107.5 Cheltenham spogging
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radionortheast
- proppa neck!
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- Joined: Wed Sep 09, 2015 1:38 pm
107.5 Cheltenham spogging
107.5 from cheltenham has been sprogging, mentioned on another forum, wonder if this will become common now, perfectly good transmitters can start doing this without warning http://skywavesdx.org/viewtopic.php?t=10867
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Albert H
- proppa neck!
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Re: 107.5 Cheltenham spogging
Unfortunately, "perfectly good" doesn't apply to a transmitter that spontaneously begins spewing rubbish!
Adjacent spurs are usually caused by low frequency instability. Many RF transistors have 10 - 15dB of gain at Band II frequencies (many of the more recent FETs have much more gain). These RF transistors will have spectacularly high gain at 1 or 2 MHz. If the design of the PA is poor, a power transistor can start to oscillate at low frequency. If this happens, the low frequency oscillation mixes with the intended carrier, producing images of the main carrier either side of the intended frequency.
Infamously, back in the late 70s or early 80s, Roger (later of Broadcast Warehouse infamy) built a rig for "Bromley Sound" on 94 MHz. They used an old RFL field site at Keston, in the woods, which had a vertical "H" pointing north-westwards into London. It was quite a high site, and used to get out remarkably well. Unfortunately, Roger had tuned the rig up for the last Watt of RF he could squeeze out of it, without using a Spectrum Analyser (or even a Wavemeter), When they put it on the air at about 10am one Sunday morning, it appeared roughly every 3 MHz across the band! A quick check with a scanner showed that it was every ~3 MHz across the airband too. I was in Bromley (about 6 miles away) at the time, and discovered the mess almost as soon as it was switched on.
We tried to get hold of the lads running Bromley Sound, but nobody was answering their phones - despite trying to get hold of three or four of them. My friend Andy M and I hopped in his car and raced up to Keston. We arrived at the site to find a big black rig at the foot of the tree, with a cassette player providing the audio, and a couple of car batteries powering it all. We disconnected the battery, and left a note
"Please clean up this piece of junk!!" on top of the rig, held down by the Battery clip we'd disconnected.
Bromley Sound didn't reappear for a couple of weeks, but the next time it came on, it was only on one frequency!
Adjacent spurs are usually caused by low frequency instability. Many RF transistors have 10 - 15dB of gain at Band II frequencies (many of the more recent FETs have much more gain). These RF transistors will have spectacularly high gain at 1 or 2 MHz. If the design of the PA is poor, a power transistor can start to oscillate at low frequency. If this happens, the low frequency oscillation mixes with the intended carrier, producing images of the main carrier either side of the intended frequency.
Infamously, back in the late 70s or early 80s, Roger (later of Broadcast Warehouse infamy) built a rig for "Bromley Sound" on 94 MHz. They used an old RFL field site at Keston, in the woods, which had a vertical "H" pointing north-westwards into London. It was quite a high site, and used to get out remarkably well. Unfortunately, Roger had tuned the rig up for the last Watt of RF he could squeeze out of it, without using a Spectrum Analyser (or even a Wavemeter), When they put it on the air at about 10am one Sunday morning, it appeared roughly every 3 MHz across the band! A quick check with a scanner showed that it was every ~3 MHz across the airband too. I was in Bromley (about 6 miles away) at the time, and discovered the mess almost as soon as it was switched on.
We tried to get hold of the lads running Bromley Sound, but nobody was answering their phones - despite trying to get hold of three or four of them. My friend Andy M and I hopped in his car and raced up to Keston. We arrived at the site to find a big black rig at the foot of the tree, with a cassette player providing the audio, and a couple of car batteries powering it all. We disconnected the battery, and left a note
"Please clean up this piece of junk!!" on top of the rig, held down by the Battery clip we'd disconnected.
Bromley Sound didn't reappear for a couple of weeks, but the next time it came on, it was only on one frequency!
"Why is my rig humming?"
"Because it doesn't know the words!"
"Because it doesn't know the words!"