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@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@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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>
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