antenna-theory.com

[BatBen78]

Hi@all,

i got a question about an 868MHz monopole antenna:

my antenna is formed by a screw. The srew is mounted directly on the pcb using a nut. This pcb is surrounded by a metal case(grounded). My screw-antenna breaks through the metal case without contacting it. Now my question:
which part of the screw forms the antenna(lambda /4) the whole screw lengh or only the part of the screw outside the case?

thanks for help and greetings

Ben [/img]

[bigSteve]

Do you have a picture or can you make a quick diagram? I'm not sure what you're describing

[BatBen78]

|top
|
|
|
_______ | ________metal housing
|
|
_______ |_________pcb

the vertical line is my antenna. How long should my antenna be? lambda/4 from pcb to top or lambda/4 from metal housing to top?

hope you understand my question now

[BatBen78]

hmmm. my vertical line has been autoformated.
all vertical lines should be in a line...

[BatBen78]

Hi@all,

is a my lambda/4 or is b my lambda/4?

greetings Ben

[bigSteve]

Ok I see. So the housing and the PCB are grounded?

Your dimension A should be close to lambda/4. But what is the distance (B-A)? I would imagine pretty small?

Also be careful that nut with a thin washer will introduce a lot of capacitance, if it is too thin your antenna will have small bandwidth and not much radiation.

[BatBen78]

Hi bigSteve,

b-a is appr. 10mm-15mm. Thanks for the tip with the capacitance, i will try to use a plastic nut.

Thank you!

Ben