crossband linear ssb transponder project

Discussion in 'Homebrew and Kit Projects' started by KC4UMO, Aug 7, 2008.

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  1. KC4UMO

    KC4UMO Ham Member QRZ Page

    My good friend and Elmer N4CZT that recently past away have been tossing a project around for the last 6 months or so. He was a diehard satellite operator at one time then got very heavy into 6 meters.

    He and myself had been discussing building a linear ssb transponder. ( like one of the older Oscar birds) Just an analog type with a small bandwidth.. Since he passed away I have been thinking more and more about it. And one day, after a lot of study, I like to do this in memory of him.

    I have already played around with a cross band repeater. That was fun. 70cm receive and 2 meter transmit. Very simple and most dual banders will do that.
    This one was to have a 70cm ssb receiver, a 2-meter ssb transmitter with one watt, along with a shared IF circuit for linear operation. No on board computer controlling it. It was going to be in one of the old satellite frequency ranges, put on a small tower and activated at certain times by a computer. We even had a fellow that said if we made it small enough he would fly it over several counties in his hot air balloon but that was not the importance of the project.
    The biggest reason was to learn more about it and experimentation.

    I have spent the last few weeks searching different articles on the web about ssb transponders. I have found this to be a big challenge for me. He was the brains behind it and probably forgot more in the first few years of his Amateur career than I will ever know.

    I know it sounds maybe like a “why do that” type of project but it was important to him. And if it takes me rest of my life to do it then that is how it is. I just can not seem to get it out of my mind.

    Open for discussion?
    Last edited: Aug 7, 2008
  2. AB8RO

    AB8RO Ham Member QRZ Page

    I've never thought about "transponders", but, what is a transponder other than a mixer, a local osc and input and output bandpass filters (plus output RF amp of course)? I suspect that it's more practical to convert to a common intermediate frequency. It seems to me that it's basically a receiver without product detector connected to a transmitter without an audio modulator.

    A common i.f. would allow you to set input and output frequencies independently and give you better control over the bandwidth.
    Last edited: Aug 7, 2008
  3. AB8RO

    AB8RO Ham Member QRZ Page

  4. KC4UMO

    KC4UMO Ham Member QRZ Page

    That was one of the links he showed me a month ago while visiting him.
    Thanks for finding that!.

    I am going to give that one some thought and study. The way it is designed a stong signal will produce a stonger output, while a weaker signal will produce a weaker output. Also it is a single frequency which would be much simpler to build.

    Good find on that one.
  5. AB8RO

    AB8RO Ham Member QRZ Page

    Look at the second link which has more detail on the AGC apsect. This should be fairly easy to experiment with. Any receiver with say a 10.7 Mhz i.f. output could be used. You then just need a linear mechanism to convert that 10.7 Mhz signal into your output frequency. Ok, that's not saying much, I guess what I'm saying is that it's little more than the back end of an SSB transmitter for the frequency in question. You need everything after the crystal filter. To control your bandwidth you need a wide filter between the two i.f. stages. Many transmitters use a 9Mhz i.f. so you would have to convert the receiver or change the lo of the transmitter to match.
    Last edited: Aug 7, 2008
  6. KC4UMO

    KC4UMO Ham Member QRZ Page

    Great info!

    Ok, but your ability to get "on topic" with the info shows your knowledge.

    Going to take me a bit to soak all this in. I will print off the articles you posted for study. I only done a quick read through hitting the key points.
    These links are much better than the links I googled.

    Much appreciated.

    If you or anyone other has them, keep em coming....
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