Re:Cave-Link: Underground text communication system
by TedDoesntTalk on Wednesday November 10, 2021 @10:31AM
(#29175702)
Can the antennas be coiled to achieve a long length while not being strung , for example, 40 meters? I recall car antennas that used to have coils long ago.
Re:Cave-Link: Underground text communication system
by namibj on Wednesday November 10, 2021 @01:19PM
(#29178193)
There are coil antennas, but they work magnetically.
The 40m straight wire works electrically.
The key difference is that the electric antenna interacts with more of the wave, so it's stronger in the far-field, while the magnetic antenna interacts more localized. The latter is why/how NFC works at 13 MHz (23 m wavelength) with an antenna that fits on a credit card.
Re:Cave-Link: Underground text communication system
by bigiain on Wednesday November 10, 2021 @07:55PM
(#29183123)
Old school 27MHz CB radios would have 1/4 wave antennas 2.75m tall. In practice, for vehicle mounted use, they'd almost always have bottom or centre coils to allow overall lengths down in the 1.5-2m sort of range, with compromises accepted there for practicalities and convenience. Fixed base stations would often use a "proper" 2.75m antenna though.
Re:Cave-Link: Underground text communication system
by ajsnigrutin on Wednesday November 10, 2021 @02:19PM
(#29179204)
Not really, atleast not the classic designs and if you want to transmit. You can bent them a bit, but the total lenght has to be around the quarter wavelength
The lower you go with frequencies, the larger antennas you need (quarter wavelength is usually the minimum... for FM radio (around 100MHz) it's 75cm which is very realistic length for a car or a house radio... at 14mhz (ham radio freq), it's 5.3 meters, which is still doable if you live in a house, and at 1.8MHz (another ham band) is ~40 meters, which is hard.. going lower is even harder, and special "tricks" are needed.
The second problem is the bandwith itself... some wifi equipment supports bands up to 100MHz of width (to simplify, you use all the frequences from eg. 5.1GHz to 5.2GHz), and if you want to stay at low frequencies, it's impossible to create such a wide band, because the antenna sizes are so different at 1MHz and 101MHz, the propagation is different, etc., so usually you're stuck with narrower bandwiths at lower frequencies (and of course, many of the low frequencies are already in use.. eg for fm radio).
But generally yes, higher the frequency, more directly (in a straight line) it goes, and you get more losses in stuff between the transmitter and receiver.