First, you need to define what you mean by a "long wire antenna." I currently understand that you are referring to an end-fed random wire (EFRW) antenna, rather than an end-fed half-wave antenna (EFHW), which is commonly referred to as "end-fed antenna" in the ham radio community and typically includes a 49:1 unun. Also, it's not a long-wire antenna in the strict sense of antenna theory.
Then, you also need to define what a long-range directional antenna tuner is. I currently consider it to be an outdoor automatic antenna tuner with a direct connection to the antenna, specifically for EFRW systems, such as the icom AH-4 and yaesu FC-40.
Also, we need to define what "general automatic antenna tuning" refers to. I currently believe it includes both indoor automatic antenna tuners placed near a radio and antenna tuners built into the radio itself. For example, the FTDX10 radio has an internal antenna tuner with a matching range of 16.7～150Ω.
Differences: ① Long-line antennas are specifically designed for random lengths of non-50Ω antenna resonators with highly variable impedance characteristics, focusing on matching capabilities for such loads. ② Long-line antennas are typically installed near the antenna feed point and specifically consider issues such as outdoor environments, high RF voltages, and terminal insulation.
Common practices for using automatic antenna tuning with long wires:
Typically, a 1:9 unun should be connected at the long-wire antenna end, and line lengths should ideally avoid being close to an integer multiple of 1/4 wavelength of the operating frequency. Also, the length of the feedline connecting the unun to the antenna should be strictly controlled (ideally, keeping the antenna as close as possible to the unun).
Why "avoid making the feed point fall into a region with an impedance close to an integer multiple of 1/4 of the operating frequency"? This is to avoid placing the feed point in a region with very low or very high impedance.
When the frequency is a fractional multiple of approximately 1/4 λ (wavelength), the feed point of the antenna is located near the current null, tending towards a low impedance state. After further transformation with a 1:9 ratio, the impedance may be as low as single-digit ohms, significantly lower than the tuning range of typical narrow-band indoor antennas and built-in radio tuners.
When the resonant point is a multiple of approximately 1/2 λ, or an even multiple of 1/4 λ, the feed point of the antenna is located near the voltage peak, tending towards a high impedance state, which can reach several thousand ohms. After a 1:9 transformation, it may still be close to 1kΩ, far exceeding the 150Ω limit of the internal tuner in an FTDX10, significantly surpassing the tuning range of typical narrow-range indoor tuners and tuners built into radios.
It is important to emphasize that a ratio of 1:9 is simply a common, empirical practice.However, it is absolutely necessary to...Not best practiceThe actual transformation depends on the impedance of the specific-length resonator at the target operating frequency, as well as the matching range of the antenna. This needs to be calculated in advance, or measured after installing the resonator and connecting it to a network, where a 1:4 or 1:1 ratio might be preferable.
The reasons for bringing the antenna closer to the transmitter (the disadvantages of placing the antenna near the transmitter / the disadvantage of a long cable distance between the transmitter and the antenna):
The attenuator can only achieve a complete 50Ω match for the entire load network, which includes the attenuator output end, "attenuator feed line + unun + amplifier" system, but it cannot guarantee that the feed lines after the attenuator maintain a 50Ω impedance at any point along their length.
However, even after conversion through the UNUN, a long-distance coaxial cable can still deviate significantly from 50Ω. As long as the final load (UNUN + amplifier) deviates from 50Ω, this 50Ω coaxial cable will be in a high VSWR (Voltage Standing Wave Ratio) state. Real-world coaxial cables experience conductor and dielectric losses, and a high VSWR causes the voltage/current peaks within the cable to increase, further increasing the actual losses (primarily dissipated as heat). The longer the cable, the greater the cumulative loss, which reduces the system's transmission efficiency.
Furthermore, due to the unbalanced nature of long-wire antennas and the lack of a low-impedance RF return path at the unun end (often, people simply don't set up an effective RF Ground Plane Antenna, or even just hang a single wire), RF current tends to flow 'around' the Ground Plane Antenna, rather than returning through a low-impedance ground. This results in high-frequency common-mode currents being induced on the surface of the shielded cable, causing the cable itself to radiate. This radiation is not ideal and can make the system's radiation pattern, efficiency, and noise characteristics uncontrollable.
Furthermore, the common-mode current transmitted to the radio end and the radiation from the coaxial cable's outer surface will generate RFI (radio frequency interference), causing problems such as disconnection of the computer serial port, abnormal audio card performance, failure of the touch screen, interference with televisions and lamps, etc. Even if a common-mode inductor is connected in series at the radio end, it can only alleviate some RFI symptoms and cannot completely prevent common-mode current from entering the radio. It is best to connect a low-impedance RF return path (not a ground rod) between the u/u antenna and the coaxial cable, and then connect the inductor in series between the u/u antenna and the coaxial cable to further suppress any remaining common-mode currents coupled to the coaxial cable.
Do you feel confused after reading it?
So, instead of going through all that trouble, it's better to just buy a long-term weather forecast directly.
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