7 votes

Waveguide size for 2.4 GHz

The cutoff frequency for the $TE_{11}$ mode in a circular waveguide of radius $a$ is $$F_c={c\over{2\pi}}{1.841\over{a}}$$ so for your pipe of 16 mm diameter, $F_c= 10.99 \text{ GHz}$ So as a ...
tomnexus's user avatar
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7 votes
Accepted

Why do ham radio operators say that a full wave dipole isn't resonant?

Let's consider a simpler case by bringing the tips of a half-wave resonant dipole together so the two wires of the dipole are parallel, making a quarter-wave section of of balanced transmission line. ...
Phil Frost - W8II's user avatar
7 votes
Accepted

Why does a sine wave show two peaks in the frequency domain on GNUradio?

Because mathematically, a function like $\sin(\omega t)$ has an angular frequency of $\omega$ and $-\omega$. Consider: $$ e^{i\omega t} + e^{-i\omega t} $$ By Euler's formula this can be expanded to: $...
Phil Frost - W8II's user avatar
6 votes
Accepted

Would it be possible to light up a bulb wired one side to a transmitting antenna?

no, because the current has to flow through the bulb, and the end of the dipole is exactly the point where, through inevitable boundary conditions (end of conductor) the current is always 0. What you ...
Marcus Müller's user avatar
6 votes
Accepted

Does antenna Q cause distortion?

As far as I can see, no. The shape of the standing wave might have an effect on the antenna's spatial pattern, but the fact that it is a standing wave pretty well implies that it's steady-state, ...
hobbs - KC2G's user avatar
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6 votes

Why can a monopole antenna receive wavelengths significantly longer than its physical length?

As Marcus Müller says, a small antenna is still an antenna, just a less efficient one. Adding to that: an inefficient transmitting antenna is something we try really hard to avoid if we can. An ...
hobbs - KC2G's user avatar
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5 votes
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Is a 1/2-wave coax cable shorter or longer than the full-speed 1/2-wavelength?

Shorter. Like you said, the velocity of propagation is lower, which means that the wave travels a shorter distance in the same time, which means that the wavelength is shorter for the same frequency.
hobbs - KC2G's user avatar
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4 votes

Would it be possible to light up a bulb wired one side to a transmitting antenna?

When thinking about antennas you have to keep two views in mind simultaneously: When looking at the big picture, you have the magic of current flowing into an open piece of wire and radiating away. ...
tomnexus's user avatar
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4 votes

Wave bounce inside a resonant antenna

Now our antenna creates a standing wave. Nope, not a wave in the "propagating wave" sense. That's the fallacy here: there's not a wave propagating forth and back. What is standing is indeed ...
Marcus Müller's user avatar
4 votes
Accepted

How to calculate phase constant for a coaxial cable?

The velocity of propagation for the TEM mode $V_p = \frac{c}{\sqrt{\epsilon_r}}$, i.e. in your case about $0.626\,c$, or $1.877 \times 10^8\,\frac{\mathrm{m}}{\mathrm{s}}$. (This is a bit different ...
hobbs - KC2G's user avatar
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4 votes
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Why can a monopole antenna receive wavelengths significantly longer than its physical length?

An antenna that's the wrong length doesn't stop being an antenna; it just becomes a less efficient antenna for the given frequency. As a matter of fact, basically every conductor is an antenna. That's ...
Marcus Müller's user avatar
3 votes

Would it be possible to light up a bulb wired one side to a transmitting antenna?

Yes, somewhat. The end of the antenna is the worst place to put a bulb, in terms of getting it to light, but it can still happen if there's enough power going into the antenna, for two reasons: The ...
hobbs - KC2G's user avatar
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3 votes
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Wave bounce inside a resonant antenna

Keep in mind that when you are talking about standing waves, you are talking about voltage. Also, a systems' feedline is containing the EM energy, and the antenna is shedding that energy. With regard ...
Louis Seaman's user avatar
3 votes

Does the self inductance and velocity factor of the elements of a dipole antenna change its resonant frequency?

Notice how there are no terms anywhere in the equations for anything related to the self inductance of the elements or related to the fact that radio waves travel slower in metal than in air. Indeed, ...
Phil Frost - W8II's user avatar
2 votes

Wavepropagation through air - EMC test values 100V/m

Field strength in the far field is $E = 5.477 \frac{\sqrt{P}}{r}$. If P is watts and r is meters, then E is V/m. So 100 V/m is approximately the field strength of 100 W at half a meter 1000 W at 1.7 ...
hobbs - KC2G's user avatar
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2 votes

How can an RF coil emit at 3 different frequencies at the same time during Amplitude Modulation?

