Questions tagged [theory]
This tag is for questions of radio theory and/or the underlying physics.
62 questions
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What will the *amplitudes* of mixer products be?
The key fact about mixers, which iirc is even part of the FCC amateur radio exam(s) in the USA, is that they output "the sum and difference" of the input frequencies. And iiuc in practice it'...
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Understanding how antenna mismatch can damage a transmitter
I know it's important to tune an antenna to the same impedance as the transmitter for maximum power transfer. But I've read that if you don't do so, you not only get less radiated power, but you can ...
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How do radio jammers work?
I just want to prefix by saying I asked where to post this question on meta and no one was 100% sure but this seemed to be the most likely fit so please tell me if this is off topic and where I should ...
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Optical line of sight over long distances?
Is there a way to use radio to restrict communication to optical line of site at distances greater than 10 miles?
For example, the signal is transmitted directionally from a mountain top fire lookout, ...
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Why are there weird sounds in radio?
When I'm tuning through the radio on HF, suddenly, I hear a tone. So, I tune back, and the more I tune back, the more the tone changes pitch. For example, if I tune back 1 kHz, the sound rises by 1 Hz....
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What is the apparent AC voltage source caused by a mismatched impedance on the antenna or transmission line?
From Understanding how antenna mismatch can damage a transmitter
"The reflected wave appears to the transmitter's antenna port... as an AC voltage source... whose magnitude and phase depend on ...
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How does a mag mount antenna achieve a ground plane?
I've been thinking for some time about magnet mount mobile antennas and wondering how they establish their ground plane. Consider that many mag mount antennas have some kind of protective coating on ...
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How does single sideband work theoretically?
How does single-sideband (SSB) work theoretically? If a theoretical SSB transceiver is a "black box" and only its inputs and outputs can be analyzed, not the way it works internally, what is ...
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Is FM effectively spread spectrum?
My understanding is that FM achieves better audio quality (under the right circumstances) than AM by spreading the narrow audio signal's information into a wider RF signal. Does this mean FM is ...
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What does the term 3 kHz deviation relate to in the FM world?
I am confused when someone says "your audio levels are set right and has good deviation at 3 kHz" when talking about wide-band 25 kHz FM signals.
First confusion: Why are audio levels measured in ...
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Why do we use decibels in radio?
We can measure power in watts, and gain or loss in unitless ratios. So why bother using decibels for these things?
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Is a UHF connector any real disadvantage off the transmitter circuit board or at an antenna feedpoint?
I have a discone antenna intended for use in the 25-1300 MHz frequency range, but it uses a SO-239 connector.
Now, we know that the so-called "UHF" i.e. the PL-259 male plug and SO-239 ...
- 6,960
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What do you call antenna that combines multiple Hustler resonators (using a VP-1) above a shared mast?
I know that a Hustler MO-3 54" mast with something like a RM-20S resonator on top is officially an example of a center-loaded quarter-wave vertical...
... but Hustler also allows you to mount ...
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What does a SSB frequency actually indicate?
An SSB signal is an AM signal with the carrier and other sideband filtered out, if I understand correctly.
As such, when specifying an SSB frequency do you specify the carrier frequency, then state ...
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How should I think about "extra" impedances inserted within/across a transmission line?
I'm worried this might be two separate questions — or perhaps not any coherent question at all? — but some recent thinking about transmission lines made me realize I still don't know how to think ...
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How does an antenna transform impedances along its length?
If I insert some impedance (like a loading coil, or a trap) at the base of a monopole antenna, the effect on the feedpoint impedance is easy to predict: the impedance is in series with the antenna, so ...
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EIRP - Conservation of energy
I have a theoretical question about EIRP.
EIRP is calculated by summing the power of the radio (the intentional radiator, IR) plus the gain of the antenna (dBd/dBi) minus the sum of the losses. The ...
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How much is the greatest distance of a communication link that man can reach? Intergalactic? (ascertainable)
I have no data to add, just to say that I would like an answer that includes data and calculations to verify Shannon's law in that case.
Concrete examples of this type of communication: http://en....
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What cause `multiply` has difference result between `float` and `complex` signal?
Construct 2 similar flow graph in Gnuradio,just different with data type.
complex type data has a correct result.But ...
- 52.5k
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How does IQ modulation work (intuitively)?
I'm trying to get my head around IQ modulation. What I understand so far is that each of the I and Q branches of the mixer produce two sidebands with the same frequency components, but in the Q branch,...
- 13.6k
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Modeling a regenerative receiver
What is the signal processing model of a simple regenerative or super-regenerative receiver?
Is it just a linear 2-pole IIR filter with gain? How does a single regen control knob control both ...
CommunityBot
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Why do PSK modes have "bandwidth"?
My understanding of a naive PSK scheme is that you have some frequency(s), f(n), at baseband, and you modulate information by shifting the phase (what "point in time") the signal is at. Isn'...
- 13.6k
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SDR: How are I and Q determined from the incoming signal in quadrature sampling on the receiver side?
I'm new to digital radios and signal processing, so I apologize if this question is trivial but I haven't been able to find an answer here or by googling. Also, some terminology might be off, please ...
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Transmitting Video Underwater
My brother and I are considering designing and building a remotely controlled submarine. As far as experience, I have some knowledge on RC systems, and I am almost done with my General license. My ...
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What does antenna frequency range mean in practice?
I'm trying to wrap my head around software defined radios and antennas and noticed that lots of antennas are described in terms of a frequency range. For example, this one is described as operating ...
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Can a receiving antenna really be "as small as you can imagine"?
