# Questions tagged [theory]

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

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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 ...
• 193
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....
• 51.9k
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 ...
• 12.2k
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 ...
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905 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 ...
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 ...
• 51.9k
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 ...
• 51.9k
209 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?
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35k 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 ...
• 862
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 ...
• 51.9k
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 ...
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330 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?
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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 ...
• 12.2k
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 ...
• 7,620
246 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 ...
• 55
618 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 ...
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