Serial Port Splitters in Amateur Radio Applications

Serial port splitters are software utility programs that can turn one physical serial port into multiple virtual serial ports. When serial port splitters are being discussed, the first question asked is usually "why would I want to do that?".  To answer that, you need to know that serial ports can generally only be used by one program at a time. Let's suppose that you use DxLab software for logging and rig control. All you had to do is just connect a computer to the radio to read the frequency, log it, and QSY the radio when you click on a spot from the Dx cluster.  

OK, simple enough. But suppose you want to run another program at the same time and you want that program to also communicate with the radio. For example, you might want to enter a Dx contest and use a dedicated contest logger like N1MM and at the same time run DxLab SpotCollector in order to watch for new band countries. and QSY the radio to the spotted frequency. Or, you might want to run WSJT-X and a logging program at the same time. Well, now you have a problem. You shouldn't set two programs to use the same serial port, nor can you use a Y cable. If you do, both programs will try to communicate with the radio on that port and neither will work. What you need is a way to "split" the radio's serial port into two distinct ports so it can communicate with more than one program. You might even need a third serial port for an amp or antenna switch. Once you start automating the urge to expand is natural. Fortunately, it's very convenient to create a virtual serial port for each program you run that needs access to the serial port connected to the radio. Once a separate virtual port is assigned to each application you don't have to worry about which programs happen to be open before you open another. They could all be open and it wouldn't matter.  

That is basically a description of my setup. I have used the Eltima Serial Port Splitter successfully for several years to split a physical serial port (Com3 in my case) into three virtual serial ports (Com8,9,10). Each of my dedicated programs is set to run on one of those virtual ports and all three programs can run simultaneously.  You simply identify the physical port you want to split and create the virtual ports that your programs will use. Your computer and software applications treat these virtual ports as if they were physical ports. They even show up in Windows Device Manager. 

20210416...Update!!  I recently purchased an SDR radio. These radios generally do not have physical serial ports. Instead, they use a virtual serial port (on the ethernet connection) to communicate with software programs. That presents a problem for most serial port splitting utilities which can only split physical ports. That's the situation I ran into with Eltima. When I tried to split the radio's virtual com port the SDR's control program could no longer communicate with my logging program. Fortunately, I found a utility that can split not only physical ports but virtual ports, as well...Eterlogic VSPE (virtual serial port emulator). While this program requires a bit more geeking ability than some other solutions like Eltima, it is very flexible and powerful as well as very affordable at only $25 (try before you buy). Eterlogic is now my recommended solution and is an essential utility for SDR owners who want to split a virtual port. Eterlogic's VSPE allows up to 8 programs to utilize one virtual serial port. If you decide to try VSPE, here are a few tips to make things go smoother:

• Click on the white space to create a split com port
• Note that the port you want to split should be selected on the right side of the panel
• The single virtual port you create on the left side can be used on up to 8 applications
• Since you will want to run VSPE every time the computer boots up, you should make it a "Windows Service". Click the "helper" tab and follow the instructions. Be sure to run the Windows CMD screen as an administrator. 
• You can download a trial version of VSPE and try it out before you buy. It is fully functional with this caveat: Windows Service cannot be set up. The program must be run manually. 

160 Meter Shunt Fed Tower System - Top Band Vertical Antenna

I wanted to use my tower as a vertical on 160 meters. The tower is an LM-470D motorized 70 foot crank-up supporting a ten foot mast and the following antennas: KT34XA, 40-2CD, 3el 6m, and 80 meter quarter wave sloper This system is designed to allow the tower to be raised and lowered each day. The 3 foot metal stand-off arm is attached at the third level, approximately 50 feet fully extended, and the pipe is 15 ft long. The only purpose of the pipe is to stabilize the gamma wire so that it doesn't get tangled in the tower when it is raised or retracted. There are other ways to accomplish it, but this system is simple and has proven reliable over a 20 year period. If a fixed height non-retracting tower is used, the pipe is unnecessary. I mounted the omega match inside on the barn wall to get it out of the weather. Prior to moving it to the barn, I had it at the base of the tower, first in a Tupperware style container and then in a heavy duty irrigation box. In both cases, I had to regularly deal with animals, insects, and moisture. It worked, but it was high maintenance compared to this system, which has been "no maintenance". There is about 100 sq feet of aluminum sheet tied to the base of the tower, which is earth grounded. Two 1/4 wave elevated radials are attached at the top of the first section, about 15 feet off the ground. These radials are not grounded and a tap wire runs from the radials to the top feed through insulator. This system has been in use in more or less this configuration for 20 years with about 120 DXCC countries confirmed through casual DXing on the Top Band. See photos of this setup at QRZ.com.

