CHEMANDY ELECTRONICS

FLEXI-BOX RF Bridge

The RF Bridge is built in a FLEXI-BOX on a dedicated PCB. The connectors can be selected from BNC, N and SMA. The bridge (they are sometimes referred to as a coupler) is a 50Ω device and has an Input, Output and a Coupled Output. It is intended as a measurement aid and is also part of the Standing Wave Kit. We can build and test these at Chemandy with an insertion loss of 0.5 dB + 0.4 dB, coupling 16.5 dB + 1.0 dB and directivity better than 20 dB from 0.3 MHz to 300 MHz.  See figures 1&2.   

Figure 1

Figure 2

Bridge Principal

    The RF Bridge uses two transformers which can be wound on two separate toroidal ferrite cores or one binocular ferrite. These can be connected with phase relationships as per Figure 3 and this gives a 180⁰ coupler. By altering the phase of the windings, a 0⁰ coupler can be configured. The bridge measures power with the (top, in Figure 3) transformer inserted in the line detecting current and the lower transformer detecting voltage. This is sometimes called a Tandem Bridge. 

    If a signal is passed from the Input to the Output, it will also appear on the Coupled output, but at a lower level. The difference in input level and the level at the Coupled output is the Coupling. A signal passing from the Output to the Input will appear on the Coupled output but at a very low level and a perfect bridge would have no output in this direction. The difference in level at the Coupled output between a signal applied to the Input and a signal applied to the Output is the Directivity. The value of the termination determines the impedance of the bridge and in some designs, this is can be provided by an external load. The use of an internal load allows the circuit to be adjusted for optimum performance, mainly directivity.

Figure 3

Experiments

    The PCB supplied with the FLEXI-BOX RF Bridge kit is suitable for experiments and has good isolation as can be seen in Figure 4. It is of course very easy to work on as are all of our projects. A selection of ferrites are included with the kit version and larger quantities can be obtained from http://rswww.com/ . If you require high performance from your own design then be prepared to spend hours tweaking wire positions, winding ferrites and making screens and possibly screened coaxial links. It is a fascinating process and very educational but extremely time consuming. There is a very good article on these devices at http://www.members.tripod.com/michaelgellis/direct.html 

Figure 4

      This test aid is available Ready Built or in Kit form. The Kit contains the hardware only; i.e. box, pcb, connectors, screws etc. Components are not included because they are surface mount and  it is impossible to supply the correct amount because of the 'ping' factor. Anybody who has fitted surface mount components will know that when a component is being placed with a pair of tweezers there is often a 'ping' and the component disappears into thin air. We would have to supply at least two of every component thus doubling the cost.

A selection of escutcheon drawings are available for downloading as .zip or .bmp files.

Pricing

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