Syllabus Sections:-

SWR  ( also know as Return Loss )

4E1   33 Understand that the standing wave ratio (SWR) is a measure of the signal travelling back down the feeder expressed in terms of the standing waves caused by the reflected signal voltage (or current).

The formula from the exam equation sheet is :-

When there is a mismatch between feeder and antenna we proved in the earlier section (click here to check back) that to satisfy some of the transmitted power is reflected, ie returned, to the transmitter. Moving back from the antenna we find that the reflected voltage is alternately in phase or out of phase with the transmitted wave.

When in phase the voltages ADD, when out of phase they subtract.

The highest voltage is VFORWARD + VREVERSE, and

the lowest voltage is VFORWARD - VREVERSE

These points of maximum and minimum are at steady positions along the feeder and are called STANDING WAVES.

The STANDING WAVE RATIO which is a measure of the mismatch is VMAX / VMIN which is (VFORWARD + VREVERSE) / (VFORWARD - VREVERSE), on the equation sheet is as below

it is also incidentally equal to ANTENNA RESISTANCE / LINE CHARACTERISTIC RESISTANCE or to be more accurate, the impedances.

Effect of line loss.

Losses along the feeder reduce the voltages of the reflected wave as it travels towards the transmitter, which increases the VMIN and decreases the VMAX, so that the ratio VMAX / VMIN falls as it travels back to the transmitter, ie the Standing Wave ratio reduces.

So if you have a long feeder and a great deal of loss, the SWR at the transmitter might well look ok but in fact there could be a major mismatch at the antenna!!!!

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4E2   33 Recall that return loss is the ratio of the forward signal power to the return signal power; normally expressed in dB.

A reflected signal from your antenna is called for this section "RETURN". Return Loss is a statement or measure of how small the amount is reflected signal. To have more power going out of your antenna you need a small amount of return power which equates to a large loss of reflected power hence a large "Return Loss"

The equation given in the Equation Sheet for the exam does not take that statement at face value. Instead it takes it to mean

Return loss = Forward Signal Power  divided by Return Signal Power

and the syllabus shows the equation as

we can show this as a diagram to further help explain it to you ...

RETURN LOSS is really nothing more than another way of saying SWR.

A high RETURN LOSS result of the equation is actually a good thing, when using the equation on the equation sheet.

The lower the return loss or put it another way the "Greater the REDUCTION in power returning to the transmitter" the better the antenna is radiating.

We are considering the REFLECTED POWER with respect to the ARRIVING INCIDENT POWER, to determine how much power is reflected with respect to that which is arriving. Another way of putting it is to say "How much power is left of the INCIDENT POWER to be reflected after the power has been consumed by the load resistance.

So if the incident power into the antenna is 100W and reflected power back to the transmitter as 1W, ignoring for a moment the dB equation and using simple maths, the RETURN LOSS is a ratio 100:1 = 100 / 1 = 100 actually  HIGH RETURN LOSS.

AND if we have incident power into the antenna is 100w and 75 Reflected power back to the transmitter RETURN LOSS = 100 / 75 = 1.33 actually a LOW RETURN LOSS.

So where does the equation given in the syllabus fit in how to use is with our calculator ?

For this instruction we are using the Casio fx-83GT but similar are available and in fact a newer version Casio FX-83GTX is now available.

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All we need to know for Return Loss is how to use the   button.

Switch on your calculator and press the button   appears on the screen

but for the Return log calculation we need 10 log so clear the screen using the AC orange button and then press 10 followed by the log button and you will see

we have incident power of 100W and reflected power of 10W

so after the bracket enter  100 then divide sign then 10 and then )

press =   and that is Return Loss = 10dB

Try working out 100W incident and 75W reflected ...

and we get is 1.3dB when rounded to one decimal place.

So Low Return loss means a high loss in power into antenna.

Try working out 100W incident and 1W reflected ...

and we get is 20dB .

So HIGH Return loss means a LOW LOSS in power into antenna .

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4E2   33 continued Understand that a low SWR equates to a high return loss and a high SWR equates to a low return loss.

You are aware, or should be that a low SWR means that very little of your signal is being reflected by a mismatch in the antenna / feeder system and vice versa. So take a look at the following examples.

If the load is replaced by an open circuit, then no power is radiated by your antenna and 100% of the INCIDENT POWER is reflected, and there is NO RETURN LOSS.

So from this you can see that a HIGH SWR and a LOW RETURN LOSS is BAD and a LOW SWR and a HIGH RETURN LOSS is GOOD. Seems a crazy way of looking at things but that what you have to learn !!!

AND LASTLY on Return loss the answer is normally expressed in dB but as you do not have to do any maths on this point just be aware that answer would be in dB and not as a unit mentioned above !!

So remember the Higher the RETURN LOSS in dB becomes, the better the performance of the feeder and aerial system if there are no line losses.

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4E3  33 Understand that the loss in the feeder will reduce the SWR and increase the return loss as measured at  the transmitter and that the SWR at the antenna is unaffected.

If we have a system where there is no feeder loss and the match is perfect then 100W from the Tx will be 100W at the antenna.

But Feeders (what ever type) are not loss free (and also called line loss) and as you move up away from HF to VHF and UHF and higher then the losses increase.

The feeder loss will affect SWR reading, making it lower, in the shack and in consequence also the Return Loss will be lower.

Thus the Feeder loss can conceal problems with the antenna for example if the feeder loss is 50% of the power then even with the antenna end not connected to an antenna only 50% of the power will be returned back to the transmitter  and give you a lower SWR reading than it should be.

Over time your feeder is likely to deteriorate and thus there will be increased line losses and the reading on your SWR meter will appear to improve, which is a false reading.

Now thinking in Return Loss terms if no antenna is connected then the Return Loss will be 0dB as all the power will be returned. However in actual fact if you have a 50% feeder loss only 25% will be returned to the transmitter.

So if we have 100W transmitted and line loss of 50% (3dB) so only 50W reaches the antenna and that 50W looses another 50% (3dB) in line loss going back to the transmitter is reduced from 50W to 25W making a total loss of 6dB there and back.

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4E3  33 continued Recall that Return Loss at transmitter = Return Loss at antenna + 2 x (feeder loss)

Whilst not on the Equation sheet on page 71 states

" Transceiver return loss = antenna return loss + (2 x line loss) "

So if a feeder has a return line loss of 3dB and the antenna has a return loss of 10dB :

Tx return loss = 10 + (2 x 3 ) = 16dB

Remember that a HIGH Return Loss is GOOD  as is a Low SWR but in fact it is all false readings DUE to the LINE LOSSES