By
inserting a matched (nominal system impedance)
attenuator in front of a mismatched load impedance,
the mismatch "seen" at the input of the
attenuator is improved by an amount equal to twice
the value of attenuator. The explanation is simple.
Return loss is
determined by the portion of the input signal that
is reflected at the load (due to impedance mismatch)
and returned to the source. A perfect load impedance
(complex conjugate of the source impedance) would
absorb 100% of the incident signal and therefore
reflect 0% of it back to the source (return loss of ¥
dB).
For the sake of
illustration, assume that the load is an open (or
short) circuit, where 0% of the incident signal is
absorbed by the load and 100% is reflected back to
the source. The reflected signal would therefore
have a return loss of 0 dB. Insert a 3 dB
attenuator in front of the load. Now the incident
signal is referenced to the input of the attenuator.
As signal at the
input of the attenuator will experience a 3 dB
reduction in power by the time it reaches the load.
That 3 dB less power will be 100% reflected by the
load and experience another 3 dB reduction in power
by the time is returns back to the input, for a
total loss of 6 dB. The same principle applies for a
load anywhere(§)
between zero and infinite load impedance (short and
open circuits, respectively).
Calculate the
improved VSWR as follows:
 Convert the load
VSWR to load return lossper the following
equation:
RL_{LOAD}=20*log
dB
 Add twice the
attenuation value to RL_{LOAD}: RL_{NEW}=RL_{LOAD}
+ 2*ATTEN dB
 Convert back to
VSWR per the following equation: VSWR=
Of
course, the method can be reversed to predict the
attenuator required to improve a load VSWR by a
predetermined amount. To do so, calculate the
desired return loss and subtract the known load
return loss. Divide the answer by two to get the
attenuator value needed.
§
Actually, the attenuator is
only rated for its specified attenuation level when
it is connected between two nominal impedances.
Therefore, the attenuator will either have to be
designed to closely match the two impedances at its
input and output (source and load, respectively), or
an adjustment will need to be made in the specified
attenuation value to compensate for the mismatched
load impedance.
