Antenna connector

Thread Starter

Zeeus

Joined Apr 17, 2019
546
Trying to connect an antenna (a wire>10m) to a bandpass filter (BPF)

It's important for me to know the impedance of the antenna

Thinking of using a 51 ohm resistor - using this resistor across the end of the wire to ground and then coupling to the BPF (this way the BPF sees 51 ohms- assuming the impedance of the wire is much larger)

But think not a good idea to add a resistor there.

Use an emitter follower?

Please what am I missing/ what can I do?

Thanks
 

Delta prime

Joined Nov 15, 2019
330
It's important for me to know the impedance of the antenna
With the information you have provided this will steer you in a general direction you can improve this direction by accounting for the operating frequency the operating frequency is all you require just chug and plug. Do not talk about emitter followers bandpass filters or confusing a DC resistance to an AC impedance ;) here you go.
Impedances in radio-frequency transmitters must be matched to pass maximum power from stage to stage. Most impedances include inductances and capacitances that must also be factored into the matching process
Antenna impedance must equal the transmitter output impedance to receive maximum power.

The transmitter output is usually connected to the antenna via a transmission line, which is typically coax cable. In other applications, the transmission line may be a twisted pair or some other medium A cable becomes a transmission line when it has a length greater than λ/8 at the operating frequency
where:λ = 300/fMHz
λ = 300/fMHz = 300/433 = 0.7 meters or 27.5 inches​
A connecting cable is a transmission line if it’s longer than 0.7/8 = 0.0875 meters or 3.44 inches. All transmission lines have a characteristic impedance (ZO) that’s a function of the line’s inductance and capacitance:
ZO = √(L/C)​
 

DickCappels

Joined Aug 21, 2008
6,384
Your emitter follower idea is one I have seen done in a receiver that needed to distribute the incoming signal to many different circuits on different bands. The trick is estimating or calculating the emitter follower's impedance to you can match it to your filter.

Chapter 4 of this book covers impedance matching nicely.
https://archive.org/details/BOWICKC.1997.RFCircuitDesign/page/n175/mode/2up

This person demonstrates a home-made antenna matching network at 40 meters and 2 meters.
https://www.google.com/search?client=firefox-b-d&q=random+wife+antenna+tuner#kpvalbx=_CgbFXvHhDLDTz7sP0t2B-As53
 
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