Common Mode Current Choke

Common mode current (CMC) is current that flows along the outer shield (braid) of coaxial cable used as a feedline for driving certain types of antennas.  It is undesirable because it can introduce EMI inside the home environment affecting not only your transmitter/transceiver but other nearby electronics as well.

By increasing the impedance along the outer shield, the current is effectively "choked", and the problem is reduced or eliminated.  This can be accomplished by placing ferrite beads around the outside of the coax or wrapping the coax itself around a toroid to increase the inductive reactance.  Note that the inner conductor and inner shield current paths of the coax are not affected.

I decided to go the toroid route for my project.  I used a 4' length of RG58 and wrapped it around an FT240-43 toroid twelve times.  This particular toroid is a type commonly used for the HF amateur bands (3-30MHz).

Next, I measured the attenuation by connecting it to my vector network analyzer.  From 3 to 20MHz the attenuation was at least 30dB (ie a 1/1000th reduction). The minimum attenuation occurred at 30MHz but was still greater than 25dB.  Objective achieved.

Attenuation achieved with RG58 coax w/ toroid (12 turns)

As a baseline for comparison I then took a short piece of the same type of coax and measured its attenuation.  The maximum value was under 7dB.  Obviously, quite useless as a choke.

Attenuation achieved with RG58 coax only (no toroid)

Finally, the completed choke was installed inside a water resistant case and inserted in-line with my feedline.

Completed CMC choke in water resistant case

 

WSPR Transmitter

WSPR (Weak Signal Propagation Reporter), is a digital radio protocol designed for probing potential propagation paths using very low-power transmissions. Developed in 2008 by Nobel Prize-winning physicist Joe Taylor, K1JT, it is a useful tool for amateur radio operators to test antennas and global band conditions.

After reading about this and finding out how little it would cost to take the plunge, I started looking into setting up a basic station and eventually came across a series of kits from QRP-LABS.  Unfortunately, once it arrived it sat on my workbench for quite some time while I moved on to other things. Well, I have finally completed the assembly and wrapped up my initial tests.

My particular kit consists of the control unit (which provides a user interface for entering and customizing various parameters such as operating modes, transmit frequencies, etc.), a clock generator for square wave synthesis, a GPS receiver for precise timing of the transmissions and several filters for the various amateur bands of interest (I chose 10m, 15m, 20m, 30m, 40m and 80m).

After winding the toroids for each filter, I made a final check of their inductance values before soldering them into place.  I made these measurements with a NanoVNA-H4 that I purchased as a gift to myself for Christmas. This amazing device can be used for numerous tasks around the workshop (measuring antenna characteristics such as resonance, SWR and return loss, measuring unknown L/C values and evaluating filters, to name just a few) and all for under $100.

The output power for each band is summarized below (measured with a scope and terminated with a 50-ohm dummy load):

80m - 160mW

40m - 170

30m - 180

20m - 140

15m - 125

10m - 080

Below are images of three of the six filters (10m, 15m and 30m) showing the passband characteristic of each along with the corresponding sinusoidal output.  The vertical gray bar in each passband image shows the actual transmit frequency range.

10m LPF

28.126MHz carrier

15m LPF

30m LPF