27
It's a feature not a bug.
When turned on, the weather alert function checks the local NOAA station for its alert tone every 5 seconds for half a second... doing so takes over the receiver.
I'd turned it on, and forgotten I'd done so; and how that feature is implemented never registered in my mind. Turning it off solves my "problem."
Thanks to ...
23
It'll be easier just to buy a programming cable and use the free Chirp software to turn off transmit permanently. I did this, and used my Baofeng as a receiver only before getting my licence.
EDIT - When I mean permanently, I mean the radio will not respond to the transmit key being pressed. If you get your license in future, you can reenable this feature ...
21
My two favorite locations for listening online are:
http://websdr.org/
WebSDR was first conceived as a means to make the 25 m radio telescope at Dwingeloo available to many radio amateurs for EME reception. In order to test a preliminary version of the software without using the 25m dish, a shortwave WebSDR was set up on Christmas Eve 2007 at the radio ...
17
When the tone is present, it is because the antennas are out of phase. One antenna's signal will be leading the other; which one is leading depends on which one is closer to the transmitter (provided that the antennas are less than 1/2 wavelength apart, so that the phase difference will always be less than 180°).
When we switch from the lagging signal to ...
15
The earliest Soviet satellites, like Sputnik 1, transmitted what amounted to a CW stream -- just pulsed RF at pretty low power (later Sputniks transmitted data by pulse width modulation, still essentially CW). In order to hear it, you needed a Beat Frequency Oscillator -- a BFO -- and with a common radio receiver, the signal generator was standing in for ...
14
The answer is simple: filtering.
For example, let's say the desired signal is at 800 MHz, and the intermediate frequency (IF) is chosen to be 100 MHz. Mixing the 800 MHz signal with a 900 MHz local oscillator (LO) would get the signal into the desired 100 MHz IF, because 900 - 800 = 100.
But also the image frequency of 1000 MHz would end up at the IF, ...
12
It doesn't sound like you're doing anything wrong. Most likely, you are simply only hearing strong stations; broadcasters put much more power into their transmissions than amateurs are legally allowed to, so you can hear them over a much wider range.
Advice on the practice of listening:
For finding signals, first of all, always use SSB. Even if it's the ...
12
Your “B” is the stereo difference signal of broadcast FM stereo.
It is placed at twice the pilot frequency so that it can be recovered by having the receiver lock onto the pilot signal and frequency-double it to obtain the subcarrier signal marking the position of the difference signal. The receiver uses this subcarrier to shift it in frequency down to the ...
11
If you haven't seen them, the RTL-SDR dongles are really hard to beat for scanning. They let you see a waterfall display so that you can see visually which frequencies are active. It's wonderful!
10
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 ...
10
Intersymbol Interference (ISI) — or analog equivalents, like ghosting — are relevant only when the difference in time of arrival between the primary and reflected signal are significant compared to the symbol duration.
For example, PSK 31 has a symbol rate of 31.25 baud, meaning each symbol is 32 ms long. If the difference in arrival time is significantly ...
9
I'm working on a system (theoretical at the moment) whereby I wish to receive data from multiple sources all transmitting at the same time. I've read that a helical antenna is capable of this.
I'm afraid you're misinformed. No single antenna, no matter what the design, can be better than another at receiving multiple signals at once. A receiving antenna ...
9
Yes, you can do this. There are two issues: connecting it to the radio, and the effect on reception. The latter might be problematic enough that using two receivers is actually a better option.
Connecting two antennas to the radio
If you simply connect two antennas in parallel, the math is just like connecting two resistors in parallel: the impedance is ...
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^...
9
In short: yes, it's possible, but it's a bit harder than that. You want to drive all of the receivers from a shared clock, and sample them in phase with one another, to form a "coherent receiver". This requires some hardware work; you can't just plug multiple regular RTL-SDR sticks into a computer and make it work.
There are some coherent RTL-SDR ...
8
"Pole" comes from the Laplace transform, or it's discrete (for digital filters) equivalent, the Z-transform. Without delving into the mathematics of it, these transforms enable a filter designer to express the response of a filter in terms of some number of poles and zeros on the complex plane.
