[AMRadio] Low Power AM?


rbethman rbethman at comcast.net
Mon Nov 24 12:05:04 EST 2008


This does not cover the question of matching OUTSIDE the limits of the 
impedance "SPECIFIED" 50 to 75 ohm antenna system.

This isn't a TRICK question!

I said it very succinctly.  You WILL need SOMETHING outside the radio to 
achieve resonance.  Otherwise your VSWR is going to be WAY out of whack 
and plate current/plate power displacement WILL be way out of parameters.

Hence antenna tuners, antenna matching devices at their base, and etc.

Bob N0DGN

sbjohnston at aol.com wrote:
>
> Yes, of course you can reduce the loading and redip the tuning to get 
> lower output.   You can also choose various settings of the loading 
> capacitor to match various load impedances for higher output power.   
> Another way to think of the loading adjustment is to imagine if varies 
> how much of the load is coupled to the finals - how much the are "loaded"
>
> If there was only one correct value for the loading capacitor, why 
> would anyone ever spend the money for a variable?   (Cheap rigs don't 
> - but they are stuck matching to a fixed output Z and degree of loading.
>
> Steve WD8DAS
>
> sbjohnston at aol.com
> http://www.wd8das.net/
> -----------------------------------------------------
> Radio is your best entertainment value.
> -----------------------------------------------------
>
>
> -----Original Message-----
> From: rbethman <rbethman at comcast.net>
> To: Discussion of AM Radio in the Amateur Service 
> <amradio at mailman.qth.net>
> Sent: Sun, 23 Nov 2008 5:57 pm
> Subject: Re: [AMRadio] Low Power AM?
>
>
>
>
>
>
>
>
>
> Jim,
>
>
> I addressed this earlier with my statement that when doing this
> "reduction of antenna loading" that THIS puts things into a NON-RESONANT
> state.
>
>
> The only way to do this AND stay resonant would be SOMETHING outside the
> radio that DOES resonate.
>
>
> The 35 ohm antenna you refer to is typical of certain antennas.  That is
> WHY they have matching or loading devices that USUALLY go with=2
> 0them.
>
>
> It is also why 1/2 wave dipoles have a VSWR RANGE where it works - AND
> frequencies where it will NOT
>
>
> Or am I wholly insane?
>
>
> Bob - N0DGN
>
>
> Jim Wilhite wrote:
>
>> Ok I will ask the stupid question(s).  What is the purpose of a 
> output
>> (Pi, Balanced) network?  Is it not to transform the plate load
>> impedance (everyone knows how to calculate this right?) to whatever
>> the antenna presents to the output of the transmitter.  Is not the
>> rated power calculated at 52 or 72 ohms?
>
>>
>
>> So if you have a final that is looking into 35 ohms (say a vertical
>> antenna) and you dip the plate, what happens?  Do you see 50 ohms
>> anywhere?  Is the system matched?  Just what is happening?  What is
>> the efficiency and HARMONIC suppression at this particular setting?
>
>>
>
>> Anyone explain this to me?
>
>>
>
>> Jim/W5JO
>
>>
>
>>
>
>>
>
>>
>
>>
>
>>> I really did not say to maintain the dip with the load capacitor.
>
>>>
>
>>> When I tune the final of any Pi-network Tx I first set the dip with 
> the
>
>>> PLATE capacitor. Then I adjust the coupling with the LOAD capacitor.
>>> And I
>
>>> always recheck the dip with the PLATE capacitor after making any
>>> adjustment
>
>>> to the LOAD capacitor.
>
>>>
>
>>> And when I am done I DO RECHECK THE GRID DRIVE!
>
>>
>
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>
> -- 
> Bob - NØDGN
>
>
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-- 
Bob - NØDGN



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