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Before I break something …
What will happen if I connect the outputs of two transmitter together (double, cancel, fry the TX’)?
Total posts : 45366Ken, I think it would be the same as outputting two audio amps together. With, say, two 50-ohm transmitters feeding into each other the actual impedence would be halved to 25-ohms NOT counting an antenna, and each output transistor would be handling not only its own current but the current absorbed from the other.
To do it properly I would add a 50-ohm resistor in series with each output and have the two resistors join at the antenna, I think that would match.
What it would do to your total power outputI cannot imagine, since the resistors would lose some of the power from each transmitter.
Total posts : 45366I believe they have some plans for splitters and combiners that maintain the proper load and prevent the XMTRs from blasting into each other. I’m thinking you’d want to do that at the oscillator stage and not modulate until after the combine. I can’t see how two modulated signals would be intelligible if combined.
Just my random thoughts…
Total posts : 45366“I’m thinking you’d want to do that at the oscillator stage and not modulate until after the combine. I can’t see how two modulated signals would be intelligible if combined.”
That makes sense. The idea was to see if I could boost the signal to test CC in the marina. Probably easier to build an rf amp.
Total posts : 45366What I think we meant to say earlier, not just me, but also everyone else, is that the two combined transmitters would need to be syncronized to avoid a “beating of the frequencies” which could be a low “wah wah wah” or a higher pitched tone signal.
Total posts : 45366Its been done before.
I can’t remember the stations exactly. But I have read where they combine two AM TX for double the RF output.
Total posts : 45366I don’t think so … they will be getting the exact same signal in the exact same place at the exact same time. The audio output with a Y-connector to the two TX’. No sync problem.
The problem is the two TX’ backing up signals into each other. The only solution thaty makes sense to me is to combine them prior to modulation.
Therefore, I concluded it would be simpler to just to replace the RF amp, in one unit … if I went that route.
But many thanks for all the thoughts so far. The main problem is that right now purchasing a viable dedicated CC transmitter is out of reach. However I’ve conversed with PhilB offlist about a 7-watt mod for the SSTRAN for such purposes. That’s much more affordable.
I’m also looking into two other possibilities:
1) Webphones, tablets, and in-car Internet Radio is a fast-growing genre for audio and video. Therefore, as the their use expands, affordable-to-build WiFi LANs will become a more viable alternative for day-to-day community programming services, saving the WWW for more appropriate specialty programming.
2) Nothing seems to be happening with the local LPFM Community radio station, even though they have had a construction permit for over 2 years, so I’m thinking about putting together a LPFM station in a partnership. The toughest test will be using the FCC’s engineering data requirements … I’ll have to get help to walk through it, or learn it all step-by-step, which may just barely be doable to get into the next window, if there are any frequencies left to use in this area.
Total posts : 45366Perhaps the LPFM with the construction permit needs you to come in and take control. If so, think about a lengthy contract so they can’t brush you aside once you’ve made it happen.
Total posts : 45366Having talked to several and past (hey quit) board members, I don’t think the current president is about to let anyone else have control … consequently I’d rather put together another effort. Also, they have too many board members … anyone who knows about such things will tell you the smaller the board the easier it is to get something done.
You can spend all your time worrying about paper interference issues with Canadian stations, and push those papers around at meetings (where there are no engineers present), but you can’t test for the (perhaps non-existant) issues that way, especially considering the wider latitudes allowed under the most recent law. At some point you need to just call it good, build your station and turn it ON for testing. Why they haven’t done that is cause for grief. Plus, as I’ve previously mentioned, they don’t have open meetings, and have lost credibility and support. That’s a shame.
So maybe I can get a partnership together, write up a contract, and begin sifting through the FCC engineering parameters with Prometheus and the folks at KLOI on Lopez (who have been on the air for more than two years, with nowhere near as much trouble).
Total posts : 45366Ken,
If you want to pursue carrier current the Ramsey AM-25 kit may be a starting point. RFBurns provided a link to an article on this subject which I have copied here:
http://teslapress.com/am25.html
This is not a “plug and play” project but it may provide an affordable means to begin experimenting with CC if you are comfortable in dealing with the technology it takes.
Neil
Total posts : 45366The carriers of the two transmitters have to be synchronized, not only in frequency, but in phase. Otherwise, the phases of the two carriers will vary at random with each other, causing time-varying constructive and destructive interference, called “flutter.” Using the same RF source to generate the carriers of both transmitters will eliminate the flutter, but there will still be interference between the two transmitters. Both transmitters should be adjusted for the same phase at the outputs, so that the interference is constructive. Destructive interference would result in one transmitter reducing or cancelling out the output of the other transmitter.
As for adding the outputs of two power amplifiers, this is rarely simple, and usually requires RF circuit design expertise.
Total posts : 45366Yep, definitely not plug’n’play, but wow! That’s a great mod for CC ops. I must’ve missed RFBurns message. Thanks to both him and yourself.
Total posts : 45366Thanks Ermi,
Yep, I looked over the situation and decided it wasn’t worth the time … there are better solutions to experiment with.
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