Get your tone encoders ready!

GET YOUR TONE ENCODER READY!!

 

 

Well, folks, as bad as I hate to do so, it appears I am going to have to place a 100 Hz tone decoder on 145.47 in order to get it back on the air.  As I am sure most of you know, I had to turn 145.47 off a week ago due to a LOUD squealing noise that was keeping the repeater keyed up. I suspected a 5 megawatt TV transmitter that is located at the same site, since the noise heard thru the repeater was TV video, also known as “ sync buzz.”  Last Thursday, I was able to confirm that suspicion when the TV stations transmitter was turned off at 5 AM for a test. 145.47 was crystal clear, and I worked a station in Lexington, North Carolina without any noise whatsoever. As soon as they turned the TV transmitter back on, back came the noise.  Several of you were up at that wee hour to help me, and I want to say thanks. But the problem came back with the return of the TV transmitter to the air…so I had to turn it back off.

 

How is this happening?  Well, here’s a little unplanned blog entry on intermodulation interference, sometimes called “intermod.”  Not all interference that is generated is “intermod,” but I will explain why this case is. When two RF signals are combined, they produce a sum (added together)  and a difference (one subtracted from the other) of the two individual frequencies. This combining, or “mix,” can happen in numerous places, from one of the two transmitter amplifiers or a rusty joint that is ripe for RF rectification. The interference that we are receiving on 145.47 is a result of a RF carrier (coming from the TV transmitter) on 290 Mhz. The 290 MHz signal is “mixing” (combining) with the output of my repeater on 145.47. There is probably a “sum” frequency that is around 435 MHz, but the problem is the “difference” frequency, which falls right on 144.87 MHz, which is, of course, the input to my repeater. When two signals combine, you will hear modulation components of BOTH signals. That’s why you hear the video noise AND the squeal. The squeal is from my repeater feeding back into itself. Think of it as the same thing as when a PA system is too loud and the output from the speakers gets back into the microphone(s) and squeals.

 

There are other ways to have intermodulation interference. Mixes can occur on a transmitters harmonic frequency. For those of you who have been around a while, you may remember when 146.625 would come up randomly and play the weather station. After thinking it was a QRM’er at first,  I decided that no QRM’er was dedicated enough to do this 24 hours per day at various intervals.  So the search began. What I found was that a paging transmitter that was being used by our local fire department had a nice second harmonic, and that harmonic was mixing with the weather service transmitter and showing up on my repeater input. Here’s the math: 154.250 X2= 308.500. 308.500 – 162.475 equals 146.025, which is dead on the 146.625 input. This is referred to as a “third order” intermodulation problem or 2 times freq A minus freq B equals freq C. There are also fifth order intermodulation products, but I have never had to deal with them.

 

So, now we know what the problem is, what can you do to fix it?  The easiest thing to do is eliminate the mix. I am working with the TV station to eliminate the 290 MHz frequency that is causing this problem. Bear in mind that this is a FIVE MILLION WATT TV transmitter. The offending frequency doesn’t have to be very strong and the TV station still be considered legal. The TV station engineer has traveled  to the transmitter site twice so far to try to find where this signal is coming from.  Today, he went with a spectrum analyzer, which is an absolute MUST when troubleshooting this kind of problem. However, he can’t “see” this signal on his analyzer. However, we know the signal exists, because several of you have confirmed it for me by listening on your receivers wherever you may be located. So, I am planning a trip to the transmitter site with *my* spectrum analyzer and a receiver capable of receiving 290 Mhz.  I fully expect to receive the signal on my receiver, and point it out to the engineering staff of the TV station. However, until they locate the signal and reduce or eliminate it, 145.47 will still have the lovely noise that you have heard until I take another course of action. That action, unfortunately, is a CTCSS (or “PL” or tone board) installed on the repeater receiver.  I had to do this recently on 146.625 until another stray video carrier finally went away. I wish I had thought to check the same source as this problem…they might have been the same thing.  

 

Now, here’s another lesson to be learned from this type of problem: When I install the tone board, the problem will not be heard anymore. Notice that I did NOT say that the problem would go away. It will merely be “muted” until a signal with the proper tone is received at the repeater. Then, if the signal (from you, the repeater user) happens to be weak enough, then you will STILL hear the video noise as it “scrubs” with the legitimate repeater user. It will affect the weaker signals the most. Some of you that were used to getting into 145.47 with low power from a distant location may find that you can’t get into the repeater, or that your signal isn’t as good as it once was. This is because the tone decoder only hides the problem of the signal that it is still on the input frequency of the repeater.  This is also one of the reasons that I do NOT like tone squelch amateur repeaters, along with other reasons. A repeater with a tone squelch is not friendly to a traveling user, unless the user happens to know the tone frequency and has the tone encode enabled on his or her radio. Most modern radios have tone encode (and decode) capability, but you have to KNOW what the tone frequency is first.

 

Lastly, what happens if the TV station can’t or won’t eliminate the interference?  There are two options. I can attempt to eliminate the “mix, “ IF (that’s a big IF) the mix is happening in the 145.47 transmitter. Unfortunately, its not always easy to determine where the mix is happening. Another problem…the device used to eliminate this problem is called an isolator.  You probably don’t have one of these laying around in your ham shack. They are around $1000 each and they are finely tuned fro the exact “pass” frequency (in my case, 145.47). If you *do* have one, I would like to talk to you J !!

 

Have you been hearing about the “Digital TV conversion?”  Well, if you have, there is some good news for this problem that relates to the changeover to digital television.  In February, ALL analog TV stations are mandated to shut down in favor of their digital signal. All of the local stations (this one included) already have their digital signals on the air. When February rolls around, guess what…this transmitter will be off the air.  HOORAY!!

 

 It is my sincere hope that I can restore 145.47 to useable service long before February.  I don’t like having one of my busiest and most active repeaters crippled, especially if its something that is not the fault of the equipment I have on the aire or a situation that I have control over. Please bear with me and I will do my very best to correct the 145.47 problem…the best way I can, as soon as I can.

 

NOW, FOR SOME GOOD NEWS! 

 

I have saved enough shekels for a new duplexer for 146.73!  I will be making the final payment on a six-cavity repeater duplexer that I have had in “lay away”  for several weeks.  I hope to have it here and tuned up within a couple of more weeks. If you think .73 is doing good now…wait until it can actually hear better!!!

 

73 de Tim WB4GBI