# RF's output impedance to couple antenna

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#### julian403

##### Full Member level 5
If I look the next circuit, the real antenna's impedance, it's 1/hoe???

So, if the coaxial and the antenna are 50 Ohms. What can I do?

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#### Osman Ceylan

##### Junior Member level 1
You can calculate the impedance but it seems it is not easy. You can try to find with an EDA tool such as AWR or pspice. If it is not high power (it seems it is not), no problem, just use it. There is a small trick. If it is a tv related circuit, it is probably 75ohm. otherwise it is usually 50ohm.

#### chuckey

The output impedance of the transistor is modified by the two PI sections, so the actual output impedance could be anything, but I guess its 50 ohms or lower depending on the length of the aerial in wave lengths.
Frank

julian403

### julian403

Points: 2

#### julian403

##### Full Member level 5
But, how it can be a real impedance (like 50 Ohms) if there are just capacitor and inductor in the output?

##### Super Moderator
Staff member
I'm willing to try to break down the terminology by process of speculation, not being a radio expert. To send a broadcast requires that electrons be turned into photons somewhere in the antenna. The impedance in this case does not involve current through a particular component. I think it has to do with the ease of sending electrons out of the circuit. Therefore if voltage is sufficient, and the antenna is tuned properly, then impedance drops, and electrons depart the output stage without returning.

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The output impedance is found in the voltage & current which must be applied in order for the antenna to transmit.

#### chuckey

Suppose you have a tuned circuit with an inductor and a capacitor across it. At some frequency when the capacitive reactance equals the inductive reactance, the circuit resonates and appears to have a high restive value. Splitting the capacitor into two each of twice the value and they are in seriies(so the circuit is still at resonance) will result in a load across one capacitor appearing as the same load across the other. If you earth the join of the capacitors, you have the PI network, with Rin = Rout. Changing the ratio of the capacitors, while keeping their effective value the same, will transform Rin to any value of Rout.
Frank

#### Osman Ceylan

##### Junior Member level 1
You were right if there were only L and C. But there is transistor collector impedance seen into collector. Depending on your bias condition, a real impedance requirement appear. R(class A)=2Vce/Imax

julian403

### julian403

Points: 2

#### julian403

##### Full Member level 5
But that real impedance which it's in series whit the antenna impedance (the antenna's real part is the loss and the energy radiated), it's hoe? there is not another resistance. The reactance make of capacitors and inductors are filters and make the correction antenna reactance. Because the tank circuits, which resonate is in other step (in the first step).

The resonance circuit it's the BB115 and L1 whit the trimmer. It's in the first step. So it's not the transmition's output impedance. The elements in the output are L5 L5 and the 15p , 1n, 56p and 47p capacitors. It's all reactance, there is not 50 Ohms (real impedance) likes antenna which has 50 Ohms, there is j Xc.

#### chuckey

That 56P should be two capacitors, one to terminate the first PI section, the second as the input C of the next PI section. For obvious reasons the twp capacitors are combined into one.
Frank

#### julian403

##### Full Member level 5
And the real output impedance it's hoe. Where hoe= Vce /ic

#### FvM

##### Super Moderator
Staff member
And the real output impedance it's hoe. Where hoe= Vce /ic
No. The real output impedance at low frequencies is set by the Early voltage Va, see https://en.wikipedia.org/wiki/Early_effect
At radio frequencies, other parameters like collector bulk resistance and transit frequency in combination with the reactant transistor impedances are forming the real output impedance.

julian403

Points: 2