RadioBanter - View Single Post

November 11th 03, 08:19 PM

On 11 Nov 2003 19:03:32 GMT, ojunk (G. Doughty)
wrote:

sorry. meter is between icom and tuner. meter is an rs model so not so sure
how accurate it may be. It does allow for calibration though.
73
G. Doughty

Hi OM,

Calibration is simple enough. Load into a good dummy load and observe
any residual SWR. Residual SWR is either the product of poor
connections, poor calibration, or a poor load.

Using a known, good wattmeter in series with the SWR meter, adjust the
drive of your source to obtain full scale deflection of the SWR meter.
Observe its power reading and compare to the known wattmeter.

If it is off, there is generally a range adjustment for correcting
that error. However, as power measurement has easily the worst chance
of being confirmed, there is very little merit (and only ego to be
served) in making any change on the basis of accuracy. You may if you
wish, but it offers every prospect of upsetting the SWR display.

If you are testing scales, it also is incumbent upon you to reverse
the SWR meter connections and confirm there is no Residual SWR in that
direction too. Few aficionados travel this path and rely wholly on
manufactured mismatches to perform to worse performance. You can also
repeat the power scaling in this direction too (be aware that it will
be a smaller full-scale swing). There may, or may not be another
range adjustment available to you.

You don't even need a second wattmeter if you simply discard the
notion of absolute "accuracy." You can use the forward scale to
calibrate the reverse scale (after having reversed the input/outputs
too) to sufficient relative "accuracy" for most needs (like measuring
SWR).

If you had no residual SWR, and did make an adjustment, you should
then repeat the step above to confirm no residual SWR has crept into
the mix.

Needless to say, there are a variety of meter designs. Some will have
up to 3 adjustments: one for each range (forward/reverse) and one for
overall gain (actually a variable cap to adjust for the meter's
frequency characteristics, usually at the high end of its service
range). No calibration is done with three tweaks. You will have to
repeat the steps above to drive out the interactions of each
adjustment with the other. You will have to repeat all the steps
above on each band. You will encounter "some" error somewhere (you
WILL measure Residual SWR). What this means is that you will have to
spread that error such that all operating scenarios will contain some,
but not all of it. On the other hand, if you use a wide range meter
on one or two neighboring bands alone, you can indeed drive out the
error.

Focus on the meter with the most readable scale. The Radio Shack
meter's I've seen have fairly good 'tronics even in those specifically
for the low end, CB market. What you should look for is complete
shielding, bypassed meter wiring, and robust connectors. You can even
replace their resistors with small Cermet trimmers (choose a
replacement value of 80% to 120% of original value).

73's
Richard Clark, KB7QHC