I often see enthusiasts building their own antennas in the VHF/UHF band, such as dipole, inverted-V, or horizontal antennas. After connecting a power meter and SWR (Standing Wave Ratio) meter, if the SWR is out of range, they immediately try to match it using a balun. However, the matching effect is often not very good.
The problem actually lies with the magnetic core. Some people use manganese-zinc iron oxide magnetic cores (the cheap, green ones that are often included with TB), but in reality, due to their material properties, these cores are only suitable for medium wave applications.(MF)In this low-frequency range, it has little to no effect on shortwave and high-frequency signals. Some people also use a PCB to install an antenna amplifier, and then solder a high-frequency patch antenna onto the PCB (as shown in the figure).
While this particular BAE system does perform reasonably well in the VHF/UHF band, its maximum tolerable power is very low, only around 0.1 watts, which is less than 1 watt. Therefore, it is not suitable for use.
Also, it is important to note that the magnetic permeability (μ) of the toroidal core should be moderate. If it is too high, it will result in a lower usable frequency range and higher losses at high frequencies. If it is too low, it may require more turns of wire to achieve good matching and conversion efficiency.
After online research and investigation, the NXO-100 magnetic loop antenna offers a good balance of performance and price. However, its operating frequency range is 1 MHz – 50 MHz. Therefore, if you are not using the 6m band, its performance will be somewhat suboptimal (especially in the 70cm band), but it can still be acceptable. If you have modest requirements, this magnetic loop antenna is a good choice.
After searching on TB, I also found a magnetic ring (the one I'm currently using) that is suitable for frequencies from 1MHz to 150MHz. It's very good, as shown in the picture below (although it costs 9 yuan for shipping).
Product link (Note: I have not received any benefits from the seller, nor am I a pyramid scheme promoter. This is just the link I use for my antenna [sweat]):
https://m.tb.cn/h.hWIbQ06?tk=5CZeVvDmMip CZ007
When coiling, if using a single VHF antenna, you can use multiple strands of wire to coil it 5-6 times; if using a single UHF antenna, you can use multiple strands of wire to coil it 3-4 times. For V/U dual-band antennas, you need to split the signal and adjust accordingly based on specific conditions.
Of course, there is also demand for the winding of Balun cables and the specific winding details. It should not be used with excessively thin cables: According to the skin effect you learned in elementary school (grade 2), high-frequency RF signals pass through conductors almost only on the surface, and the current inside the conductor is almost zero, so the actual resistance of the conductor will be much higher than DC resistance (due to reduced cross-sectional area, calculate the conductivity using the formula). Excessively thin cables will reduce transmission efficiency and cause current to become heat, which is very unsafe for high-power machines. Therefore, it is recommended to use Litz wire or multi-strand copper wire. When winding, it should be wound evenly: If the bundles of conductors are too close together or overlap, they will create parasitic distributed capacitance, forming an LC parallel circuit. This will cause impedance changes at certain frequencies, affecting the standing wave ratio. According to the low-pass filtering effect, this may make the effective bandwidth of the Balun narrower and unable to cover the V/U frequency band. In addition, according to the characteristics of capacitors, parasitic capacitance in the V/U band is likely to have a high capacitance, which will cause heat generation and loss, and also potentially cause phase changes. Phase changes have little impact on lower frequencies (such as MF and LF), but can significantly affect the directionality and standing wave phase of the V/U microwave band, affecting the antenna's performance.
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