On this business of resonating an inductive dummy
load, I decided to put some numbers into the thing
so we can sort the wheat from the chaff.
First, I am sure that we are all in agreement that if
the inductance of the dummy load is really low then
the bandwidth of it will be really great. Duh!
I got out my network design program (Elsie) and
set up the discussion, using a frequency of 1 MHz
and a system impedance of 50 ohms.
Then I added in 1 microhenry in series with the 50
ohm load and added a capacitor in series with it
to bring the thing to resonance. With 1 uH it took
.02533 or so microfarads to resonate. And the
return loss was 20 dB or better from 500 kHz
to 2000 kHz.
In a 50 kW load I wouldn't be surprised to see
one microhenry of inductance with all the interconnecting
wires and so on. For a ham rig (smaller physical size
load) it might very well be less.
BUT - if I run the inductance on up to 10 microhenries
and so ran the capacitance down to 2533 pF, then
the bandwidth shrunk a whole lot: 20 dB return loss
from 920 to 1080 kHz. And using wirewound
resistors might easily produce 10 uH inductors.
Bottom line in my mind is that we reallly have to
find out what that load resistor's inductance is in
order to calmly and rationally talk about bandwidth.
- Jim WB6BLD
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