This calculator computes the SWR of an antenna under test by accounting for the error introduced by a lossy cable
that is included in the measurement set-up. See the diagram below.
It is assumed that you are measuring the system with a reflection meter that is connected to a coaxial cable that is in turn
connected to the antenna under test. The lossy cable introduces and error measurements.
The error increases dramatically as the cable loss or antenna SWR increases.
There is no error if the antenna SWR = 1:1.
Reflection meters come in several varieties. Some measure only the SWR, some measure the forward and reflected power
levels, and some measure Return Loss
This calculator can accommodate all types of meters.
The information required is the measured SWR or Forward/Reflected power levels or Return Loss, the length
of the cable in metres or feet and the loss data for the cable being used. The loss data can be
obtained directly from a Web link given below or from manufacturers data sheets.
How To Use The Calculator
If You Measure SWR
(1) If your meter measures SWR, use the input area for SWR Measurements.
(2) Click the check box.
(3) Input the Line Length, the Cable Loss and the measured SWR.
The Line Length can in units of metres or feet and the Loss can be in units of dB per 100 metres or dB per 100 feet.
NOTE: Do NOT mix metres and feet.
(4) Click the Calculate button and the Antenna SWR will be displayed.
Here is an example:
. Line length = 15 metres.
. Line loss = 3 dB per 100 metres.
. Measured SWR = 4.0.
. Then, the Antenna SWR = 4.98
If You Measure Forward and Reflected Power
1) If your meter measures Forward and Reflected Power use the input area for Forward/Reflected Power Measurements.
(2) Click the check box.
(3) Input the Line Length, the Cable Loss, Forward Power in Watts and the Reflected Power in Watts.
The Line Length can be in units of metres or feet and the Loss can be in units of dB per 100 metres or dB per 100 feet.
NOTE: Do NOT mix metres and feet.
(4) Click the Calculate button and the Antenna SWR will be displayed.
Here is an example:
. Line length = 15 metres.
. Line loss = 3 dB per 100 metres.
. Measured Forward Power = 10.0 [Watts]
. Measured Reflected Power = 3.6 [Watts]
. Then, Antenna SWR = 4.98
If You Measure Return Loss
1) If your meter measures Return Loss use the input area for Return Loss Measurements.
(2) Click the check box.
(3) Input the line length, the cable loss, and Return Loss [dB].
The Line Length can in units of metres or feet and the Loss can be in units of dB per 100 metres or dB per 100 feet.
NOTE: Do NOT mix metres and feet.
(4) Click the Calculate button and the Antenna SWR and Return Loss will be displayed.
Here is an example:
. Line length = 15 metres.
. Line loss = 3 dB per 100 metres.
. Measured Return Loss= 8 [dB]
. Then, Antenna SWR = 8.72
and the Return Loss = 2 [dB]
How to Collect Cable Loss Data
The cable loss data can readily be found on the
" Times Microwave Calculator "
Web page by clicking this link.
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