Coverage Issues from Phase-locked Part 15 AM Systems

[rfry]

Recently I have written here about the coverage pattern issues when using two Part 15 AM systems on the same frequency -- even when their carriers are phase-locked (synced).

The link below shows this in graphic form for two phase-locked systems at a physical separation of one mile.

http://rfry.org/Dual Part 15 AM Antenna Az Pat - 1 mile Sep.gif

This pattern is valid for receiving locations about the same distance from each transmit antenna. The pattern would be different for receivers much closer to one transmit antenna than the other. But it shows how coverage can vary in this situation.

Even though the transmitters are phase-locked, the systems don't reinforce each other in every direction -- in some directions they cancel each other.

 

[KeithHamilton]

This is interesting but virtually is never is encountered in the field. There really is no reason that I see to sync 2 Part 15's RF carriers that are separated 1 mile apart.

Here is what people ARE doing:

Transmitters that are 1 mile apart and more are audio synced, (exact audio syncing without delays is only important when the coverage areas are next to each other, audio delays translate into echos. If the transmitters are many miles apart then there is no problem. There is no need for the RF carrier to be synced at this distance (1 mile or more), doing the syncing is a costly process at this distance. What people do is just cystal control the transmitters and zero beat them. Also I tell people to be sure the transmitters have similar enviromental circumstances to limit temp drift. You will get some phase flutter if you are in a car driving between two coverage areas with this method but it typically isn't a problem (not severe, sounds like going under a power line).

When people DO sync the RF carriers it is when the transmitters are hundreds of feet apart, like at either end of a shopping mall. Then there is possibility of a cable connection. The RF sync requires RS485 which is wide bandwidth.

 

[rfry]

> This is interesting but virtually is never is encountered in
> the field. There really is no reason that I see to sync 2
> Part 15's RF carriers that are separated 1 mile apart. ...
> When people DO sync the RF carriers it is when the
> transmitters are hundreds of feet apart, like at either end
> of a shopping mall. (Keith Hamilton)
____________

I recalculated the net pattern for 500 feet of separation, and posted the plot at the link below.

http://www.rfry.org/Dual Part 15 AM Antenna Az Pat - 500 ft Sep.gif
(For some reason this URL isn't linking. But you can see the plot by copying and pasting the line starting http into your web browser.)

At all radiator separations greater than 1/2-wavelength there will be azimuth directions where the net pattern is reinforced, but also where it goes to zero -- even when the two transmitters are phase locked.

 

[MonsterFM-KBKH]

> No matter what the separation, there will be directions
> where the patterns add, and directions where they cancel --
> even when the two transmitters are phase locked.
>



In essence, a directional antenna with a power ratio or 1:1 and a phase relationship of 0 degrees...


Forgive me for being unfamiliar with the linking of images into the board but, were one to operate a non-da at 1 watt from our FM site, (3 miles outside of town), the 1mv./mtr. contour (into a .25 wavelength radiator) would look like the contour in


http://www.monsterfm.com/927/files/1wattnda.gif


It would almost completely cover the city.

Now, add a second "tower" to the equation with a power ratio of 1:1 phased at 180 degrees and spaced at a distance of 90 degrees from the original "tower" in an easterly (or westwardly) direction and look at the pattern at


http://www.monsterfm.com/927/files/1wattda.gif


As you can see, it covers the city by about the same amount and a few miles east and west on the interstate!(This would also effectively DOUBLE the effective power as these - utilizing two transmitters - would add whereas a "normal" AM directioal array actually devides transmitter power up among towers.)

Sorry, I modeled it as 1 watt because, for whatever reason, when I try to punch in .0001 watts, my Comstudy goes gaga with "invalid floating zero" errors...

I can see, however, that with the transmitter outputs ADDING, where there could be some coverage to be gained from this but, as Richard indicates, there will be SOME cancellation in SOME direction always. Not necessarily a bad thing *if* you can control where the cancellations land in relation to your listeners. BUT, by this stage of the game, you're beginning to get into some serious money for still limited coverage. But, I will grant you, as an experimenter, it looks like fun to try!<P ID="signature">______________
Terry Keith Hammond

Message Boards: http://www.monsterfm.com/cgi-bin/ultimatebb.cgi</P>

 

[KeithHamilton]

I think it is true that the field strength will vary somewhat as phases add and cancel but I don't feel you can take a model and easily apply it to a given situation, for example a mall. The program doesn't take into account the terrain which will affect the signal. There won't be a neat dividing line where you will get the signal & then you won't, it will be more complex then that. I have installed the systems and have seen the results. Also you assume 0 phase, we don't always adjust to 0 deg.

 

[rfry]

> I think it is true that the field strength will vary
> somewhat as phases add and cancel but I don't feel you can
> take a model and easily apply it to a given situation, for
> example a mall.

The patterns in my example plots will be very close to the plotted value for the conditions stated. This same technique is used to design directional arrays used by AM broadcast stations, and such calculated patterns have been confirmed by careful field strength measurements going back over 60 years.

> The program doesn't take into account the
> terrain which will affect the signal.

The patterns I plotted show the shapes of the radiation envelope from the antennas, themselves, and will remain true no matter where they are sited. Once the signal is launched from the antenna system, its value in a given direction can be modified by many factors, including terrain. But the terrain variations along the path lengths in an area as small as a mall parking lot or even a small city aren't going to change the shape of the calculated pattern significantly for Part 15 applications.

> There won't be a neat
> dividing line where you will get the signal & then you
> won't, it will be more complex then that.

This is a misinterpretation of the plot. The lines don't show areas where you will or won't receive the signal -- just the boundaries where the net antenna gain crosses the given value. For example, on the plot for 500 foot separation, the signal strength at about 73 degrees will be about 3 dB below its peak value. But that doesn't mean the station won't be heard at 70 degrees (for example), it will just be weaker there.
In the plot for 500 foot separation, the only place there will be zero signal (or essentially zero) will be in/near the directions of the four nulls seen there.

> Also you assume 0
> phase, we don't always adjust to 0 deg.

Adjusting the phases of the two radiators to ANY fixed relationship will determine the directions of the lobes and nulls, but they will still be there.
//

 

[rfry]

> Now, add a second "tower" to the equation with a power
> ratio of 1:1 phased at 180 degrees and spaced at a distance
> of 90 degrees from the original "tower" in an easterly (or
> westwardly) direction and look at the pattern at
> As you can see, it covers the city by about the same
> amount and a few miles east and west on the interstate!(This
> would also effectively DOUBLE the effective power as these -
> utilizing two transmitters - would add whereas a "normal" AM
> directioal array actually devides transmitter power up among
> towers.)

But aren't you driving this directional antenna system with a total of 2 watts?

> Sorry, I modeled it as 1 watt because, for whatever
> reason, when I try to punch in .0001 watts, my Comstudy goes
> gaga with "invalid floating zero" errors...

Maybe try 0.1 watts? The value ".0001 watts" is only 1/1000th of Parts 15's 100 milliwatts.
//

 

[MonsterFM-KBKH]

> But aren't you driving this directional antenna system with
> a total of 2 watts?


That was what I meant when I stated that the two add.


> Maybe try 0.1 watts? The value ".0001 watts" is only
> 1/1000th of Parts 15's 100 milliwatts.
> //
>


Sorry. My typo. That should've read .0001 KILOwatts...

I apologize.<P ID="signature">______________
Terry Keith Hammond

Message Boards: http://www.monsterfm.com/cgi-bin/ultimatebb.cgi</P>