Antenna Briefs #2 - Power, Range, and Licensing
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- Опубліковано 4 лип 2024
- How far can radio / wireless signals transmit? In this second video in the series, we look at the relationship between power and range, and the antennas used. While the topic can be quite complex, the aim is to illustrate the core issues involved. Transmitter range values from 130 meters to millions of miles are possible once the concepts are understood. However, there are restrictions on antennas and power levels allowed, and transmitting in general requires a license. The key reasons for licensing are briefly discussed.
- Наука та технологія
Always clear and straight forward, thanx a lot.
You're very welcome ! Thanks.
Very good
Thank you for wonderful information in the videos and a interesting channel. I have just recieved my amateur license and my signal. I just want to say that I would love see and hear much more about mathematics and equations. Do not be ”afraid” to show and explain the mathematics in depth. I understand that you have to compress the information in the videos. I would love to see a video explaining how to design and calculate the properties of an antenna. How do the antenna design software work? Thank you again and keep up the good work you do!
Thanks, and congratulations on your license ! Putting in math is something I've debated a lot. Have tried to limit things to real-number algebra-based material, since many people don't have the complex-number math for addressing phase (for circuits or antennas). But it is amazing how much can be done even without it. I like your suggestion of doing something with antenna software. Maybe I can setup the phase concepts in a diagram and then turn to sim software to skip to the results and avoid the need to derive things. Not perfect, but it would be a good transition into the software and antenna pattern concepts without complex numbers. FWIW, we have used commercial software like "ADS" and "EMpro" in our university classes, but I've also used the free version of "EZNEC" to illustrate dipole antenna currents and patterns. It can do a lot. I'd recommend it to anyone who hasn't looked at it already (or has something more powerful). The user interface is kinda clunky, but it does some of the same basic things as the commercial software does in terms of setting up the analysis and using finite-elements and superposition to get the far-field pattern. It's just limited to wires rather than handling surfaces... www.eznec.com/ 73
Great video! very well explained why the theorical distance are very different from the one achieved in real transmissions
Thanks for the feedback. Glad the video helped illuminate that. As you point out, that's always an important issue, especially for commercial radio product marketing/buying...
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Hi, generally the receiver comes with input sensitivity like -95dBm for example. What does it realy means? Thank you very much
The unit "dBm" means decibels relative to a milliwatt. So it is a power level. -95 dBm is 95 dB (9.5 orders of magnitude) less than 1 mW. So, it's 0.316 picoWatts. Sometimes sensitivity is quoted in terms of voltage instead of power. Assuming a 50 Ohm antenna and a 50 Ohm input for the receiver, we can convert 0.316 pW to voltage, say, in microvolts (uV), using P = V^2/R, which can be rearranged to V = sqrt(P R). Here, 0.316 pW (i.e. -95 dBm) gives 4 uV. Hope that helps.
@@MegawattKS always helps. Thank you very very much.
My NanoVNA has arrived. Can I use it to measure input power of a wifi signal?
Sir, first of all you are owsome.
Secondly I have an issue and need little help I have two monopole antenna 0.32m each i connect one to a function signal generator and the other one to the oscilliscope adjust the signal generator to give 1 volt signal peak to peak on 50 Mhz and use that as a transmitter and receive this signal throw the other monopole antenna attached to the oscilliscope distance between antennas is 3 meter the problem is that the voltage signal value appears on the oscilliscope is much lower than the value calculated using fersii formula after that i used telescopic dipole antenna similar to what you used in this video for same frequency and still getting much lower value (around 100 mili volts) on the scope against 1.2 volts when applu fersii equation calculations please advise
Interesting. We used to do something similar in our Antennas course lab and the results were typically off some - especially at close range. Here, I can think of a couple possible reasons. First - if the antennas are 0.32m at 50 MHz, that's only about 1/20th of a wavelength, so they're not resonant monopoles. So they're not going to transmit or receive as well as assumed by the Friis equation. In addition, it is hard to make measurements at these frequencies. 50 MHz means lambda is 6 meters. At 3 meters away, we're in the "near field" - but the Friis equation assumes far-field patterns. In the near field, there is still a strong radial component to the E field. Finally, there is the issues of what kind of "counterpoise" is present in the monopole and what objects are nearby distorting the field pattern. In our class labs, we found that even at 900 MHz with resonant dipole antennas, things would depart from theory at a distance of a few wavelengths due to "multi-path" in our indoor setting. That's partly why the video shows things done at 900 MHz. Hope that helps. Antenna measurements are hard, but interesting to do !
@@MegawattKS what confused me is that i found website called everthing RF which provides online calculater for calculating the far field based on formula far field starts at distance greater than 2D2/lamba and when I put D as 0.32 m and lamba as 6 meter calculater gives result of 4 cm so I thought at 3 meters I am in the far field I donot know why online calculator give 4 cm as near field border while you mentioned in the video it should be greater than 2 wave length ?
@@user-rr6lb6bn1w I normally use that formula for things like dish antennas that are "electrically large" (> lambda/2 in size). It may work for electrically small antennas, but that would only mean the fields are radiating and not reactive. Perhaps the main problem here was the antenna was only 0.32/6 = 1/19th wavelength in size and you are seeing the effects of a very small radiation resistance (I calculate about 0.59 Ohms) plus a large negative reactance (maybe -j200), instead of 50 Ohms resistive. So most of the signal from the generator was reflected by the antenna and didn't get radiated. I.e. the antenna radiates, but it's not getting much power from the generator. Similar issues (though perhaps not as bad) on the receive side...