[Antennas] Return Loss Bridge Construction

[Charles Greene]

At 03:57 PM 5/25/2002 -0400, Harvey&Bessie wrote:
>"Return loss" is a telephone industry term for VSWR, expressed in dB,
>rather than a ratio. Construction of such an instrument would be
>identical to an ordinary SWR bridge. Only difference would be the
>calibration.
>Harvey/W4TG


Harvey and All,

I'm quoting from the data sheet of the Eagle model RLB150 return Loss Bridge:

"Return loss bridges are useful in measuring VSWR, or return loss of 
filters, mixers, antennas, and amplifiers.  The return loss bridge has 
three ports, SOURCE, LOAD and REFLECTED.  A signal generator or sweep 
generator is applied to the SOURCE port.  The device under test (DUT) is 
connected to the LOAD port and a measuring device is connected to the 
REFLECTED port.  ----  Return loss bridges may use aninternaldetector at 
the REFLECTED [port or provide an RF output.  The RLB150 bridge provides 
and RF type reflected port, thus allowing use with network analyzers, 
spectrum analyzers or service monitors."

I have the Eagle RLB150, and I use it with a service monitor that has a 
spectrum analyzer and tracking signal generator.  It will measure SWR over 
a wide frequency range, like 3 to 30 Mhz, or an equivalant range from .04 
to 1000 mHz.  It is difficult to read the SWR because you are looking at 
loss in DB, which you have to convert to SWR using a chart; a meter or 
antenna analyzer is much easier to read the SWR, but of course on one 
frequency at a time.  I use it to measure performances of baluns, antennas, 
feed lines, tuned circuits etc.  It's advantage is that it displays a wide 
frequency range all at once and you can see unwanted resonances, and other 
things that would be very time consuming if you did it with an antenna 
analyzer.  However, it is a precision instrument, is expensive because of 
the precise isolation requirements between the ports, and it requires a 
couple of expensive pieces of test equipment to use with it, namely a 
spectrum analyzer and tracking  signal generator.



73, Chas, W1CG