[R-390] Impedence Matching Question

[Roy Morgan]

At 02:55 PM 6/9/2006, n4buq at knology.net wrote:
>While searching for some replacement coax for my PTO output cable (thanks
>again, Roy!), I got to wondering what the correct impedence of a cable
>would be for this application.

Barry,

"correct" depends on the criteria you use.

Case A:
You apply a very firm requirement that the cable be the same as exactly as 
possible as the original.   This is impractical perhaps.  Don't bother.

Case B:
You are practical and use what you can get. It will work fine.

Case C:
You are practical, but inquisitive. You investigate the topic of 
"Characteristic Impedance" of coax cables and come to the conclusion that 
the impedance of a cable can be important when the length of the cable is 
greater than perhaps a tenth of a quarter wavelength at the operating 
frequency, IF the system is unmatched.  Then you calculate the wavelength 
of 3 mc and decide that your one-foot piece of coax is WAY less than about 
60 feet and you realize that worrying about coax impedance is not 
needed.  You revert to Case B.

>   I realize the short length cable and the
>relatively low frequency make this not as much of an issue as it might be
>in other situations, but I was still curious.

The cable impedance is simply not an issue. The capacitance it adds in 
parallel to the circuit *may* be an issue, if the adjustments cannot 
compensate for the difference between the original cable and your substitute.

>I calculated the reactances of the output coil and the parallel capacitor
>at 3 Mhz...very close to 40 ohms.  Is this a correct method to
>calculate the output impedence of this circuit

Sounds about right. A way to test for output impedance is to run your PTO 
open circuit and measure the RF output voltage, then load it with common 
resistors till the output voltage drops to one half the original. The value 
if the added resistance is about the output resistance of the PTO.  An 
Allen Bradley Type J pot of 250 ohms or so is useful for this experiment.

>(and possibly why the output cable is most likely 50 ohm coax)?

Possibly, but it's my guess that the Collins engineers understood very 
clearly that the coax characteristic impedance did NOT matter, and other 
factors were used in choosing it and the connectors.

Note: the R-390/URR uses BNC connectors and larger coax.  It's quite likely 
that the MB connectors and smaller coax used in the A version, were part of 
the "Cost Reduction Program" that resulted in the R-390A/URR as we know it.


Roy


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