[Amps] 8877 input Z

[TexasRF at aol.com]

Heck you could just use a .57 uH inductance in parallel with a variable C  
(or switched fixed C) adjusted to resonate on the band in use. About 57pF 
(less  42pF) at 28MHz up to about 883 pF (less 42pF) at 1.8 MHz.
 
Q=.5 at 28 MHz, up to 8.6 at 1.8 MHz.
 
But wait! If a proper L is connected across the band selected C we can  
maintain the Q at 3 or higher. Sounds kind of familiar, huh?
 
73,
Gerald K5GW
 
 
 
 
 
In a message dated 3/28/2014 2:42:23 P.M. Central Daylight Time,  
km1h at jeremy.mv.com writes:

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 at gmail.com>
To:  <amps at 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 at 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 at 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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