Noise Bridge...Measuring Receiver Input Impedance.

[Yorkie]

Many years ago I built a noise bridge from a kit - quite a simple thing. It just generates white noise, and you place it between the receiver and antenna, rotate the control until there's a null in the noise, which is quite a sharp null, then read off the impedance on the scale of the bridge. Obviously if the input of the receiver is 50 Ohms, you want the antenna to match that, and by inserting a noise bridge between an ATU and the receiver you can tune it for best match. Nothing like as relevant for receiving as for transmitting of course, but you can't transmit through it, so you do the matching on receive.

When I was active in amateur radio I used to experiment a lot with antennas - I built a G4MH style mini-beam and used to make my own traps for trapped dipoles using double sided PCB as the capacitor in the traps, and winding coils on fibreglass spacers, tuning them with a home-brew grid dip oscillator that I made that went from 1.6MHz to 150MHz with a range of six plug-in coils. I calibrated the dial on my homebrew nixie tube digital frequency counter.

Later I bought an MJF 249 Antenna analyser which you could connect to the antenna feeder and sweep across the bands to see where you got a 1:1 match. Handy for drawing an SWR graph to tune the antenna for best match at the desired portion of the band - CW end or SSB. MFJ was not universally highly regarded, some (libellously) saying that the initials stood for 'Made From Junk'. I always felt that they made neat innovative products, nicely designed and built, and at competitive prices. When I bought the MFJ 249, it was the only analyser on the market and includes a digital frequency counter. I put it to a lot of use in the days when I was active in amateur radio.

The MFJ249 antenna analyser manual is here - I dare say that other models have since overtaken it:

http://download.qrz.ru/pub/hamradio/sche...Manual.pdf

I've attached a few pics which might be of interest. Pics of the noise bridge built into a little ali box 3" x 2" x 1" showing the inside and outside. A pic of an antenna trap ready to be housed in a 2" diam plastic tube with end caps, and the GDO that I made, with the set of coils alongside, and a 7.1MHz test coil in front, to check that the GDO is working and on frequency. (It was from a Practical Wireless design, back in the days when PW was mainly a technical and constructional mag).

All my yesterdays!

Cambridge kits do a kit for a noise bridge, but bizarrely, they include a piece of plain PCB and a pattern for you to etch your own PCB rather than supplying a PCB with the kit.

http://cambridge.eu.pn/2012/kits/kithtmjs/kitanb3.htm

Hope that's on interest.