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[nunoradioportugal]

Hi!
I'm Nuno from Portugal.
I've got a tower with two fm antenna arrays with 2 and 4 bays each. If you please, someone who works in broadcast area, can help me with the spacing calculation between two arrays (not between bays in each array) in wavelenghts?

Thank you in advance

[R. Fry]

Hello, Nuno. Are these two arrays on different frequencies? If so, what are they?

Is the purpose of your question to find a vertical spacing between the closest bays of the two arrays to keep r-f intermodulation* between the two systems below some limit? If so, what is that limit?

* without sufficient decoupling, each co-located FM system will tend to produce an in-band spurious signal at twice its carrier frequency minus the other carrier frequency.

Paper 7 at http://rfry.org has more information about co-sited FM systems.

[nunoradioportugal]

Hi Richard

For the IM I will install two cavity filters in each transmitter. The frequencies are 92,6 MHz (2 circular bays) and 104,6 MHz (4 circular bays) in a 230 feet tower.
I usually use a 1,5 wavelength of two freq's average for separating. Is this a good compromise?
I'm just concerned about lobes and coverage.

Thank you

[R. Fry]

A vertical spacing of ~1.5 wavelengths between the closest bays of these two arrays is a good starting point (at least). Re-location may be needed depending on the IM levels permitted by the cavity filters installed to control that IM -- as measured in the transmitter room(s) when both systems are in operation. An alternate to relocating the arrays might be to install cavity filters with more attenuation at the frequency of the other system.

The design/adjustment of each transmitter's final r-f amplifier also has an effect on the production of IM in these installations.

The arrays will have the least mutual coupling when they are supplied by a single manufacturer, and their bays are installed with their vertical axes in vertical alignment.

Half-wave vertical spacing of the bays in either array (or both) also will reduce the mutual coupling of those arrays, however more bays will be needed in order to retain about the same gain as with 1-wave bay spacing. Adding bays will increase the weight/windload ratings needed by the supporting structure. Or more transmitter power will be needed to produce a given radiated power from that two- or four-bay 1/2-wave spaced array.

The radiation pattern of each array should not significantly be affected by the other array except for depression angles exceeding 10 degrees. If these arrays will be installed at very high elevations above nearby areas having high population density then this subject might need evaluation by a consulting engineer having specific knowledge of the terrain, and the coverage requirements from this antenna site.