[AMPS] RF chokes -- My 2uH worth...

[km1h@juno.com]

Marv, I save very few messages but this one is a keeper. I have heard
mention of that circuit but this is the first real info....tnx for
sharing it.

73  Carl   KM1H




On Thu, 29 Oct 1998 20:59:54 EST wc6w@juno.com (Radio WC6W) writes:
>
>Hi all,    I've been following the choke discussion and I would just 
>like to share a technique which I observed in a 2-36 MHz Hughes 
>Aircraft airborne
>kilowatt amplifier that was designed circa 1958.
>  The fixed portion of the pi coil is fabricated from what appears to 
>be a section RG-402.  The center conductor is connected directly to 
>the plate of the tube and the plate blocking capacitor is connected in 
>"conventional" fashion between the plate and the outer conductor of 
>the semi-rigid cable plus the plate tuning capacitor.  The other end 
>of the center conductor that emerges where this coil is connected to 
>the bandswitch and the lower frequency coils is connected to the HV 
>supply through a rather ordinary looking 1mHy choke of sufficient 
>current
>capability.
>  When this arrangement is operating at it's highest frequency, the 
>effective impedance presented at the top of the choke is only 50 
>ohms!!!!
>  
>  Of course, as more coils are switched in the impedance level rises 
>but, since the operating frequency is simultaneously decreasing, the 
>applied stress on the choke probably remains somewhat constant.   An 
>amplifier employing a pi-L output network would raise the impedance 
>level somewhat though the stress on the choke would still be greatly 
>lessened as
>compared to the conventional connection.
>     RG-402 is rated for 5KV or thereabouts DC, so this technique 
>would be applicable to a legal limit amplifier.   I suppose the method 
>could be extended to higher power levels by employing RG-401 (7.5KV) 
>or if that isn't available, just running some high voltage / high 
>temperature wire
>through a piece of tubing before fabricating the coil.
>  I've never seen this exact circuit employed in any amateur equipment
>nor even described in the literature.
>  There was a similar concept presented in June '78 Ham Radio, p.98, 
>which suggested placing the plate choke after the pi coil and 
>relocating the RF coupling cap to that point.  This would keep the 
>impedance at the top of the choke low and constant over the entire 
>operating range in exchange for increasing the current through the 
>coupling cap and doubling the required voltage rating for the plate 
>tuning capacitor.  These are
>likely uneconomic tradeoffs!
>
>73,
>  Marv  WC6W
>
>
>
>
>*  ___________________________________________________________________ 
>You don't need to buy Internet access to use free Internet e-mail. Get 
>completely free e-mail from Juno at http://www.juno.com/getjuno.html 
>or call Juno at (800) 654-JUNO [654-5866]  -- FAQ on WWW:              
> http://www.contesting.com/ampfaq.html Submissions:              
>amps@contesting.com Administrative requests:  
>amps-REQUEST@contesting.com Problems:                 
>owner-amps@contesting.com Search:                   
>http://www.contesting.com/km9p/search.htm   

___________________________________________________________________
You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at http://www.juno.com/getjuno.html
or call Juno at (800) 654-JUNO [654-5866]

--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm