# How to calculate the impedance and inductor value of an antenna?

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#### microlab

##### Full Member level 5
dear friends

how can we calculate the impedance of an natenna?
i would like to do this for the antenna of the fm radio arial
how can i calculate the impedance of the same?
and how can i use the same as an antenna with the impedance of 50 ohm?
i understand it is possible by adding an additional inductor to the same.
but how can i calculate the value of the inductor?
should i reduce the antenna length?
is there any equation to find out the antenna impedance and the inductor value?
i want to use the same for the rf transmitter working at the frequence of 433 mhz

thanking you all

ml

#### beetro78

##### Member level 3
wsvr meter

you can calculate the antenna impedance by measuring the WSVR (ROS) on the coaxial cable feeding the antenna. Pay attentions to the cable length..it should be to much long, otherwise you the measured can be affected by the cable attenuation. On the other hand if the cable is to short you cannot admit a TEM wave only, you should consider also cut off modes...2/3 guided wavelength after/before each connection should be ok. Clearly you need a VSVR-meter (ROS-meter in some country).

If the antenna impedance mismatch wwith the cable intrinsic impedance you need an impedance adaptor...normally you can buy a tunable adaptor..obviously you can also buy capacitive/inductive and resistive elements and construct it buy yourself...

Matching the load to the cable impedance with passive elements is tipically not a broadband solution (in principles it perfecly matches at only 1 frequency). With tunable elements you can tune the elements value to work on differents frequencies...

Often the VSWR-meter integrates a tunable impedance adaptor.

Yes, you can use equation of the simple circuit made of the source generator, the trasmission line modelling the cable and the load antenna (once you calculated the antenna impedance). But, probably if you ask the question you do not know the theory and you'll need a few of time to get inside the matter...anyway you can find all the equation u need on Microwave Engineering, Pozar (search on EDABOARD).

I don't understand why you want to make the antenna shorter..to work at higher frequencies? to make it compact?

I don't know if you antenna can work on the 433 Mhz...maybe it can, it depends on the specifications...post it. Anyway, 30 cm seems to me to be more or less half wavelength at 433 Mhz....so I would say yes, without any other info...

Bye

#### microlab

##### Full Member level 5
Re: ANTENNA

dear friend

thank you for the mail
i am using this antenna because it's available here
the system look cute than using a wire hanging from the system
the pcb antenna design is too tricky
the 1/4 wave length is about 16.5cm
if there is any other solutions,pls let me know

ml

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