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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. Note that by my convention the loss value
is returned as a positive number, since the word "loss" implies the negative. If
it were to be termed "return gain," then the result would be reported as a negative
number. Equally qualified experts will disagree on whether return loss should take
on a negative value or a positive value; the important thing is to keep the sign
correct in your calculations; i.e., if you use a positive value, then subtract it,
and vice versa.
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.
See
Espresso Engineering Workbook (free) for a calculator.
§ 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.
Related Pages on RF Cafe - VSWR - Return Loss
- Γ Conversions - VSWR Mismatch Errors
- VSWR Reduction by Matched Attenuator
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