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**SWR Variations**

Discussion in 'General Technical Questions and Answers' started by W0LC, Nov 26, 2003.

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  1. W0LC

    W0LC Ham Member QRZ Page

    Here is an interesting experience.

    Ever hook your antenna up to a tuner with a cross needle meter and then read the SWR on it, and then look at the radio SWR meter and note that sometimes, the difference is quite large?

    Ever wonder what causes that (all coax is good).
     
  2. K7KBN

    K7KBN Ham Member QRZ Page

    Few, if any, built-in SWR meters will give anything more than a relative indication. Cross-needle meters are a little better, but since they are subject to inaccuracy in both the forward and reflected power meter movements, I prefer my Palomar M-840. They didn't sell too many of these, for some reason. It uses LEDs to indicate forward power and SWR, and it gives clear proof that SWR doesn't change when you increase or decrease power.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Sure, all the time.

    Lots of reasons for this, but the obvious one is that unless the line is perfectly matched (SWR = 1.0), forward and reflected power measured anywhere along the line will vary depending upon where in the line you make the measurement.

    The only important place to measure (or attempt to minimize) SWR is at the transmitter (or amplifier, if one is used) itself, not someplace else, if the reason for "tuning" is to maximize power transfer from source to line.

    Besides, the indications provided by most transceivers are inaccurate. That really doesn't matter, they don't have any rated accuracy. The only important thing is that the tuner is adjusted for minimum reflected power, or highest incident power available at the signal source (transceiver). Doesn't matter what the reading actually is, as long as the reflected reading is minimized.

    I don't own any HF transceiver whose internal metering agrees with external metering, except when the match is already perfect -- in which case, I surely don't need the tuner.

    WB2WIK/6
     
  4. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Hi to all,With reasonably low loss transmission lines and practical lengths, the VSWR, as measured anywhere on the transmission line, will remain quite constant. The reflections are established by the interface between the transmission line and the Load and interference pattern should remain the same anywhere along the line. So, Steve, WB2WIK, why would the measurements change or are you referring to changes caused by losses? (the losses will cause the VSWR to diminish as the measurement is made far from the load) 73, Pete
     
  5. W0LC

    W0LC Ham Member QRZ Page

    I generally agree with some of the comments. I use an antenna tuner primarily to keep the SWR , where applicable, to a minimum since I run an amplifier. A standalone transceiver with internal tuner is fine, but not when you run an amp.

    I appreciate the feedback!

    Interesting reading.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Hi Pete,

    Well, it doesn't.

    Measure any increment of an electrical 1/2WL back from the load and the Z will be the same as that at the load; measure any odd increment of an electrical 1/4WL back from the load and see what you measure. An open is transformed to a short, and a short to an open. In between, other transformations occur with a mismatched load, and all are calculable if you know the load impedance, line impedance and the line's electrical length.

    And, as you pointed out, as cable loss increases by mismatch, frequency and other variables, the SWR measured close to the source will always be lower than that at the load.

    WB2WIK/6
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Oops, re-read that last post and realized I forgot to mention that, "yes," the SWR remains the same number but as you rotate around the chart, the complex Z elements change, e.g., a short and an open are both SWR = infinity, but are obviously two very different impedances (zero and infinity), as the easiest example.

    The typical Monimatch coupler design* behaves differently at these extremes, and I've never seen one that worth a darn at >20dB return loss (they all indicate 1:1 at this point) or <3dB return loss (they all indicate infinity at this point), so the circuit, using common components, only has about 17dB dynamic range -- if we're lucky.

    So, it's not that the SWR changes in a lossless line (it doesn't), but the indication certainly does. And, of course, if return loss were 0 (SWR = inf), using any amount of line between the source and load will greatly improve SWR at the source (due to line loss).

    A very sticky wicket, indeed, and one that frequently results in varying measurements along a line.

    (*Used in virtually all SWR bridges, including the ones built into transceivers...)

    WB2WIK/6
     
  8. W8JI

    W8JI Ham Member QRZ Page

    </span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (WB2WIK @ Nov. 26 2003,09:18)</td></tr><tr><td id="QUOTE">Sure, all the time.

    Lots of reasons for this, but the obvious one is that unless the line is perfectly matched (SWR = 1.0), forward and reflected power measured anywhere along the line will vary depending upon where in the line you make the measurement.

    The only important place to measure (or attempt to minimize) SWR is at the transmitter (or amplifier, if one is used) itself, not someplace else, if the reason for &quot;tuning&quot; is to maximize power transfer from source to line.

    Besides, the indications provided by most transceivers are inaccurate.  That really doesn't matter, they don't have any rated accuracy.  The only important thing is that the tuner is adjusted for minimum reflected power, or highest incident power available at the signal source (transceiver).  Doesn't matter what the reading actually is, as long as the reflected reading is minimized.

    I don't own any HF transceiver whose internal metering agrees with external metering, except when the match is already perfect -- in which case, I surely don't need the tuner.

    WB2WIK/6[/QUOTE]<span id='postcolor'>
    SWR does not vary along a line unless the meter is not calibrated to the same impedance as the line impedance.

    A 50 ohm meter will change SWR readings along a 75 ohm line, a 60 ohm meter will change along a 50 ohm line, but a 50 ohm meter won't change when moved along a 50 ohm line.

    Common mode currents, in some cases, can change the tuning of an antenna when a meter is moved or a feedline perturbed. But the real cure here is to fix the antenna system.

    The most common reason for a difference between a tuner and radio is one or both meters are not calibrated correctly.

    73 Tom
     
  9. W8JI

    W8JI Ham Member QRZ Page

    </span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (w8ji @ Nov. 26 2003,16:00)</td></tr><tr><td id="QUOTE">[/QUOTE]<span id='postcolor'>
    Let me correct this a bit:

    SWR does not vary along a line unless the meter is not calibrated to the same impedance as the line impedance *and the line is mismatched or very lossy*.

    A 50 ohm meter will change SWR readings along a 75 ohm line and a 60 ohm meter will change along a 50 ohm line *if the line is mismatched or very lossy*, but a properly working 50 ohm meter won't change when moved along a 50 ohm line *no matter what the SWR*.

    Common mode currents, in some cases, can change the tuning of an antenna when a meter is moved or a feedline perturbed. But the real cure here is to fix the antenna system.

    The most common reason for a difference between a tuner and radio is one or both meters are not calibrated correctly.

    73 Tom
     
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