Saturday, February 16, 2019

Resistive antenna bridges

You're listening to a weak signal. There's fading, there's static, there's a station a little too close for comfort. But you can just about make out all of their transmission.

That is until the carrier appeared. Your S-meter swings wildly between S9+20 and S9 +40 as adjustments were made. When you thought it was over it appears again.  Followed by speech.  "Haarlo Harlo 1 2 3". Apparently oblivious of or uncaring towards the activity on the frequency.

All that could have been prevented if the other station used a resistive antenna bridge. This is a small device that allows your antenna coupler to be adjusted without radiating a signal.  True, they don't measure VSWR or RF power output. Nor do they replace an antenna analyser. But they are cheap, self-powered, and, if you wish to build one yourself, requires fewer than ten or fifteen parts. They also protect the transceiver's final from wild impedance swings that can occur when adjusting the antenna coupler.

A resistive antenna bridge is just a few resistors, capacitors and a potentiometer. Oh, and some sort of indicator that shows the null you get when you've tuned up correctly. That could either be an (increasingly rare) meter movement or LED. Or, if a voltage controlled oscillator is added, an audible indication.

 Here's a few bridge ideas if you'd like to build your own.

GQRP Club resistive SWR bridge by G3ROO & G4WIF (PDF)
Determine SWR with a resistive divider Circuit Salad
ZS6AZP's investigation of a resistive bridge
G0WQR resistive bridge with LED from Sprat
L-match antenna coupler with resistive bridge from my site
Inside a resistive antenna bridge my video

PS: The items below may assist your experiments.  They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.

No comments:

Post a Comment

Upgraded website

Enjoy reading about diverse facets of amateur radio? Like building projects? Sometimes find my videos hard to find? If any of these applies ...