I'm tuning a wire dipole.
Cross-needle SWR meter shows the same SWR on the lower end of the band as on the higher end. However, the needles move much higher on the lower end. They cross on the same SWR line so the SWR reading doesn't change.

What does the "apparently higher power output" mean in this case?
Note that:

  • The transmitter shows that it's Txing full power, not falling back.
  • "Apparent power transmitted" is higher than actual radio output (8 or 6W while radio is 4W). I understand that is the normal behaviour; the meter's manual says "do not look at the actual numbers, just read the SWR".
  • When I connect a dummy load, the meter shows SWR of 1:1 and correctly displays my radio's power output, so the meter seems to be working.

Alexander, this is very common and is in fact normal behaviour for the type of SWR meter you are using.

For these SWR meters, the relative forward and reflected power readings change if the transmission line impedance changes. Even though the static characteristic impedance of the transmission line is fixed at say 50 ohms, as soon as you start transmitting, if the SWR isn't exactly 1:1, then standing waves appear on the transmission line and this has the effect of changing the line impedance, or the dynamic impedance seen looking into the transmission line during operation.

If there are standing waves on the line due to the SWR not being perfect, then the line impedance becomes a function of the frequency you are using and a function of the length of transmission line. As you change frequency, the line impedance changes and so the relative power readings on your SWR meter change. If you move the SWR meter along the transmission line whilst leaving the frequency constant (you normally wouldn't do this), then you get the same effect and the line impedance and meter readings will change.

The SWR meter power readings are only accurate when the SWR is exactly 1:1. In this special case, the line impedance of the transmission line is equal to it's characteristic impedance, because the characteristic impedance of the transmission line is equal to the impedance of the radio and of the antenna, and so there are no standing waves on the transmission line.

Having said all that, the SWR reading shown on the meter will always be accurate, because the SWR reading is not affected by the line impedance. The SWR reading is the ratio of forward to reflected power, which is directly related to the ratio of maximum and minimum amplitudes of the standing wave voltage, and ignoring losses, these ratios are determined solely by the impedance of the radio, transmission line characteristic impedance, and antenna impedance.

Hope that all makes sense !

  • 1
    $\begingroup$ It does Andrew, thanks! I still have one practical question though... am I right in suspecting that high "apparent Tx/Refl power" means that there's a larger mismatch, and, as a result, higher amplitude of the standing wave? Which, in practice, means that I shall try to tune the antenna to LOWER the apparent Tx/Reflected power?.. $\endgroup$
    – Alexander
    Jul 5 at 22:43
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    $\begingroup$ Hi Alexander, yes, but i think that the power reading could be higher or lower than the real value depending on the frequency, so for both cases the closer you get the SWR to 1:1 the closer the power readings shown on meter will approach the real value of power. $\endgroup$
    – Andrew
    Jul 5 at 23:03
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    $\begingroup$ Lowering the SWR will lower the peak power of some location on a long transmission line, but will raise the peak power at other locations on the line. So it depends on the length of your coax. $\endgroup$
    – hotpaw2
    Jul 6 at 3:54
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    $\begingroup$ Thanks @Andrew. Pretty much what I thought... but wanted to have a second opinion and confirmation! Couldn't find it anywhere else, neither in the meter manual nor in the ARRL books! $\endgroup$
    – Alexander
    Jul 6 at 9:30
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    $\begingroup$ You say the "line impedance changes", but it's not clear if you mean the characteristic impedance of the transmission line, or the impedance seen looking into the transmitter end of the transmission line. $\endgroup$ Jul 6 at 18:13

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