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Questions tagged [rf-power]

Questions about power ratings and restrictions, including maximum power by law or for certain components, or about calculating power in certain situations.

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19 votes
3 answers
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What makes cellphone transmissions so energy cheap?

The modern cellphone can carry on a full duplex conversation for a time period measured in hours on a battery half the size of my handheld transceiver. My transceiver can barely receive for that ...
Adam Davis's user avatar
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18 votes
1 answer
6k views

How does an SWR meter really work?

For ham radio operators, the SWR meter is a ubiquitous piece of equipment. There are dozens of standalone models on the market and most modern transceivers and antenna tuners have this functionality ...
Glenn W9IQ's user avatar
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12 votes
2 answers
1k views

What are the (US) rules about unlicensed low-power transmissions?

It is common wisdom that transmissions of any sort (perhaps unidentified, encrypted, in the FM broadcast band, etc.) are permitted if they are sufficiently low power. Where are the FCC regulations ...
Kevin Reid AG6YO's user avatar
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12 votes
1 answer
6k views

How do I discover APRS coverage?

I'm considering adding simple APRS transmitters to my vehicles. I'd rather just put one in and forget about it, but I understand there are areas of the US where a stronger transmitter would be ...
Adam Davis's user avatar
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11 votes
4 answers
9k views

What is dBm and why is it used rather than watts?

Reading through an RF power amplifier datasheet, I found the sheet only referenced dBm output power, rather than watts. What is dBm? How do I convert it to watts? Why, or when, would you use dBm to ...
Adam Davis's user avatar
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10 votes
2 answers
4k views

What determines power handling capacity of an antenna?

I just bought a cheap 1/4-wavelength, 2m. mag-mount antenna. The package says it can handle 50 watts. I can think of three things that could limit the capacity: $I^2R$ losses in the connection to the ...
Pete NU9W's user avatar
  • 950
10 votes
1 answer
496 views

Why bother to have a high power base station when mobile units are generally low-power?

I want to find out why, say in a GSM/cellular system, a base station can be up to 50 watts however the mobile units can be only 100mw (for example). Surely if the base station ever uses 50w to reach ...
code_fodder's user avatar
10 votes
2 answers
3k views

What is the actual loss in a feed line with high SWR?

It is well-known that it is unwise to have standing waves on the feed line between one's antenna and one's transmitter, unless it is a very low-loss type (e.g. ladder line), because the transmitted ...
Kevin Reid AG6YO's user avatar
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9 votes
5 answers
2k views

How can I prevent my mobile radio from draining my vehicle's battery?

I'm a very absentminded ham. I have a radio mounted in my car, wired directly to the vehicle battery. Fuses and high-gauge wire and all that good stuff. But, if I accidentally leave it on, my battery ...
Dan KD2EE's user avatar
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9 votes
2 answers
3k views

Importance of Ferrite Size in Coax Choke Balun

The question is quite simple: When making a coax choke balun, does the size of the ferrite matter, and if so, why? I have seen design notes that say to use a single FT240-43 toroid for powers up to ...
M1GEO's user avatar
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9 votes
1 answer
456 views

How do you choose an inductor wire gauge for high-power RF?

At 100 watts in a 50-ohm system, the current flowing through an inductor would be about 1.41 A, thus a 26-AWG wire should be plenty since they are chassis-rated for 2A and for just a few windings that ...
KJ7LNW's user avatar
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8 votes
2 answers
1k views

What is the point of a SWR/power meter where the needles cross over each other?

There are SWR/power meters with two separate gauges, and others that have them mounted so the gauge needles cross over each other. Aside from looking cool, does the cross-over point of the two meters ...
Adam Davis's user avatar
  • 12.1k
8 votes
2 answers
2k views

dBm signal minus dB

I've got a 27dBm signal that I'm going to put through a 6dB attenuator. How do I work out the resultant signal in dBm? I've tried Googling that without any luck.
Chris Pavey's user avatar
8 votes
4 answers
6k views

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

Studying for a Pivate Pilot's Licence (PPL) and I've hit something in the study material that doesn't make sense to me. Typical of the PPL study material, it's quite shallow, so the full explanation ...
Matt's user avatar
  • 81
8 votes
1 answer
4k views

Power limit of 50 ohm BNC connector?

Is there a limit to the power a 50 ohm BNC connector can handle?
Timtech's user avatar
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7 votes
3 answers
3k views

decibels as ratio and dBm as absolute values

I am just trying to understand the nitty gritty of using decibels and absolute measurements in dBm. I see them referred to routinely in RF, and using reference sheets I can understand enough to make ...
nanker's user avatar
  • 173
7 votes
2 answers
5k views

What is inside a 50 ohm termination?

