22 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 ...
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  • 18.1k
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 ...
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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 ...
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  • 8,922
9 votes
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decibels as ratio and dBm as absolute values

Trying to work it out I did: ...
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  • 23.1k
9 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. ...
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  • 8,171
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 (...
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  • 3,881
8 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 ...
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  • 23.1k
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 ...
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7 votes
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How does power received by a dipole relate to electric field intensity?

There are a lot of ways to approach this problem, but here's one: we can calculate the power density of that field, and determine the area from which the antenna captures power, and multiply them ...
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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.
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  • 1,023
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 ...
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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 ...
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6 votes

Yaesu exceeding power limit under certain conditions

Think of the "power" setting on the radio as more of a guideline than a rule. It should be more or less accurate, given reasonable inputs. Few radios actually measure the output power and adjust gain ...
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6 votes
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Does amateur radio induce a health risk?

Sure, there are plenty. Unfortunately, they all seem to be selling something. The scientific consensus is quite clear: no known risk, beyond the obvious risk of being cooked which MPE limits are set ...
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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 ...
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6 votes
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Why bother to have a high power base station when mobile units are generally low-power?

The link is actually balanced, because the base station receiver has diversity gain. 2G and 3G cellular systems use two antennas at the base station (for each coverage direction). They are either ...
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  • 8,171
6 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 ...
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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 ...
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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 ...
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5 votes
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Can I touch the ground of a PWR/SWR meter?

There's no problem with what you're proposing, usually. Most of your other equipment probably has exposed metal connected to “ground”. If it's not causing trouble, this won't either. But if you have ...
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  • 23.1k
5 votes

What does radiation power mean?

This means that the transmitter output power (to the antenna) is 10 dBm (which is $10^{1.0}\, \rm{mW} = 10\, \rm{mW}$). Normally this is valid at the rated output impedance. If the transmission line ...
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  • 1,099
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'...
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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 ...
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  • 18.1k
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 ...
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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. ...
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  • 1,208
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 ...
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  • 1,022
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 ...
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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 ...
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  • 4,248
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 ...
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