Questions tagged [theory]

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

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63 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 ...
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3answers
110 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?
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103 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 ...
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4answers
143 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 ...
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38 views

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 ...
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2answers
79 views

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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1answer
57 views

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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1answer
477 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 ...
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2answers
85 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 ...
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1answer
107 views

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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2answers
1k 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 ...
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1answer
95 views

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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2answers
323 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 ...
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85 views

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 ...
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3answers
445 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 ...
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3answers
76 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 ...
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4answers
1k 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 ...
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2answers
153 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\...
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6answers
489 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 ...
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1answer
27 views

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 ...
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1answer
286 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 ...
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1answer
187 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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4answers
270 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 ...
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2answers
3k 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 ...
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2answers
683 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 ...
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4answers
2k 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 ...
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1answer
344 views

Strange image of concentric circles…magnetic field? [closed]

I woke up to find a series of "images" on the window of my kitchen slider. They were concentric circles. They appear to be images of a magnetic field. I was trying to link a photo of them onto this ...
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1answer
242 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 ...
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1answer
342 views

What are the signal loss mechanisms between a ground station antenna and a satellite receiver?

What are ALL the potential signal loss mechanisms between the output of the earth station antenna and the input of the satellite receiver. With a bit of explanation for each if possible please.
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55 views

Interpreting the Friis transmission equation [duplicate]

In the Friis transmission equation, can you please explain the significance of the factor $$\left(\frac{\lambda}{4\pi R}\right)$$ and what it helps us achieve? Specifically in terms of path loss.
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4answers
910 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 ...
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2answers
4k 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....
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1answer
545 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 ...
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2answers
145 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 ...
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4answers
2k 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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1answer
150 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 ...
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3answers
7k 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 ...
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1answer
146 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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1answer
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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1answer
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 ...
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2answers
951 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 ...
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1answer
22k 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 ...
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3answers
4k 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 ...