Active Antenna Bridge Project

I have been thinking for a while about building an Antenna Analyser, I wanted something fairly simple, at a reasonable price and reproducible.

After a lot of searching the webb i couldn’t really find what i wanted, i liked the idea of using a DDS signal generator to keep it simple and stable. The Pic micro DDS control software i have used is from the website of Terry VK5TM, Terry made changes to the original software by Craig Johnson AA0ZZ, designer of the Pic-El board,  .
The software fitted the requirements for controlling the  AD9850 DDS very well so a big thanks to Terry for making the hex file available to download free from his website,  all the files on Terry’s site are for personal use only. If you want to use the software visit his site. .
The pcb was derived from Terry’s with some modifications for the Antenna Bridge.

The RF bridge was a harder choice, it could have been all singing and dancing with SWR, Resistance and reactance all calculated by a pic micro and displayed on an LCD display.I wanted a simpler bridge that would do all of the above but with just one cheap analogue meter.
Many years ago along with many other amateurs i built a Noise bridge, as the name suggests it had a wide band noise generator as the signal source and a bridge circuit, and the idea was to be able to adjust your ATU without transmitting QRM on the Bands. The Noise Bridge had the ability to read Reactance, Resistance, and it used the S meter of a receiver tuned to the correct frequency to show when there was a deep null in received signal from the Bridge, with this information you could tune an ATU, check the resonance of antennas and filters etc.
In other words it was what is called today an Antenna Analyzer.
The very thing i wanted, an RF bridge with a cheap analogue meter, A control to read reactance and a control to read resistance. Below is a photo of what i came up with.
There is no need for a receiver as the DDS signal generator provides a good sinewave at any frequency from 1 to 30 MHz, the output from the bridge is read on the analogue meter.

The end result is a simple Antenna Bridge that is easy to build and simple to use. it uses 8 x AA batteries or external psu.
We have five club members building it at the moment and i will update this post with the results of their labours.


Above is a picture of the inside, the whole thing apart from battery box and external psu socket is built on the lid of the box.
You will have noticed that the analogue meter has an SWR scale on it but this meter does not read SWR, when in use you just tune for a minimum needle movement (a deep null) which is  virtually no movement at all when this RF bridge is balanced.
The resistance and reactance controls balance the bridge, if the reactance is zero and the resistance is 50 ohms +/- a little and the meter is zero your halfwave dipole that you are testing will be at resonance, simple.

More soon,  Colin  G0NQE