14 votes

How do PTT buttons work?

The basic PTT (push to talk) button on the HT is a simple switch that is normally open and when it is pressed, the switch is closed (conducting). Internally to the radio, this applies power to the PA (...
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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 ...
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10 votes

How To Join Packet Radio BBS

A summary of requirements: Licensing: Technician - very minimal requirements Equipment: Transceiver, Terminal Node Controller and terminal Transceiver - traditionally a VHF/UHF FM radio Terminal ...
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  • 580
10 votes
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What does the spark-gap do in a spark-gap transmitter?

The spark gap is a simple, low-tech way of generating high frequencies. It was the only way of generating them before valves and transistors were invented. As C1 charges up through R1, eventually the ...
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9 votes
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How to calculate power transmitted by monopoles?

10⁻⁴ which I think is too low power. Why? Gut feeling says that is actually pretty good a transmission for such a distance. Notice how your free space path loss is $$\left(\frac{\lambda}{R}\...
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8 votes

Share wide band antenna with multiple transceivers

The basic concept is to use RF bandpass filters for each frequency range. This is frequently done by hams for VHF and UHF applications. The common term for a grouping of these filters is diplexer, ...
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8 votes
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What replaces the T/R switch in modern transceiver

Receivers and transmitters are still separated today, they are just in the same box. There is still a T/R switch too, integrated in the box. In many radios, it's a relay, same as they have been for ...
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7 votes
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Detecting the listeners

No, that's not possible. There is no practical way to determine whether the transmitted power is usefully received by an antenna, or just absorbed by the ground. It is possible to detect that a ...
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7 votes
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Share wide band antenna with multiple transceivers

A wideband antenna is not what you're looking for – you really don't care about anything between 900 and 1500 MHz, or between 1600 and 2400 MHz. Wideband antennas are inherently hard to make, and ...
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7 votes

How to create a simple 1296MHz transmitter?

So, there's a bit of disappointment I'll have to spread here: Building microwave circuits like the one you need isn't per se as easy as just scaling up a circuit for, say 3 MHz or 100 MHz. Let's ...
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7 votes
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Where does this common spurious transmission come from? Is there a quality difference?

This is very likely to be be due to overload of the receiver. A quick, rough way to tell the difference: Tune your SDR receiver so the waterfall center frequency is not the same as the transmitter's ...
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7 votes
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Can these in-band spurs from rpitx be eliminated somehow?

I can't dig up my source for this, but I'm pretty sure these spurs are just the result of rpitx using a fractional-N PLL (that isn't in any way optimized for phase noise) to generate a square wave. ...
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  • 8,730
6 votes

How far does a watt equal in range in the FM band?

This is a link budget question. Transmit power is just one factor: others are terrain, antennas, frequency, quality, noise, and so on. If we restrict the variables to typical values for FM commercial ...
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6 votes
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How to transmit images and video through a hurricane?

In general, the HF bands (1.8 MHz through 30 MHz) do not have sufficient bandwidth to support live transmission of a video signal. In the US, the FCC does not authorize an emission mode for live video ...
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6 votes

Will I get cleaner keying by keying the oscillator, or interrupting the amplifier B+?

Highest frequency stability is generally achieved when the oscillator is kept running while some following stage (or stages) is (are) keyed. However, the very high Q of a crystal-controlled oscillator ...
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  • 7,685
6 votes
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How does one know how a signal has been modulated

Yes, you'll have to look at it. I can't go into detail about every possible modulation, because there's just too many. But typically, a look at the spectral representation gives you an idea of ...
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6 votes
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If i transmit on the same frequency using two separate transmitters joined together will they both add up in a reciever to produce a stronger signal?

You certainly can, in theory, use multiple transmitters together to produce a signal of higher total power. However, there is a catch: the signals must be in phase with each other. Otherwise they will ...
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6 votes
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Is this "floating" output single-ended or differential and against GND or VCC?

The other part of me thinks that the DC blocking capacitor will be a low impedance at RF so that the RF is actually "pushing against" GND or VCC but I don't know which one. This appears to ...
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6 votes

Reasons for an antenna tuner at the transmitter other than transmitter protection?

Just because the finals don't go up in smoke when driving a mismatched load, doesn't mean they're working well. They will probably deliver less power to the antenna than they would in a matched ...
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  • 8,730
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

Audio Morse Code to CW converter?

First, I'd start by separating the idea into smaller logical parts. I'd have an audio input section, a detection section that would test to see if we actually have a Morse code tone and a section that ...
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  • 2,715
5 votes

What is a "dip relay" and how does one use it to build a transmitter?

This design could be using the key to control a tiny relay (possibly inside a Dual Inline Package) to connect and disconnect a tiny low power oscillator from the antenna feed.
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  • 12.4k
5 votes

Weird Harmonic Issue on Raspberry PI FM Transmitter

There are many articles which describe how to use a R-Pi to emit (=transmit) a signal. However what most, if not all, articles fail to describe is that this signal is a very rough signal, usually not ...
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5 votes

Weird Harmonic Issue on Raspberry PI FM Transmitter

You are observing a practical example of harmonics. Harmonics of a Square Wave You do not mention how you are generating the signal with your Pi but it probably uses a simple square wave generator. ...
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5 votes

Unlicensed TV broadcasting power limit (Band I)

Since you mentioned part 15, I'm guessing you're asking about the rules in the United States. If you look at part 15 subpart C in detail, you'll see that the frequencies in the VHF I band (44 - 87.5 ...
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5 votes
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What happens when you directly couple the transmitter antenna to the receiver antenna?

If your transmitter emits enough power to be heard more than a few meters away, the likely result will be destruction of components in the receiver and possibly in the transmitter as well. Receivers ...
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  • 3,871
4 votes

How To Join Packet Radio BBS

A Technician Class license or better is required. What equipment you need very much depends on the solution or solutions you pursue. There exist well-baked schemes for creating relatively long ...
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4 votes
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Ettus B210 USRP Is it possible to transmit on both Tx Ports either with same or different application?

UHD requires a USRP to be owned by one process only, so no, you can't share the same USRP across two flow graphs, but you could use for example the ZeroMQ PUB/SUB sink/source pair to stream data in ...
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4 votes
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How to tune an FM transmitter to a specific frequency?

I will assume you will get the legal issues sorted so I will address the circuit itself. If you look at the second circuit you referenced, you will find L1 and VC1 in parallel. These form a tuned ...
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