# Tag Info

14

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 (power amplifier) stage of the transmitter, it idles the receiver, it switches the frequency synthesizer to generate the correct transmit frequency and a number ...

10

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 than exposure you are likely to get from cell phones, commercial broadcast stations, WiFi, and unintentional radiation from all kinds of electronics.

9

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 air in the spark gap breaks down, becoming a short circuit. The breakdown of a spark gap happens over just a few nanoseconds. (It turns off much more slowly ...

9

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 $$\left(\frac{\lambda}{R}\right)^2\cdot\frac1{16\pi^2}\approx \left(\frac{\lambda}{R}\right)^2 \cdot \frac1{160}$$ and $\frac\lambda R=3\cdot10^{-3}, \left(\frac\lambda R\right)^2=9\cdot10^... 8 A summary of requirements: Licensing: Technician - very minimal requirements Equipment: Transceiver, Terminal Node Controller and terminal Transceiver - traditionally a VHF/UHF FM radio Terminal Node Controller - Partly a modem, partly an AX.25 layer manager Terminal - Any serial terminal will do, but most likely you will use a personal computer A ... 8 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, triplexer, or quadplexer as appropriate. Sometimes the term "duplexer" is used although this creates confusion with a different device that is typically much more ... 8 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 decades. Relays are hard to beat for isolation and maximum power ratings. Their downside is relatively slow speed, which prevents really good QSK operation. They ... 7 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 superheterodyne receiver with a known intermediate frequency is powered on and tuned to a particular frequency, because the receiver's local oscillator radiates a ... 7 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 even harder, even impossible, to make uniformly good across their whole range. What you much likely will rather want is a multi-band antenna. For example, I'd ... 7 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 assume we start with CW and hence just need an oscillator (and a switch). So, let's do, say, a Colpitts, right? Well, sure, but the problem is that it's ... 7 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 frequency. Transmit. Check whether the spurs you see are symmetric about the transmitter's frequency or the receiver's frequency. This tells you which side ... 7 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. You get pulses of different (discrete) widths, which means you get spurs at predictable intervals. There isn't really anything you can do to make them go away, ... 6 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 broadcast stations, we can use the FCC's rules to make some estimations. By that reckoning, you need a transmitter of around 50kW with an antenna 150 meters ... 6 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 on HF. The lowest available band in the US that supports live video is 70 cm (~440 MHz). There are, however, options for transmitting pictures on HF. One of ... 6 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 permits direct keying. While the cathode, grid or plate of any stage can be keyed depending on design goals and available parts, an oscillator should be keyed ... 6 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 whether you're dealing with a straightforward single-carrier signal a spread-spectrum signal or a multicarrier signal. One place to see some examples is the Signal ... 6 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 cancel each other out instead of adding, half the time. To have the same phase, they would have to share the same RF oscillator (or be coupled to a master ... 6 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 be a kink in your understanding. RF doesn't "push against" one or the other. Let's be more specific about what we mean by "pushing against": ... 5 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 would trigger the output device. In the input section, I'd place some sort of galvanic isolation that would separate the input jack from the part connected ... 5 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. 5 W8JI has a great writeup on this topic. I'll summarize the key points. Key clicks are generally undesired "clicks" or "thumps" generated by a CW transmitter as the key is put down or let up. They can be caused several things. The most obvious is that a fast transition from carrier on to carrier off requires a wide bandwidth. You can think of this by ... 5 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 even a sinus, and has harmonics. It is these harmonics (=spurious emissions) which can cause interference to a whole range of services and devices when not ... 5 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. Square waves have the characteristic that in addition to the fundamental frequency (the frequency that is being directly generated by the program in your Pi) ... 5 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 MHz) have some specific unlicensed uses, mostly related to audio transmissions (wireless microphones, cordless phones, etc.) Since the question is about TV ... 5 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 are designed to amplify, detect, and decode signals in the range of a few to a few tens of microvolts; transmitters as weak as a few watts will put tens of ... 4 Transmitting on the FM broadcast band with any kind of power is illegal pretty much everywhere. Getting your ham license gives you some other bands to play with but music is only allowed on certain bands for very brief periods, ie, read "no pirate stations allowed". That being said, here's a 15W FM transmitter that will cover one bloody large area with the ... 4 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 or antenna are not matched to the transmitter output impedance, less power will be effectively irradiated by the antenna.

4

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 distance vhf/uhf ham-radio-exclusive networks where radios are used to tie stations together over short range, and then many hops are used to pass traffic across ...

4

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 circuit that sets the output frequency of the transmitter (ignore the polarity symbol on VC1, it is a remnant of the circuit editor on this site): simulate this ...

4

the spark gap's purpose is to very suddenly change the current in the LC circuit to which the antenna is, in fact, directly connected. the sudden change produces a very strong inductive kick in that circuit, producing a suddenly-rising voltage in it. that sudden rise gets filtered by the capacitor and inductor, causing them to resonate at the transmitting ...

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