First off, an RF coil does not oscillate. It can have an oscillating current in it, but it is just one part of an oscillator circuit. In an AM transmitter, the sidebands are not generated in the ...
GodJihyo's user avatar
  • 252
2 votes

Wave bounce inside a resonant antenna

Don't forget: The antenna is a 2 port device: power transferred from the transmission line to the antenna (#1), and power further transferred from the antenna to free-space (#2). The power ...
webmarc's user avatar
  • 4,105
1 vote

Wave bounce inside a resonant antenna

Consider a single finite length wire, bent in an L shape, 1/4 wavelength of some given frequency F0 from the end. A current impulse, say driven by a single spark from a spark transmitter, will indeed ...
hotpaw2's user avatar
  • 13.3k
1 vote

Why can a monopole antenna receive wavelengths significantly longer than its physical length?

A small antenna can be as efficient as a half wave one, but with a much smaller bandwidth. As an example I made a crystal receiver for MW, about 1 MHz, using a whip antenna of length 0,5 meters (0....
sm5bsz's user avatar
  • 1,009
1 vote

Wavepropagation through air - EMC test values 100V/m

In terms of power density, a signal traveling with a field strength of $E_0$ V/m will have an average power density of $$\left<S_i\right> = \frac{E_0^2}{2\eta_0}$$ where $\eta_0$ is the ...
AG5CI's user avatar
  • 542
1 vote

How can an RF coil emit at 3 different frequencies at the same time during Amplitude Modulation?

It's a mathematical equivalence. One of the trigonometric identities. If you amplitude modulate a sine wave, or sum 3 pure sinewaves of certain frequencies and amplitudes together, you get exactly ...
hotpaw2's user avatar
  • 13.3k
1 vote

Why does a sine wave show two peaks in the frequency domain on GNUradio?

The output of the FFT you used is complex. When you input a strictly real signal to a FFT that produces a complex result, half of the complex result is the complex conjugate of the other half (this ...
hotpaw2's user avatar
  • 13.3k
1 vote

Wavepropagation through air - EMC test values 100V/m

The test severity level is something that will be requested, or specified, between the purchaser and the supplier. In other words you don't just "comply to ISO 11452", you will have a ...
tomnexus's user avatar
  • 11.1k
1 vote

How can an RF coil emit at 3 different frequencies at the same time during Amplitude Modulation?

The wave form is complex. It can be thought of as an infinite number of sine waves that when added together, form a complex wave. Imagine a sheet of rubber, a vibrating membrane. This membrane can ...
wbg's user avatar
  • 301
1 vote

How can an RF coil emit at 3 different frequencies at the same time during Amplitude Modulation?

In the real world, there is no such thing as a zero width wall. Similarly, you can't have a radio transmission with zero bandwidth. Let's say, hypothetically, that you have a transmitter that is only ...
user10489's user avatar
  • 6,230
1 vote
Accepted

Broadband dipole with no standing waves?

Any change in impedance at the end of a wire, different from the rest of a transmission-line-like wire, will reflect some current. An impedance matched resistor at the ends of a dipole won't reflect a ...
hotpaw2's user avatar
  • 13.3k
1 vote

Why do ham radio operators say that a full wave dipole isn't resonant?

I hope it's not bad form to reply at this late date, but there are a few issues I did not see directly addressed. First, any radiator of a capacitive type antenna (not just a dipole) is resonant at ...
Lou-in-USA's user avatar
1 vote

Is the phase difference between the voltage and current of the standing wave on a dipole antenna constant along the dipole's length?

@Andrew, I wonder if you are interpreting this type of drawing: as a depiction of current and voltage being out of phase. To be clear, it is not; this image depicts the relative current and voltage ...
Lou-in-USA's user avatar
1 vote

Is the phase difference between the voltage and current of the standing wave on a dipole antenna constant along the dipole's length?

The total current (according to physics) across any region around the antenna is the sum of the conduction current within the elements plus (according to Maxwell's equation) the displacement current ...
hotpaw2's user avatar
  • 13.3k
1 vote

Does the self inductance and velocity factor of the elements of a dipole antenna change its resonant frequency?

Note the velocity factor of the metal also doesn't appear in generic equations for inductors. The inductance is more about the permeability of the free space surrounding the conductive elements, not ...
hotpaw2's user avatar
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