Tag Archives: Homebrew

General, Homebrewing, Station Equipment

A Broadcast Interference (BCI) Notch Filter for General Coverage Receivers

General coverage short-wave receivers may lack preselection against strong AM broadcast stations, and these broadcast stations may overload the receivers. SDR receivers, particularly the USB type[1], that plug into your computer are examples of this receiver type.

When I was designing my 10-band QRP transceiver, I wanted to incorporate an AM notch filter into the receiver for general coverage. In this mode of operation, the bandpass and lowpass filters are bypassed leaving the front end of the receiver wide open.

Since there are no FM broadcast transmitters nearby, I chose to include a notch filter for AM only because the receiver gain in my QRP transceiver from 88 to 108 MHz is greatly attenuated.

Rather than design and build a suitable notch filter, I looked for a suitable commercial off-the-shelf  (COTS) product that I could package into what I wanted.  The solution was the Nooelec Flamingo+ AM – High Attenuation Broadcast AM Bandstop (Notch) Filter[2].

My transceiver is based upon the N3FJZ software and hardware architecture [3], and Rick’s software architecture provides for general coverage whenever the receiver is tuned to other than one of the 10 designated ham bands. Thus, under these conditions, all of the filters in the transceiver are switched to bypass mode. I made use of this feature to incorporate the Nooelec Flamingo+ AM Notch filter in the bypass path.

Should I decide to incorporate an FM notch filter into the design in the future, the Nooelec Flamingo+ AM notch filter is easily disconnected from the printed circuit board carrier, and it may be replaced with a Nooelec Flamingo+ FM – High Attenuation Broadcast FM Bandstop Filter [4].

The schematic of what was built is shown in Figure 1. Optically coupled, 2-channel Arduino relays[5] are employed. When a bypass command is asserted in software, the receiver RF is routed around the bandpass and lowpass filters in the transceiver when it is in receive mode, only, i.e., the bypass does not function when the transceiver is in transmit mode.

Figure 1. Filter Bypass With BCI Notch Schematic. When the software command is asserted to place the transceiver in bypass mode, the relay modules will bypass the bandpass and lowpass filters in the receive path and insert the Nooelec Flamingo+ AM – High Attenuation Broadcast AM Bandstop (Notch) Filter. Since the AM BCI notch filter is connected to the PCB with right angle SMA connectors, it may be replaced by an FM BCI notch filter, as desired. Please click on the figure to enlarge it.

A printed circuit board, Figure 2, was designed as a carrier for the optically coupled relays and AM BCI notch filter. Pin headers are used for all connections to the 2-channel relay modules. Dupont[6] wires provide easy interconnects for power and logic inputs while pin headers are used for relay connections into and out of the printed circuit board. Dupont wires may be homebrewed with suitable component parts and a crimping tool, or they may be purchased at predetermined lengths.

Figure 2. As-Built AM BCI Bypass Printed Circuit Board. The AM notch filter may be replaced by an FM notch filter of similar form-factor by disconnecting the right-angle SMA connectors. Please click on the figure to enlarge it.

Finally, the response of the Nooelec Flamingo+ BCI Notch was measured on a spectrum analyzer with integral tracking generator. The result obtained is shown in Figure 3. The measured notch depth is close to 70 dB over most of the AM broadcast band. This result compares favorably with the plot provided by Nooelec[7] in Figure 4.

Figure 3. Measured Nooelec Flamingo+ AM BCI Notch Depth. The performance of the AM BCI Notch printed circuit board was measured on a spectrum analyzer with integral tracking generator. The notch depth is approaches 70 dB over most of the AM band. Please click on the figure to enlarge it.

Figure 4. Nooelec Data. The plot found in the Nooelec data sheet compares favorably  with the measured data of Figure 3. Reproduced with permission from Nooelec. Please click on the figure to enlarge it.

Anyone wishing to duplicate this printed circuit board may contact me for a Gerber file. I will not be offering any printed circuit boards.

References:

1. https://www.nooelec.com/store/sdr/sdr-bundles/hf-bundles.html

2. https://www.nooelec.com/store/sdr/sdr-addons/flamingo-plus-am.html

3. http://www.remmepark.com/circuit6040/MAX-SSB/MAX-SSB.html#110

4. https://www.nooelec.com/store/sdr/sdr-addons/flamingo-plus-fm.html

5.https://www.amazon.com/gp/product/B081MVCS8F/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

6. https://www.amazon.com/TOAPPNER-Multicolored-Breadboard-Arduino-Raspberry/dp/B089FZ79CS/ref=sr_1_2_sspa?crid=IB4C4C33XMM4&dib=eyJ2IjoiMSJ9.tjHxIQLJsk16_0YVtUGN6Tqnr8euWNsWVjpSaq5RQkYtxZ9Cezy7x5qOhagKvYtMzwlO3bKCBbaL1aW0gvt6neKoy9ihFziKKV1XaMgGsZAE8xRYaSTrpxQdRvB0pAUE20gJVd3C2KcNPIu-KcdICH9n984YMZgPEz0KU8pLTtGa-RcD9BD6ef2DqvC9xEyQTaj2b0LmfNg1lNr1V_BlptXMnJAI1jqwkYqPQCB5h5I.fVWAD3xtI6a-TS73_L9fQ9c26h3fo70muKyIhPmYqA4&dib_tag=se&keywords=dupont%2Bwires&qid=1732556600&sprefix=dupont%2Bwires%2Caps%2C99&sr=8-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1

7. https://www.nooelec.com/store/sdr/sdr-addons/flamingo-plus-am.html. Op cit.