Using these transforms simplifies many tasks of the filter ...
8
A scanner is a specialized receiver with features for effectively monitoring many frequencies, classically by tuning to each in rapid succession (scanning) and stopping when a signal is found, then continuing when the signal ends. The scan function can of course be disabled to use it as a receiver for a single channel.
Scanners as commonly discussed are ...
8
Back in the olden days, before transceivers took over, the transmitter and receiver were separate units, and the receiver had to be protected from the transmitter. The solution then, as now, is an electronic T/R switch. One approach is an RF sensor that triggers a relay; the MFJ-1708 is one example. A more sophisticated approach uses an active element to ...
8
Essentially, it is an FM to AM converter, followed by an AM detector, followed by an audio amplifier.
The heart of this circuit is C1 and L. Together, these make a parallel LC circuit. At resonance, this looks like a very high (for ideal components, infinite) impedance. Away from resonance, the impedance decreases and becomes more inductive or capacitive.
...
8
The efficiency of any antenna has a direct impact on the gain of the antenna. The gain of the antenna equally effects transmit and receive - a characteristic known as reciprocity.
The highest gain dipole is a 10/8 wave dipole. When you begin to shorten that dipole, its gain drops. When the dipole becomes very short (<<1/10 $\lambda$) it is called an ...
8
The story is about Eric LeMarque who got lost after snowboarding out of bounds in the Sierra Nevada mountains.
Radio receivers usually contain some kind of oscillator. Detecting the unintentional radiation from the oscillator or the mixing products in the receiver is technically possible, but in modern receivers the power is so weak this power is so weak it ...
7
WWV and WWVH broadcast time and frequency reference information continuously, using AM on 2.5, 5, 10, 15 and 20 MHz, so you should be able to pick up all of them using your receiver, given an adequate antenna.
At my location (San Francisco Bay Area) with lousy equipment, these signals are only slightly stronger than shortwave broadcast stations, but more ...
7
These things will significantly help under one condition: the RF noise floor is below the receiver's noise floor. This is almost never the case on HF, where low-noise amplifiers are easy and atmospheric noise is high. On VHF and UHF the natural noise floor is lower, and low-noise receivers are more expensive, so it is possible but not guaranteed. It will ...
7
No, the receiver does not try to determine a carrier frequency, you do. When receiving SSB, you must adjust the VFO (main frequency control), not just until you hear a signal, but also to adjust to match the suppressed carrier frequency by ear.
Slightly-off SSB signals have a distinct sound, which is not just being too low or high in pitch, because the ...
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
Would the portable ~$40 Tecsun get damaged somehow by a 100-foot antenna because it's designed to work with shorter, portable antennas?
Unless you are within a kilometer or two of a very high power transmitter, there is no chance of your antenna collecting enough power damage your receiver.
You are more likely to cause damage to your receiver from static ...
7
CW is one reason. CW is on-off keying, so if the AGC reacts at a timescale similar to or shorter than the transitions of the signal, it will partially cancel the signal, making it "muddy" and more difficult to copy. 15wpm morse has a dit length of 0.08 seconds, making a dah or an inter-letter space around 0.24 seconds, so a fast 0.1s AGC will do it quite a ...
7
Yes, this most certainly was a thing, and people actually did it.
This type of communication has been done at ELF frequencies in the 3-30 kHz range. Their linear amplifiers were ordinary Hi-Fi audio amps. I don't know if LF frequencies higher than that are useful for that, but the hams on the 2200m band (137 KHz) use above-ground antennas.
A lot of ham ...
7
The roofing filter is the filter through which the first IF must pass. The roofing filter is an IF filter, specifically the first one. AB4OJ provides a block diagram:
As for why it's called a "roofing" filter, Elecraft suggests it's about protecting the subsequent stages from spurious signals:
The term "roofing filter" has most often ...
6
It is possible in principle to detect a receiver.
Almost all designs of receivers have a local oscillator (LO), which oscillates at the same frequency as the carrier frequency of the signal it is receiving, or at some close offset to it. The signal from the LO can leak back out the antenna. However, in a well-designed receiver this leakage is minimized and ...
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