April 27, 2021 Update: Choke Failure and Conversion to an L Matching Network

Lesson learned: a choke on an antenna mounted on a shunt-fed tower can experience substantial RF stress during operation on the top band.

During operation on 160 meters at 1,500 watts, the coiled-coax choke feeding my 40-meter Yagi failed. The photograph shows the damaged choke and burn marks on the boom.

I suspect that coupling among the tower, Yagi, and feed line subjected the choke to excessive RF voltage or current. I was unable to establish the exact failure mechanism.

VE6WZ documented a similar balun failure and suggested that shunt feeding his tower on 160 meters might have been responsible. His experience and mine illustrate a potential problem in these installations, but they do not establish that every coiled-coax choke will fail. 

I replaced the damaged choke with a DX Engineering Maxi-Core 20 1:1 balun and its associated mounting hardware. After the replacement, the tower system’s impedance changed enough that I could no longer obtain a match with the existing omega network. My interpretation is that the original choke and its interaction with the Yagi and feed line had contributed to the loading of the tower system. Without before-and-after impedance measurements, however, I cannot say precisely how that loading changed.

A conventional two-capacitor omega network requires a suitable inductive load at its connection point. Whether it can match a particular tower depends on the impedance presented by the complete installation, including the shunt conductor and tap location. Physical tower height alone does not determine whether it will work. However, the tower must be electrically long to achieve a match. 

 

click to enlarge

Boom Burn

In my installation, replacing the choke shortened the electrical lenghth of the system such that the Omega network could not produce a match. I replaced the two-capacitor omega network with an inductor–capacitor L network (below) which resulted in an indicated SWR of 1:1 at 1.825 MHz. Subsequent high-power contest operation has shown no observed failure or deterioration.

The original work described here preceded my ownership of a vector network analyzer (VNA). A properly calibrated VNA can measure the resistance and reactance presented at the matching-network connection point. Those measurements can then be used to calculate suitable matching-network configurations and component values.

The L match mounted on the wall of the barn. 

The 2kw inductor is tapped at 7 uH and the capacitor is a 90-150 pF variable

My conclusion is that stacks under 80 feet in height may require the L match and not the Omega match to resonate. I have gotten some inquires about how to build the L network.  The variable capacitor is frequently available on eBay used. The coil can be purchased, built on a form, air wound or on PVC. If you need to build the coil, an inductance/capacitance meter is helpful These are available on-line at low cost. 

Adapting a Two Pin Speaker Mic to the Elecraft KX2 and KX3 (Kenwood, Baofeng, Pofung, etc)

Finished Portable Ear bud-Mic

Following up on the "no-weight microphone" post (below) is an adaptation of two pin speaker mics often included with VHF HT's and readily available inexpensively on-line. These mics have the advantage of a PTT (push to talk) button. Shown above is an ear bud/mic that was included with a Pofung HT. I chose to modify this because the Elecraft headphone jack has low output and I didn't want to hold a speaker mic to my ear during reception. However, this adaptation will work for with a Kenwood style speaker mic, as well as with the ear bud mic shown above.

Parts required:

• glue gun
• one 3.5mm stereo to 3.5mm stereo 1 meter cable
• perf board
• ear/mic or speaker mic
• heat shrink tubing

Instructions:

• Cut the stereo cable in half. Strip the ends and determine the color coding to the tip, ring, and sleeve of the plugs.
• Cut the two pin connector from the ear bud mic. Leave 3 inches of cable with the plug. Strip the cable and determine the color coding for the tip, ring, and sleeve of each plug. These wires are tiny and cloth type, so put a dab of hot solder on the ends to make them conductive. 
• Solder the wires from the ear mic in a row on the perf board and then match up the appropriate wires from the stereo cables in the adjacent row and solder them. Bridge the lands between the stereo cables and the ear mic with solder. Note that the speaker ground (sleeve on the small plug) will have two connections and that not all wires will have a connection. 
• Test the assembly with you radio.
• When it tests OK, use hot glue to affix the cables to the perf board, wihich provides strain relief. 
• Slide a piece of heat shrink tubing over the perf board and shrink it to provide protection. 
• Adjust the mic gain and compression on the radio to match the mic characteristics. 
• Note: because ground is provided by the speaker plug, both mic and spkr must be plugged into the KX for this to work. If you want to use the KX speaker instead of the earbud,you must put the unused earbud plug into the aux jack of the KX to provide ground in order for the circuit to function.
  

  

  Hot glue for strain relief

  

  Wiring diagram for two prong speaker mic to Elecraft