# Tag Info

15

QAM modulates both the phase (= frequency) and amplitude of the signal.

13

It turns out that we DO simultaneously use frequency and amplitude modulation: our everyday term for this is "single sideband." As described on Wikipedia, single-sideband is a special case of quadrature modulation in which the quadrature component (Q) is derived directly from the in-phase component (I) of the modulating signal. This effect becomes evident ...

7

(This is a purely theoretical answer; I have no experience in repeater building. Sorry.) The components of a repeater are: an antenna, a duplexer, a transmitter and a receiver, and a repeater controller. The antenna and duplexer are passive devices and aren't affected by what modulation you're using. The repeater controller is on the audio side of the ...

7

The capacitor and resistor make a low-pass RC filter, with a frequency cutoff around $\frac{1}{2\pi RC}$ (in hertz). Do keep in mind $R$ is not just the value of the resistor, but rather the entire load on the circuit which includes the input impedance of the next stage. The idea is to strip out the RF component, while passing the baseband component. If ...

7

Since you said you're using an RTLSDR, it's very likely that you aren't receiving a station on 22,695 kHz, but rather one on 6,105 kHz. NHK World Radio does broadcast on that frequency (according to the schedule I'm looking at, they broadcast between 0200 and 0400 UTC from a transmitter in France). The RTLSDR "direct sampling" mode is a hack, and ...

6

In the picture there is only ONE frequency and the amplitude changes Ah, but this actually causes a second frequency component, the one that you wish to transmit: (Please excuse my rudimentary paint skills.) It is this second, implicit frequency which combines with the AM carrier to then form a strong carrier signal in the centre of the spectrum, and a ...

6

Swapping I and Q reverses all the frequencies. For example, a signal 5 kHz above the mixer's LO will appear at -5 kHz, instead of 5kHz. CW and AM are symmetrical in the frequency domain, so it doesn't matter for the purposes of demodulation, though your software is likely to display the wrong frequency. SSB is not: reversing I and Q will make USB look like ...

6

FM modulated radio is much more resistant to noise and can deliver better sound quality than AM radio. Under certain conditions, that is! Namely, additive noise, and a fading channel. or is this noise resistance also an inherent property of FM? Huh, that surprisingly can be a pretty philosophical question about what bandwidth actually is, but let's ...

6

What if I wanted to run a radio broadcast station on a commercial SW band? Say, I am on a boat in international waters. What you're suggesting is known as pirate radio. That Wikipedia page provides plenty of examples of unlicensed stations, many of which operated for years before either being shut down or getting licensed. Many of those stations operated ...

5

I'll try to address your concerns a bit! I'll be simplifying here and there, but I think you'll notice where I'm doing that. I'll be doing my best to explain to him how this works. I understand that when a radio ray passes over an antenna, it generates an alternating electrical field in the antenna wire. Close, but not quite :) I'll correct you here, ...

5

That's a lot of questions! Would there be any SNR advantage if each sideband was detected seperately? Would the correlation of the LSB and the USB be usefull because the noise in general is not correlated? Well, that's exactly what envelope detection does: It takes advantage of the fact that the correlated signal energy in the two sidebands doubles the ...

4

The waves (to be correct: the signals) are exactly as suspectable to noise for both modulations! What's different is the receiver: As the name suggests, Amplitude Modulation (AM) works by taking the input (audio) signal, and using its strength, to modify the amplitude of your carrier wave. The AM receiver hence only has the amplitude of the received signal ...

4

EDIT to be more specific about the input circuit per Phil's comment: The antenna (aerial) is connected to the the oscillator of the first transistor stage formed by VC, L and the self capacitance of T1. This forms a classic regenerative circuit where the oscillator essentially samples a portion of the signal from the antenna as the oscillator builds up its ...

4

Groundwave field intensities in the MW broadcast band depend almost entirely on radiated power, frequency, and Earth conductivity. Below is a map of the 0.5 mV/m groundwave contour of radio station KOA, Denver (50 kW transmitter, 850 kHz, omnidirectional transmit antenna). Their transmit site is close to the eastern edge of the Rocky Mountains. The ...

4

Say the unmodulated carrier amplitude (voltage) is 1. At 100% modulation, the envelope will vary between amplitude 0 and 2. So at peak, the amplitude is twice an unmodulated carrier, but because power is proportional to the square of amplitude, power is $2^2=4$ times the carrier power. So I think your CBer friend has it backwards. If you are looking at the ...

