Was browsing through some 2004 post on the now gone "Community Radio USA" forum, where MLR was having some kind of issue with a transmitter he was building.. his original post is not intact, but I had asked what I now guess was a silly question in that thread that had quoted a quip of his original post.
Below is my question followed by a very interesting response from an Andrew_MacKenzie. I don't remember this discussion, and have never considered the possibility that music programming puts more wear and tear on a transmitter than talk programming does. But it seems to make perfect sense.
What do you think?
> >" and I ended up frying the final stage when trying it out at a low frequency (680)."
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Is this to indicate that using the lower frequencies generally means more wear and tear to a transmitter? -- and (therefore) that transmitting on the higher frequencies is less abusive to a transmitter?
Subject:Re: I just can't get this - Suggestions? [re: RPowers] Posted by:Andrew_MacKenziePosts:79Status:OfflinePosted on:10/02/03 11:05 AM
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Not necessarily. It's just harder to get a decent match at lower frequencies when using Part15 antenna specs. You can set up a Gizmo to run at the low end of the dial, but the efficiency of the 3M antenna will only be about half that of what you would get at the upper end of the dial, and thats not much at all to begin with (IIRC, it less than 2 percent at 1600khz). The end result would be less overall transmission range.
Now, my opinion always was that whats 'abusive' to an AM transmitter is whats being pumped through it. The TX has to work harder to broadcast music than it does to broadcast talk programming, for instance. Bass heavy music is more work than non-bassy music (hip-hop or Top 40 vs. classical for example). Pushing the modulation to maximum will also stress the TX harder than if you backed out your levels a little. That's one of the reasons why when you check your output stage power levels, you do it with the TX unmodulated. The levels can change slightly when the TX is actually broadcasting audio. In my case, my audio is about as loud as it can get since I use pro processing equipment (designed for FM, but works well in the non-filtered AM enviroment too), but it also makes the TX work about as hard as it can to transmit that audio. A double edged sword sorta speak. You need maximum modulation to get maximum range, but you're working the equipment pretty hard to achieve that. Not much of a big deal for a broadcast transmitter, but for these little Part-15 rigs thats asking quite a bit, especially in 24/7 operation scenarios.
To sidetrack for a moment: Back in the days when we DX'ed AM, particularly in the 80's when programming was mixed, you could hear the difference in a stations signal when it went from a talk show to a music program or vice-versa. The skywave signal was always stronger & faded less during the talk show, because the TX didn't have to work as hard to broadcast it. Generally, that was a sign that the TX was getting old or fatigued and needed some tuning up. Engineers would tell you that thier AM transmitters would throw off twice as much heat during a music program, a sign that the TX was working much harder than it did for news or talk programming.
Good TX designs anticipate being operated in a demanding fashion and the units will be 'overbuilt' to compensate for it. The Rangemaster is a good example. The Gizmo, while a good design in it's own right, is still fairly basic and my experience with the unit showed that it didn't like to be pushed too hard. The newer Metzo seems to have a much beefier final stage and should be able to handle a poorer antenna match and higher mod levels better than it's little brother Gizmo.
-AM