return loss & gain

[mhtplsh]

return loss efficiency

Better the matching lower the return loss.
Better matching, better gain.
Then better return loss means better gain also.
Kindly comment on this.

 

[snkhan]

return-loss is negative ?

Better return loss is associate with how well the design is matched to the input impedance usually 50Ω and also higher radiation efficiency.

Gain is a relative measure associated with some ideal reference design (extracted from the radiation pattern not with return loss) and also depend on the type of antenna and . Like if u have and omnidirectional antenna then gain is almost same e.g in azimuth plane. And if u have direction antenna it has gain in some preferred direction.

 

[SagSag]

mhtplsh,
you are correcr in some cases,
for a power amp I match once, I had bad
matching and the gain was bad, when I corrected
the matching the gain increased.
you can also look at it in another way,
matching is relative to S11
if the matching is poor than a lot of power is
reflected, hence the S21 (which is relative to gain)
is small.

 

[bkd]

good mathching -> good total efficency
and you know gain= directivity* total efficency. So in one kind of antenna( the same radiation pattern) when you get good mathching, of course you get good gain

Added after 1 hours 9 minutes:

You can check the equation in CSt ( simulate any antenna ) and check if gain = directivity * total efficency

 

[jallem]

The two are interraleted, lets see an example:
Lets have a 0 dBm input power to a amplifier,
if the return los is such as the 90 % of the
energy is reflected then only 10 % pass trough
the second port. You will have RL= 10log(0.90)
RL= - 0.45. It is bad. So the real input power is then
-10 dBm.

Now lets say the amplifier has a gain of 10 dB.
This means the output power is again 0 dBm but
the gain referenced to the input power is 0 dB.

Even though the gain of the amplifier is OK the
total gain is bad because losses due to reflection.

Lets say now that the RL is such that only 10%
of the power is reflected, that means that the
RL=10log(0.10)= -10 dB, this is good. So the
input power is 0 dBm - 0.45 dB = -0.45 dBm.

Since our amplifier has a gain of 10 dB then
output power is -0.45 dBm + 10 dB = 9.55 dBm.
So the injected power is 0 dBm and the output
power is 9.55 dBm and the gain is G=9.55.

In the first case the amplifier boosted the
power at its imput(-10dBm) to 0 dBm and in
the second case the amplifier also boosted
the power to 9.55 dBm. In both cases the gain
is good, but input matching makes the difference.

 

[mhtplsh]

But when tuning preamplifier with bandpass filter some times the gain increases but also return loss decreases. That's why i was puzzled.

 

[jallem]

Maybe you are in a situation
like the graphic below.

 

[Jone]

good mathching -> good total efficency
and you know gain= directivity* total efficency. So in one kind of antenna( the same radiation pattern) when you get good mathching, of course you get good gain

Good matching doesn't mean good efficiency. For example with conjugate mathing, only half of the power is delivered to the load. Minimizing load and generator impedances will maximize the efficiency.

 

[biff44]

Yes, but it is easy to prove that the MAXIMUM power is delivered to the load when conjugate matched.

 

[snood]

Maybe you are talking about the difference between Gain and Realized Gain which is proportional to (1-|S11|^2)
I.e. a -4dB S11 >> ~ 0.7dB drop in 'realizable' gain.
The match cannot be thought of as a simple 'attenuator' of power. It is integral to the transforming nature of the antenna structure.
I hope this is of some help.
:arrow:

 

[Azulykit]

Return loss is a measurement of power reflected at the antenna port. The greater the reflection (less negative in dB's) the more that is lost and the lower the power available for radiation and thus the lower the gain.

Gain is a little harder to discuss intuitively. It has two aspects: one aspect is like an amplifier where it can boost the apparent signal in a particular direction. It also has an efficiency characteristic and has a flavor of efficiency or insertion loss as well. these two pull in opposite directions and are often difficult to separate without extensive 3D pattern measurements.

One thing to keep in mind is that a good antenna has a good match, but often so does a poor, inefficient one. Rarely is an antenna useful with a poor match. A good match is necessary but not sufficient for good performance. If you look in the technical literature you may be surprised to see how often authors tout the matching they have accomplished without discussing gain. I always get very suspicious when I see that.

 

[plasma]

Hi mhtplsh

It is not obvious that good match lead to higher gain most of the time you connect antenna thru cable and if it's long cable the loss of the cable give you better match. Like connecting attenuator to the antenna.
Also in some cases some energy can absorb in another propagation mode. You will get good match but pure gain in the preferred mode.

pl

 

[snkhan]

Hi mhtplsh

It is not obvious that good match lead to higher gain most of the time you connect antenna thru cable and if it's long cable the loss of the cable give you better match. Like connecting attenuator to the antenna.
Also in some cases some energy can absorb in another propagation mode. You will get good match but pure gain in the preferred mode.

pl

Your are quite right, Gain is nothing much to do with return loss. There is no doubt that better match leads to a higher efficiency. but the scope of gain is limited by the influence of modes, grounding effect etc.