The net is full of instructions on how to build excellent reflection bridges for UHF/SHF frequencies. However, my requirements are slightly different and as such I'm going to show you how to make a very nice bridge using a commercial BI-Directional Coupler with outstanding LOW frequency return loss. I've chosen a MiniCircuit PDC20-3BD+ Bi-Directional Coupler which, according to the datasheet is usable from 0.2 - 250 MHz with about 20 dB coupling and directivity between 38-50 dB, as the base for the reflection bridge. It is small and will be housed in a Schubert Filter Case using BNC Connectors. No need to go for N or SMA connectors at these Frequencies. To be able to compensate and get the best directivity I've added two small trimmer C's and made sure that the termination resistor is as close to 50Ω as possible.

This means that if the directivity of the bridge is 30 dB, we cannot measure a return loss > 30 dB. Even if the measurement approaches 30 dB say about 28 dB or there about, the measurement becomes ambiguous. The displayed measurement could be reading better or worse than the actual value. So to improve measurement accuracy the directivity should be as high as possible and the termination resistors should be as close as possible to 50Ω. The following design will be a bridge with a directivity of > 40 dB at up to 146 MHz, i.e. from HF to VHF.

Below is a picture of the unit I've build Pic 2. The little rectangular block is the PDC-20 on its back with the 8 pins sticking into the air. The two trimmers are 3-20 pf and the resistor is 49.9Ω 1% size 805 (measured 50.06Ω).

So to get the directivity of the bridge, I measured -70 dBm return loss (Pic 3), and according to the manufactures datasheet the coupling is approx. -19 dB. With that knowledge at hand we can now calculate the directivity of the bridge. It is approx. 51 dB (-70 dBm - -19 dB). And, as we can see in the below picture (Pic 4), the bridge is pretty flat up to 150 MHz (see testing a bi-directional coupler).

Pic 3 shows the bridge measured up to 500 MHz after the bridge case had been closed. I had to re-compensate the bridge after I put the case lid on. Drilled two small holes above the trimmers for access whilst the case is closed. The measurement in photo 3 shows the normalised result to cater for the bridge coupling factor. Looks pretty good and better than anticipated. Nearly 50 dB directivity up to 146 MHz. See the below Spreadsheet to give you a better understanding of what that mean for our VSWR/POWER measurements.

Pic 5 shows the coupling of the bridge which is certainly flat to 150 MHz and in spec with the datasheet. Well, I'm happy with the result, this will be my main bridge for any further measurements (until I move into the UHF spectrum).

Parts:

• • • 1x Minicircuit PDC-20-3BD+

    • 1x Schubert Case FG2B

    • 2x BNC Chasis mount connectors

    • 1x BNC Chasis mount plug (you need to do some searching they are not that easy to find)

    • 2x 3-20pf trimmers (as small as possible, 1-10pf is probably better)

    • 1x 50Ω resistor (2x 100Ω 1% or 1x 49.9Ω 1% or better)