If the load is 50 ohms (using 50 just for example purposes) in parallel
with 42pF a series inductance matches well enough that a given
inductance will be good for more than 1 band. However if you add
another 42pF shunt capacitor from the exciter side of the inductor and
change the inductance to about 91nH the circuit will match from 1.8 to
30MHz with a maximum exciter SWR of 1.15:1 with NO switching required at
all. If you want it even lower a combination low pass and high pass "L"
network can be done (only 1 more component).
73,
Larry, W0QE
On 3/28/2014 11:54 AM, TexasRF@aol.com wrote:
Hardy, the 54 ohms is a nominal figure that will vary with rf drive level.
Certainly close enough for designing input networks. Higher drive levels
will result in a lower input impedance; lower drive levels will result in a
higher input impedance.
The 42 pF is the capacitance between the cathode and grid primarily. So,
you will need to provide an inductance to cancel that out. The inductance
will be different for each band of course.
73,
Gerald K5GW
In a message dated 3/27/2014 7:55:49 A.M. Pacific Daylight Time,
n7rt@cox.net writes:
I am looking for the input impedance of an 8877. I found one source and
that was K6DC's (SK) article years ago that said it was 54 ohms with no
frequency dependency or reactive component. The input C is 42 pF according to
the data sheet. So I will assume I can use 54 ohms in parallel with 42 pF.
Anyone have anything different?
73 Hardy N7RT
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