> 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.
> >
> >
> >
> _______________________________________________
> Amps mailing list
> Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps
>
>
>
>
> **************A Good Credit Score is 700 or Above. See yours
> in just 2 easy
> steps!
> (http://pr.atwola.com/promoclk/100126575x1222846709x1201493018
> /aol?redir=http://www.freecreditreport.com/pm/default.aspx?sc=
> 668072&hmpgID=115&bcd
> =JulystepsfooterNO115) _______________________________________________
> Amps mailing list
> Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|