how to verfiy that antenna is resonant and perfecty matched

[shahid78]

i am just wondering how can we verify that antenna is resonant at any particular frequency and at the same time properly matched to 50ohm feed line?
this is what my understanding is:

1: if its return loss is less than 0db only for a range of frequencies then it means it is resonant for that particular band

2: for it to be properly matched it must have a very small return loss ( -10db) otherwise not

can any one comment on this

thanks

 

[Azulykit]

Re: how to verfiy that antenna is resonant and perfecty matc

A low reflection coefficient is necessary for good antenna performance but it can also indicate an antenna with very low efficiency. Think of an attenuator following an antenna, the reflection coefficient will be low and the antenna will be inefficient.

The radiation characteristics including pattern shape and gain should be examined as well as s11 before concluding that the antenna is performing well.

 

[pollop]

If the radiation pattern is 'good' but shows a -ve gain (eg -2dB), does this mean the antenna is inefficient?

 

[jian]

Re: how to verfiy that antenna is resonant and perfecty matc

Hi:

There are mainly 2 types of losses in a typical antenna:

1. Material Loss: It includes conductors, substrates and surface waves. Ideally, surface wave is not material loss. However, surface wave will travel on top of the surface of substraet. It will be absorbed by the substrate eventually in reality because substrates always have loss in reality.

2. Return Loss: It is the loss due to mis-match.

If an antenna is well matched but its maximum gain is low, it means that the antenna has little return loss but much material loss. It is neither a good antenna. Regards.

 

[DrunkBear]

Re: how to verfiy that antenna is resonant and perfecty matc

A low reflection coefficient is necessary for good antenna performance but it can also indicate an antenna with very low efficiency. Think of an attenuator following an antenna, the reflection coefficient will be low and the antenna will be inefficient.

The radiation characteristics including pattern shape and gain should be examined as well as s11 before concluding that the antenna is performing well.

hi Azulykit,
i'm wondering why the efficiency would be low if the reflection coefficient is low? A low reflection coefficient means good match, e.g -20dB, isn't it?
BTW, in which case a attenuator is put after an antenna?

 

[Azulykit]

Re: how to verfiy that antenna is resonant and perfecty matc

My comment about the attenuator was intended to show that a small reflection (negative dB) coefficient could indicate a well performing antenna, BUT it could also be caused by a very lossy antenna. Placing a pad in the feed line will lower s11 but does nothing for the efficiency.

Occasionally one encounters an antenna where the match (s11) has been "improved" with a lossy element in the antenna structure. A sophisticated user will understand that although the s11 is improved the antenna has been degraded.

If a radiation pattern shows maximum gain levels that are all negative I would take this as an antenna with marginal efficiency. It might still be usable as in the case of an electrically small antenna for example. In general I would like to see positive gain in at least some directions.

Did the idea get through this time? I was not suggesting that one should add a pad to an antenna to improve the performance. The idea is that the pad would improve the appearance of good performance by decreasing s11 and making it appear that the antenna match had improved. It in fact damages efficiency. File this in the cheap trick file.

Another way to think of this is that low (very negative dB) s11 is necessary for good performance but clearly it is NOT sufficient.

 

[Mosieur_Oiso]

Re: how to verfiy that antenna is resonant and perfecty matc

Hi there,
For what kind of application is that antenna? Is it fixed or integrated in a mobile device?
You can make a quick check for your antenna when measuring the return loss with a VNA: S11 should be very sensitive when you approach your hand just next to the radiating element. If nothing happens then it means that the power is dissipated somehow, but not radiated...
Mosieur_Oiso