In fact, there are several companies on the market that sell finished GP antennas. They look very nice, have branding, and receive good reviews. Although they are a little more expensive (which is understandable given the costs of molding, stamping, and drilling), they are a good option for those who don't want to assemble them themselves.
However, I looked around and found only M connectors. This makes it unusable for those who prefer BNC connectors. Therefore, I'm introducing a simple, low-cost DIY solution that performs just as well as commercial antennas, although its appearance is a bit rough. The cost is approximately 100-110 yuan. I apologize, but I don't have any pictures to show you right now; I will provide them later.
Required materials:
- Pull-out antenna (M10 head, approximately 15 sections, total length of 5.6m. The material should be 201 stainless steel, retracted to about 53cm). (It is best to buy one with secondary reinforcement for stability and durability), 55 yuan.
- Insulators (e.g., SM7120 M10), less than 8 yuan. – These are used to isolate the resonant part from the ground network.
- BNC female-to-male quick connect adapter, approximately 3 yuan/piece – BNC female connector for connecting to a BNC male connector via cable. The other end (quick connect section) connects two wires ① and ② (2.5 square millimeters BV wire is sufficient, approximately 15cm long).
- Two circular terminal strips, approximately 6 yuan/10. One strip is soldered with ① wire, used to feed RF signals (placed at the base of the pull-out antenna, crimped onto the top of the insulator), and the other strip is soldered with ② wire, used to connect the feeder cable's insulation layer to ground (crimped onto the bottom of the insulator).
- Ground anchor (ground stake), M10 interface, approximately 8 yuan per piece.
- Ground (16-stock chart)* 5 meters, with circular connectors and heat shrink tubing), 20 yuan. – This can be purchased as a finished product and then cut into two pieces as needed after receiving it.* 8 shares, or 4 lots.* 4.
When used, the ground anchor will... Circular patch for ground network SameCircular terminal block with ② line connection.We should twist and clamp them together, which essentially involves connecting the shield of the cable to the ground plane through the quick connect - ② - circular connector.
The entire kit, except for the two circular connectors, requires wiring, but everything else is designed to be easily disassembled and carried. Different frequency bands can be switched flexibly by pulling out different lengths of the antenna elements. It is recommended to pull out the thinnest one first, followed by progressively longer ones, to avoid having the telescoping antenna head pointing in the wrong direction.
If possible, use a laser to engrave the position markings for each frequency band on the antenna elements. (Otherwise, you can mark them with an oil-based marker, or fine-tune using a VNA during installation.)
Please ensure that the retractable mast, insulators, and ground stake have consistent dimensions. I chose M10 for all components because the retractable mast uses an M10 head. If you are not using it in the ground but instead mounting it on a camera tripod or lamp stand, you will not need a ground stake. However, you will still need to purchase something like an M10 to 3/8 inch adapter to allow for flexible modification.
Additionally, I saw a type of ground anchor that used punched flange pads (approximately 10cm in diameter, with 4-8 holes drilled around the edge and one M10 hole in the center). The holes on the edge were used to press the corresponding number of ground anchors into place, and it looked similar to the inside of an umbrella. This type may require custom fabrication through negotiation with the vendor.
Regarding ground plane length, there are varying opinions, ranging from one wavelength to 1/20th of a wavelength. Personally, I believe that a 5m ground plane is sufficient for operating frequencies between 7MHz and 28MHz. Furthermore, the ground plane doesn't necessarily need to be made with a solid wire; it can be customized using individual wires or even, in coastal areas, HAM operators can utilize natural beach sand as a ground plane (but be careful of corrosion from salt).
Furthermore, the maximum length of the oscillator (5.6m) determines that it can only operate in a frequency range of at least 14MHz. If you need 10MHz, you would need to add another oscillator. For 7MHz, the oscillator is too long and unsafe, so adding an inductor is the only option. The design calculation for the inductor can be referenced from BG8NSK's Excel table, which is very detailed.
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