That sounds like you can use an old Drake, Johnson, B&W, etc LPF right at
the socket and replace the fixed output cap with a trimmer for a perfect
match.
Carl
KM1H
----- Original Message -----
From: "Vic Rosenthal K2VCO" <k2vco.vic@gmail.com>
To: <amps@contesting.com>
Sent: Friday, March 28, 2014 2:00 PM
Subject: Re: [Amps] 8877 input Z
You can design the input circuit as a pi-section lowpass filter with a
cutoff above the highest band (I used 32 MHz). You can use standard tables
for a 50-ohm input/output impedance because 54 ohms is close enough. Then
subtract 42 pf from the computed value of the output capacitor.
This will result in an input circuit that will work on all bands without
switching.
On 3/28/2014 10:54 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
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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