AM Transmitters Intermodulation.

[Robert]

Guys I need some help to prevent Intermodulation between my two Part 15 Am Transmitters (Rangemasters). I am Broadcasting @ 1620 and 1680 Khz.

Both Transmitters are mounted 20'apart on two Wooden Towers about 40'high.
I am getting good range on both transmissions, except that you can hear a leak of the 1620 Music mixed with the 1680 Audio or vice versa.

I know the Big Stations transmits various frequencies either from the same Tower or from nearby Towers. (like a typical AM Antenna farm) How do they prevent this. Thanks.<P ID="signature">______________
Robert Dillon</P>

 

[JasonW]

My first guess (not having more detailed information on your installation) is that RF may be getting into the audio running to each transmitter.

I'd use shielded audio cable and wrap the audio cable to each transmitter through a ferrite toroid to act as an RF choke. -- JasonW

> Guys I need some help to prevent Intermodulation between my
> two Part 15 Am Transmitters (Rangemasters). I am
> Broadcasting @ 1620 and 1680 Khz.
>
> Both Transmitters are mounted 20'apart on two Wooden Towers
> about 40'high.
> I am getting good range on both transmissions, except that
> you can hear a leak of the 1620 Music mixed with the 1680
> Audio or vice versa.
>
> I know the Big Stations transmits various frequencies either
> from the same Tower or from nearby Towers. (like a typical
> AM Antenna farm) How do they prevent this. Thanks.
>

 

[CW]

> Guys I need some help to prevent Intermodulation between my
> two Part 15 Am Transmitters (Rangemasters). I am
> Broadcasting @ 1620 and 1680 Khz.
> Both Transmitters are mounted 20'apart on two Wooden Towers
> about 40'high.
> I am getting good range on both transmissions, except that
> you can hear a leak of the 1620 Music mixed with the 1680
> Audio or vice versa.
> I know the Big Stations transmits various frequencies either
> from the same Tower or from nearby Towers. (like a typical
> AM Antenna farm) How do they prevent this. Thanks.

AM stations usually do NOT use the same towers for mulitple stations (It can be done in a diplexed mode for two stations but it requires LARGE multiple filters at EACH tower to do so; PITA overall...and done ONLY when they have to). FMs are easier to do. Have you tried killing the audio to one xmtr and then seeing if you still hear the opposite's audio on it? 100mw Part 15 should not cause that...

 

[radio8z]

> Guys I need some help to prevent Intermodulation between my
> two Part 15 Am Transmitters (Rangemasters). I am
> Broadcasting @ 1620 and 1680 Khz.
>
> Both Transmitters are mounted 20'apart on two Wooden Towers
> about 40'high.
> I am getting good range on both transmissions, except that
> you can hear a leak of the 1620 Music mixed with the 1680
> Audio or vice versa.
>
> I know the Big Stations transmits various frequencies either
> from the same Tower or from nearby Towers. (like a typical
> AM Antenna farm) How do they prevent this. Thanks.
>
Hi,

This is probably not IMD but rather, as mentioned in other replies, pickup of one transmitter by the audio circuit of the other.

In transmitters, IMD happens when the foreign strong signal enters the transmitter final amp and mixes with the transmitter signal due to non linearities. The difference frequency of your system is 60 KHz., so IMD would appear at one or both carrier frequencies +/- 60 KHz. This mixing could also be happening in your receiver. Try monitoring from a distance and see if it goes away.

My experience with true IMD was at 146 MHz. in a repeater and it took a notch filter to eliminate it. Not a trivial task at 1600 KHz.

Neil