[Amps] CB amp directly coupled via cap to antenna

Sun Apr 11 07:44:09 PDT 2010

Bill, I think you missed part of the discussion:we were not talking about  
tuned circuits.

We were talking about an amplifier with no tank circuit being coupled  
directly (no transmission line) to an antenna. Normally when we think about  
amplifier tank circuit Q, we do not include any part of the antenna R +j;  
considering it to be 50 +j0.

In the case under discussion someone mentioned a Q of 1 and my reply was to 
 counter that statement.

This is all hypothetical of coarse as there is always some C across the  
tube plate to cathode plus any stray C lurking around. But, in the 
hypothetical  case of no tank circuit, there is infinite reactance, other than what 
might  be present due to the plate rf choke, which we will ignore.

The discussion went on to state that the antenna would be resonated at the  
operating frequency. Antenna resonance is accepted to R +j0 customarily.

In the hypothetical case, the parallel circuit is infinite reactance  
shunted by the antenna resistance. Q =  R/infinity is a Q of zero. If we  
substituted a dummy load of R=antenna there would still be infinite tank  reactance 
and a Q of zero.

In this hypothetical case, I stand by my suggestion that Q =   zero.

73,
Gerald K5GW
\

In a message dated 4/11/2010 2:04:25 A.M. Central Daylight Time,  
wlfuqu00 at uky.edu writes:

Sorry, I  clicked on the wrong email.
I was responding to Gerald's (K5GW) email where  he was in error about 
resonance and Q. Also, most of us are familiar with R,X  and Q and also 
Bandwidth, Center Freq. and Q. But there is another  relationship which has to do 
with the energy stored in a resonant system  relative to the energy flowing 
through it during each cycle.   
Q is proportional to the stored energy in the tuned circuit  divided by the 
energy going into it on each cycle. That is why high Q circuits  get hot or 
have large wire sizes etc. Also, that is why the in the old days a  tuned 
circuit was often called a "tank" circuit. It  actually stored  oscillating 
energy like a tank.  I think some of the SKs from many years  back understood 
resonance better that today's engineers.

73
Bill  wa4lav

________________________________________
From: Roger  [sub1 at rogerhalstead.com]
Sent: Saturday, April 10, 2010 3:57 AM
To:  Fuqua, Bill L
Cc: TexasRF at aol.com; dezrat1242 at yahoo.com;  amps at contesting.com
Subject: Re: [Amps] CB amp directly coupled via cap to  antenna

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 at 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 at yahoo.com writes:
>>
>> ORIGINAL   MESSAGE:
>>
>> On Thu, 08 Apr 2010 17:09:08 -0400, Ron   Youvan
>> <ka4inm at 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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