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Questions tagged [theory]

This tag is for questions of radio theory and/or the underlying physics.

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19 votes
3 answers
7k views

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 ...
Adam Davis's user avatar
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19 votes
3 answers
34k views

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 ...
Peter KB1AVL's user avatar
14 votes
1 answer
2k views

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 ...
Adam Davis's user avatar
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12 votes
4 answers
3k views

Are antennas circuits?

For an antenna to work, doesn’t charge have to flow through it? A current must be present. To me it indicates it must be a circuit, but how is a dipole a circuit? If they are not circuits, then how ...
mikew's user avatar
  • 335
12 votes
2 answers
2k views

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 ...
Adam Davis's user avatar
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12 votes
5 answers
1k views

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 ...
Phil Frost - W8II's user avatar
11 votes
6 answers
893 views

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 ...
Dominick Pastore's user avatar
10 votes
7 answers
2k views

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,...
John B's user avatar
  • 201
10 votes
1 answer
326 views

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?
Phil Frost - W8II's user avatar
10 votes
1 answer
3k views

How does a switching mixer multiply the two signals?

I understand what a mixer does, and when I see it in a block diagram it's obvious what purpose it serves and how it fits in the overall design of the radio. However, when I look at the schematics and ...
Adam Davis's user avatar
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9 votes
2 answers
971 views

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 ...
DarcyThomas's user avatar
9 votes
1 answer
796 views

Why must twin-lead conductor spacing be small to avoid radiation?

A transmission line does not radiate when the spacing between the two wires is very small when compared to the wavelength, but the transmission line begins to radiate when the spacing between the ...
Danny Paul's user avatar
8 votes
2 answers
3k views

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 ...
Aleksander Alekseev - R2AUK's user avatar
8 votes
1 answer
288 views

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'...
natevw - AF7TB's user avatar
7 votes
4 answers
2k views

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 ...
Phil Frost - W8II's user avatar
7 votes
1 answer
6k views

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 ...
mrog's user avatar
  • 1,023
6 votes
3 answers
10k views

What is the velocity factor of a wire?

This is Belden 9223. It is a 50-ohm coaxial cable, velocity factor 0.56 This is Belden 8524. It is a 22 AWG hookup wire. No velocity factor in the datasheet. Why does the coax datasheet have a ...
Phil Frost - W8II's user avatar
6 votes
4 answers
4k views

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 ...
DarcyThomas's user avatar
6 votes
2 answers
7k views

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....
Phil Frost - W8II's user avatar
6 votes
3 answers
5k views

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 ...
Skyler 440's user avatar
  • 7,600
6 votes
2 answers
183 views

How is RF a radio receives just not a mishmash of eletromagnetic energy?

So at any given time, electromagnetic waves of many different frequencies are flooding the space around me. I have a little 2 meter radio. Are not all these electromagnetic waves exciting the ...
mikew's user avatar
  • 335
5 votes
5 answers
2k views

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 ...
Someone's user avatar
  • 604
5 votes
3 answers
667 views

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 ...
Steven's user avatar
  • 205
5 votes
3 answers
780 views

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 ...
Bitbang3r's user avatar
  • 401
5 votes
2 answers
949 views

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 ...
Phil Frost - W8II's user avatar
5 votes
6 answers
599 views

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 ...
kj7rrv's user avatar
  • 179
4 votes
2 answers
257 views

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\...
Jerald Thomas's user avatar
4 votes
1 answer
162 views

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 ...
hotpaw2's user avatar
  • 13.4k
4 votes
3 answers
845 views

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 ...
Richard Fry's user avatar
  • 2,922
4 votes
0 answers
186 views

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 ...
F4BJH's user avatar
  • 61
3 votes
3 answers
245 views

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 ...
377ohms's user avatar
  • 55
3 votes
3 answers
143 views

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 ...
jford's user avatar
  • 33
3 votes
1 answer
218 views

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....
o'o's user avatar
  • 390
2 votes
3 answers
418 views

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 ...
Galaxy's user avatar
  • 199
2 votes
3 answers
461 views

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'...
Expectator's user avatar
2 votes
2 answers
777 views

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 ...
pavja2's user avatar
  • 123
2 votes
2 answers
464 views

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 ...
SRobertJames's user avatar
2 votes
1 answer
65 views

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 ...
SRobertJames's user avatar
2 votes
4 answers
356 views

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 ...
mrog's user avatar
  • 1,023
2 votes
3 answers
297 views

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 ...
Ethan's user avatar
  • 129
2 votes
3 answers
244 views

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 ...
natevw - AF7TB's user avatar
2 votes
2 answers
60 views

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 ...
natevw - AF7TB's user avatar
2 votes
2 answers
300 views

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 ...
Allergenfree's user avatar
2 votes
1 answer
591 views

What is shape factor of a resonant circuit?

I have seen the shape factor of a resonant circuit defined in different places as both "the ratio of the -3dB point to the -60dB point", and elsewhere as "the ratio of the -6dB point to the -60dB ...
Harry Weston's user avatar
2 votes
1 answer
206 views

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?
Phil Frost - W8II's user avatar
2 votes
1 answer
298 views

Quarter Wave Transmission Line transformer

Can someone explain to me exactly how a quarter-wave transmission line transformer is put into action? I mean, how does it operate and what values are usually taken into consideration, calculation ...
Max's user avatar
  • 123
2 votes
1 answer
180 views

ISM band bandwidth compared to other bands for theoretical WLAN use, performance?

As a purely theoretical question, what would it be like if wireless computer networking equipment was redesigned and allowed to operate at some other band than the unlicensed ISM band which is ...
AttributedTensorField's user avatar
1 vote
4 answers
352 views

There is no reverse flow of energy in a mismatched transmission line, is this correct?

Is this text correct? I use it when helping people to study for their Licence, and get a lot of indignant scorn for my assertion "It is important to realise, that, after the first brief flutter, no ...
Harry Weston's user avatar
1 vote
2 answers
122 views

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, ...
W6M's user avatar
  • 113
1 vote
3 answers
145 views

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?
user17230's user avatar