Saturday, May 4, 2019

The T-match antenna coupler

One of the simple and common forms of antenna coupler is the T-match. So called because its circuit element form the letter T. You can sort of see it in the diagram below.

In some ways the T-match is the opposite of the pi-network (also named after how its parts look on a diagram). Whereas with the pi network both variable capacitors have their frames earthed, in the T-match they don't. Conversely the T-match has one side of its inductor earthed while in the pi-network it's floating. 

Another case where they're different is their behaviour above the frequency to which it is adjusted. A pi network acts as a low pass filter. Your harmonic suppression should really be done in the transmitter but if it isn't the pi network can help suppress them. 

Whereas a T-network acts as a high pass filter. So don't expect any harmonic suppression from it. Instead it behaves like a high pass filter. But that can sometimes be good. For example if you live near a 530 - 1700 kHz broadcast station your receiver may be overloaded by strong signals from it if its front end is weak or unselective. A T-match coupler offers attenuation at these lower frequencies so should lessen or eliminate the problem. 

The T-match can match antennas that present a wide range of impedances. It's unbalanced. So you can use it with end-fed wires and the like. Or even coax-fed antennas. That's fine if the antenna is only slightly off. But otherwise bear in mind the risk of significantly increased losses if there is a large mismatch and a long run of coax.
As you can see from diagram both sides of both variable capacitors are floating. That makes construction harder. You need to find a way of insulating the frame from the coupler's case. Instead of metal you could use an insulated wood or plastic case. Also the shafts will be hot with RF so make sure knobs are plastic and well insulated with no protruding grub screws etc. 

If you'd like to build a T-match antenna coupler here's a few ideas to get you started:

* KC8AON's mini T-match
* VK5AJL T-match antenna couplers
* VK6YSF T-match antenna coupler
* W4ULD Getting the most out of our T-match (pdf)

PS: I have written five books on various facets of amateur radio. They are available in electronic or paperback form. Find out more here.

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