A Splitter/Combiner

A while back I build a combiner for some two tone tests on some older type transceivers. Well the transceivers have now gone and so I found new use for the combiner as a splitter. 

The combiner has pretty good port isolation of over 50dB and an insertion loss of about 6dB. To test some of my SD-Receivers I use the splitter to divide the receive signal from the antenna and feed two SDR's so I can compare rx-prowess. 

The toroid I used is my trusty old workhorse, a Jaycar Model LO1230 (with the dimensions of 18x10x6mm) which has very similar characteristics as Mix 43. A FT50(A)-43 might do a slightly better job (maybe), but the LO1230 is available locally at a reasonable price and does the trick nicely. The resistors are 50Ω resistors (green gold white white yellow) (I have a few left overs from other projects) but a 49.9Ω 1% resistor should do the trick as well. 

Now, the 100nF capacitor is more or less an insurance policy, it is used as a DC blocker. You could leave it out if you so desire. I've put 12 bifilar windings around the toroid and that's it. Nothing to elaborate. 

Here is a quick sketch of the unit:

And here are some measurements of the unit.

1. Attenuation:

2. Port isolation:

As can be seen the isolation is very good from 80-6m (3.4MHz - 50MHz) but is still good enough for rx testing below 3.4MHz and above 50MHz. I might test again to see what the specs are for below and above frequencies. Anyway as it is it works for me at the current use, which is being used as a splitter and not a combiner.

3. Photo of build unit:

I currently have the unit setup for my SDR-IQ and RSPdx to compare rx prowess at ELF, VLF and HF using SDRConsole, SpectraVUE, SDRuno and HDSDR. The 6dB attenuation is only a problem at the higher end of the spectrum which I can compensate with an assortment of low noise amplifiers (LNA's) from Minicircuit and homebrewed units.