On a 4 bay short spaced FM antenna...

[TXengineer]

What purpose would having the #2 and #3 bay out of phase of #1 and #4?

I know that typically on a half-wave spaced, you put each bay out of phase of the other... but why this configuration?

 

[Andy_Brown]

> What purpose would having the #2 and #3 bay out of phase of
> #1 and #4?
>
> I know that typically on a half-wave spaced, you put each
> bay out of phase of the other... but why this
> configuration?
>


Beam tilt, null fill and polarization tweaking (varying from circular to elliptical) are all effected by phasing the various elements. Also directional FM may be achieved this way as well, but I'm not sure on that.

<P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>

 

[ironbear]

> > What purpose would having the #2 and #3 bay out of phase
> of
> > #1 and #4?
> >
> > I know that typically on a half-wave spaced, you put each
> > bay out of phase of the other... but why this
> > configuration?
> >
>
>
> Beam tilt, null fill and polarization tweaking (varying from
> circular to elliptical) are all effected by phasing the
> various elements. Also directional FM may be achieved this
> way as well, but I'm not sure on that.

Not useful for directionalizing of the main antenna pattern, but up and down, yea.

Many broadcasters are concerned about complying with ANSI, which is a standard adopted by the FCC which limits RF exposure to people working close-in at transmitter sites. 1/2 wave antenna spacing is often used to keep RF off the ground, where people are. ANSI exposure limits are set pretty low, just to be on the safe side. Is RF a hazard? That's controversial...but the major concern ANSI is set up to protect involves prevention of direct heating of persons in close contact with the antennas, as in a microwave. (eek!) If you're very far away at all, heating is not in any way an issue.

 

[TXengineer]

> Many broadcasters are concerned about complying with ANSI,
> which is a standard adopted by the FCC which limits RF
> exposure to people working close-in at transmitter sites.
> 1/2 wave antenna spacing is often used to keep RF off the
> ground, where people are. ANSI exposure limits are set
> pretty low, just to be on the safe side. Is RF a hazard?
> That's controversial...but the major concern ANSI is set up
> to protect involves prevention of direct heating of persons
> in close contact with the antennas, as in a microwave.
> (eek!) If you're very far away at all, heating is not in any
> way an issue.
>
It's most likely being used for a beam tilt. Which is what I was sort of thinking originally.

 

[Goat Rodeo Cowboy]

> >
> It's most likely being used for a beam tilt. Which is what I
> was sort of thinking originally.
>
I did administrative work for a company building antennas for the cell phone and PCS industry. Based on the conversations I listened in on, to get beam TILT you would probably have to introduce some phase shift and some variation in power distribution between the elements to achieve beam tilt.

AS the other postings indicated, putting the elements at half-wave and reversing phase on every other element could achieve a much more focused lobe, minimizing not only the signal on the ground beneath the tower, but also minimizing the energy toward the coulds and airplanes. Focus on the horizon.