FM Station

[restrada27]

Hello,

Just wanted to see if anyone can help with this question we are installing two jampro antennas for a radio station and wanted to see if LDF4 coax was sufficient for a 170ft run. It will be one run with a power splitter at the top to feed both antennas the station is FM with 90 W of power.

Thank you

 

[Ylli]

Can't comment on the antennas. Jampro appears to be a major manufacturer and I would assume make quality products. Just be sure to install them as per manufacturers recommendations. Your feedline is 50 ohms, so I have to assume the splitter and the two antennas are designed for 50 ohms. There will be some loss in the splitter, perhaps as much as 0.5 dB. The feedline loss at 90 MHz will be about 1.1 dB for a total loss of 1.6 dB.

90 watts into the feedline will give you about 62 watts at the antennas - 31 watts on each. Should put out a reasonable signal.

 

[ronsimpson]

Why two antennas? How will you mount two antennas. Are you trying to make a directional antenna?
A splitter will have losses.
I think two antennas is a bad idea unless you really know how antennas work.

 

[Nigel Goodwin]

Why two antennas? How will you mount two antennas. Are you trying to make a directional antenna?
A splitter will have losses.
I think two antennas is a bad idea unless you really know how antennas work.

He specified a 'power splitter' - which 'may' be a phasing harness? - it's not going to be a crude lossy resistive splitter.

It's very common practice to use multiple aerials fed from a single transmitter, a great many TV transmitters are built like that - and connect via a 'device' that splits the power to each aerial.

Where I work they had a fault on the local TV relay a number of years ago (I rang the broadcasters to report it), where the signal was weak in one direction - it was OK where I worked

After it was fixed they reported back (I had good contacts with the transmitter service people), and apparently there were four aerials (all in a vertical white tube), and one had failed. I've actually climbed that mast twice - and I wouldn't like to be replacing an aerial up there!. I was, and still am, somewhat bemused why the telemetry didnt pick it up? - I would imagine one of the aerials failing would mess up the SWR, and surely the transmitter would notice that and report it?.

 

[ronsimpson]

It's very common practice to use multiple aerials fed from a single transmitter, a great many TV transmitters are built like that

I know, I have built many radio stations. AM radio is very often directional. FM is much less likely. Of all the FM stations I built or worked on, none were directional. (by that I am talking North/South/E/W) Vertically they were all directional. The big stations had 11 or 12 antennas stacked vertically to get a very tight pattern. We often had to put a dead spot downward because people living near a tall tower don't want a strong signal.

Based on the questions in post #1 I assumed he/she did not know what to do with two antennas. I did not mean to be insulting and I am sorry for that.
Picture of 4 circular polarized FM antennas spaced vertically at 1/2 wave length.
My antennas did not have a "phase splitter" in the way most people think. The coax has a "T" at all but the top elements. The top element had a right angle corner in the coax.

My coax looks like this:

Considering " Jampro antenna" here is a picture of a low power 2x antenna. Here they use "T" in the coax. I might consider center feeding the antenna, which I have never done for FM.

 

[Nigel Goodwin]

My antennas did not have a "phase splitter" in the way most people think. The coax has a "T" at all but the top elements. The top element had a right angle corner in the coax.

Which is presumably a phase splitting arrangement?, like a phasing harness - essentially just accurate lengths of cable - presumably half wavelength, which was why your aerials were spaced that far apart.

For TV receiving aerials, we used to just buy the manufacturers ready made phasing harness, in order to stack 2 or 4 aerials together.

 

[ronsimpson]

The 1/2 wavelength is so the signal going up/down will substract to nothing and the signal going parallel to the earth will add.

 

[restrada27]

Per the installation manual they want two bays installed one top of the other with 130-3/4" separation between them. Polarization is circular and they suggest a 7/8 coaxial cable with power divider near the center of the two antennas. Currently they have a 1/2 andrews coax line will this affect the performance ?and would it help if it was changed to 7/8 andrews coax or is this the wrong coax to use for this application

Thank you

 

[ronsimpson]

Attached is a data sheet for 1/2 in coax and 7/8 inch..

 

[Ylli]

Coax Calculator

www.qsl.net

With 170 feet of cable, and 90 watts in, you will deliver to the splitter:
70.4 watts with the 1/2" cable and
78.7 watts with the 7/8" cable.

Or you could go to 1 1/4" cable and get 81.9 watts at the splitter.

Really not much difference on that short a run, but spend what you have.....

 

[ronsimpson]

What country? Do you have permits to do this?

 

[restrada27]

Thank you all for the helpful info
Appreciated