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Re: [Amps] Asynchronous coax matching transformer

To: Carl <km1h@jeremy.mv.com>
Subject: Re: [Amps] Asynchronous coax matching transformer
From: Larry Benko <xxw0qe@comcast.net>
Date: Sat, 01 Aug 2009 18:59:45 -0600
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Carl,

I asked myself what a mated PL-259/SO-239 connector "really" looked like 
RF wise many times after hearing all kinds of horror stories.  Finally 
after procuring a couple network analyzers and the understanding of how 
to make the measurements I decided to make a bunch of measurements with 
many different connectors.  Just about every measurement produced 
slightly different results but after averaging them I came to the 
conclusion that a mated UHF M-F connection was a piece of 30 ohm 
transmission line that was 4.2 electrical degrees long at 100 MHz.  
Using those numbers would equate to a 222 MHz return loss of 21.2dB 
(1:19:1 SWR) which is pretty close to your experience.

73,
Larry, W0QE

Carl wrote:
> UHF connectors have varying effects but for all pratical purposes show a 
> 20dB or better return loss thru 222 mHz before climbing. Many versions do 
> show RF heating due to poor quality insulation, even at HF.
>
> I refuse to buy imports and search out Amphenol, Kings, and other mil spec 
> marked ones at hamfests, sometimes attached to a piece of junk coax.
>
> Carl
> KM1H
>
>
> ----- Original Message ----- 
> From: "Joe Subich, W4TV" <lists@subich.com>
> To: <TexasRF@aol.com>; <gm3sek@ifwtech.co.uk>; <amps@contesting.com>
> Sent: Saturday, August 01, 2009 4:35 PM
> Subject: Re: [Amps] Asynchronous coax matching transformer
>
>
>   
>>     
>>> If the sections are joined via coax connectors, you have to
>>> take into account where 75 ohms ends and 50 ohms begins. The
>>> connectors are invariably of a 50 ohm type and their internal
>>> length must be added to the 50 ohm part of the matching section.
>>>       
>> It depends on the type of connector.  UHF connectors are often
>> closer to 35 Ohms if I recall correctly.  N connectors can be
>> either 50 or 75 Ohms ... if using a barrel for splicing, it is
>> probably better to use a 75 Ohm barrel with a 75 Ohm center pin
>> on the 50 Ohm cable (removing one stand of the center conductor
>> if necessary).  If using a male/female connector pair then one
>> would use 50 Ohm connectors and "build up" the center conductor
>> of the 75 Ohm cable.
>>
>> For HF work - even through six meters - the connector length
>> is less than 1 degree and can probably be ignored.
>>
>> 73,
>>
>>   ... Joe, W4TV
>>
>>
>>
>>
>>
>>     
>>> -----Original Message-----
>>> From: amps-bounces@contesting.com
>>> [mailto:amps-bounces@contesting.com] On Behalf Of TexasRF@aol.com
>>> Sent: Saturday, August 01, 2009 1:38 PM
>>> To: xxw0qe@comcast.net; gm3sek@ifwtech.co.uk
>>> Cc: amps@contesting.com
>>> Subject: Re: [Amps] Asynchronous coax matching transformer
>>>
>>>
>>>
>>> Hi all,in addition to the examples Larry offered, the scheme
>>> will also
>>> match impedances less than 50 ohms, down to 22.2 ohms if I
>>> remember  correctly.
>>>
>>> A two way power divider where two 50 ohm loads are in
>>> parallel requires a
>>> 25 to 50 ohm transformer which is easily handled with the
>>> series matching
>>> method.
>>>
>>> The only down side is the difficulty in adjusting the
>>> matching sections. On
>>>  the other hand, transmission line calculations via Smith
>>> Chart or computer
>>>  program will give the exact lengths in degrees needed so
>>> tuning is not a
>>> real  requirement anyway.
>>>
>>> If the sections are joined via coax connectors, you have to
>>> take into
>>> account where 75 ohms ends and 50 ohms begins. The connectors
>>> are invariably of
>>> a 50 ohm type and their internal length must be added to the
>>> 50 ohm part of
>>> the  matching section.
>>>
>>> The last issue is velocity factor of the coax and connectors.
>>> A serious
>>> builder will be prepared to confirm these quantities before
>>> cutting the
>>> matching  sections to length.
>>>
>>> It is a real joy to go through the numbers and have the
>>> transformer turn
>>> out right on frequency.
>>>
>>> 73,
>>> Gerald K5GW
>>>
>>>
>>>
>>>
>>>
>>> In a message dated 8/1/2009 10:59:13 A.M. Central Daylight Time,
>>> xxw0qe@comcast.net writes:
>>>
>>> Hi Ian  & Carl,
>>>
>>> Actually given only 50 ohm and 75 ohm coax and not
>>> paralleling any coax
>>> you can match all resistive impedances between 50 and  112.5
>>> (75*75/50)
>>> ohms.
>>>
>>> e.g.
>>> 50 ohms ---- 29.3 deg. 75 ohm coax -----  29.3 deg 50 ohm
>>> coax -----  75
>>> ohms (as shown by the 1/12 wave  match)
>>> 50 ohms ---- 40.5 deg. 75 ohm coax ----- 23.8 deg 50 ohm coax
>>>  -----  85
>>> ohms
>>> 50 ohms ---- 58.3 deg. 75 ohm coax ----- 14.9 deg 50  ohm
>>> coax ----- 100
>>> ohms
>>> 50 ohms ---- 90.0 deg. 75 ohm coax -----  0.0  deg 50 ohm coax ----- 
>>> 112.5 ohms (std 1/4 wave match)
>>>
>>> Also many  reactive impedances are also easily matched
>>> e.g.
>>> 50 ohms ---- 42.6 deg.  75 ohm coax ----- 10.3 deg 50 ohm
>>> coax ----- 75 -
>>> j20 ohms
>>>
>>> For a  matching over a small range of frequencies
>>> transmission lines work
>>> well.
>>>
>>> 73,
>>> Larry, W0QE
>>>
>>>
>>> Ian White GM3SEK wrote:
>>>       
>>>>  Carl wrote:
>>>>
>>>>         
>>>>> Does anyone remember the formula  for matching 50 to 75
>>>>>           
>>> Ohms with 2
>>>       
>>>>> very short lengths of  coax? If I remember it used a 75
>>>>>           
>>> Ohm section
>>>       
>>>>> at the output of the  amp, and a
>>>>>           
>>> 50
>>>       
>>>>> Ohm section to the CATV hardline. Frequency  dependent of course.
>>>>>
>>>>> Its been about 25 years since I  built one.
>>>>>
>>>>>
>>>>>           
>>>>  http://www.tuc.nrao.edu/~demerson/twelfth/twelfth.htm
>>>>
>>>> By  paralleling two lengths of 50 or 75 ohm line, it can
>>>>         
>>> also match to
>>>       
>>>> 25
>>>>  or 37.5 ohms; or to even lower impedances by paralleling
>>>>         
>>> three lengths
>>>       
>>>> of the same line.
>>>>
>>>>
>>>>
>>>>         
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