[R-390] Alignment question
Bob Camp
[email protected]
Sat, 11 Jan 2003 18:04:54 -0500
Hi,
More or less the answer on the dummy load on the URM-25:
Most signal generators are 50 ohm devices. That unfortunately can mean two
different things. Either they are designed to run into a 50 ohm load -or-
they are designed to run into a 50 ohm load AND they have a 50 ohm output
impedance.
A Measurements model 80 is a good example of a generator that is set up to
drive 50 ohms but does not provide a 50 ohm output impedance. You have to
put a pad on the output to make it have a 50 ohm output impedance.
This turns out to be important for a rather odd reason. Changing a resistor
in parallel with part of a tuned circuit probably will change the resonant
frequency of that circuit. This was one of the things they used to teach
everybody in school. Then along came Sputnik .....
In the case of the R-390 what the dummy load is doing is providing a match
between the generator and the input of the radio. It's there to ensure that
the radio is set up running out of the same impedance as the antenna
distribution system it is connected to.
The next question obviously is - are you running a 120 ohm balanced antenna
distribution system ? If so - WOW! - you need the dummy load. If not -
join the club - you need something different.
Most of us these days seem to use 50 ohm single wire coax rather than 120
ohm balanced coax to hook things up. The 120 ohm balanced stuff is great and
very useful it's just not what we use anymore.
In order to properly set up a R-390 for 50 ohm coax you need to strap the
input of the radio or use a matching transformer. There are threads on each
back in the archives.
Once the radio is more or less set up to run 50 ohms you then use a 50 ohm
output impedance generator to align the radio. If you decide to set up for
75 ohms and use cable TV stuff then you need a 50 ohm to 75 ohm pad between
the generator and the radio.
Chuck's good old R390 videos explain all this (except for Sputnik) a lot
better than I can.
Hope that makes at least some sense ...
Enjoy!
Bob Camp
KB8TQ
----- Original Message -----
From: "Jim Brannigan" <[email protected]>
To: "R-390 list" <[email protected]>
Sent: Saturday, January 11, 2003 12:53 PM
Subject: [R-390] Alignment question
> Hello All,
>
> I changed the 3rd mixer back to a 6C4, checked the mechanical alignment
and
> am now warming up the URM-25 and R-390A.
>
> For the RF alignment the manual refers to using an electrical dummy load,
a
> DA-121/U. This is not part of the URM-25 pack. Is this necessary or
should
> I just use a 50ohm load?
>
> If it is necessary, does anyone have the values used in the dummy load?
>
> Jim
>
>
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