflector as well. This is in an unoccupied area in the coopola. The issue is the church doesn't want any additional antennas mounted to the building and I want to try to balance the use as a RX site and as there back up TX site. I would assume the corner reflcector will concentrate the RF to a certain degree. Some of the models I have looked at cover 30-60 degrees.

My next question would be, If I were able to get an omni antenna mounted on the outside of the brick building, and stand it off about 2-3' on the opposite side of the needed coverage area, how bad of a null do you think would occur, as opposed to doing a corner reflector.

There is another agency that has a UHF RX site at the location, and they used waht appears to be a telewave 4 bay folded dipole with the loops set as a fin. They stood it off the side of the building with large house brackets, but I question its effectiveness.

Thanks, Rob

PS I have pictures of the site and current set up, if any one wants them via e-mail. I don't know how to post them on here.

[Bill_G]

I was going to suggest the folded dipole set in cardioid pattern if the noise floor proves acceptable. The dipole will have a wider aperture and greater surface area than a corner reflector. Since someone is already using one, do an effective sensitivity test on that rcvr to see how well it performs. Cell sites throw off a lot of energy especially when your antenna is in the same horizontal plane as theirs. You'll need a window filter or a set of cavities to keep them out of your front end. A wooden building will be relatively transparent to UHF.

[FMROB]

Bill,

Unfortunately this building is built like a brick s_it house. It is brick, block, plaster and lathe and is very old. If you can PM ur e-mail I will send you a pictures. I normally use three filter cavities with a preamp from Angle Linear if the nosie floor is acceptable.

Out of curosity, what procedure do you use for the easiest way to measure noise floor at the site.

Thanks, Rob

[Bill_G]

I use an iso-T to do an effective sensitivity test. Check the direct sensitivity through the iso-T, then check sense injecting through the isolated port first with the rcvr ant port terminated to 50 ohm load, and second terminated to the antenna line. The difference is the site noise.

For example - take off the ant line at the rcvr port on the radio, connect the iso-T, inject carrier through the non-isolated port, squelch opens at -115db. Totally normal. Connect a 50 ohm load to the non-iso port, and inject carrier into the 45db isolated port on the tee. Squelch opens at -70db. Numbers agree. No weird cable or loading issues going on. Take off the 50 ohm load, and connect the antenna line. Squelch opens at -65db meaning you have 5db of site noise putting your effective sensitivity at -110db.

5db site noise ain't bad especially at a busy site. Greater than that, you'll want to look at your filtering, your preamp, your antenna, it's proximity to transmit antennas, adjacent channel systems, distance to broadcasters, tower lighting, loose crap on the tower, etc, etc, etc. Some cost benefit analysis has to be considered. You may have to walk away from 8db of constant site noise because there is nothing you can do about it, or you can become hyper-vigilant and call in the FCC if you suspect the LPTV on the same hilltop is running barefoot, or the paging company needs to fix their transmitters.