Comrades, good morning!
Although I am a newcomer to amateur radio, just starting out, I have already become fascinated by shortwave communication. Therefore, I began to build some feed devices that I could use in the future. The reason for this project was to create a 1:1 balun specifically designed for a vertical antenna, using videos from several UP (YouTuber) on Bilibili as my learning materials. As expected, it failed initially due to a high VSWR (Voltage Standing Wave Ratio). After posting and asking for help online, I was able to resolve the issue and successfully built a balun with a VSWR of less than 1.1 across 6-50MHz. I am now sharing my experience in hopes that it can be helpful to other friends.
First attempt
Materials used:
Magnets: I bought about 20 FT240-43 magnets on Taobao.
Winding wire: 1.2mm insulated wire
Load: 50 Ω / 3 W precision resistor
Winding method: Heat shrink tubing is divided into segments and tightly wrapped with insulating wire, resulting in a total of 10 turns around the magnetic ring. The ends of the insulating wire are all scraped clean of any remaining coating.
Measurement method: Use a quick-connect clamp to connect the insulated wire and the resistor load, and use a NanoVNA with an M female to SMA male adapter for measurement.
Production results:
Operating frequency (4-30 MHz, VSWR of 1.29-6):
Asking for help in a new post.
Help request URL:"What's the reason behind the 1:1 Balen-to-Bobbi high DIY project?"
Thank you to all my senior colleagues who responded!
Here's a summary of the reasons, which can be broadly categorized into 4 areas:
- Regarding the magnetic ring: Suspected of having a problem.
- Regarding cables: twisted pair cabling; three-wire twisted cabling; coaxial cable twisting; the impedance of 1.2mm insulated wire is not 50Ω.
- Regarding load: Calibrate a VNA for zero-intercept resistance; directly measure the standing wave ratio of the resistor.
- Regarding wiring: Poor contact; Shorten the connection; Resistance welding at the header
Exclude modification
After carefully researching magnetic rings, I discovered that legitimate Fair-Rite FT240-43 magnetic ring prices start at least 60 yuan. The ones I bought from TB were likely not genuine Fair-Rite products. Therefore, I purchased the FT140-61 (Fair-Rite part number 5961002701, LC Mall part number C21884958) and FT14-43 (Fair-Rite part number 5943002701) magnetic rings from the Liuchang mall. I chose the -61 because it was in stock, while the -43 had a longer shipping time due to overseas procurement. I started with the -61 for testing purposes. Although there might be slight differences in materials, at least this one is genuine.
(We recommend the article written by BH5UNT)"Understanding and Selecting Magnetic Rings"）
Test results: Using the same type of insulated wire and winding as in the first test, and measuring with the same method, the standing wave ratio was slightly lower, but the improvement was minimal.
(Since I forgot to take a picture, just imagine there was one here. Hehe)
To eliminate potential issues with the cable, I used a 50Ω RG316 coaxial cable as the winding wire and soldered SMA connectors to both ends. The VNA was connected using two BNC cables, and the resistor was directly inserted into the SMA connector.
Test results: No noticeable effect.
Since the SMA connectors have already been soldered on both ends of the coaxial cable, this time the load was directly screwed onto the SMA connector using the 50Ω calibration component provided by the VNA. The VNA then connected to it using a dual-head cable.
Test results: Excellent performance, with standing wave ratio directly reduced to 1.027-1.089 for frequencies between 6 and 50 MHz.
Regarding the issue of connecting cables, two separate sets of crocodile clips connected to SMA connectors were prepared for comparison, one used as a control and the other used directly. The load was connected using the VNA's built-in 50Ω calibration component, which was connected via a twin coaxial cable.
Test results:
① Use a cable with SMA dual connectors and crocodile clips to connect at the VNA port – directly screw onto the dummy load: 6-30 MHz, standing wave ratio is 1.11-1.56
② Use alligator clips to connect the cable at the VNA connection – SMA dual-head connector. The dummy load should also be connected with alligator clip cables: 6-30 MHz, standing wave ratio of 1.18-1.78.
Summary
Through a series of control variable experiments, it can be largely determined that the primary reason is that the 50Ω resistor I had previously used was not suitable. This could be due to factors such as inductance or capacitance values affecting the results. Although I have also used this resistor for VNA calibration, it did not have much effect.
Secondly, secondary factors such as impedance and contact resistance in the bypass line further exacerbate the increase in standing wave ratio (SWR).
In conclusion, the cost of materials and time spent on DIY projects will always be higher than buying finished products. However, it is a very helpful way to learn relevant knowledge. If you don't want to spend too much time and effort, it's best to buy finished products instead.
If you plan to DIY, it's best to purchase the magnetic rings from large platforms like LiCrea Store, Mouser, or Digikey. I recommend buying from LiCrea Store for overseas purchases because the other two have minimum order requirements, which may not be suitable for individual enthusiasts who buy in small quantities. Buying directly from LiCrea is more cost-effective. I also discussed this with LiCrea's customer service and they suggested keeping some commonly used magnetic rings in stock to make it easier for enthusiasts to purchase.
Next, I plan to either replace the existing jumper wire or switch to a coated wire. After all, the RG316 cable is slightly thinner (I also need to consider the possibility of using it for 100W transmission in the future), and I'll also get a waterproof enclosure and other accessories so that I can set up my vertical antenna with a shortwave receiver and communicate freely.
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