Can 800 MHz cause intermod to VHF?

[a1emt]

I will preface this question by saying I am not an RF engineer.... I have been given the blessing to utilize an unused feedline and antenna to install a 146.xxx P25 amateur repeater. The tower is question is also home to two, 800 MHz trunking systems, with antennas located in the same horizontal plane as the VHF antenna. The public safety administrators have no concern with interference between the two systems and VHF.

However, a local amateur radio coordinator is saying this:
the 800 MHz trunked systems create an "intermodulation problem, which upon further consideration and consultation, I feel creates an unworkable situation for the operation of any amateur repeater with a 600kHz split (i.e. 2 meters). "

"The more significant products affecting any 600kHz split VHF repeater are:
854.2125 - 853.6125 = 600kHz
854.2125 - 853.6375 = 575kHz
853.6375 - 853.0625 = 575kHz
851.6500 - 851.0875 = 562.5kHz"

As mentioned above, in addition to these pairs mixing with any VHF amateur repeater transmitter, there are additional products which affect individual channels."



I have a hard time believing that any properly installed 800 MHz system is going to have a negative affect at 146.xxx. Is this information for real?

Jim

[qball]

The answer is yes. 800Mhz can cause problems for you, and just about anything other set of frequencies as long as the math adds up. I have a site that will never support a UHF repeater that uses a 5MHz offset because of offending 800MHz transmitters are 5MHz apart.

860.7125 - 855.7125 + 460.300 = 465.300

What all that means is that when those 3 tx frequencies above are active at the same time, they generate harmonics the equal the receive frequency of the UHF repeater...the repeater will hear itself and go into self-oscillation. As you may notice, you can add in the transmitter frequency for ANY standard offset UHF repeater and the equation shows the rx frequency for that UHF repeater...This stinks! In your case, if the product of the transmitters at your site equal the +/- 600KHz offset of your repeater, sooner or later you will have a problem when all of the mixing transmitters are active.

I did a quick Google search for an intermodulation calculator and came up with this:

http://www.tcstx.com/software/Intermodulation.cfm

Enter your frequencies for the site and you will see what works and won't. I use a different calculator program, but I can't remember the name of it....I will send it when I get to work tomorrow.

Hope this helps!

Q

[a1emt]

Thanks for the detailed explanaion Q !

Jim

[Bill_G]

Q - don't IM calcs mean that interference is possible, but not guaranteed? Without a mix point, the product is not created. At busy sites, and/or poorly maintained sites, the probability increases, it becomes a consideration, but should it prevent new service from constructing?

[MikeOxlong]

There was a 6m repeater cosited with an 800Mhz trunking system that had a voice repeater 1Mhz offset from the primary control channel.

Repeater was basically useless when that voice repeater fired up.

Only solution was to move the repeater to another site.

[qball]

Q - don't IM calcs mean that interference is possible, but not guaranteed? Without a mix point, the product is not created. At busy sites, and/or poorly maintained sites, the probability increases, it becomes a consideration, but should it prevent new service from constructing?

Bill,

I will give you that...possbility it is. There are things that can be implemented at a site to help reduce intermodulation; grounding, antenna separation, circulators, and high quality filters. There is also the possibility that the offending channels never transmit at the same time (one of them could be protected or system loading is light). However, if the intermod study shows that there is a product that could be generated on your RX frequency and you are getting ready to dump thousands into coax, antennas, labor, and repeaters, I would avoid it. Since the antenna is already there, try it! It may work just fine.

I have two most unuseable DB420s at 350' to prove how much of a waste it can be. Unfornately, there was no study done in my case because we were there 1st. The 800MHz stuff came on the tower after me, and they were only concerned with affecting themselves. The tower owner won't do anything about it. Changing 800MHz channels isn't an option as there aren't anymore available in the region. Lastly, I am not a paying tenant. In my case, one 800MHz channel is the primary control and can't be changed for logistical reasons, and the voice channel is protected which helps tremendously. However, the protected voice channel still does a transmitter check every so often, and the noise is present for a couple of seconds.

I really would like to see it work, but before big money is spent, I would err on the side of caution! Just don't want anyone to end up in the same unfortunate situation.

[qball]

A1EMT,

Here is a link to the free intermod calculator that I have played with. It is called Passive RF Intermod Calculator. The download link for the program is at the bottom of their page.

http://www3.telus.net/PassiveRF/

I wish you the best of luck getting your repeater in operation! P25 is pretty neat.

Q

[a1emt]

Thanks guys for all the experience shared! We may go UHF instead of VHF. Just too good of a site to pass up !

Jim