Intermod Product On My Rx Freq

[Birken Vogt]

In a very nicely setup site I recently discovered 20 dB of desense to my customer's high band receiver when another tenant transmits on high band 3.4 MHz away. This happens the same whether we are on the 120' master rx antenna or another antenna at 40'.

After doing some study I discovered that the intermod product of two broadcast stations in the low end of the band plus this high band transmitter exactly fall on my customer's RX. One broadcast is 200w and the other is 1000w. They are both on the same mountaintop but not the same tower. The high band tx is on the same tower.

Neither FM tx has a band pass filter on the output, though we have figured that their internal LPF should not be allowing the mix to be occurring inside their PAs. Is this a safe assumption?

It is not practical to move my customer's RX nor the high band TX. The 1000w station probably cannot move frequency. The 200w station is a translator and might be able to move frequency but it is doubtful.

I am interested in experiences here. Is there any hope of finding the point of the mix and eliminating it? Would moving to a different tower likely change anything?

Thanks
Birken

[Bill_G]

Who are the incumbents, and who is new? If the two low power FM stations and the affected rcvr were co-existing fine before a new VHF transmitter was added, then the new guy will probably have to leave. Or he can pay you to determine where the mix is occurring, and possibly fix it. It could be a fence, an old abandoned antenna, or any weathered surface where two dissimilar metals can form a non-linear junction. If you have access to a PIM analyser, it might be helpful.

[Birken Vogt]

The high band tx is old. The 1kw tx is old. Our rx is from about 2001 and the 200w tx is from about 2005. We just recently took over this customer so we have just discovered this issue. Is there any hope for a situation this old?

[VA3HF]

This sounds like you have a 3 transmitter 3rd order intermod product where the 2 Broadcast transmitters are also 3.4MHz apart.

You say you are on a Master RX antenna - is this just an antenna with a preamp and power dividers with maybe a wide band preselector? Likely cause is the amplifier or any nickle plated or gold over nickle plating on the transmission line connectors. It can also me the RX antenna itself.

Can you actually see the noise floor/product increase with the transmitter keying up 3.4MHz away?

Make sure it isn't your receiver front end producing it inside. Use some bandpass cavities in front of the receiver.

It could also be anything in the vicinity picking up all 3 transmitters and producing the product that you are receiving. Good luck....

[Birken Vogt]

I haven't gone into full investigative mode yet. I did not spectrum analyze what the receiver is seeing.

The rx setup is as follows:

The new setup: DB616 at 120' comes down to Telewave starpoint (just a glorified tee) into a 1 MHz wide preselector for some other radios, and 3 qty TWPC-1504-2F for discrete frequencies for this radio and some others. All this was engineered directly by Telewave, and all ports perform pretty much flawlessly ordinarily.

The old setup: SRL-235 at 15' comes to half of a TXRX/Bird 28-37-02A (separate TX and RX antennas). The notches are set for our own transmitter which does not figure into this problem at this point.

Both rx setups give the same 20 dB of desense when the offending tx comes on. Both RX setups are totally separate if that was not clear from the above.

We are going to try moving the notch of the TXRX to the offending transmitter and see if that changes anything.

We are running a MTR2000 receiver with the external preselector also.

Any ideas appreciated.

[Bill_G]

For clarification - Is the interference constant or intermittent? By your description, it seems intermittent.

At 20db of desense, you might be able to see it on a spectrum analyser, but probably not. Most analysers don't go deeper than -100 very well.

One way to verify whether it is IM or not with a scanner or portable on the freq of interest. If you can hear it outside, it's IM. OTOH, if you only hear it on the rx line, the mix could be occurring in the front end of your rcvr.

[Birken Vogt]

Let me define constant interference: the two broadcast stations are always transmitting, of course, so when I hit the tx button on the transmitter in question (located in the same vault) it seems to knock my receiver down a steady 20 dB.

Good thought about the front end, although this is an active site so it could be challenging to use a scanner or portable to get any useful info.

[Bill_G]

No. If it's an IM hit caused by an outside mix source, a scanner can help you DF it. Super useful tool. If the scanner doesn't hear it, the mix is more probably in your ant and/or transmission line.

[VA3HF]

The 3 transmitters are local so a scanner or even a portable outside will receive the signals and produce the IM product in its fromt end. You will need a low IM antenna connected to a bunch of bandpass cavities tuned to the RX frequency an the whole setup needs to be a LOW PIM system too to be able to move around to find the source of the intermod if it is generated outside the systems. But then again, just because you find one source doesn't mean it is the source you are being interfered by. The most likely source is the SRL-235 antenna. They were notorious for being very poor PIM response unless you got the newer generation of them that were PIM rated. I would definitely only use a LOW PIM rated antenna in that high RF level environment.

But all it would take is a nickel plated connector or a Gold over nickel plated connector anywhere in the RX system to generate the IM between the antenna and any filter.

