[R-390] Impedence Matching Question

[n4buq at knology.net]

On Mon, 12 Jun 2006 11:54:57 -0400, Roy Morgan <roy.morgan at nist.gov> wrote :

> 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
> 
> 
> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Cell 301-928-7794
> Work: Voice: 301-975-3254,  Fax: 301-948-6213
> roy.morgan at nist.gov --

Thanks, Roy.  No, I wasn't worrying about whether or not the cable's 
impedence was a close match to the output.  I was just curious if this was 
the correct method to determine the output impedence of the oscillator.  I 
suppose if one were to worry about this, the impedence on the other end of 
the circuit would also be a factor (if they were different).  For this 
short run, the coax is there to provide shielding and cable impedence isn't 
much of a factor.  It was more of an academic question than anything else 
so Case "C" applies.

Thanks again for the cable.  It's been pretty hot here the last couple of 
days and my workshop (aka the utility room) is not very comfortable, but 
hopefully I'll get this done very soon.

73,

Barry - N4BUQ