What you need are TWO Birds. Running them cascaded you will discover
that the input Z of a Bird feeding a matched load isn't necessarily a
matched load. E.g. the Bird changes the impedance seen. To minimize
that, Bird in their manuals and catalog, recommends that a cable be used
with the Bird to make the total added line length any integral multiple
of a half wave in coax. So what you are seeing might not be faulty cable
construction, but rather just a poor length of cable. These impedance
changing effects are greater the more sensitive the slug because the
more sensitive slugs are more tightly coupled to the main transmission
line.
The input Z of a Bird 43 feeding a matched load is also a matched load,
as long as we are talking about a 50 ohm impedance system. The input Z
of a Bird 43 feeding an unmatched load is an unmatched load, and that
load is not the same Z as without the Bird in line. Just like any added
length of feed line when there is significant SWR on the line, the Z
changes along the line.
It is true that Bird recommends using a specific cable length, depending
on the frequency you are using the instrument on, which "When added to
Bird 43 Thruline to Equal 1/2 Wavelength". Their chart for determining
the length of cable to use covers zero to 500 MHz, but only has data
above about 110 MHz where the length to be used is 35 inches. The idea
is that, when adding or removing a 1/2 wavelength of line, the SWR
conditions, and what the source (transmitter or amplifier) "sees" will
be the same either with or without the meter and jumper cable in line.
If the SWR is pretty good, that is very little reflected power, then it
doesn't much matter. And for the HF bands the five inches of line length
added by the Bird 43 is pretty insignificant.
I keep a set of cables with my Bird 43, for use on VHF and UHF. I have
never made or used a special cable for ten meters or any of the HF
bands, and I don't think it is necessary.
DE N6KB
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