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Thread: Tuning

  1. #21

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    Email to the article author has been sent! The following is a copy of my email to Mr. Purdum:

    "Mr. Purdum

    In the Nov. QST regarding the dummy load project, you mention in the conclusion using the dummy load to tune on a band without affecting other hams. This information has led to a debate (and arguments) on a ham forum.

    Could you please clarify what benefit tuning to a dummy load (that is already 50ohm) has with respect to tuning to an antenna? Tuning to a dummy load surely cannot be of any benefit when the radio and the dummy load are already matched... None of us on the forum (except one) see how it has any use with respect to tuning to an antenna with a different impedance. Perhaps everyone is misunderstanding what you mean.

    Thank you!
    73, KE0KOY Brandon"

    Im sure we all look forward to his explanation!

    BTW, your last post started with "Just to be perfectly clear I never use the internal tuner" , while your initial post started with "Easy question: Let's say I use the tuner in my FT-450D". Don't change your story!
    Last edited by brandon lind; Tue 18th Dec 2018 at 00:40. Reason: Added quoted email text

  2. #22
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    b-lind - actually my story did change in that I realized what I was doing with my Yaesu's internal tuner didn't make any sense. But the real question in the op was do I need to tune again with a band change and of course the answer is yes. But then what got muddy was that I was being told that tuning to a dummy load made no sense at all. I was being told that it was a troubleshooting tool and nothing more.

    Here's what I've noticed: if I'm properly tuned to a particular frequency on 20m, for example, and I switch to the 40m band, I have to perform some re-tuning to get to or get close to resonance. If after switching to 40m and a frequency at or near what I want to use I first tune to the dummy load, then when I check to see how close I am to resonance it turns out I'm much closer than without employing the dummy load. It kind of 'gets me in the ballpark.' And this is what I believe was being stated in the article.

    But hopefully you'll receive a response and some clarification.

  3. #23

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    I did get an explanation that makes some sense, but it does not apply to your situation. The author was speaking of a scenario where a tube type transmitter is driving an amplifier with a 50ohm input impedance. He states that if a person places a coax switch between the tube transmitter and amplifier, the plate and load adjustments can be fine tuned to the amplifiers input impedance of 50ohm without being transmitted on the air as the dummy load has the same impedance as the amplifier input. But unfortunately, his reasoning only extends to cases involving tube transceivers that operate into a 50ohm amplifier.

    (EDIT: he did also claim the procedure useful for modern transceivers as there are sometimes slight mismatches between a solid-state transceivers output and the input to a power amplifier, but typically, in my opinion, if there is more than a few ohm difference, there is probably something seriously wrong with the equipment. His notion also relies on the erroneous assumption that it is the transmitter that is not 50 ohm. In many cases it is the amplifier input that is not exactly 50ohm and tuning to a 50ohm dummy load would thus be a fruitless effort. A few ohm difference between a transmitter and amplifier shouldn't break the gain bank)

    In cases where the transceiver is of the solid-state type having a constant output impedance that never needs adjusting, this operation should not be required. His remark to me regarding one whistling into their mike and playing with the mike gain is more of a radio etiquette issue than a tuning issue.

    (EDIT2: I plead guilty to being the edit champ, in my defense, i'm usually adding things i forgot, not backpedaling... I forgot to address your "it gets me close" thing. When it comes to matching a 50ohm radio to a 50ohm dummy load using a tuner, there are an INFINITE number of combinations of reactances that do that. In the case of my tuner, which is a T configuration employing a series capacitor, shunt inductor and then another series capacitor, as long as the two capacitors are set to the same capacitance, there is an inductance that will return the output to 50ohm. You should be able to find positions of equal capacitor settings that satisfy any shunt inductor setting to match 50ohm to 50ohm. I cannot see how doing this gets you close to what the antenna is because tuning to a dummy load should be possible (depending on frequency and tuner range) on all inductor settings given equal capacitor settings. This is why tuning to a dummy load first should have no bearing on getting you near the antenna impedance. The fact you experience this is out of sheer luck!)
    Last edited by brandon lind; Tue 18th Dec 2018 at 13:07.

  4. #24
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    OK! I'm a lucky guy!

  5. #25

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    Although he uses an L matching network, this video is an excellent way to visualize how reactive components affect impedance.

    https://www.youtube.com/watch?v=gw1TYWwfvGk

    If you would like, I can use smith chart software to demonstrate visually how 50ohms can be matched to 50ohms an infinite number of ways using a T matching network (like an MFJ941E tuner) and post a video if that would help clear it up

  6. #26
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    No need - I'm sure you're correct and I can see the math on that. Thanks.

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