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DenTron Jr Monitor antenna coupler SWR modification

Discussion in 'Radio Circuits, Repair & Performance' started by AA7EJ, Apr 10, 2012.

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  1. AA7EJ

    AA7EJ Ham Member QRZ Page

    I bought this antenna coupler years ago and it works just fine.
    According to the schematic the original has only relative power metering circuit.
    On my copy there seems to be forward / reverse switch ( unmarked) for true SWR metering.
    The modification should be simple to trace, however, if someone has a schematic of this modification it would be helpful.
    As far as I know SWR pickup circuit need two diodes and there is only one diode in the coupler.
    Needless to say – it works only in forward direction.
    Something must be missing.
    73 Vaclav
  2. K7MEM

    K7MEM Ham Member QRZ Page

    DenTron had several models. You may be looking at a schematic for a slightly different model.

    The DenTron Jr only has a diode tapping off of the "Input" coax connector. So it is effectively seeing both the forward and reverse power. The forward power becomes more prominent as you reduce the reverse power. You can usually work pretty well with that setup, but you never know what your real match is.

    I have a MFJ 16010ST tuner. It is effectively the same as the DenTron Jr., except there is no meter. It's a good tuner but lacks any metering. When I am looking at an antenna with the MFJ tuner, I usually feed it with my MFJ-259 Antenna Analyzer. That way I can adjust the match without bothering anyone on the band. Then I just switch the cable to the transmitter or transceiver I want to use.

    What the DenTron Jr. is missing is the Forward/Reverse sensing element. There are several different types. In the early 80's I made a sensor out of a piece of single sided PC board. The picture is below. If you zoom in on the image, you can see that I cut it with an X-Acto Knife. A couple of resistors, diodes, and capacitors was all that was needed. I ran the forward and Reverse outputs through some shielded cable to the display head. As you can see in the picture, not counting the connectors on the back, all it is is a meter, a switch, and a variable resistor. The rest of the box is empty. I think I got the dimensions for the sensor from a ARRL handbook. Remoting the sensor from the display head lets you place the sensor at a more convenient place.

    The box for my sensor is about 5-1/2" long, so if your cramped for space you could make the sensor from a short piece of coax threaded through a small toroid. The old Heathkit SWR/Power units, like the HM-102, used this kind of sensor. Just go to Google for details. Google has about a zillion types of SWR bridges. Just pick one that works for you.

    100_0803.jpg 100_0805.jpg
  3. AA7EJ

    AA7EJ Ham Member QRZ Page

    this is embarrassing, but I just spent last 10 minutes tracing the modification. It is nothing more than add of another pot to adjust the full swing of the meter. Basically moving it from the back to the front!
    I have another high tech unit – Micronta CB tester and that one has an external coaxial pickup for the SWR measurements.
    I am getting ready for a lengthy field trip and was just trying to reduce the amount of “junk” I have to carry on my back.
    I really do not need SWR meter since it is integral with FT857 anyway HI HI HI.
    Many thanks for your reply and photos. The PCB pickup looks really nice.
    I also see you own the old indestructible Heathkit keyer. They seems to be plentiful at swap meets.
  4. K7MEM

    K7MEM Ham Member QRZ Page

    Thanks. At 2 Meters it is off very slightly, but for HF it's as accurate as any SWR bridge.

    Yes, I am a Heathkit fan. I built the keyer you see 30 years ago while I was living in Germany and running as DA2EU. It has always worked great. Last summer I went to the Williams, AZ Ham Fest and bought two more. I got them both for $10. They were pretty dirty and neither of them worked. After I cleaned out the spider webs and crud, they were pretty easy to get working. I removed the key mechanism from one and combined it with a TicK keyer module. It's nice to have the key adjustments readily available. Once in a while I need to open my original keyer to adjust the key mechanism. But that has never been a big issue with me. I am using the electronics from that unit for another small project.

    Good luck on your field trip.
  5. KE3WD

    KE3WD Ham Member QRZ Page

    Those who find their Heathkit keyer to be indestructible, never fear, just ship the damn thing to me and I will destroy it for you.

    All work guaranteed.

  6. AA7EJ

    AA7EJ Ham Member QRZ Page

    You did not state your rate, but I am sure the shipping will discourage anybody to take you seriously.
    There is a reason why they call stuff like that "boatanchor".
    I think I will start a new thread – post the worst looking boatanchor before and after restoration. HI HI HI
    73 Vaclav
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