Common Mode Chokes for RX Antennas
Challenged by the QRM picked up by my omni directional active RX vertical when trying to work TL8TT on Topband, I decided it was time for a directional RX antenna pointing south in addition to my N/W and N/E Pennants. I had a transformer and termination resistors collecting dust so I decided to throw out some wire to make a short BOG, Beverage On Ground. Since I didn’t have any proper common mode chokes at hand I just wound about 10-12 turns of the coax around a FT-240-31 core. How does this quick fix compare with a proper RF choke?
The BOG works, but it is a bit noisy and seems to pickup more than it should from the sides. I suspect the 25 meter long feeder between the transformer and the RX antenna switch pick up signal, so decided to make a few new chokes of my preferred type to improve the situation.
A few years ago I found a test report with measurements on different common mode chokes by N4IS. Looking through the options and what cores I had laying around at the time I decided that a model with 20 turns of RG-174 or RG-316 on a FT-140-77 core was the best solution enough solution for Topband. I made a few of those and put into plastic boxes, but I never had an instrument capable of verifying they worked as advertised.
Recently I bought the MetroVNA, a tiny Vector Network Analyser developed specially for ham radio. With this little unit it is extremely easy to test and measure antennas, filters and transmission lines. Testing my chokes revealed that they did work well and performs close to the measurements by N4IS.
I also compared the difference between my preferred choke with 20 turns of RG-316 on the FT140-77 core with 12 turns of RG-58 on a FT-240-31. It turns out that the 20 turns of RG 316 on FT140-77 is superior on low bands.
The difference is biggest where it matters the most to me! On 1.8 MHz the advantage of the 20T/FT140-77 choke is 10 dB higher attenuation. On 3.7 MHz the 20T/FT140-77 choke still better but the difference is less, around 2db. On 7.1 MHz the 12T/FT240-31 choke provide around 3 dB more attenuation than the 20T/FT140-77, but 40 meter is less important to me as I more seldom use the RX antennas there.
It was easy enough to verify the difference using the internal screen of the MetroVNA. But things gets even more exiting when the unit is wirelessly paired with a PC or Mac using Bluetooth and then used together with one of the desktop applications available. The graph above was created in Excel after exporting MetroVNA data to a comma delimited format using the vna/J software developed by DL2SBA.
Winding the choke is pretty easy. If you have a choice between RG-174 or RG-316, go with RG-316 as it has a teflon jacket making it a tenth of a millimeter thinner than RG-174. You will easily fit the 20 turns on the core and leave a small gap between the first and last turn. With RG-174 you might only fit 19 turns depending on the cable quality.
Now there is a handfull of new chokes waiting to be installed and used at SE0X. Hopefully the BOG will be a little more quiet. It seems like there can never be too much ferrite added to the setup! 🙂
N4IS test of chokes: WF_CHOKES_N4IS_REV01.pdf