[Lowfer] Return loss bridge questions.

[Dave Brown]

Is it a commercial unit?  Google may be useful  if it is. Many do not claim 
to cover frequencies lower than a few MHz but a few tests will show if it 
can be used down there.
Often commercial units have no detector built in (but there are exceptions) 
and require either a 'receiver' on the test frequency (a spec an will do 
nicely)  or an externally connected detector unit to produce rectified DC 
output that can be either measured with a dc millivolt meter or displayed as 
a swept pattern (using a sweep generator as the source) on a scope. While 
many are designed for 50 ohms and have a 50 ohm load built in, some will 
require the impedance reference load to be externally connected. If there 
are 4 ports then this is almost certainly the case. Only three ports would 
suggest the reference load is built in.
 If its a non-commercial unit then you might need to have a look 
inside-there's not a lot to them, just a resistor bridge arrangement and 
perhaps a transformer for input or for output.

 If not using a receiver with calibrated level capability, you will need a 
way of calibrating the bridge output and this is usually done using standard 
resistive loads on the test port (e.g. 50 ohms, 75 ohms, 25 ohms, 150 ohms 
etc-corresponding to match and various degrees of  mismatch) with a fixed 
level applied to the source or input port. It's not a bad idea to do this 
anyway and check the level calibration on the receiver or spec an.  It's 
also possible to use a variable attenuator on the input port for calibration 
where, after setting a reference level with either a short or open cct, 3dB 
inserted corresponds to 6 dB additional return loss, etc etc. Regardless of 
how you do it, the aim is to build up a calibration chart of DC levels or a 
graticule with various return loss levels (horizontal lines) on a scope to 
compare the unknown to. Keeping the bridge input constant while measuring 
is, of course, essential.

73
 Dave, ZL3FJ


----- Original Message ----- 
From: "KD7JYK DM09" <kd7jyk at earthlink.net>
To: "Discussion of the Lowfer (US, European, &amp;UK) and MedFer bands" 
<lowfer at mailman.qth.net>
Sent: Wednesday, January 15, 2014 4:45 PM
Subject: [Lowfer] Return loss bridge questions.


>I have a "return loss bridge".  Figure I could use it for analyzing
> antennas.  I have various oscillators from Hz to many 10's of GHz, much 
> more
> than the bridge can handle, but I figure I can test most everything with 
> it.
>
> Input is obvious, also device under test, the "unknown", what I am not
> familiar with is what do I use to read the output?  What do I put here?  A
> Voltmeter?  RF detector of some sort?  Something to convert RF to Volts 
> and
> look for the lowest reading/less return?
>
> I'm just not familiar with thing at all!
>
> Any info appreciated.
>
> Kurt
>
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