 

Homebrewing

A Simple Set of QRP Calibration Mismatches That You Can Build

I enjoy homebrewing, and when it was time to calibrate the Stockton Bridge for measuring forward and reflected power in my QRP rig, I realized that I didn’t have a set of mismatches that could be used for that purpose. A set of mismatches is also useful for checking an SWR meter, or a nanoVNA. There are off-the-shelf mismatches that you can buy to test your nano, but they do not provide the power handling capability that this design does.

If you would like to get started with PCB design, this might be a perfect project to begin with. I am not endorsing any particular PCB design tool that you might find online, but I have found that EasyEDA is easy to learn.

The mismatches that I describe here can be used for QRP. All resistors in the design are 51-ohms and 2 Watts. Since we are not using 50-ohm resistors, this will result in a small error. Also, the layout is distributed (spread out), and this will result in a bit of extra capacitive reactance for the larger mismatches. SMA connectors were chosen because all of the RF interconnections within my QRP rig consist of RG316 terminated with SMA connectors.

Figure 1 is the schematic of what was built. It seemed pretty easy to construct the mismatches from a single value of resistor, but there is nothing to stop you from building mismatches from one or two parallel values of different resistors. What I have here seemed like a good idea because it made calculations simpler, and it resulted in power-handling capability large enough for any QRP project that I envisioned. Values of 3.0:1, 2.5:1, 2.0:1, 1.5:1 and 1.0:1 were chosen as data points for the Stockton Bridge that I was calibrating. For continuous operation at 5W, a cooling fan is recommended, particularly for the 1.0:1 load bank as it has the fewest number of resistors.

Figure 1. A Simple Set of Mismatches. Use of a single resistor value, 51-ohms and 2 Watts throughout, makes construction easy and economical. Use of series and parallel combinations result in higher power dissipations. Please click on the figure to enlarge it.

Figure 2 shows the virtual 3-D layout as provided by the EasyEDA PCB layout tool. The tool shows you what you are going to get once the PCB is assembled. The EasyEDA library is extensive, and it provides additional capability to import manufacturer’s symbols and footprints not already in the library. Although this set of mismatches was built as a PCB, there is nothing to stop you from building the same design on a piece of copper-clad perforated board.

Figure 2. Calibration Mismatches 3-D View. EasyEDA provides a 3-D viewer so that you can see what the final product will look like before the PCB is fabricated and before the PCB is populated. The mismatch values are shown near the SMA connectors. Please click on the figure to enlarge it.

Figure 3 shows the shows the final product after PCB fabrication and population. It closely matches what is shown in Figure 2.

Figure 3. As-Built Calibration Mismatches. The actual PCB closely resembles the 3-D model shown in Figure 2. The overall dimensions of the completed board are 6.50” x 3.60” including the connectors. Please click on the figure to enlarge it.

S11 values for the mismatches were measured from 1.8 to 29.7 MHz and stored on an old MFJ-226 Graphical Analyzer. The values were converted to rectangular form in Excel spreadsheets to facilitate calculations. Smith Charts were also plotted. In order to make this article more concise, the Smith Charts are provided in the Appendix that follows.

If you would like the S11 data in the form of Excel, please contact me. The spreadsheets contain all of the formulas required to convert S11 magnitude and angle to other useful forms including S11 rectangular, impedance rectangular, impedance polar, VSWR and return loss.

If you would like the Gerber file for PCB fabrication, you may also contact me.  I will not be providing any bare boards, although you may wish to pool a PCB order to distribute the shipping costs among a few hams. For the JLCPCB supplier, the minimum number of boards is 5.

Appendix A. Smith Charts

Figure A-1. Smith Chart 1.0:1 Mismatch.

Figure A-2. Smith Chart 1.5:1 Mismatch.

Figure A-3. Smith Chart 2.0:1 Mismatch.

Figure A-4. Smith Chart 2.5:1 Mismatch.

Figure A-5. Smith Chart 3.0:1 Mismatch.

Antennas, General, Homebrewing

An Antenna for the Vertically Challenged

No, I’m not talking about short people! I’m talking about hams who may not have tall trees or the capability to put up a tower on their property. I want to tell you about a fascinating antenna design – a low-profile, high-performance solution ideal for those of us who might be challenged by restrictive antenna regulations or limited space. I’m referring to the Magnetic Radiator, specifically the Multiple-U (MU) design detailed in this article. This clever design comes to us from the inventive mind of Paul D. Carr, N4PC (SK) who had a column in CQ Magazine many years ago.