I see RF applications requiring 50 ohm termination to avoid any open port reflections. Curious what is actually inside a 50 ohm termination connector. Is it simply a 50 ohm resistor? But then it is ...
eecs's user avatar
  • 159
7 votes
3 answers
16k views

Decibel subtraction in dBm and dB

If i have equations for subtraction of 27dBm-3dBm=27dBm how can this be written as 24 dB. I convert the 27dBm and 3dBm -> 10^2.7-10^0.3.But how can we simply ...
Mayank Mehra's user avatar
7 votes
1 answer
999 views

Using more than 400W in the UK

With a full amateur license in the UK, up to 400W can be used - however, there are cases that might call for higher power, such as bouncing signals off the moon. Is there any way I can obtain ...
berry120's user avatar
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7 votes
1 answer
1k views

Can I touch the ground of a PWR/SWR meter?

I'm planning to build a 5W PWR/SWR meter very similar to this design by G3YMC, but with two meters: I'd like to put it in this metal box, and use the box as ground. However, the bottom of the box ...
user avatar
7 votes
2 answers
1k views

distance between shack and antenna

I am putting a 40m antenna in my attic and I was wondering would it be safe to have a shack in the room right below the attic (10ft of coax or less) or should I have my shack in the basement/ground ...
Bogdan YU2DBC's user avatar
7 votes
1 answer
1k views

Double power-handling ability of RF amplifier output transformer

I want to base the output network for a 500W push-pull HF amplifier on "A 250W Broadband Linear Amplifier" described in Chapter 17 of the 2014 ARRL Handbook. I need advice on whether and/or how the 1:...
Brian K1LI's user avatar
  • 7,878
6 votes
2 answers
379 views

Power rating of a selfmade air coil

A colleague was winding an air coil himself. It's a 110µH coil with 200 windings for an 80m antenna, intended to be used on a 600 W transceiver. He used a copper lacquer wire with 0.6mm diameter. ...
Thomas DK3TU's user avatar
6 votes
2 answers
1k views

Can an oscilloscope be used as an RF voltmeter?

I just recently acquired a slightly functional Icom IC-735 HF radio, and am trying to resurrect it. The service manual outlines procedures that specify the use of an RF Voltmeter, which I do not own. ...
user avatar
6 votes
2 answers
523 views

Does amateur radio induce a health risk?

Electromagnetic Exposure wise, does Amateur radio (Particularly 1.8 to 440 megahertz) induce a health risk due to RF radiation? Other than the Maximum Permissible Exposure (MPE) limits, which will ...
Skyler 440's user avatar
  • 7,600
6 votes
3 answers
13k views

How do I power my radio in a vehicle mobile install?

I want to install a 30W mobile UHF radio in my car. I already have an antenna and mounting bracket - but how do I get power to this radio? It has an SAE bullet connector on the back - do I just need a ...
Dan KD2EE's user avatar
  • 3,966
6 votes
1 answer
384 views

Why do some antennas list modulation type alongside power rating?

Looking at HF antennas I've noticed that occasionally I'll see power ratings similar to "200W (SSB)". Why would the modulation matter for an antenna's power output rating, and in the absence of other ...
Adam Davis's user avatar
  • 12.1k
6 votes
3 answers
8k views

How should this SWR meter's directional coupler work?

I'm wrapping up a "home build" of the MFJ-941EK antenna tuner kit and having trouble getting the wattmeter calibrated. I can't use C4 to get the reflected power reading calibrated down to 0 as is ...
natevw - AF7TB's user avatar
6 votes
2 answers
900 views

Power dissipation in transmission lines?

I was wondering, does the impedance of a transmission line have any effect on the power it dissipates? I'm familiar with the fact that an impedance equal to the load will provide maximum power ...
s3c's user avatar
  • 837
6 votes
2 answers
569 views

Yaesu exceeding power limit under certain conditions

I noticed something fairly interesting today, under certain operating conditions my FT857D will exceed the power limit, the highest I saw it go was around 150W! This happened when the power was set to ...
s3c's user avatar
  • 837
6 votes
1 answer
290 views

Is it safe to run two barefoot rigs on one antenna with an AB switch?

I have a Daiwa CS-201A two position switch. The single side is connected to by quarter wave vertical (40m). The A and B sides are connected to an IC-7300 and TS-830S. The switch is claimed to have ...
Paul's user avatar
  • 781
6 votes
1 answer
386 views

SNR measurement for a BPSK/QPSK signal

I have created a software (gnuradio) that performs modulation/demodulation and supports a number of modulation schemes and error correcting codes. I'm in the process of evaluating the BER vs SNR for ...
Moses Browne Mwakyanjala's user avatar
5 votes
3 answers
2k views

Cheap antenna for new HF HAM

I just got a 100W HF radio and I am new at this. What is the cheapest antenna I can hook up without burning up my radio or shocking myself?
Pugz's user avatar
  • 151
5 votes
2 answers
2k views

What is the purpose of limiting radio output power?

