Radio Station Project Inquiry 2

[SamerSaed]

Hi Engineers,

I'm Starting a new FM Station Project in BENGHAZI - LIBYA which is 60km2 in area, i was advised to use a transmitting system of a 600w Transmitter attached to 4 double dipole antenna (

http://www.labelitaly.com/website/akk2.html)

as my tower specs are : tower angle 4 leg (leg size = 10 x 10), parameter at 96 m height = 2.5m x 2.5m. attached a picture of my tower.

i want to get an omni-directional coverage as my tower is located in the center of the city and I've been told that unless a panel antenna the tower will make a shadow or something like that to the broadcast signal.

i want to know if i'm really forced to use a panel antenna in my system, why i can't use a high power regular dipole antenna.

and please advice if the transmitter power i'm using will be enough to cover the 60km2 area.

hope to find help here.

Thanks in advance.

 

[BrianG]

It's not clear from the google earth link whether the arrow is pointing to where your new tower will be located or whether its pointing to those existing tower structures.

However, I can't see how the manufacturer can claim it produces an 'almost omnidirectional' field pattern on his web page: the polar diagram in the attached link shows that each antenna has a beam pattern that is, at most, 60 degrees wide at the -3dB points. Even with four of them mounted on the sides of the tower there is no overlap in the beams, so you will have dead spots in the broadcast pattern at four different compass bearings.

 

[SamerSaed]

Thanks brian for help.

The picture from google earth is pointed to the tower site in general, i'll be using one of the two towers.

Then you recommend not using a panel antenna as there will be gabs in my boroadcasting area's sector.

can you recommend me an antenna system that gives me an omni-directional coverage for my area with respect to my towers specs??

 

[BrianG]

I only advised that I did not think that four of the panel antennas of the type you showed would not give you the coverage you want. If there a reason why you can't put an omnidirectional dipole on the top of the tower then there may be no alternative to panel antennas.

I think the type of panel you were considering in your first post would need at least eight panels (four sides and also each corner of the tower) to get something like the 360 degree field from an omnidirectional antenna.

Looking at the picture again, however, the two towers appear to be designed primarily for microwave links, which has different requirements than broadcast VHF. With the towers appearing to be relatively close to each other, if you mount all antennas on one tower the panel directly facing the other tower may suffer from serious reflections, causing a shadow in that direction. A dipole on the top of the tower really will be the simplest solution!

 

[Higgler]

here's a monopole FM antenna, 50 watts only. Broadcasts omni.

http://www.antennas.us/store/p/296-VL-9801-383-88-108-MHz-Omni-FM-Band-Antenna.html

a monocone antenna for FM would be best. More energy downward and longer range.

If your area is 60 Km^2, you only need 10 watts, that's 6 miles x 6 miles. or 60 km x 60 km?

Here is a 500 watt version. I assume you only transmit at one frequency in the FM band.

http://www.zcg.com.au/fm_radio_collinear_antennas.htm

 

[VEBill]

I believe that Broadcast FM signal quality contours are much higher than the sorts of field strengths required for VHF communications. Ten watts in FM Broadcasting would be considered to be ultra low power neighborhood coverage.

Mr. Higgler's 2nd link mentioned the magic word "collinear". By stacking antennas in the correct manner one can provide omnidirectional-in-azimuth antenna gain. Basically concentrate the RF power towards the distant horizon, not up into space.

There's also the question of FM Broadcast polarization. Here's a webpage that explains it all:

http://fmbroadcastantenna.com/polarizationinfo.html