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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