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 1:100 = 1 / 100 = 0.01** a LOW RETURN LOSS.

AND
if we have incident power into the antenna is 100w and 75
Reflected power back to the transmitter RETURN LOSS = 75 /
100 = 0.75 a HIGH RETURN LOSS.

So where does the equation given in the
syllabus fit in. Well properly, loss quantities, when
expressed in decibels, should be positive numbers. However,
return loss has historically been expressed as a negative
number, and this convention is still widely found in the
literature and is where it is considered the Syllabus has
used the equation but that equation taking the ratio of
reflected to incident power results in a negative sign for
return loss.

So when using the syllabus equation you
will result in a NEGATIVE number rather than the positive
and greater than 1 figure you would expect.

So to fully understand the next part you must adjust you
brain to think that negative numbers are bigger than
positive numbers but before moving on try out a few sums on
your calculator and see the difference results for in the
exam your will have to use the equation

** 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
consumed and 100% of the INCIDENT POWER is reflected, and
there is NO RETURN LOSS.

Let's
look again at a diagram that you saw earlier in the
section on SWR.

Power
arriving is 200v x 2 amps = 400 Watts

Power
Reflected is 100V x 1 amp = 100 Watts

The
reduction in the power arriving to the power reflected is
100 / 400 = 25% = 1/4 or we can say 6dBs, this is the
return loss at SWR of 3:1

If we
try Power Reflected / Power arriving as 16 / 400 we get
1/25 or 4% and the new RETURN LOSS becomes 14dBs and the
SWR works out to be 1.5 :1, a better performance.

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 RETURN LOSS in dBs becomes, **the
better the performance **of the feeder and aerial
system.

####

SWR
return loss and Reflected Coefficient

SWR
return loss and Reflected Coefficient are all related
functions, RETURN LOSS is much more convenient to use in
assessing systems performance, as manipulation in dBs is
simple and especially at microwave, power meters are much
used.

NOTE:
Reflected coefficient is the ratio on INCIDENT WAVE
VOLTAGE (or CURRENT) to the REFLECTED WAVE VOLTAGE (or
CURRENT).

It
can be shown that it is equal to in our example above and that it is equal to = 1/2 ( or (300-100)/(300+100) = 1/2

** 5d.3 Understand that the feeder loss will
reduce the SWR
and increase the return loss at the transmitter. **

** Recall that Return Loss at transmitter =
Return Loss at antenna + 2 x (feeder loss) **

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
nothing is perfect so let's now assume that we have 100W
from the TX and a mismatch so we have 10W reflected.

so
from this equation :

RETURN
LOSS at antenna = 10 log 10 / 100 = -10dB

But
if the feeder has a loss of 6dB then :

RETURN
LOSS at TX = -10 + (2 x -6 ) = -22dB which appears to make
the antenna not so bad !!!

**Antenna matching units**