Fm antenna separation question

[fl-lpfm]

Good day Radio Pros!!!

I have question. What will be the correct separation for 2 ( OMB MP1) antennas at 95.7 FM.

Thanks

 

[RadeoEngineer]

10.28 feet for full wave spacing.

 

[fl-lpfm]

I just got this from the manufacturer

D = 225/MHz

D = 225/95.7 = 2.36 meters

Inches

D = 8859/MHz = 8859/95.7 = 92.57”

Feet:

D = 739/MHz = 739/95.7 = 7.73’


Is a little off from yours. What do you think?

 

[oldiesstation]

did they tell you this is basically a spiral dipole disguised as a cp radiator?(same as Nicom model) i would have gone with something like a shively 6812-2.are you sidemounting off a tower leg or pole mounting? For towers with less than 24 inch face i would mount 1 foot away off tower leg, 2 ft if tower face is more than 24 inches.as far as spacing 5 ft minimum to 10 ft max would be ok.do not use metal ties on the cable(use plastic).Have fun and good luck.If it came with instructions,PLEASE READ CAREFULLY.

 

[rfry]

I just got this from the manufacturer ... Is a little off from yours. What do you think?

If these two bays are part of the same antenna array fed by a coax cable harness of some kind, then the physical spacing of the two elements may depend on the velocity of propagation of the cable harness (both bays most likely need to be driven in the same relative r-f phase).

The spacing of the bays will have an affect on the elevation pattern and gain of the array, and so the input power it needs to produce a given ERP.

Normally this information would be supplied with a new antenna.
//

 

[fl-lpfm]

This antennas will be mounted to a mono-pole and then a vertical pole where the antennas are going to be mounted. They both connect with a power divider (splitter) and then to the transmission line.

 

[RadeoEngineer]

I just got this from the manufacturer

D = 225/MHz

D = 225/95.7 = 2.36 meters

Inches

D = 8859/MHz = 8859/95.7 = 92.57”

Feet:

D = 739/MHz = 739/95.7 = 7.73’


Is a little off from yours. What do you think?

You might want to ask your manufacturer for some clarification on what their numbers are, but the standard formula for wavelength at FM is 300/95.7=3.135 meters X 3.28 = 10.28 feet center to center of the elements for full wave spacing. If they're showing you a shorter spacing it may be because you need less gain or a different elevation pattern and/or both. It all depends on the information you provided to them on what you're trying to do with TPO and ERP, along with line loss and other factors.

 

[Chuck]

Good day Radio Pros!!!

I have question. What will be the correct separation for 2 ( OMB MP1) antennas at 95.7 FM.

Thanks

As the spacing changes, so will the gain of the antenna system. Most manufacturers of this type antenna seem to recommend 7/8 wave spacing, which does not give unity gain. If you run it full wave, you will get higher gain (unity, or close to it) , but you may find that most of that energy is going "up" rather than horizontally. As you decrease the antenna spacing, the pattern "squashes" towards the horizon, resulting in less energy going straight up toward space and more toward the horizon. That is what you want in most cases. The down side is the efficiency decreases, so you will need more transmitter power to make your assigned ERP. There is no free lunch.

I'd go with the manufacturer’s recommendations. Thy probably have a fairly decent idea about how their product performs. Further, they will be able to tell you how efficient the antenna is at a given spacing, which is necessary information to calculate your transmitter power output. Don’t forget to include feed line loss plus any other passive loss (like lightning arrestors, connectors, etc) in those power calculations. They can eat up quite a bit of power. At LPFM levels, every watt is critical.

 

[fl-lpfm]

Good day Radio Pros!!!

I have question. What will be the correct separation for 2 ( OMB MP1) antennas at 95.7 FM.

Thanks

As the spacing changes, so will the gain of the antenna system. Most manufacturers of this type antenna seem to recommend 7/8 wave spacing, which does not give unity gain. If you run it full wave, you will get higher gain (unity, or close to it) , but you may find that most of that energy is going "up" rather than horizontally. As you decrease the antenna spacing, the pattern "squashes" towards the horizon, resulting in less energy going straight up toward space and more toward the horizon. That is what you want in most cases. The down side is the efficiency decreases, so you will need more transmitter power to make your assigned ERP. There is no free lunch.

I'd go with the manufacturer’s recommendations. Thy probably have a fairly decent idea about how their product performs. Further, they will be able to tell you how efficient the antenna is at a given spacing, which is necessary information to calculate your transmitter power output. Don’t forget to include feed line loss plus any other passive loss (like lightning arrestors, connectors, etc) in those power calculations. They can eat up quite a bit of power. At LPFM levels, every watt is critical.

Hello Chuck,

I talked to the manufacturer and they told me everything you just said. This setup is for an lpfm. Thank you, and thank you all for all your comments and suggestions!