antenna loading effect

[geppei]

how to estimate to loading effect of antenna?

and how to solve it?

for example,
a monopole was put in handheld application, but there were earpiece, LCD ... i found the input impedance of the monopole was incresed and resonant frequency was shifted.

one more important discovery is that, my monopole was orginally 3X ohm, by putting a conductor nearby, say a earpiece magnet, the input impedance increased to 50ohm (it became well matched automatically ). However, from the spectrum ana experiement, the performance was worse than before although it was 50Ohm matched.

So in concluded, if there is a load near the antenna, the impedance will change. although the load make the antenna become 50Ohm, it is confusing to let one believe the antenna working better. The fact is, worse than before


Loading can never be good. beside putting away from the antenna, what else we can do to elimate the effect?

Any suggsetions or comments are welcome

Thx

 

[boy]

loading effect

if you have a EM simulation tool, you can try to do full wave simulation with all loads you mentioned.

 

[spikeyang]

Loading near the antenna may absorb energy from the antenna port,so the S11 parameter is good ,you could see a low VSWR and a 50 Ohm input impedance, but the overall antenna efficiency is not.

Maybe it explained something.

 

[leeiantze]

I found this information interesting. Does it meant that when we measure a good low VWSR, it may and maynot relate to our end performance.
I have measured a portable with monopole antenna, when I am holding it in hand held and when I am placing it on my shoulder, the VSWR shift alot. On the shoulder, the VSWR tend to shift to lower frequency. issit because the antenna port is absorbing the energy from human body? thanks.

 

[shahid78]

hi

you havent mentioned whether VSWR is increasing or decreasing when you bring it closer to the shoulder.

the human body becoz of its high permtivity will shift down the resonant frequency .

also as it is a lossy medium it will absorb energy and reduce the radiation efficiency

i hope this helps

regards

 

[vfone]

Going back to the initial question. How to estimate the loading effect of antenna? You can’t (without a EM simulator), but you can avoid.
Try to keep clear the Reactive Near Field of the antenna, from any components, especially metals.
Reactive_Near_Field = 0.62*SQRT(D³/λ), where D is the antenna dimension.