[Amps] Ouch - 23 Ohms ?

[Karl-Arne Markström]

The only way I know about to find out the design impedance
of a directional coupler if you can't "reverse engineer" it, is to
terminate the main line with a variable resistance, connect a test signal
and vary the resistance until the signal coming out of the
decoupled port goes through a null, when the coupler is oriented to read
reflected power.

Then the resistor setting is = the design impedance.
There are coupler designs where the resistance of the internal terminating
resistors are unrelated to the design impedance.

73/

Karl-Arne
SM0AOM



----- Original Message ----- 
From: "Dan Sawyer" <dansawyer at earthlink.net>
To: "David Kirkby" <david.kirkby at onetel.net>
Cc: "amps" <amps at contesting.com>
Sent: Sunday, December 11, 2005 5:10 PM
Subject: Re: [Amps] Ouch - 23 Ohms ?


> Thank you. The pickup tap reads about 23 Ohms. If I string 6 of them 
> together, out to in, and terminate the last one with 50 Ohms the 
> impedance is about 22 Ohms.  The only negative effect is they appear to 
> take about 2 db off the signal (this is measured across a broad 
> frequency range with a spectrum analyzer.)
> 
> As you said, in an antenna circuit there appears to be little or no 
> effect. I am using two of these near an antenna. The first one is 
> connected to pick up forward, and the second one is reversed to pick up 
> reflected. They seem to work without much effect in this configuration.
> 
> Thanks - Dan
> 
> David Kirkby wrote:
> 
> >Dan Sawyer wrote:
> >  
> >
> >>Ouch. I picked a set of couplers that are apparently not 50 Ohms. They 
> >>appear to be 23 Ohms.
> >>
> >>What aspects of the construction would make a coupler 23 Ohms over 50, 
> >>or for that matter anything else? These are 20 db couplers so the 
> >>majority of power is passed straight through. How much impact would this 
> >>have on a 50 Ohm circuit? When I insert one in a 2 meter antenna path 
> >>the effect on SWR appears to be negligible.
> >>
> >>Thanks - Dan
> >>    
> >>
> >
> >Are you saying these couplers have a design impedance of 23 Ohms, or are 
> >you measuring 23 Ohms with a DVM or something?
> >
> >If they are a transmission line of 23 Ohms inserted in a line of 50 
> >Ohms, they will transform the impedance to a value you can only 
> >calculate if you know the length and frequency. But for short lengths, 
> >the impedance in your system will not change much from 50 Ohms. As the 
> >electrical length is increased (either by increasing the frequency or 
> >having a physically longer coupler), the effect will be more pronounced.
> >
> >But without knowing more information, in particular what makes you think 
> >they are 23 Ohms, then its hard to say much else.
> >
> >dave k
> >_______________________________________________
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> >Amps at contesting.com
> >http://lists.contesting.com/mailman/listinfo/amps
> >
> >  
> >
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