[R-390] R-390A sensitivity measurements

[w9ya at arrl.net]

Hey to all the gang;

Um....put a 6db pad 'tween the generator (50 ohm sourced) and the load (at
whatever Z the r390 may be at that particular freq.) and forget the
transformer. This is a "close enough for g'nment use" solution. In fact
you probably will not be able to measure the difference compared to
"ideal" solution.

Vy 73;

Bob
w9ya


> 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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>>
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