[Bill_G]

That is actually the point of using a scanner and/or portable. If you have three different receivers all agreeing, it's real, and will be fairly easy to locate. If they don't agree, but each has their own problems, it's still there, it's more complicated, and more difficult to locate, but not impossible.

[VA3HF]

That is actually the point of using a scanner and/or portable. If you have three different receivers all agreeing, it's real, and will be fairly easy to locate. If they don't agree, but each has their own problems, it's still there, it's more complicated, and more difficult to locate, but not impossible.

Not necessarily.

[Bill_G]

That is actually the point of using a scanner and/or portable. If you have three different receivers all agreeing, it's real, and will be fairly easy to locate. If they don't agree, but each has their own problems, it's still there, it's more complicated, and more difficult to locate, but not impossible.

Not necessarily.

Okay. There was a time prior to PIM analysers when the physics of the problem were not understood, but the problems still existed, and we had to figure it out on our own. Simple tools gave me insights that I used to nix the problems. Wasn't always successful. Unfortunately, they were not skills easily passed on, and often looked at more as wizardry than science. Not that PIM testers are appliances. They have their competence hurdles.

[Birken Vogt]

I have some questions of a more general nature:

1. If we do find all the rusty pieces of metal or PIM generating antenna or cable and fix it, with this intermod product being right on our RX is it likely it will stay fixed, or will some other piece of hardware start generating some more intermod within 6 months?

2. I have seen some Anritsu or other equipment that was advertised as able to find PIM, possibly by distance, maybe mentioned above...can anyone point me to some models that work, literature on them, experience on them, and suggest prices? Available used?

3. The 200W FM broadcast translator has been on their freq since about 2005 and our rx has been there since about 2001. Do we have any leverage to cause them to move frequency? There are several open frequencies nearby.

Thanks
Birken

[Bill_G]

If you run an intermod study, do the math, and it shows a direct hit, then you're goners. A hit is a hit. Since they are a broadcaster, they should be open to discussing the problem. That doesn't guarantee cooperation. Have your ducks in a row before you make your case. And their engineer may be willing to help figure out the problem.

As for PIM tools, Anritsu is a good place to start. Be ready for sticker shock. If you have one of their more modern devices already, it might be upgradable.

[Birken Vogt]

Since the 1000w broadcaster, the other VHF transmitter, and we, were there first, presumably an intermod study would have shown their proposed frequency in 2005 was a hit. Do we have a leg to stand on with the FCC or whoever 10 years down the line, or is it too late for that angle?

[Bill_G]

I don't know. Contact the FCC.

[VA3HF]

Intermod problems depend on the actual power levels at the non-linear junction. If the source of the IM is in your RX antenna system, the trick is to reduce the level of one or more of the offending transmitters getting received at the antenna or improve your PIM of your system. Vertical separation of the transmitter antenna to the receiver antenna works best for getting the signals lower but the maximum on a tower is about 60dB on VHF.

To check if your equipment connected to your antenna is not the source, add a 3dB attenuator in line and the IM product should drop by 3dB too. If it drops by much more, the source of the IM is in the stuff past the pad to your equipment.

Was the problem there for 10 years or did something change or did you just notice the problem? 20dB is too large not to notice it easily.

[Birken Vogt]

We just took over the system from another vendor. We find new troubles all the time.

[Bill_G]

+1 on using attenuators to help diagnose these issues. Bring a handful at values of 3, 6 and 10db. You have to be on your game though, and think it through. Let's say you put your spectrum analyser right on the rx mainline, and lo and behold - there it is - the direct hit on your receive freq. If you put a 3db pad in line, and the signal drops 3db, it's real. If it drops more, or disappears entirely, it was a birdie in the front end of your test equipment. ie: a false positive. These non-linear responses give you clues which direction the problem is. It's good practice to insert a pad between your test equipment and any line you intercept to keep your test equipment out of the equation, and to keep you from chasing your tail.

PS - taking over from a previous vendor has it's challenges. Just state your findings, leave the other guy out of it, and let the customer draw his own conclusions. Your success will speak volumes. In this specific case, the other guy may have understood the problem, but couldn't get the cooperation of the customer, or couldn't explain it well enough to get the customer to react. Something like this isn't going to be cheap. I would get a commitment of a man week (40 hours) from the customer before proceeding, with no promise that it will be resolved, but merely diagnosed. It may be beyond your control especially if it's gone on for a decade with no resolution.

[VA3HF]

Also make sure you don't have some LMR type foil transmission line in the system. It has poor PIM in the presence of strong carriers. Use only silver plated RG214/U or solid copper hardline. Copper over aluminum centre conductor like that of LDF4-50A is acceptable but don't nick the copper plating.

It may be easier to just change the customer's RX or TX frequency.

[Birken Vogt]

I would love to change the customer's RX. It seems suceptible to interference, but we have a bunch of receiver sites and lots of subscriber units inside and outside the agency that would need to be reprogrammed. The work would be challenging. I suspect we might have to go that route someday.

The customer's TX does not contribute to the intermod; it is other transmitters not related to us.