When attempting to measure the voltage standing wave ratio (VSWR) for an antenna, the measurement is often made by measuring the VSWR at the transmitter, where the antenna transmission line is connected to the transmitter. When assessing the antenna VSWR in this manner, the measured VSWR will ALWAYS be slighter better--that is the VSWR will be a lower numerical ratio--than the actual VSWR at the antenna due to the effect of loss in the transmission line.
Measurement of the VSWR at the transmitter is the simplest way to measure the VSWR, and that VSWR is what the actual transmitter will be seeing.
The reason the VSWR will be higher at the antenna is the effect of the line loss is to attenuate the amount of forward power that arrives at the antenna, and also to attenuate the amount of reflected power that returns to be measured at the directional wattmeter.
A common practice is to accept some transmission line loss in exchange for the convenience of avoiding having to move the transceiver to the antenna location and directly connect the transceiver to the antenna. The loss in the transmission should be kept to a minimum as much as possible, but a loss of 1-dB is probably still considerable tolerable. Many VHF Marine Band antennas will be connected to their transceivers with a 15-foot length of RG-58C/U cable, which at 150-MHz will be inserting a loss of about 1-dB.
The effect of a 1-dB loss in transmission line on the VSWR that will be measured at the transmitter end will be demonstrated in the example below
EFFECT ON VSWR OF TRANSMISSION LINE LOSS
--Assume the actual VSWR at the antenna is 2:1; this means 11.12-percent of the power is reflected.
--Assume the transmitter produces 20-Watts of power into the transmission line. This is the forward power for the VSWR measurement. Then:
--Because of a 1-dB loss only 15.9-Watts reaches the antenna.
--The power reflected at the antenna will then be 15.9 x .11121 = 1.77-Watts due to the antenna having an actual 2:1 VSWR.
--This reflected power will also be attenuated by 1-dB on the return path through the transmission line.
--The reflected power at the transmitter will then be 1.41-Watts due to the 1-dB loss in the transmission line.
--A directional wattmeter measuring 20-Watts forward and 1.41-Watts reflected will indicate a VSWR of 1.72:1
When someone notes that their VHF Marine Band antenna seems to have a better VSWR bandwidth than specified, the improvement is probably due to measuring the antenna VSWR with some transmission line loss inserted into the test set-up.
If the transmission line loss increases, even greater masking of the VSWR at the antenna occurs.
For example, assume the transmission line has 3-dB of loss, and there is nothing connected at the far end.
If 100-Watts of power is input into the line, only 50-Watts reaches the end.
With nothing connected, all power is reflected back, all 50-Watts.
But the reflected lower is also attenuated by 3-dB, so only 25-Watts returns to be measured.
The directional wattmeter indications are 100-Watts forward and 25-Watts reflected, for a measured VSWR of 3:1.
In this extreme example we see that an INFINITE VSWR is converted to a 3:1 VSWR due to measuring with a transmission line loss of 3-dB.
This relationship leads to the conventional wisdom that if a VHF antenna at the end of a long transmission line has a VSWR approaching 3:1 there is probably no antenna connected to the transmission line.
A further problem with VSWR masking is the rated attenuation of a transmission line is typically for the condition of a matched load, or a VSWR of 1:1. When a high VSWR exists on the transmission line, the attenuation will increase from the effects of the high VSWR. For proper analysis of the masking effect from line loss, the loss factor for the transmission line must be adjusted to a higher loss actor to account for the effect of the higher VSWR, which in turn will create more masking of the actual antenna VSWR when measured at the transmitter end of the transmission line.
The reason a high VSWR causes an increase in line loss compared to the matched condition is the creation of the standing waves; at some part of the line there will be higher current and resistive losses will increase, and at some parts of the line there will be higher voltage and dielectric losses will increase.
The calculation of the added loss due to high VSWR is rather complicated, so I will not delve more deeply into how to calculate it. However, it is a real loss. This further adds to the masking effect of measuring the VSWR at the transmitter end, as now the transmission like will have even more loss that its rated loss for a matched condition, and more transmission line loss causes more error when the VSWR is measured at the transmitter end of the transmission line.