Factual argument and feed-line analysis software

Transmission Line + Antenna + Two SWR Meters

Author: R.J.Edwards G4FGQ © 6th July 2005

Factual SWR Argument & Supporting Software IMPORTANT: Input power to the transmission line is standardised at 100 watts. This assumes the tuner, if there is one, has been adjusted to transform line input impedance to the value of load impedance required by the transmitter. That is the sole purpose of the tuner. But, in general, the internal resistance of the transmitter is not known and that part of the system has been excluded from the program. This has no effect on behaviour of the rest of the system.

There is an SWR meter at each end of the line. Both meter readings are exactly calculated by the program including reflection coefficient indications.

As is usual, SWR readings apply to whatever is situated between the meter and the transmitter. Reflection coefficient readings apply to the impedance seen looking immediately towards the antenna. Reflection coefficient readings are therefore more meaningful, better defined, and less ambiguous than SWR readings.

Meter Zo can be selected to match line Zo or can be the standard 50 ohms. It is useful, or sometimes necessary in practice, to make measurements under nonstandard conditions. The presence of meters has no operational effect.

Note that when line length is a whole number of 1/2-wavelenghts, the line input impedance is the same as the antenna impedance provided line attenuation is not too large. Say 0.5 dB or less. This is a valuable property of 1/2-wavelength lines.

Line length, in conjunction with line Zo, can be used to transform antenna impedance to more suitable values at the transmitter end of the line. However line loss, if appreciable, complicates matters and adjustments become experimental.

When the line has a loss greater than about 15 dB, input impedance converges on line Zo regardless of length in waves. This is interesting but not of much use.

Note the lower SWR and reflection coefficient at the transmitter end of a line having any loss.

Note the increase in line loss as SWR on the line increases. To observe changes in SWR and line loss vary antenna feedpoint input resistance or reactance.

Don't forget to change Zo of the SWR meter to line Zo if needed.

Note: Notions of reflected power do not enter an analysis or even into the internal calculations. Only amps, volts and Ohms Law are involved. Notions about reflected power only serve to confuse. Keep things simple.

In the author's opinion the name "SWR meter" should be abolished:

  1. The instrument can't measure SWR. It is a fixed-ratio RF resistance bridge.
  2. Any conclusions drawn from SWR 'measurements' are ambiguous.
  3. Entirely different values of transmitter load impedance give the same SWR.
  4. The meter's usual location does not allow it to indicate SWR on any line.
  5. The line intended to be measured does not, in any case, usually exist.
  6. The only way to measure SWR is by two voltage measurements on an actual line.
  7. The line must be long enough to make two measurements 1/4-wavelength apart.
  8. SWR measurements tell you nothing about line Zo or terminating impedances.
  9. It is just a voltage ratio between two different points spaced 1/4-wave apart.
  10. The meter wastes at least half of the information available to it.
  11. The angle of the reflection coefficient is discarded in the process.
  12. So any value in the magnitude of the reflection coefficient is also lost.

The so-called SWR meter only indicates whether or not the transmitter load impedance equals its design value of 50 ohms. If not equal to 50 ohms it cannot tell you what it actually is. However, it is a valuable indicator nevertheless.

Solution: Change the meter's name to TLI. (Transmitter Loading Indicator).


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