The half waves of coax rule for measuring VSWR accurately

Lots of hams recite a rule that accurate measurement of VSWR can only be made at the feed point or an integral number of electrical half waves from the feed point.

It is one of those ‘rules’ that the proponents cannot usually explain… they would regard themselves as experts, but blindly follow folk-lore that they do not understand.

I have asked many where exactly the electrical half wave should be measured, antenna feed point to:

  • VSWR meter output connector;
  • middle of the VSWR meter;
  • VSWR meter input connector; or
  • somewhere else.

Note that a VSWR meter may be a significant electrical length,  a Bird 43 is more than 60° at 440Mhz.

The answers usually reveal that they haven’t a clue, they are blind followers of folk-lore… and that is the stuff of which ham radio is made.

The possible origins

When an inline power meter is used to adjust the output tuning of a narrowband tuned PA, the tuning is very dependent on the load presented to the PA, and in turn, if there are standing waves, that depends on the length of cable from load to PA.

The impedance seen looking into a transmission line with standing waves varies with displacement from the load.

If you need to temporarily insert transmission line to make this measurement, you will get the best results by inserting exactly an electrical half wave of line (including the VSWR meter) so that the load impedance seen by the PA is similar with the instrument in line as without, and so that having tuned it with the instrument in line, the settings are most appropriate to the line length load to PA when the instrument is removed.

Bird gives this advice in their manual for the Bird 43:

Load Matching
When a Bird 43 is used to tune a load to a transmitter and a good match is obtained, removing the unit will not change the match quality. A good 50 ohm load can terminate a 50 ohm transmission line of any length without altering conditions at the transmitter. The 43 is just an extra length of 50 ohm line in series with the measurement.
When the load is not well matched (an antenna with a VSWR of 1.5 or 2.0) the line length between the load and the transmitter will transform the load impedance as seen at the transmitter. Removing the wattmeter shortens the total line length by four inches plus two connectors. This is still not significant at low frequencies where five inches is a small fraction of a wavelength, but at higher frequencies the frequency or power output of the transmitter may be affected.
Transmission line theory shows that if the line length changes by exactly 1⁄2 wavelength, the impedance is unchanged. To have identical match with the wattmeter in or out of the circuit, insert or remove 1⁄2 wavelength of line (including the unit). To do this, use a length of cable which, when added to the unit, equals a 1⁄2 wavelength at the frequency of interest. If multiple frequencies are needed, a separate cable length is required for each. See Figure 11 for sample cable lengths.
NOTE: Cable length is measured from end to end of the connector’s outer conductor, except for UHF or mini-UHF plugs where the length is measured from tip to tip of the center pins.

NOTE: Dimensions shown are for solid polyethylene cable like
RG-58C/U or RG-8/U, which have a velocity of propagation 66% of that of air. If RG-58 or RG-8 type cables containing foam polyethylene (velocity of propagation of 79%) are used, the dimensions in the graph must be multiplied by the ratio of the relative velocities; 79% ÷ 66% = 1.2 in this case.

Readers will note that the length recommended extends the line by an electrical half wave in total, it has nothing to do with the location of the Bird element, nor is it the actual transmission line on one side of the Bird.

Whilst this practice was relevant to PAs like valve transmitters with medium Q push pull tanks, or pi couplers, it is rarely needed with modern transmitters with broader output stages

Nowhere in that manual does Bird suggest that the VSWR measurement must be made at the feed point or integral half waves from the feed point.

The failure of the rule

The rule probably has its roots in the specific application to tuning a narrowband output stage in older generation equipment, and application to VSWR measurement or power measurement is nonsense.

In the absence of understanding, people learn rote rules, and follow them without understanding. They then share these ‘secrets of success’ with gullible new learners who accept them without reasoning as it makes them sound like pros when they can sprout the BS.

Few hams understand how directional wattmeters (the basis of many VSWR meters) work and their correct application, and this gives rise to all sorts of nonsense rules… subjects for posts another day.

References / links

  • Bird Electronic Corporation. 2004. RF directional thruline wattmeter model 43 – instruction book. Cleveland: Bird Electronic Corporation.