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[AMPS] 50:75 ohm matching

To: <amps@contesting.com>
Subject: [AMPS] 50:75 ohm matching
From: km1h@juno.com (km1h@juno.com)
Date: Thu, 24 Dec 1998 17:25:12 -0500


On Thu, 24 Dec 1998 18:58:37 +0200 "John Fielding"
<johnf@futurenet.co.za> writes:
>
>I asked him why he had done that and he replied "Well you see the 
>antennas
>are actually designed for 60 ohms and so they work equally well on 50 
>or 75
>ohms".


I wrote an article in CQ several years ago and contacted all the larger
antenna and rig manufacturers and received similar answers.


>
>Later I discovered that in Germany the standard impedance for many 
>years
>(maybe still is?) was 60 ohms and I saw some old test equipment at a 
>German
>amateur flea-market which had 60 ohm Dezifix connectors.
>
>There may be a moral to this story, design your antennas for 60 ohm
>impedance!
>
>Incidentally the 75 ohm CATV cable is in my opinion the best for the 
>price,
>75 ohms has the lowest loss per unit length, whereas 50 ohm cable 
>came
>about as a quirk of chance during WW2 as a convenient way of making 
>low
>loss solid transmission line using stock size copper tube.  I read an
>interesting article in an old American magazine as to how it came 
>about,
>maybe someone has a copy lying around?

The way I heard it is that 25 Ohms is best for power transfer, 75 for low
loss and 50 was the compromise.
RG-8 came out sometime during WW2 as a result.

73  Carl  KM1H



>
>John   ZS5JF
>
>----------
>> From: David A. Pruett <k8cc@ix.netcom.com>
>> To: amps@contesting.com
>> Subject: [AMPS] 50:75 ohm matching
To: <amps@contesting.com>
>> Date: 24 December 1998 06:03
>> 
>> 
>> Carl, KM1H wrote:
>> 
>> >Why on earth do that? Any rig or amp worth using will load directly 
>into
>> >75 Ohms with no problem. Not all the world has 50 Ohms as a 
>standard and
>> >the mfg designs for the variations.
>> 
>> Up until I installed my 80M four-square using a ComTek switchbox, I 
>would
>> have agreed.  My entire station (four towers and all monoband 
>antennas)
>> uses
>> CATV hardline.  For most bands I use asynchronous transformers made 
>of 50
>> and 75 ohm cable to calm any mismatch concerns.  With the 80 
>four-square,
>> I figured "why bother - its eighty meters, right?"  Well, with a 
>properly
>> installed array centered on 3650 at the end of approximately 270' of 
>1/2"
>> CATV hardline, the SWR at the shack end was near 1:1 on in the phone 
>DX
>> band
>> and 2:1 on CW.  At the load end it was near 1:1 in both places.
>> 
>> I suspect that the length of the feedline was approaching being a
>quarter-
>> wave multiple down near the CW end although I did not confirm this.
>> Installation of a pair of homebrew 50:75 toroidial transformers from 
>N8CC
>> brought the SWR to near unity on both ends of the band.
>> 
>> (Note: for those who are not familiar with the ComTek box, it uses
>quadra-
>> ture feed and a dump load to drive the array.  While the resulting 
>SWR on
>> the feedline is near unity, the normal SWR variations vs. frequency 
>are
>> masked by the drive system.  I'm NOT claiming that the 50:75 
>transformers
>> made the antenna broadbanded.)
>> 
>> In my case, the 50:75 transformers allowed the system to function
>correctly
>> rather than forcing me to "tune" the feedline (a task with somewhat
>limited
>> flexibility seeing that the feedline is buried).
>> 
>> 73,
>> 
>> Dave/K8CC
>> 
>> 
>> --
>> FAQ on WWW:               http://www.contesting.com/ampfaq.html
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>> Problems:                 owner-amps@contesting.com
>> Search:                   http://www.contesting.com/km9p/search.htm
>
>--
>FAQ on WWW:               http://www.contesting.com/ampfaq.html
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>
>

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