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11 votes
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Why are longer antennas not suitable to pick higher frequencies?

This is sort of two questions, but they are related. The first question asks why a car AM receiving aerial works to receive 500kHz when mathematically speaking the electrical length of a $1/4\lambda$ ...
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6 votes

How do I rewire a simple a.m. loop antenna that became unwound?

There is no special precise construction to these antennas — they are just wire wrapped around an inert plastic frame. It does not matter (significantly) how the turns (wraps) lay next to each other. ...
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6 votes
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VNA basics, antenna coax cable

You want to use the VNA against the final system configuration... or at least as much as practical. Why? The antenna's environment and transmission line will impact its characteristics. It's pretty ...
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6 votes

VNA basics, antenna coax cable

If you want to understand what your radio "sees", which is probably the case, then you want to measure the entire system including the cabling to the antenna rather than just the antenna ...
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5 votes
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Transmission power of Apple Wi-Fi antennas (EMF and health)

Maximum permissible power output for Part 15 operation is 1W; Apple devices are known to use up to that limit. e.g. https://fccid.io/BCGA2589/Test-Report/WLAN-OFDMA-Test-Report-1-5691887.pdf The above ...
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  • 186
5 votes

Why are longer antennas not suitable to pick higher frequencies?

A long antenna acts like a lot of little short antennas, all wired together. Note that depending on the angle of the signal to the big antenna, the distance to each short antenna from the source can ...
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4 votes

Radio without antenna?

Radio without antenna? That's like a sound card without a speaker. The radio has to have an antenna or there is no way to receive or transmit the radio waves. However, there's lots of ways to hide ...
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  • 4,188
3 votes

ELF antenna receiver build instructions?

Transmitting and receiving aren't the same problem. You don't need a resonant antenna, or a high-efficiency antenna, to receive perfectly well. You just need enough signal from the antenna to get ...
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  • 8,730
3 votes

ELF antenna receiver build instructions?

There are ELF antenna's designed for this purpose, such as https://www.stormwise.com . They are not half-wave dipoles, so the dimensions can be a bit smaller than the size of the planet. Usually ...
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2 votes

Why do ham radio operators refer to current flow on the outside of coax as common mode current?

Common mode current occurs when current in the shield flows in a direction common with current in the center conductor. "Common mode current" is just a simple descriptive label to ...
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2 votes

Why do ham radio operators refer to current flow on the outside of coax as common mode current?

RF Current on the outside of a coax will radiate an EM field. You can test the amount of this EM with an RF field strength held up next to a coax at various locations along its length, and testing ...
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1 vote

YouLoop antenna for amateur satellite reception

It looks good for HF and probably OK at 145 MHz. The benefits of the balanced loop are mostly below 50 MHz. At 435 MHz I don't think it will work - they don't say it will. If the balun works at all, ...
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1 vote

Radio without antenna?

Depends on what you mean by the terms "radio" and "antenna". But one can connect an RF transmitter circuit with an RF receiver circuit, both in shielded metal cases, using coax. ...
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1 vote

ELF antenna receiver build instructions?

For true ELF the earth is often a significant part of the antenna. This usually means really good and deep grounding systems, and very long leads. The usual experimental approach would be ground ...
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  • 1,242
1 vote

How do I rewire a simple a.m. loop antenna that became unwound?

A similar-looking (Sony) antenna (12.5cm x 10.5cm) was measured on a 1kHz bridge as 24 microhenry. The bridge "Q" was very low as expected at 1kHz (well below 1), but gave a good null for ...
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  • 516

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