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43 votes
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Why does 1/4 wavelength have a ground plane and 1/2 wavelength needs none?

I can't understand why we need to use a ground plane at 1/4 wavelength and 1/2 wavelength not needed. A 1/2 \$\lambda\$ dipole has voltage and current waves like this: - Picture from Wikipedia. ...
Andy aka's user avatar
  • 462k
36 votes
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Why do microwave ovens use magnetrons?

Magnetrons are cheap, reliable, pretty efficient (65% or so- and they tolerate high temperatures so heat sinking is easy) and made with mature technology. They are also reasonably tolerant of VSWR ...
Spehro Pefhany's user avatar
21 votes
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Why do we need to use transmission line theory?

Why do reflections happen only when the wire is at that length? Reflections always happen, it's just that when the line is really short compared to the frequencies involved the effect of the ...
TimWescott's user avatar
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20 votes

Are TV coaxial cables compatible with WiFi antennas?

So you want to transport that 2.5 GHz (or even 5 GHz ?) Wifi signal over TV COAX cable ? Indeed to the non-RF people you'd just think that would work. And it does BUT there will be almost no signal ...
Bimpelrekkie's user avatar
20 votes
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Radio frequencies, what are the limits?

Radio waves and light waves are both electromagnetic (EM) waves. Visible light is physically the same thing as radio, just at a different frequency/wavelength. All EM waves are a coupled electric and ...
Matt S's user avatar
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18 votes
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What piece of technology is able to record the exact phase of microwaves?

The phase of a signal is meaningless without a reference. We are therefore only ever talking about the relative phases of 2 (or in the case of VLBI radio telescopes) N signals. We use a local ...
Neil_UK's user avatar
  • 168k
17 votes

90° Bend: Mitered vs Curved

Neither a mitred nor curved bend is as 'good' as the equivalent length of straight track. There are two main aspects to goodness, S11 and S21. S11. Other things being equal, width and thickness of ...
Neil_UK's user avatar
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14 votes
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What causes loss of power over lifetime of a magnetron?

A Magnetron is a "vacuum tube". One limit on vacuum tube lifetime is cathode emissivity - the ability to provide electrons for the 'tube' to 'modulate'. Decay mechanisms can be complex but ...
Russell McMahon's user avatar
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13 votes
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How dangerous is a magnetron?

Some magnetrons use beryllium oxide as the "ceramic" looking insulators inside of the ring magnets on both the "Stem" and the "Antenna" ends. Reference the image below, ...
JRaef's user avatar
  • 3,685
13 votes

Why does 1/4 wavelength have a ground plane and 1/2 wavelength needs none?

A quarter wave monopole antenna doesn't have to have a ground plane... unless you want it to radiate EM energy with a certain efficiency and pattern. EM radiation requires accelerating electrical ...
hotpaw2's user avatar
  • 4,831
12 votes

What size of hole do microwaves eventually pass through?

Wah! WAH! Ok, you're putting yourself and your friend in deadly danger. First of all, your 29$ device is not a substitute for proper RF qualification. It simply isn't. You might be able to find a ...
Marcus Müller's user avatar
12 votes

Why do microwave ovens use magnetrons?

The domestic microwave oven needs high power to cook the meal and high frequency to excite the water molecules. What is not needed is high stability because the water energy absorption spectrum is ...
Autistic's user avatar
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12 votes
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Glass of microwave oven

It isn't the glass in the microwave oven door that blocks the microwave radiation. The microwave radiation is blocked by the metal plate with holes behind the glass. The inside of a microwave oven is ...
JRE's user avatar
  • 72k
12 votes
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What's the use of an impedance matching network when the transmission line has the same impedance of the antenna?

The reference design doesn't include a matching network, it only reserves a spot for one on the PCB. If you later figure out that your transmission line doesn't actually have exactly the same ...
Jonathan S.'s user avatar
11 votes
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Material that doesn't let microwaves through, but is transparent to IR

For bulk material, you might try a slab of germanium. It's not a metal, but a metalloid, resistivity about 1 Ωm in its pure state, but will improve dramatically with doping, which may not affect its ...
Neil_UK's user avatar
  • 168k
10 votes

How dangerous is a magnetron?

