[QCWA] Baluns Part 8 (Mark McGilvray)

[Mark]

I have use 300 ohm ladder line 50ft
length into a W9INN BALUN BOX
MODEL MFJ-912 with a 6ft RG-213
Coax and it works great.

73
Mark/k5mrk



----- Original Message ----- 
From: <qcwa-request at mailman.qth.net>
To: <qcwa at mailman.qth.net>
Sent: Saturday, August 18, 2012 11:00 AM
Subject: QCWA Digest, Vol 100, Issue 11


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> Today's Topics:
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>   1. Baluns part 8 (Jeffrey D Angus)
>   2. Re: Baluns part 8 (William Pasternak)
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> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Fri, 17 Aug 2012 20:59:01 -0500
> From: Jeffrey D Angus <jdangus at att.net>
> Subject: [QCWA] Baluns part 8
> To: qcwa at mailman.qth.net
> Message-ID: <502EF6E5.1040009 at att.net>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> 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
> 
> 
> 
> ------------------------------
> 
> Message: 2
> Date: Fri, 17 Aug 2012 22:18:44 -0700
> From: William Pasternak <newsline at ix.netcom.com>
> Subject: Re: [QCWA] Baluns part 8
> To: Jeffrey D Angus <jdangus at att.net>,qcwa at mailman.qth.net
> Message-ID: <E1T2bQm-0006cw-Vs at elasmtp-spurfowl.atl.sa.earthlink.net>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
> 
> 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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> End of QCWA Digest, Vol 100, Issue 11
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