[R-390] r390a balanced input connector

[Bob Camp]

Hi

There have been numerous threads on the 125 ohm input impedance of the 
390. The simple point is that if you align the radio out of a 120 ohm 
generator then you will have a 120 ohm radio. If you align the radio 
out of a 50 ohm generator then you will have something other than a 120 
ohm radio. That's not to say you will have a 50 ohm radio, just that it 
won't be a 120 ohm radio.

All this is even more confusing when you get into receiver design 
theory and they show that a low noise receiver does not general apply a 
matched load to the antenna. The logic goes more or less that a matched 
load dumps 6 db of the signal and 3 db of the thermal noise. Things get 
even more strange when you notice that most antennas only provide a 
specific matched impedance over a very narrow portion of their useful 
frequency range.

The bottom line seems to be that if you align the radio out of the same 
source impedance as the antenna you will be using then the radio works 
just fine. There does not seem to be any real advantage from using a 
120 ohm input versus a 50 ohm input.

The only exception I can see to this is a true 120 ohm balanced input. 
In order to run this you would need to have 120 ohm balanced coax. I'm 
not at all sure what kind of balanced antenna you might have running 
around with a 120 ohm impedance. A full wave loop comes close but 
that's not a real popular antenna these days .... If you are going to 
do a true balanced input then there is an extra step to the alignment 
procedure related to properly balancing the input.

An time you connect an antenna to a radio and the output noise level in 
the receiver goes up, you have more sensitivity in the radio than you 
need. At that point you are going to worry more about things like front 
end selectivity and overload performance of your RF amplifier(s). 
Fortunately with the R-390 you don't have to settle for two out of 
three, it's got it all ..... The minor differences between 120 ohm and 
50 ohm inputs aren't going to change this much at all.

None of this relates to the high impedance input to the radio. It 
bypasses some of the front end selectivity in the radio and is 
generally held to be a bad thing except when you have a short run to a 
small whip antenna. Even in this case I'm not sure this is a real good 
idea.

Hope this helps ...

	Take Care!

		Bob Camp
		KB8TQ



On Jun 21, 2004, at 9:11 PM, JGolden365 at aol.com wrote:

> Does anyone know the UG number for J104, the balanced input connector?
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