I previously posted a thread about DIY GP antennas (https://forum.hamcq.cn/d/6809), but later realized that a matching coil was also needed to operate at either 10MHz or 7MHz. There are many ready-made matching coils available online, some made of aluminum and precisely machined, while others have logos stamped on them, which looks quite nice. However, overall, the prices haven't been significantly reduced due to economies of scale = =
If you want to experience the fun of DIY.Save moneyHere are some DIY solutions to consider:The total cost of purchasing materials for a single batch should be no more than 25 yuan (or RMB), and the actual cost of each finished product should not exceed 10 yuan.
The core structure consists of two threaded bolts, each inserted into the ends of the insulating column. Then, the coil is wound around the insulating column, with both ends connected to the threaded bolts. One bolt and the lower half of the extension rod are connected via a wire screw.
The two ends of the coil are M10 connectors. It is recommended to use an antenna extension pole with a length of approximately 50cm and a 5.6m pull-out antenna. The coil is installed at the bottom of the antenna to achieve a balance between power loss and structural stability. There are several similar structures available on the market, such as MA-12 or PAC-12. I will also call itUNT-12Okay.
(The insulated wire in the picture was generated by AI, as this is a partially finished product that I am currently working on and has not yet been wired. It is only for illustration purposes.)
Once the complete antenna system is assembled, the main components, arranged from lowest to highest, are:
Ground stake + Ground wire reel - Insulator - Extension pole + Wire screw post + - Add sensing coil - + Drawer-mounted spring
(Parts before and after)+The symbol represents a screw head.-The symbols represent screw holes. All are M10 specification.
Materials List:
- One ABS plastic isolation column (35mm high, inner diameter 12.2mm, outer diameter 18mm)
- 304 Stainless Steel Tooth Bar/Wire Spool, 1 piece (M10, 30mm long)
- Bolt with a flat head Two pieces (plate flange, width 37mm, thickness 2.5mm, nut inner diameter M10, nut outer diameter 13mm. Overall height 16.7mm. It has 3 holes on the flange)
- 1 roll of insulated copper wire (diameter 1 mm, 10 m per roll – sufficient)
- (Optional) Suitable heat shrink tubing (for protecting insulated wires), if not available, can be used instead with electrical tape or left un-protected.
- 1 sheet of sandpaper (I used a 240 grit, which is coarser and better for sanding)
The materials mentioned above can be easily purchased on Taobao. It's best to purchase them in the sizes carefully selected above.
Manufacturing process:
0. Pre-processing: Roll the sandpaper into a roll and insert it into the ABS isolation column hole. Repeatedly rub it to expand its inner diameter. Occasionally, use a metal plate screw to test if it can be inserted with some force, which indicates that it has been properly ground.
This step can also be replaced with manual friction using a small electric grinder or a drill with a sanding head. In summary, always wear a mask to avoid inhaling plastic dust that is rubbed off. The inner diameter of the plastic isolation column is slightly smaller (0.8mm) than the outer diameter of the steel plate screw, so this step is necessary. If you have a 3D printer at home, you can also try printing suitable insulation columns directly using high-performance plastics.
1. Insert one of the steel retaining nuts into the hole in the ABS plastic support column. Press it in as far as possible.
2. Scrape off the protective layer of the enameled copper wire by about 2 cm using sandpaper or a small knife, then use tin soldering to connect both ends of the copper wire to the corresponding nuts on the two metal plates (using the holes in the metal plates).
At the preparation welding points of the steel plate screws, and at the ends of each copper wire.Pre-tinProcessing to prevent penetration.
3. Single-layer, tightly wound enamel wire, 32~34 laps on the ABS plastic barrier column.
(Optional) Impregnate the coil with epoxy resin, and allow it to cure patiently. At this point, you can use electrical tape or heat shrink tubing to protect the coil.
4. Remove the protective coating from the other end of the enameled copper wire. Also, insert an ABS insulating column into the other screw on the iron plate, and perform tin soldering connection as before.
- Assemble the antenna, and use devices such as nanoVNA to test its performance characteristics. The SWR (Standing Wave Ratio) should not exceed 1.2 at 7 MHz.
Important notes:
1. When purchasing steel plate nuts, there may beIron white zinc, iron colored zinc, iron black zinc While surface treatment options are available, I don't recommend buying "Iron Oxide Black" as the electrochromic oxide layer may hinder conductivity.
2. The suggested steps for polishing should be expanded further to avoid the situation where the bolts on the steel plate cannot be removed after being pressed in.
3. Although the ends of the threaded rod are open, if the thread is tightened too deeply, it may cause the two sides to connect. However, in practice, one end of the threaded rod is screwed onto an extension bar, and the remaining length is not sufficient to reach the other side's nut.
4. Regarding the number of turns: I used a "BG8NSK antenna inductor calculator" obtained from a HAM group (download link at the end), and entered the data such as target frequency 7 MHz, total antenna length 6.1 m (5.6 m for the upper pull-out part + 0.5 m for the lower extension rod), coil diameter (outer diameter of the ABS plastic isolation column) is 18 mm, resulting in the need to use 1mm insulated wire.Approximately 30.76 turnsThe cable length should be approximately 2.07 meters (excluding the connector part). You can actually wrap it a little tighter, leaving some extra slack (or even shorten the telescoping antenna slightly), so I recommend 32.~33 turns. If the inductor is designed for 10 MHz, or if the length of the lever part, the outer diameter of the isolation column, etc., are changed, then the number of turns also needs to be recalculated.
Download link for the gain coil calculator
File shared via cloud storage: Calculation of antenna inductance - BG8NSK.xls
Link: https://pan.baidu.com/s/17JSBWYLXbJUDwIkrMenOCg?pwd=7043 Extraction code: 7043. Copy this content and open the Baidu Netdisk mobile app, which will make it easier to operate.
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