Impedance of a simple wire antenna for an FM receiver

[aht2000]

In lot of the FM receivers I see around, come with a simple black wire that you connect to the antenna input of the FM receiver and dangle it. If I use the same wire with an FM receiver I am trying to build, and knowing that the antenna impedance changes depending on a lot of things (length, position near by objects, distance from ground,... etc), so I assume that if I measure the impedance of such wire in the FM band with a VNA, it will never be a single figure that I can use to calculate my matching network.

So, how do engineers deal with such case, and what assumptions do they put in place to proceed with their front end designs?

 

[betwixt]

I would say the majority of domestic FM receiver designers have little or no knowledge of electronics and simply follow the component manufacturers data sheets. They consider a length of wire to be an antenna without any concern for impedance matching, it is essentially a cosmetic item they are expected to supply to the end user.
You are quite right to assume there is an optimum length for an antenna and an optimal impedance matching circuit but there are so many variables and different situations that it would be impractical to cater for them all so compromises are made.

Brian.

 

[FvM]

A wire antenna of 75 cm length corresponds to a lambda/4 monopole with about 40 ohms impedance, presumed sufficient ground is present. It would be pretty good matched to a single ended receiver input (coaxial antenna jack).

Unless you try to receive distant stations with a signal level near to the receiver noise floor, matching isn't so important for a FM receiver,any wire of some length will ususally work.

 

[Externet]

...impedance changes depending on a lot of things (length, position near by objects, distance from ground,... etc),

And when you find the corresponding impedance for that length, position, distance, ground... you tune another receiving frequency and all gets void.