According to article 25, section I, § 4 (25.7) ITU's Radio Regulations: The maximum power of amateur stations shall be fixed by the administrations concerned. In my country this is 500W for CEPT ...
Hex's user avatar
  • 153
5 votes
1 answer
396 views

Does the Crystal Radio really work?

I am in 12th std, the reason why I am mentioning this is that we are given a PROJECT to do in Physics subject. And then one day I searched the internet and found this amazing thing called 'Crystal ...
radiantshaw's user avatar
5 votes
1 answer
2k views

How does power received by a dipole relate to electric field intensity?

Let's say there's a transmitter far away, and I'm going to receive the transmitted signal. To keep things simple let's further say the transmitted signal is a pure, unmodulated sine wave. Further, let'...
Phil Frost - W8II's user avatar
5 votes
2 answers
269 views

Can I have a signal generator on while it's not connected?

The unit in question is an Elecraft XG3 test signal generator. It puts out anywhere from -107 dBm to 0 dBm. Is it safe to have these signal generators on and transmitting while not connected to ...
Paul's user avatar
  • 781
5 votes
3 answers
4k views

How to find or derive the maximum safe input power to a receiver?

A number of my RF devices have a specified safe maximum input power, for example: "The maximum RX power of HackRF One is -5 dBm. Exceeding -5 dBm can result in permanent damage!" source "If you look ...
natevw - AF7TB's user avatar
5 votes
1 answer
92 views

Acceptable RF radiation exposure to electronic equipment

I'm wondering if there are any general guidelines or rules of thumb about the exposure of electronic equipment to RF transmissions, 150 MHz - 900 MHz. The background is that we are looking to install ...
Frank's user avatar
  • 349
5 votes
3 answers
1k views

Is possible to make small HF microstrip or helical antenna by using high permittivity dielectric material?

Is possible to make small HF microstrip or helical antenna by using high permittivity dielectric material like SrTiO3 ?
abduoman11's user avatar
5 votes
1 answer
2k views

How to calculate power loss between radio and antenna

I have a 5W HT that only kicks out about 1W by the time it hits my antenna (which I understand is the norm). How do I go about calculating power loss in various equipment?
Dan's user avatar
  • 1,754
5 votes
2 answers
652 views

What additional information do I get from an RF ammeter, as compared to a regular SWR/wattmeter?

I've grown up using SWR/power meters. They're "simple" devices: They show forward and reverse power, and show the standing wave ratio. They, however, do not show antenna efficiency. New hams are ...
AndrejaKo's user avatar
  • 2,893
5 votes
1 answer
824 views

Why does path loss decrease as frequency decreases? [duplicate]

The Friis transmission equation can be given as: Pr = Pt + Gt + Gr + 20 log10 (λ / 4πR) Where the variables represent: ...
natevw - AF7TB's user avatar
5 votes
1 answer
579 views

What would be the typical received power of an "S9" signal?

Obviously, I would expect the amount of power at a receiver would vary greatly based on distance from, and the power/efficiency of, the transmitter. But I don't really have a good sense for what "...
natevw - AF7TB's user avatar
5 votes
1 answer
115 views

Signal generator giving DC output?

My signal generator is giving DC at its RF OUT port. At 500 MHz & 1GHz it's giving nearly 1.5V DC when I measure the voltage between the centre pin & outer conductor with a multimeter. I don'...
Geoabs's user avatar
  • 51
4 votes
2 answers
330 views

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

My son KJ7NLL just got his general, and while we don't (yet?) have plans to transmit at 1.5kW, he asks if the 1.5kW limit is per-antenna? If it is per-antenna, and you use an antenna array of 8 ...
KJ7LNW's user avatar
  • 755
4 votes
3 answers
500 views

What is the difference between antenna input impedance and its radiation resistance?

I got to know that the input impedance of the halfwave dipole antenna is given as: Zin=1/Im* sin(β(H-|Z|)) where Im is the maximum current on the antenna( when excited with a 1 volt supply), H is the ...
Moulee's user avatar
  • 43
4 votes
3 answers
638 views

Why is coax loss logarithmic?

With DC hookup wire, losses are directly proportional to length. If the resistance of 100m of wire is 10 ohms then the resistance of 200m of wire will be 20 ohms. But we measure coax loss in decibels ...
natevw - AF7TB's user avatar
4 votes
2 answers
144 views

What does radiation power mean?

I would like to ask you what it means when it is written that a transmitter has 10 dBm power. Does that mean that 10 dBm is provided to the antenna, or that 10 dBm is measured at particular frequency ...
Nexy_sm's user avatar
  • 241
3 votes
3 answers
915 views

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

The Maximum Power Transfer Theorem says that maximum power transfer occurs when the source impedance equals the load impedance, when the power transferred to the load is 50%, the other 50% being ...
Andrew's user avatar
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