# How to connect a low impedance RF amplifier to an antenna?

I'm trying to maximize power transfer from my amplifier to my custom antenna, but am unsure about how to properly impedance match the system.

My amplifier has an output impedance of 4Ω, but is connected with a BNC cable, which are usually 50Ω.

My custom antenna has an input impedance of 35Ω.

My signal frequency is 1860 kHz.

To maximize power in my antenna, should I create a matching network to make my source impedance 35Ω as well (I am afraid the off-axis inductance this creates will eat away more power from the antenna)? Or is it better to keep my source impedance low? And isn't the 50Ω cable messing things up in general?

Thanks for the help!

## 2 Answers

isn't the 50Ω cable messing things up in general?

No, the important thing is the coax - it has a fixed 50 ohm characteristic impedance and therefore, to prevent reflections from the antenna (35 ohms), you should use an RF transformer to back-convert 35 ohms to 50 ohms. This then matches the antenna to 50 ohms.

A simple 1.2:1 step down RF transformer will do this because the square root of the impedance ratio is 1.195 and 1.2:1 is good enough. This transformer needs to be placed at the antenna end of the coax. The voltage step-down is towards the antenna.

So now, your transmitter will see 50 ohms and you can stop at this point. 12 turns to 10 turns seems nearly perfect at the antenna end.

However, if you truly want to maximize power transfer then you can step up (using another RF transformer at the amplifier end) the 4 ohm amplifier output to circa 50 ohms. The square root of the impedance ratio is 3.54 so a 10:35 ratio step up transformer would be good if you really wanted to push things. Of course you may be limited by power output and by legislation.

General idea: -

Transformer design needs care and I've estimated the number of turns based on good RF ferrite material and a typical pot-core. The devil, as always, is in the detail and, this design should be undertaken methodically so as to produce sufficient magnetization inductance in order to minimize unnecessary amplifier loading.

Use a 9:1 UnUN , there are tons available as a device and tons of DIY's like Steve Ellington on youtube.

Closest whole number ratio, since you can't have partial turns , is 36 / 4 = 9 :1 , is this easier to understand ? Also the Characteristic Impedance of a Coax or any Transmission Line is not the impedance at a random length of cable. Characteristic impedance is the 'Amplitude' in the sinusoidal function of the impedance along a wavelength of that cable. There are other ways to match these 2 impedances , but the simplest for these conditions is the 9:1 UnUn solution.