Tuner confusin

Discussion in 'General Technical Questions and Answers' started by KE0WXN, Dec 11, 2019.

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  1. KE0WXN

    KE0WXN Ham Member QRZ Page

    I am using an MFJ2389 Compact Vertical and it has NO counterpoise. It is in my attic. This is a long story with the HOA. In the attic, the hot water flue (metal) is about four feet away. I have adjusted it according to the enclosed instructions.

    I read the MFJ instructions for the 939 and thought that I had to open it to set switches. I wrote an email and they replied that the ICOM cable will do it all. No need to do anything else.

    The trouble that I have is in interpreting all the buttons, menus and instructions.
    If I set the ICom 7300 up and set the tuner menu to manual (only options are manual or auto) and the PTT start to OFF, then I should be able to use the 939 to tune. I push Tune on the MFJ. It clicks away for a few seconds and then lights a green SWR light. If I check the SWR on the ICOM with the SWR graph, it is between 1.5 and 1.9. If I touch the Tune button on the ICOM, and redo the graph, in now reads 1.4 to 2.0.The light on the MFJ is now out. If I don't use the tuner on either, the antenna reads 2.5

    Yes, I should climb up in the attic and redo the antenna tuning but this is Minneapolis. It is now 10 degrees in the attic.

    I appreciate all of your thoughts but I still find this thing confusing.

  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    That's perfectly fine. As long as your Icom sees less than 2:1 you've got a good match and your rig will transfer full power to the antenna.
    For all practical purposes that is the same as your readings above. The difference between an SWR of 1.4:1 and 1.5:1 is meaningless and the difference between 1.9:1 and 2:1 is pretty much the difference in measurement accuracy.

    One thing about auto tuners, whether built in to the rig or external, their software is almost always set up to stop tuning once they find an acceptable match which is usually in the range of 1.7:1 or lower. Sometimes they'll achieve a much better match and sometimes they can't even hit their target but what they don't do is try every combination of internal components till they achieve a 1:1 match or even close to it. This also means that sometimes the same tuner on the same frequency will hit a somewhat better tuning solution and sometimes it won't but as long as both results are below that threshold setting the tuner will stop tuning and you'll get the green light.

    So in the case where you just use the external tuner and it runs its tuning cycle and shows the green light, the tuner stopped trying as soon as it got below its programmed threshold. If that was 1.7:1 that's no problem, maybe next time you tune or on a different frequency or band the tuner might achieve a 1.5:1 match which is also below that threshold but it won't keep trying until it reaches a very low SWR each time so as long as it gets below its programmed threshold and the rig sees less than about 2:1 you're good to go. Running both tuners complicates things a bit as depending on the sequence and how each is internally programmed it might seem a bit more random but as long as you end up with something less than 2:1 you're fine.

    Some tuners can be programmed to attempt to deliver a better match, IOW in some models you can change that internal SWR threshold where the tuner stops trying. The manuals for your tuners should tell you if that is possible but again it's not really necessary as long as you're getting the SWR low enough that the rig delivers full transmit power and does not fold back its output power.

    If you run one of those tuners and you get your SWR as seen by the rig down below 2:1 then you're fine and should just get on the air. No reason to keep trying for lower SWR and no reason to chain together two auto tuners (the external and internal) in that case.

    Also, in the case where your antenna delivers a 2.5:1 or similar SWR without any tuner you really shouldn't need the MFJ tuner at all. The Icom's internal tuner should be fine for finding a match into a 2.5:1 SWR or even a bit higher. Basically the internal tuner in the rig shouldn't have any trouble at all finding a match into at least a 3:1 SWR. External tuners really come into play either when there is no internal tuner in a solid state rig or when the antenna system presents an SWR of higher than 3:1 and the internal tuner might struggle.
  3. KD0CAC

    KD0CAC Ham Member QRZ Page

    How long have you had the MFJ tuner ?
    I ask because if there is a chance to return it , use the tuner in the radio .
    Or sell the MFJ tuner for money for other goodies , possible future more power , legal limit tuner - for HF base I do not see any advantage to a small tuner / if your going to get one , get one that you can grow into .
  4. W9WQA

    W9WQA Ham Member QRZ Page

    i will always use the internal tuner in my 7300,or any radio.
    if my ext sg230 is happy with 2:1 i let the internal clean it up to 1;1 so the finals see the best match. its so simple

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