How to know the impedance of a random wire antenna?

[trxguy]

Hello all,

I want to use a 600 ohms : 50 ohms balun to match a 10 feet random wire to my HF receiver. From the book said, the wire should be 30 feet long for 600 ohms.

(Q1) If I reduce the length of the antenna, the impedance would be increased? If yes, how much is the impedance of that length?

(Q2) Can I use a L-C low pass circuit to make a step-up to replace the balun transformer?

Thanks,

trx

 

[youssef_zein]

no the impedance decrease. since you are talking about a single wire antenna.it is considered to be monopole.
you could simulate it using the antenna optimizer software.
and about the LC matching circuit yes it is possible as long as you are working in a receiving antenna, cuz in a high power transmitting antenna the inductor will make problems (the radiating fields could affect the operation of the circuit). you will have to use torriods and spcial cores to handle high rf powers.

 

[vfone]

The single wire antenna has high impedance at its ends (maximum voltage) when is λ/2.

The value of the impedance is dependent also by different factors as, wire height from the ground, objects near the wire, wire resistance, etc.

Changing the wire dimensions it will change the antenna impedance at specific frequency.

You can change electrically the physical dimensions of an wire antenna placing a series inductor (if antenna is too short), or a series capacitor (if antenna is too long).

Using a tunable Pi matching network (2 shunt caps and 1 series inductor) will give a broad match for different wire lengths (or antenna impedances).