[R-390] R-390A sensitivity measurements

[Bob Camp]

Hi

Simply put you have two reasonable alternatives. A one to one turns 
ratio gives you 50 ohms to 50 ohms. A two to one turns ratio gives you 
50 ohms to 200 ohms. Either way it's a mismatch. In general (though not 
always) you will do better with an input impedance that is higher than 
the characteristic impedance rather than lower.

The whole issue is made a bit more complex by having a piece of coax 
between the antenna and the radio. A high impedance at the radio may be 
transformed to a low impedance at the antenna.

Each time this thread has come up before the net result of the tests 
run has been that the radio is slightly more sensitive when run from 50 
ohms than it is when run from 125 ohms.

	Take Care!

		Bob Camp
		KB8TQ



On Mar 2, 2005, at 11:48 AM, Cecil Acuff wrote:

> I've been contemplating a balun to go from the balanced input to the 
> antenna/generator anyway.  Question is do I do a 2:1 or a 1:1.  Keep 
> the radio at it's native 125 ohm nominal impedance or take it to 
> around 50 ohms nominal which is what we are doing using current wisdom 
> on the twinax connector to coax fabrication.
>
> Cecil...
> ----- Original Message ----- From: <DJED1 at aol.com>
> To: <ghayward at uoguelph.ca>; <r-390 at mailman.qth.net>
> Sent: Wednesday, March 02, 2005 8:59 AM
> Subject: Re: [R-390] R-390A sensitivity measurements
>
>
>> What we old microwave guys are used to is having an impedance match 
>> between
>> components, so maximum power is transferred.   For newer equipment, 
>> we almost
>> always work in a 50 ohm system, and measure in dBm.   For older 
>> radios, where
>> the impedance is not 50 ohms, measurments were not so clearly 
>> defined. This
>> is true for the op-amp also, because its output impedance probably 
>> does not
>> match that of the receiver.   That's why I'm troubled by using 
>> resistors to fudge
>> the sensitivity measurement-it terminates the generator properly, by 
>> part of
>> the signal power is lost in the resistor.   I think we need a good 
>> low-loss
>> transformer to properly feed the R-390. Anyone have such a thing?
>> Ed
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