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24 votes
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How does an SWR meter really work?

Dispelling the Myth To begin with, the typical HF SWR meter does not have the ability to separately sample the forward and reverse power, voltage, or current. Any description of the device or its ...
Glenn W9IQ's user avatar
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11 votes
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Is my low-power VHF transmitter dangerous?

Electromagnetic hypersensitivity does not exist. There is no confusion. It has been rigorously refuted by science. One watt is orders of magnitude below safe exposure limits, and indeed probably less ...
Phil Frost - W8II's user avatar
11 votes
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What is inside a 50 ohm termination?

It depends on the frequency range the termination is rated for, and the power handling required. High power and high frequency is very difficult. At HF, any old non-inductive 50 Ω resistor will do. ...
tomnexus's user avatar
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10 votes
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Is the 1.5kW power limit per antenna? What about antenna arrays?

Here's 47 CFR § 97.313: § 97.313 Transmitter power standards. (a) An amateur station must use the minimum transmitter power necessary to carry out the desired communications. (b) No station may ...
rclocher3's user avatar
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9 votes
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Decibel subtraction in dBm and dB

I convert the 27dBm and 3dBm -> 10^2.7-10^0.3. But how can we simply subtract the two dBm If you convert to exponential form then you must simultaneously replace ...
Kevin Reid AG6YO's user avatar
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9 votes

Cheap antenna for new HF HAM

The cheapest antenna you can set up is a speaker wire dipole. A spool of speaker wire long enough to split the conductors and make a center fed half wave dipole ought to cost around ten dollars US (...
Zeiss Ikon's user avatar
  • 4,097
8 votes
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dBm signal minus dB

Decibels are all "ratios" at their core. A unitless dB is a simply a ratio of one number to another, perhaps input power relative to output power. We can also use decibels for absolute values, by ...
natevw - AF7TB's user avatar
7 votes

What is the purpose of limiting radio output power?

I'd speculate a large part of the justification is that amateur stations, unlike commercial broadcast stations, are not periodically certified to meet spurious emission regulations. Or any regulations ...
Phil Frost - W8II's user avatar
7 votes

dBm signal minus dB

Think about it this way. 27 dBm means 27dB above a milliwatt. Take 6dB away. Now you have something that's 21 dB above a milliwatt. Or, 21 dBm.
mike65535's user avatar
  • 1,035
7 votes

Output power from the TX/RX port of the USRP N210 decreases as the frequency increases

That's an expected phenomenon: Real-world physical systems tend to be frequency-selective (i.e. not constant over frequency), and "at large scale" low-pass systems. This applies to ...
Marcus Müller's user avatar
7 votes
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Why is there so little difference in the gains of Yagi Uda antennas with omnidirectional antennas? Shouldn't the power at the back be at the front?

An antenna that radiates in all directions equally is called an isotropic antenna. An omnidirectional antenna is one where the gain does not vary by azimuth. It does however vary by elevation. Such ...
Phil Frost - W8II's user avatar
7 votes
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How do you choose an inductor wire gauge for high-power RF?

First: yes, it's mostly about $I^{2}R$ loss, but if your inductor is part of any kind of resonant circuit, you can't assume that $I^2 = \frac{P}{50}$. The current circulating between reactive ...
hobbs - KC2G's user avatar
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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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6 votes

What is the power increase required to double the range of RADAR?

Here's what you are thinking: if the radar station and the target are 1km away, then the distance there-and-back is 2km. If the distance doubles to 2km, then the distance there-and-back is 4km, just ...
Phil Frost - W8II's user avatar
6 votes

What is output power exactly?

Yes. Just like any other electronic device producing a signal, the transmitter is “really” applying a voltage to the output, which causes a current, and so on. But we talk about power more often ...
Kevin Reid AG6YO's user avatar
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6 votes
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Signal power measurement

If the frequency counter is measuring -5.3 dBm, then before 20 dB of attenuation the power was 20 dB more than that, so 14.7 dBm. "dBm" means decibels relative to 1 milliwatt. 14.7 decibels can be ...
Phil Frost - W8II's user avatar
5 votes
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What is the actual loss in a feed line with high SWR?

Assuming no tuner losses, and assuming uniform loss throughout the line (which isn't true in practice, yet can be a practical simplifying assumption, more below), feedline losses in decibels are: $$ -...
Phil Frost - W8II's user avatar
5 votes

Measuring energy density of 2.4 GHz and 5.0 GHz signals

I'm a math and computer science student, starting to mess around with using amplifiers and antennas to build long-range wifi hardware in my spare time. You're potentially breaking the law. In fact, I'...
Marcus Müller's user avatar
5 votes

Intermittent High SWR

The MA5B is rated for 1,200 watts PEP. 200 watts is certainly well below that threshold. Things to check: Arcing, corrosion or moisture inside the match box A loose or corroded connection from the ...
Glenn W9IQ's user avatar
  • 18.6k
5 votes

Why does signal power drop every night?

That's a GSM band. If there's still GSM service in that band, it's very common to see Mobile Network Operators (MNOs) turn of base stations at night, effectively enlarging the remaining cells. I've ...
Marcus Müller's user avatar
5 votes

How does the Maximum Power Transfer Theorem relate to RF power amplifiers?

The maximum power transfer theorem tells you how to design a load to extract the maximum possible power from a given source. It does not tell you that the source will survive this treatment, and it ...
Kevin Reid AG6YO's user avatar
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5 votes
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How safe is 100 watts on 2-40 meters?

The FCC has published a bulletin OET 65B that has a table showing for VHF you don't need to evaluate the RF hazard if your effective power is 50 watts or below. For 70 cm UHF the limit is 70 watts. ...
Jim's user avatar
  • 1,688
5 votes

How to make an RF probe for accurate power measurement at QRP power levels?

How to make an RF probe for accurate power measurement at QRP power levels?                     &...
Marcus Müller's user avatar
5 votes
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Importance of Ferrite Size in Coax Choke Balun

When making a coax choke balun, does the size of the ferrite matter, and if so, why? Simplest answer: If the choke impedance is low enough to allow some common mode power through, then there is a ...
Chris K8NVH's user avatar
  • 1,042
5 votes

Increasing communication range by boosting output and input power

Is it possible to do without the power amplifier at TX ... by using a high gain antenna for RX? It is. You need another 27 dB (37-10) of antenna gain in addition to whatever you included in your ...
Graham G8URP's user avatar
5 votes

Is the 1.5kW power limit per antenna? What about antenna arrays?

The 1.5kW limit is really per transmission. But there are other limits as well that may be reached first. Some bands have a much lower power limit. Also, high on that list is safety, and this isn't ...
user10489's user avatar
  • 6,320
5 votes

Can an oscilloscope be used as an RF voltmeter?

One of the functions of an oscilloscope is actually to measure RF volts, just make sure to use the x 10 function to minimize loading on the circuit you are measuring, and keep in mind that some high ...
Andrew's user avatar
  • 3,685
5 votes
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What safety procedures should be followed when developing and testing a high power amp?

Get yourself a dummy load. If your gut is telling you your doing something stupid you're doing something stupid. Keep the power supply switch near by. Always assume it's live. If you have doubts about ...
Ben Madison KO4UXC's user avatar
5 votes
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How do you calculate peak voltage given VSWR caused by impedance mismatch in a 50-ohm system?

Before going forward I want to make a quick note: your example of 273V is the RMS voltage it takes to put 1500W into 50 ohms — but if you're concerned about dielectric breakdown then you actually want ...
hobbs - KC2G's user avatar
  • 12.4k

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