Tuesday, May 28, 2019

The pi network antenna coupler

Want a simple to build antenna coupler good for a wide range of impedances? A pi network may be the thing for you. Simple to build it's ideal for a variety of unbalanced antenna including end-fed wires. Both variable capacitors in it are earthed. This makes construction easier if you're using a metal case. Another benefit is that it forms a low pass filter. That means it suppresses harmonics. Just the thing for use with a simple QRP rig that might only have one filter section! 

Pi networks for QRP can be built using a tapped coil on some plastic pipe or the inside of a kitchen paper roll. Or maybe a tapped toroid. Either use a rotary switch or aligator clip to select different inductances.  Plastic variable capacitors can work for low power (up to about 200pF for 7 MHz or 400 pF if you want 3.5 MHz coverage) but if you have them an air spaced metal type is preferred. Use a switch and a fixed capacitor (equal to the variable capacitor's maximum) to switch in more capacitance if required. Special transmitting variable capacitors should be used for high power. You'll find the parts at any good hamfest. 

Here are some practical pi network antenna coupler designs to get you started.

* KC5LDO Pi network tuner
* AA3SJ 50 MHz pi coupler
* WF7I pi network
* DJ0IP pi network (and others)

If you've built a pi network coupler please share your experiences in the comments below.

PS: Like something to read? http://home.alphalink.com.au/~parkerp/vk3yebooks.htm. Read worldwide and favourably reviewed.  Available in both electronic and paperback.

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