[Collins] parallel amplifiers

[Dave Brown]

See below for a useful 'on line' design aid for wilkinson combiners.
 But I'm inclined to reiterate Gerrry's comments- On HF, 3 dB just 
ain't worth the effort in most cases.
 73 Dave, ZL3FJ

http://my.athenet.net/~multiplx/cgi-bin/wilk.main.cgi


----- Original Message ----- 
From: "Gerald" <geraldj at ispwest.com>
To: <collins at mailman.qth.net>
Sent: Tuesday, July 05, 2005 7:04 AM
Subject: Re: [Collins] parallel amplifiers


> On Mon, 2005-07-04 at 14:37 -0400, Drleborgne at aol.com wrote:
>> Hi: I would like to run two 30L-1  Collins linear amps in parallel 
>> to
>> increase power. Anyone in this august group would have any 
>> suggestions on how  to?
>> Thanks in advance.
>>
>>  Martin Leborgne
>
> Why? It is certainly possible, but there are virtually no HF
> communications circuits where a 3 dB increase in power will
> significantly change the quality of a signal after the vagaries of
> propagation which tend to change more than 3 dB per second. Rarely 
> does
> HF communication work at or below the noise threshold where a 3 dB 
> power
> change could make a larger difference in received signal to noise 
> ratio.
>
> Last weekend, my radio club ran two stations, one using 100 watts to
> full sized dipoles on 40 and 80, the other running 25 watts output 
> to an
> 8 foot whip fed through an L match on 20 and 15 meters. Working both 
> CW
> and phone and even though 20 was dead several hours in the night, 
> the 20
> meter station made more contacts than the 40 meter station. 80 was
> better than 15 meters, just because of propagation. 6 dB power
> difference cost -10 contacts out of about 300 total for each band 40 
> and
> 20. E.g. the 20 meter station made 10 more contacts than the 40 
> meter
> station.
>
> Besides its a lot of work to parallel amplifiers right and doing it
> wrong can lead to much amplifier carnage.
>
> What it takes is a pair of power splitters/combiners, and a couple 
> dummy
> loads. These splitters have to match but can be in phase, out of 
> phase,
> or 90 degree (quadrature). You have to match the gain and phase 
> shift of
> the amplifiers so that the sum in maximized in the output splitter
> (which also may have a difference port with dummy load) and the
> difference minimized. You can use a ratrace hybrid made of coax, but 
> it
> has the same problems of limited frequency range and the need for 
> the
> difference loads and their metering. The output difference load has 
> to
> survive half the power of the individual amplifiers when only one is
> working. Its a pain, so its not a common technique with tuned 
> amplifiers
> (using tubes) but is used to combine solid state modules for higher
> power but a couple KW gets into just a few makers.
>
> If you just can't live with less power than two 30L-1, swap them for 
> a
> Henry 2K-3 or a 30S-1. Or a 208U-3 or 3A (www.fairradio.com) and a 
> three
> phase power line to the hamshack and be sure to be cited by the FCC 
> for
> running excess power.
>
> Using good coax to a decent antenna can make up power faster than
> running lots of RF out of the PA.
>
> One of the phenomena I noticed early Sunday morning last week that a
> number of home stations had strong signals, but couldn't copy any of 
> the
> 100 watt FD stations in the pileups they caused. Because they were
> alligators, and couldn't hear "normal" stations in their local 
> noise. I
> worked out to look for the weaker signals, which answered my calls 
> on
> the first try so I added many more stations to the log than by 
> trying to
> work the home stations with big amplifiers.
> -- 
> 73, Jerry, K0CQ, Technical Advisor to the CRA
> All content copyright Dr. Gerald N. Johnson, electrical engineer
>
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