[R-390] Source for URM-25 MX-1487/U impedance adaptors?

[Bob Camp]

Hi

There are schematics out there for the adapter. Basically it is a 50 
ohm to 125 ohm resistive matching pad if I remember correctly. I 
suspect that for an enormous sum of money they are available on the 
auction sites ...

One question to ask before investing thousands of dollars in one of 
these - If you had one would you want to use it?

The pad is useful for setting the radio up to run out of a 125 ohm 
source. Most of us seem to be running antennas that are anything but 
125 ohm impedance devices. To be totally correct you would have to be 
running both a 125 ohm antenna *and* 125 ohm balanced coax into the 
radio. I suppose that if you have the antennas you probably are running 
the coax as well ....

If you are running a 50 ohm (nominal) antenna and 50 ohm coax then you 
should align the radio from a 50 ohm source impedance. The same would 
be true of a 75 ohm setup (75 ohm coax, 75 ohm antenna, 75 ohm 
generator). There have been numerous posts on this list that verify the 
value of using the same source impedance for alignment as for 
operation.

 From personal experience the biggest benefit of using the correct 
impedance is that the antenna trimmer centers up better in actual 
operation. If you want the maximum correction range from the trimmer 
then using the correct impedance is a must.

If you want to get hyper about all this then you can look at the input 
impedance of the radio. Unfortunately that only makes the problem more 
complicated since it's a complex impedance (versus pure resistive) even 
when correctly set up. Then you find that most antennas aren't 50 ohms 
over a very wide range. Ever *measure* the impedance of cheap coax 
(don't they won't take it back ...).

Heck of a lot simpler to just look at the source impedance ....

	Take Care!

		Bob Camp
		KB8TQ



On Feb 20, 2005, at 12:48 PM, w4qg wrote:

> Hello All,
>
> I receintly got my URM-25/E (uncommon variation - I understand)
> up and running at 100%. One of the first things I want to use it for
> was to do an alignment on my 390A. Well, I have most of
> the accessories but I am missing the MX-1487/U impedance
> adaptor. Does anyone know if a source for these?
> I'm sure I could whip something up that would work, but now
> that I have a nearly complete, fully functioning 25/E I wanted an 
> original.
>
> It was lots of fun repairing that 25/E, it had the common problem
> with the modulation (dead), those caps on the audio board are a
> real bugger to replace. Works beautifully now...
>
> Next I start on another 390A that is dead, more on that one to follow,
> it is an oddball (ITT nameplate, extra position in the function switch,
> looks to be all Collins sub assemblies inside...)
>
> 73,
>
> Paul W4QG
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