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Thread: Antenna tuners: where does the power go?

  1. #1

    Join Date
    May 2020
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    Default Antenna tuners: where does the power go?

    Suppose you have a 100W transmitter. You hook up a homemade antenna to it and try transmitting. The transmitter’s onboard SWR meter shows 10:1 SWR and the power amp is automatically rolled back to a watt or two to protect the output stage.

    You say, “No problem; I’ll just grab my handy-dandy antenna tuner here.”

    So you insert the tuner, plus an outboard SWR meter, and tune until the radio’s SWR says 1:1.

    Now your transmitter is pumping the whole 100W and everything looks happy...but the outboard SWR is still showing 10:1. That means 1/11 of your power is being radiated and the other 10/11 is being reflected, right? The tuner doesn’t actually make the bad antenna better, it just makes it look better to the transmitter.

    Or so I’ve read; I have no experience of my own.

    So here’s my question: where does the reflected 91 watts go? Does it heat up the antenna tuner? Does it escape somehow to ground? Is there some sort of reactance-tank magic going on in the tuner that I don’t understand?

    Thanks,
    Dan

  2. #2

    Default

    You're on the right track. The tuner presents 50-52 ohms to the final output stage of the radio to make it happy and prevent reflected power from heating the finals to the point of destruction. Since energy in = energy out, the wattage that cannot be absorbed by the antenna and feed line system is lost as heat.

  3. #3

  4. #4

    Join Date
    Nov 2019
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    Ash Fork, Arizona
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    Default

    It is a little more complicated than just being lost as heat. But you are correct, in that, the tuner doesn't fix a mismatch. It only allows the transmitter to see a good match.

    The high SWR causes much of the power reflected back to the antenna tuner. But at the tuner, there is also a mismatch and some of that reflected power is reflected back to the antenna. This happens over and over, until there isn't anything left. And, of course, some of the power is simply lost as heat in the coax.

    On my web page on UHF/VHF Antennas there is a small calculator for calculating the Loss Due To SWR. The equations for this calculation are also listed.

    So, in your scenario of 100 watts (at 146 MHZ), assume that you are feeding the antenna with 100 feet LMR-400 coax. The amount of power that will be radiated would be 35.272 Watts. The rest of that power is lost to heat, due to the resistance in the coax. If you instead had 50 feet of same coax, you would be radiating 52.926 Watts. So about half of your power would be lost in your antenna system.

    If you instead used a lesser coax, like RG-8x, your losses due to SWR will be higher and you would only be outputting 14.368 Watts. That kind of highlights the need for a good quality coax at VHF/UHF frequencies. The effect of the coax isn't as pronounced in the HF bands, but it is still there.

    When I lived in Germany, operating as DA2EU, I had a makeshift antenna. And the SWR was in the range of 10:1. But I didn't have a tuner. However, my transmitter was a hybrid (Heathkit HX-1681) and had a tube final, and I mostly operated on 15 Meters. But my radio room was on the second floor about 20 feet from the antenna feed point. So the 10:1 SWR didn't bother me. I made hundreds of contacts all over Europe, Asia, and Africa.
    Martin, K7MEM
    http://www.k7mem.com
    Ash Fork, AZ - 60 miles from the Grand Canyon on Rt-66. Elevation 5,300 ft.

  5. #5

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    Okay, so what I’m getting is that there are multiple reflections, not just one, and they form a series that is technically infinite but in real terms converges quickly. Since reflections are perfect and transmission is lossy, you’d want a high damping factor so that the series converges as quickly as possible, resulting in fewer high-energy trips down the lossy cable. (I assume nobody cares about the tail-end low-energy reflections that lose only microwatts per trip.)

    Okay, that all makes sense. I appreciate that.

    It leads to another question, though. Do these reflections smear the impulse response of the transmitter out over time? It seems like they’d have to. One very short blip of transmitter power would produce a series of diminishing echoes in the radiated signal. Yes? The length of the smear would depend on the number of reflections and the length of the cable. Does this cause a problem with a fancy name I haven’t learned yet, or is it always lost in the phase distortions caused by all the filters I n a transmitter?

    Shalom,
    Dan

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