The Bird should be appreciated for what it is - a very handy piece of
portable test equipment. What is good about it is that it is repeatable.
With repeatability, you can calibrate each particular unit and particular
slug to much greater accuracy.
It is important to remember that the Bird is a package of compromises. We
talk about a slug that goes from 2-30 MHz. However, that does not mean that
it quits operating at exactly 2 and 30 MHz. Those points are only the
factory specified points where the inaccuracy is no longer acceptable.
Although I have not run a frequency curve on one, I expect that the
inaccuracy gets greater as you approach the limits and more accurate as you
approach one or more points closer to the middle. And, in the days when I
could not afford as many slugs as I have now, I often used slugs on totally
different frequencies - simply by calibtating them on the different
frequency. They still present a 50 ohm load and still measure forward and
reflected power. It is only the absolute values that are off.
Assuming an individual Bird and slug was calibrated right on the button at a
particular frequency, all that calibration goes out the window if the load
is not the specified 50 ohms (or was it 52 ohms).
Home calibration is not that difficult. For example, we can use our
calibrated signal generator to provide a reference point, then use precision
dividers (compensated for frequency of course) to measure the voltage. A
calibrated signal generator and a scope can also be used for reasonably
precise calibration - then make up a calibration chart for each Bird element
on any desired frequency.
This is one of the great things about ham radio, that we do not need digital
readout. My favorite receiver is the old HRO, with the frequency readout on
a graph. We can calibrate it much more closely and with a lot of fun work,
it will be as accurate as a 1960 receiver.
In the process of adapting to instruments like the Bird, we incidently learn
a lot.
73, Colin K7FM
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