[QCWA] Baluns part 8

[William Pasternak]

As I recall, the Hi-Par 3 and 5 element 6 meter yagi's from the late 
1950's through mid 1960's that had folded dipole driven elements used 
the matching principle you just described, albeit many hams that had 
them simply used a 4:1 coax balum and ran 75 ohm feedline which was 
cheaper than RG-8 (or even RG-58) and easier to handle.  Others I 
knew fed the antenna with 300 ohm balanced TV twin-lead and put a 
"matching network" into a Bud Mini-Box inside their shack with a 
short run of 75 ohm coax between the radio and the matching 
network.  No problem for the transmitters of that era which used tube 
final amps with a wide-range Pi-Network tank circuit such as my own 
Hallicrafters HT-40, Clegg 99er, Gonset Communicator III and Swan 
250C. When we went to gear that had fixed 50 ohm output impedance, 
all the foregoing went away.  -- de WA6ITF




At 06:59 PM 8/17/2012, Jeffrey D Angus wrote:
>Other values.
>
>"But I want to match 300 ohm twinlead to 50 ohm coax."
>
>Ok, that's easy enough, 6:1 ratio. You're NOT going to
>do that with a choke balun. That is limited to 1:1, 4:1,
>9:1 etc.
>
>A 6:1 impedance transform is going to require a 2.45:1
>turns ratio on a voltage balun.  A reasonable method to
>accomplish this is to use two cores. First a voltage type
>to go from 50 to 300 ohms, then a 300 ohm 1:1 choke
>balun to make sure the antenna currents are balanced.
>
>You'll notice I seem partial to choke type baluns. There
>is a reason for this. They force the antenna current to
>be the same on both sides of the feed.
>
>This eliminates a majority of problems with stray RF
>due to the feed lines radiating.
>
>This is even more important when you have issues
>with a good solid RF ground right at the transmitter.
>(In my case, I'm up on the second floor and 20 some
>odd feet from ground such as it is.
>
>Jeff-1.0
>wa6fwi
>
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