4

Radio waves (such as broadcast AM and FM) can travel through a vacuum (thus do not require air). They are electro-magnetic (or EM) waves, the same as light waves (from the sun and stars, etc.), that can travel through space, but at a vastly lower frequency. The radius these EM waves can travel (unless blocked or absorbed) is the radius of the universe; but ...

3

First a bit of background on AM modulation and power levels. If we start with a 100 watt carrier and no modulation, we would measure 100 watts peak and average power. They are the same due to lack of modulation. When 100% amplitude modulating a 100 watt carrier, the PEP (Peak Envelope Power) is 400 watts. The carrier average power will be 2/3 of the 100% of ...

3

It depends... With a 100% steady modulated carrier, the PEP will be 4 times the unmodulated carrier power. The average power under this condition will be 1.5 times the unmodulated carrier power with 2/3 of this contained in the carrier and 1/3 in the sidebands (1/6 each in the USB and LSB sidebands). The real issue is - are you obtaining 100% modulation? ...

3

I find the idea of an SSB repeater quite challenging. A repeater has noteworthy the function of allowing communication between points which are mutually inaccessible along the direct line. A repeater has not the function of giving the word (the channel) to one sole participant a time (sic!). I greatly prefer the convivial chat in SSB on the HF bands to the ...

3

While I wouldn't rule out old capacitors or whatnot, if it's anything like some of the radios I've played with through the years, the solution might be a simple mechanical one: can you open it up enough so that you can hold the dial from moving while gently nudging the linkage from the FM circuit tuning mechanism until it lines up?

3

A class C amplifier indeed distorts AM and causes widespread splatter just as it does for SSB signals. BUT if you have a class C amplifier and apply modulation to it by superimposing the modulation signal on the supply voltage you can produce excellent AM modulation. The graph "150% modulation" in the question is not what one normally would see. With typical ...

3

Cascading more filters may help. Also simply adding an attenuator may help, as long as the RF noise floor is above the receiver's internal noise floor. However, spurious emissions from the station can also be an issue. In the US, spurious emissions are regulated by 73.44, which states: Emissions removed by more than 75 kHz must be attenuated at least 43 +...

3

The genuine Sony Walkman, and most of the Walkman-style radios from other makers, did indeed use the headphone cable for their antenna. My experience (with an early 1980s Sony model) was very good on FM, and even then AM was mostly talk, news, and sports, so I didn't listen much on that band. That said, AM radio isn't what it used to be. Fewer stations ...

3

I don't know that commercial class HF transmitters are still being built, but it shouldn't be very difficult to put one together. You'd need the audio mixing and control equipment that goes with any radio broadcasting station (this would be the same as a commercial MW AM or FM broadcast station), an HF transmitter that can be tuned to your chosen/assigned ...

3

Question 2 & 3 not fully answered... You probably actually are within line of sight of the FM transmitter. Broadcast transmitter antennas are typically placed on high towers specifically so that more people will be within line of sight of the tower. Of course, you may not actually be able to "see" the tower because of atmospheric attenuation ...

3

As hobbs - KC2G says, there could easily be multiple stations transmitting. Also, the frequency stability of the RTLSDR is quite poor. If you are seeing the frequency drift slowly over time (and not jump discretely) it's likely what you're observing is a side effect of the oscillator in the RTLSDR slowly changing as a function of temperature and other ...

2

Since OP requires MCU, doesn't have to be extremely sensitive or selective and the 'local' transmitter, one suggestion is Use Arduino (Atmel) to control DDS chip to generate 25MHz + - 77.5KHz Use mixer circuit taken out of Hamitup or similar. Simple transitor, FET, MosFet or diode also ok (for local transmitter) The 25MHz signal, now shifted to 77.5KHz, ...

2

The SoftRock SDR kits could qualify as "simple", if you mean simple electronics. Some of them cost as little as \$10 USD. They have fixed-frequency crystal oscillators, and you could probably swap the crystal for a nearby frequency in any of the kits as desired. The slightly more expensive and higher part count kits have a LO variable from DC to VHF and are ...

2

10 MHz WWV Receiver Experiments has a basic schematic for you as well as some good detail. However, as per Wikipedia, the 25 MHz broadcasts were discontinued in 1977. I also confirmed at the WWV website that 25 MHz is no longer used.

2

Rotorua branch 33 of NZART 2m linear repeater is still operational and has been for more than 20 years. It is a solar powered repeater and situated on a high point overlooking Rotorua Town. Output is 144.350MHz with a plus 600 shift. More details of it can be found here http://www.amalgamate2000.com/radio-hobbies/radio/Rotorualinear.htm

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