As far as I can tell, the beryllium oxide insulator scare is nothing more than a myth. Probably originates from high power radar magnetrons, which do in fact use such BeO insulators sometimes. If you ...
TiSapph's user avatar
  • 109
10 votes

Microwave starts working when door opens - how dangerous is it?

It is dangerous to humans because it heats up things that contain water. You contain water. Some parts of you can't deal with this added heat (your eyes) and will be damaged first. It also fries ...
Jeroen3's user avatar
  • 23.1k
9 votes

How to design a 10GHz PCB antenna

First of all is it possible to create 10 GHz wave on a PCB? Any clue on how to do something like, regarding the circuitry? If by "creating" you mean propagating and/or radiating, the answer is yes. ...
Enric Blanco's user avatar
  • 5,801
9 votes

should I open a solder mask over a high speed microstrip line in a PCB?

1.3GHz rectangular pulses.... is this actually digital data? If it is and the clock frequency is 1.3GHz then the actual frequency of the signal is 650MHz and I would recommend the frequency of ...
Shawn's user avatar
  • 101
9 votes

How is it possible that emitting electrons create alternating charges in the cavities of a magnetron?

There's an I. I. Rabi story here. During WWII, when the UK shipped their entire radar project to the USA (in order to avoid 1940 bombing, during Battle of Britain,) the prototypes and documents ...
wbeaty's user avatar
  • 11k
9 votes

Radio frequencies, what are the limits?

There is no standard I am aware of that defines what is considered the radio portion of the electromagnetic spectrum. There is no correct frequency range. There are only conventions that often exist ...
jramsay42's user avatar
  • 1,262
8 votes

Are TV coaxial cables compatible with WiFi antennas?

You must use coaxial cable of the proper impedance. The most common impedance for coax cable is 50 ohms or 75 ohms. If the cable you want to use matches the impedance of the interface AND the antenna,...
Richard Crowley's user avatar
8 votes
Accepted

Class F Amplifier Harmonic Termination

From the article you've linked (p.70): [...] the efficiency of the power amplifier can be limited by the transistor output capacitance [...] \$C_{ds}\$, [...] if this capacitance is not ...
Enric Blanco's user avatar
  • 5,801
8 votes

What causes loss of power over lifetime of a magnetron?

I got one possible reason from this article: "Each magnetron’s cathode has a special coating to enhance performance. Over time this material is consumed during normal operation." Although that ...
Bimpelrekkie's user avatar
8 votes
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How does a microwave oven modulate its output power?

It's really simple. They have one fixed power level, and turn it on and off. You can hear it when it is on low power. It'll hum louder for a few seconds, then have a longer period when it is a ...
JRE's user avatar
  • 72k
8 votes

A few questions on PIRs and other motion detectors

PIR sensors have quite a few advantages: they’re very cheap they use very little power (since they’re passive) – very important for battery powered/wireless sensors. We’re talking microwatts here. ...
jcaron's user avatar
  • 2,064
8 votes
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Why are transmission lines and waveguides needed for high frequency?

At low frequency, we can just use a piece of wire to connect electrical components, but why are special transmission lines and waveguides needed for high frequency circuits? Transmission lines are ...
Neil_UK's user avatar
  • 168k
8 votes

Why do we need to use transmission line theory?

Reflections always happen. They are basically the mechanism causality uses (nothing happens instantly) for different parts of the circuit to communicate with each other in order to agree on an ...
DKNguyen's user avatar
  • 56.8k
7 votes
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What is the simple meaning of TE10 waveguide

A \$TE_{10}\$ mode has one lobe in the E-field strength pattern in the x direction (the longer transverse direction) and 0 lobes in the y direction. A \$TE_{11}\$ mode has one lobe in the E-field ...
The Photon's user avatar
  • 131k
7 votes

should I open a solder mask over a high speed microstrip line in a PCB?

I am less than a year in a high frequency PCB design. So below is not my answer but advises from three different sources. Source 1. A friend of mine with a microwave experience longer than my age ...
Sergei Gorbikov's user avatar

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