Now, you might be thinking, “Another vertical antenna? What’s so special about this one?” Well, let me tell you. This isn’t your typical electric radiator. This antenna operates on a fundamentally different principle—it’s a magnetic radiator.

What does that mean? Electric radiators, like your standard dipole or vertical, generate a strong electric field close to the antenna, leading to ground losses and less efficient radiation. This design, however, focuses on creating a strong magnetic field, minimizing those losses, and improving efficiency. Think of it as radiating power through the earth rather than into it. Some key advantages of the MU design are:

  • Low Profile: The vertical elements are less than 0.1 wavelengths high, making it perfect for locations with height restrictions. We’re talking about an antenna that’s practical for even the most compact locations.
  • No Loading Coils or Radials: No need for cumbersome loading coils or extensive ground radial systems. This simplifies construction and installation considerably.
  • Efficient Radiation: The design promotes efficient radiation, even at relatively low heights above ground. This magnetic radiation pattern offers surprisingly good performance.
  • Good Bandwidth: The MU design offers good bandwidth, which is important for modern digital modes and for those who like to cover multiple frequencies in the same band without retuning.

The article provides details design specifications and construction guidelines for various bands, from 10 meters up to 160 meters, with diagrams to walk you through the process. It even offers adjustments for different antenna heights above ground.

Now, let’s be clear—this isn’t a magic bullet. The performance will vary depending on the specific location, and like any antenna, there will be some directional favoritism. In the examples provided, there is significant performance in a certain direction. However, the overall design offers impressive performance, considering its low profile and simple construction. Remember that the measurements presented are based on real-world testing, demonstrating its practical effectiveness.

If you’re looking for an efficient, compact, and relatively easy-to-build antenna that performs well for long-haul contacts, I highly recommend taking a closer look at the Multiple-U magnetic radiator. The provided charts and diagrams will help you determine your optimal design based on your specific band and location.
Back in the 1990s, I built this antenna on my four-acre property in Boulder, Colorado. Boulder County’s strict antenna regulations prevented me from using a tower despite having ample space. After extensive research, I chose this design and started with a 10-meter version, using readily available parts from my “junk box”—speaker wire and RG-59 75-ohm coax for the matching network. I improvised support using a nearby bush and my garden fence and constructed the antenna, including the spreaders, in under an hour.
Connecting my 50-ohm feedline to the quarter-wave 75-ohm balun, I was pleased to see my ATU quickly achieve a 1:1 SWR with minimal tuning effort—always a good sign. Using my old IC-745, I tuned into a busy pile-up on 10 meters. I cautiously sent my callsign, fully expecting nothing, especially with my low power output of only 100 watts. To my astonishment, the DX station from Malta immediately answered who was the reason for the pile-up.

This unexpected success initially left me stunned. After confirming the contact with a 59 report, he responded that my signal was 59+20dB at his location. I explained my simple antenna. He compared my signal to a friend’s using a 50-foot high tri-bander in Illinois, noting that I was significantly louder. Propagation undoubtedly played a role, but switching to my vertical antenna resulted in a noticeable decrease in his signal strength (about two S-units) – this proved to me the design was effective. I was hooked and decided to build a larger version for 80 meters.

Using four 25-foot supports, I constructed a much larger 80-meter version of the antenna, requiring approximately 530 feet of wire. Bamboo served as the spreaders, and a quarter-wave 75-ohm line provided the matching. I oriented the antenna east-west for broadside radiation. That evening, I monitored an 80-meter WAS net and was amazed by the clarity of the signals. Typically, 80 meters is noisy, but this antenna exhibited remarkably low atmospheric noise, a characteristic benefit of H-plane operation, which minimizes noise typically prevalent in the E-plane. The longer “skip” characteristic of this antenna meant that distant stations came in exceptionally well, making it ideal for DX but less effective for closer contacts.

I replicated the antenna design for another ham who wanted a directional antenna specifically for 17 meters. He lived in a trailer park with antenna restrictions, so we needed a lightweight, easily repositionable solution. We constructed two supports using PVC pipe, with a central section and two horizontal PVC spreaders at the top and bottom. To ensure stability, the base of the vertical PVC support was encased in cement, allowing him to easily adjust the antenna’s direction simply by moving the cement-filled buckets at the base of the supports, effectively changing the broadside direction as he desired.

The unexpected success of my initial 10-meter antenna, built from readily available materials and achieving exceptional signal clarity, fueled my curiosity for this simple yet effective design. The subsequent construction of larger versions for 80 meters and a modified model for 17 meters further confirmed its versatility and adaptability. These antennas, built to overcome challenging site restrictions, demonstrated the principle of H-plane operation in minimizing atmospheric noise while maximizing the reception of distant signals. The experience proved that resourcefulness, ingenuity, and careful design could significantly enhance signal quality in challenging operating environments.

You can learn more about magnetic radiator antennas here.

Jack, WM0G

Radio Amateurs Developing Skills Worldwide