I read this antenna theory statement somewhere:
And, for next-step or future thoughts: a receiving antenna can be as small as you can imagine, without loss of signal to noise.
Is this BS or not?
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Problem with theory of HF detector
I'm currently trying to build my own SWR power Meter. This design is based on the so known HT detector with schottky diode BAS70. For my understanding, and for easy purpose, I'm currently focusing on ...
CommunityBot
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Why do we talk about common mode current from an antenna, but not from a transmitter?
There's a lot of information available about how to block common mode current originating at the antenna end of a coax cable. But, I haven't seen any reference to common mode current originating at ...
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How can the output power of a Yaesu FT101E be increased?
I have a Yaesu FT101E which has low output because the final 6JS6C valves are old. If i increase the plate voltage on the final tubes from 600 to 800 VDC by changing the mains transformer tapping ...
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How would a time-difference-of-arrival receiver with two antennas know which side the signal is from?
An easy TDOA design by Byonics was published with the following schematic:
It appears that the antennas are switched at a rate of 640Hz, which allows a 640Hz tone to be heard in the radio when the ...
- 25k
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How to calculate combined power of two amplifiers
If I have 1 amplifier running at 95%, and another identical one running at 60%, how may I calculate the output power of the combiner? I believe this combiner is a "Magic T", but if there is a ...
CommunityBot
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Beam antenna changed directivity
My Hy-Gain TH6DXX antenna has been up on a tower about 20 years, with excellent results. Suddenly, the gain of the antenna is off the side; in other words, when the boom is broadside to the ...
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Unusual modulation
I've been writing a SciFi story, and it calls for something very unusual to happen. I'd like to check this idea for plausibility, but as I'm not a radio ham myself, I don't know if this already exists ...
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How does a Faraday cage block an EMP?
If you search the Internet for Faraday cages, you'll find a lot of information about using homemade cages to protect radios from EMP damage while in storage. A lot of that information is either ...
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Voltage wave in transmission lines
The solution for the voltage in a transmission line can be written as:
$$V(z) = V_o^+ e^{-jkz} + V_o^- e^{jkz}$$
The voltage $V(z)$ is the difference between the conductors of the line at a certain ...
- 52.5k
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How does signal strength relate to bandwidth?
Let's say I have a 25 W transmitter. First I transmit some SSB signal with 2500 Hz bandwidth (no audio compressor is used, etc...). Then I transmit a BPSK31 signal with at most 100 Hz bandwidth using ...
- 25k
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How does a folded balun work?
A folded balun is made from two quarter-wavelength, parallel conductors. A picture is worth a thousand words:
Here are some pictures of an actual construction from Gisela & Joe Noci on diydrones....
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What is Norton's transform?
This article from Itchen Valley Amateur Radio Club describes an interesting technique called "Norton's transform":
This results in a negative capacitor, but in a circuit where there are other ...
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SDR sampling bandwidth - do the bits per sample matter?
I'm looking at data converter chips for a homebrew SDR design, and I see that I can choose not just the sampling frequency, but also the bits per sample.
Obviously the sampling frequency has to be ...
- 52.5k
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Does the E-field propagate faster that the H-field?
I need some proof that the electric (E-field) and the magnetic component (H-field) of a lightning stroke both propagate at the speed of light, and that there is not a significant delay between them.
...
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Key CW on zero crossings - zero bandwidth?
I remember being surprised to learn that a CW signal has a bandwidth (albeit small), but when I thought it over, it made sense. Essentially, we are modulating the carrier with a low frequency square ...
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What's the practical significance of the source impedance in an RF filter?
I'm reading through a tutorial on filter design and Bartlett's bisection theorem. It gives an example of a filter with 2.909k terminations:
Then, the right side of the filter is scaled by a factor of ...
- 18.6k
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Is RF current always confined to the inner diameter of a braided outer conductor of coaxial cable?
Prompted by a previous question found at Using a balun with a resonant dipole -- an answer there included the following text:
"For transmission lines (twin-lead or coax) to not radiate, each ...
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Conventional or electron current
I’m reading a radio theory book, Radio Theory Handbook by Ron Bertrand VK2DQ, which suggests that electron flow (ie current flowing from negative to positive) is used in radio theory and design.
In ...
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Can a common-mode current exist on the inside of a coax shield?
People often say common-mode current flows on the outside of the shield, while the current on the inside of the shield is always opposed by an equal but opposite current on the center conductor. This ...
- 18.6k
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Why can't VHF / UHF be used with ionosphere reflection?
HF radio can be reflected by the ionosphere, giving very long range communications.
Why can't VHF / UHF used with ionosphere reflection?
Is it due to the attenuation of the atmosphere over that ...
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Can there be (long range) propagation of HF from satellites?
HF transmissions from the ground can be reflected by the ionosphere leading to very long distance propagation. Satellites in low Earth orbit (160 km to 2,000 km) fly within the ionosphere (60 km to ...
CommunityBot
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Can you create a radio beam with a Faraday cage?
Imagine we have an isotropic antenna at the origin emitting a steady sine wave at frequency $f$. Encompassing the antenna is a sphere of radius $r$ made of a fine copper mesh (with holes $\ll\frac\...
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Why are tuning and amplification in the same block?
I'm studying for my foundation level licence in the UK and the book has the following diagram for a receiver:
Why are tuning and amplification in the same block? Are they perform at the same time/by ...
- 25k
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How is impedance matching related to balance and common-mode currents?
Does a good impedance match imply good balance and lack of common-mode currents?
Does a poor impedance match imply poor balance and excessive common-mode currenst?
- 25k