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Re: [Amps] CB amp directly coupled via cap to antenna

To: Bill Fuqua <wlfuqu00@uky.edu>
Subject: Re: [Amps] CB amp directly coupled via cap to antenna
From: Roger <sub1@rogerhalstead.com>
Date: Sat, 10 Apr 2010 03:57:51 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>

Bill Fuqua wrote:
> If what you say is true then no resonant circuit has  Q.
>   

Not quite.  It's true the formula is either X/R or R/X and at resonance 
Xc and Xl are equal. But the formula says to use EITHER Xc or Xl, not 
both which would be zero at resonance.

73

Roger (K8RI)
>     In series resonance Q= Xc/R  or you can use inductive reactance. they 
> are equal at resonance.
> In parallel Q=R/Xc
> 73
> Bill wa4lav
>
>
> At 09:58 PM 4/8/2010 -0400, TexasRF@aol.com wrote:
>   
>> Q=X/R in a series circuit or R/X in a parallel circuit. Since at resonance
>> there is 0 ohms X in the series circuit or infinite X in the parallel
>> circuit,  The Q would be 0 as in zero.
>>
>> No flywheel action here.
>>
>> In a real world case, say 15,000 vdc and 5A plate current, the plate load
>> impedance would be about 1700 ohms. There would probably be 100 pf from
>> plate to  cathode which is about -58 ohms at 27 MHz. So, looking into the 
>> tube,
>> there is a  parallel circuit of 1700 ohms resistive and about 58 ohms
>> capacitive. The Q is  1700/58 = 29.
>>
>> The series equal is 2 -j58. The antenna would need to look like 2 +j58 for
>> a match. That is not a very friendly number so more help is needed.
>>
>> I will leave the rest of the design work to others.
>>
>> 73,
>> Gerald K5GW
>>
>>
>>
>>
>> In a message dated 4/8/2010 5:50:05 P.M. Central Daylight Time,
>> dezrat1242@yahoo.com writes:
>>
>> ORIGINAL  MESSAGE:
>>
>> On Thu, 08 Apr 2010 17:09:08 -0400, Ron  Youvan
>> <ka4inm@tampabay.rr.com> wrote:
>>
>>     
>>>   By  adjusting the antenna length to be resonate at the exciter's
>>>       
>> frequency the  antenna
>>     
>>> is that "tank circuit," it's just lossy by the radiation  resistance.  A
>>>       
>> variable coupling
>>     
>>> capacitor is not  necessary.
>>>       
>> REPLY:
>>
>> Upon further consideration, if the antenna  impedance matched the
>> tube's plate load impedance, wouldn't the Q be just  one? I think you
>> still need the simple LC tank circuit for the flywheel  effect.
>>
>> 73, Bill  W6WRT
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>
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