[R-390] R-390A sensitivity measurements

[w9ya at arrl.net]

Of course you would NOT want this between your antenna and rcvr., but for
making accurate measurements with your generator, the pad is the "cat's
meow".

Sorry I was not clear about this.

Vy 73;

Bob
w9ya

> 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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