9
votes
Accepted
Amateur radio practice exam question
The 15 metre band is 21 - 21.45 MHz, so the third harmonic will be above 63 MHz - too high. 10 Metres is 28 - 29.7 MHz, so the second harmonic will be 56 - 59.4MHz - looks like B is the right answer.
...
- 60.8k
9
votes
Why does this transistor oscillator not work at 100 MHz?
I think this is a common misunderstanding of the "transition frequency" characteristic of transistors / opamps. A transition frequency of 300 MHz doesn't mean the transistor "works"...
- 26k
8
votes
Buying a variable inductor
The datasheet says
Inductance values from 9 to over 280 nH
That sentence is referring to the possible ranges of inductance values for all of the part numbers listed in the datasheet.
You must refer ...
- 12.5k
8
votes
Accepted
Critical approach to RF amplifier
Will it work?
Not as you expect - there are problems:
No bypass capacitor from +12V to GND (always add one as close to amplifier as possible)
Input impedance is extremely low
Why is emitter follower ...
- 27.6k
7
votes
Accepted
What is the purpose of diodes after RF power amplifier and before LPF?
The diodes are part of a RF switch that switches between RX/TX modes of the transceiver (See also my answer to another question).
This can be accomplished by changing the DC potential at the RX LINE ...
- 16.5k
7
votes
Accepted
BJT Clapp Oscilator for 3-6MHz
The RFC radio frequency choke, in collector of Q1, will allow that collector to swing above rail. That swing above rail may saturate Q2, depending on drop across R4.
I'd bias the Q2 base at VDD/2, ...
- 35k
7
votes
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How accurate is the antenna reciprocity principle in practice?
This is accurate in itself, you will get the same attenuation of the signal in both directions, even with different antennas at different stations talking to each other.
What is not the same in regard ...
- 9,686
6
votes
Accepted
How would you combine DC power, bi-directional serial data, and an RF signal on a single 50-Ohm coax feedline?
They weren't originally designed for it, but I have seen applications that use bidirectional radio chips such as the TI CC1101 to pass control/status data in both directions through a coax alongside ...
- 178k
5
votes
Antenna for qrp pixie 2
You do not say where you are.
The radio you reference requires you to hold an Amateur Radio operator's license. If you are in the United States, you need an Extra Class license (the highest), ...
- 2,679
5
votes
Why does this transistor oscillator not work at 100 MHz?
Why does this transistor oscillator not work at 100 MHz?
In one of your previous questions, I proposed this 100 MHz oscillator circuit: -
As you can see, L1 is about ten times what you are using and ...
- 473k
4
votes
JFET colpitts oscillator - very unstable
The cutoff voltage of the jfet should be less than the supply voltage. The capacitances are rather small, and the gate-source capacitance is nonlinear and of a similar magnitude.
Increase the supply ...
- 41
4
votes
Accepted
JFET colpitts oscillator - very unstable
It turns out I did my math wrong and my loop gain was less than unity, using wrong jfet in simulation didn't help to catch that issue.
- 93
4
votes
What is the purpose of diodes after RF power amplifier and before LPF?
They are pin diode switches, when TX is enabled, current flows via R150,L40 forward biasing the diodes and effectively making them transparent to the RF power. Remove the bias (Or better reverse bias ...
- 17.6k
4
votes
Accepted
Is it possible to use a YIG tuned filter to make a YIG tuned oscillator?
YIG tuned filters use several spheres for a high order filter.
Unless you take the filter apart and assemble the amplifier on the coupling to the spheres, the electrical length in and out of the ...
- 174k
4
votes
Accepted
an/prm-10 grid dip question - how the voltage can be negative at the grid?
I was studying tube designs as a child, long before I had any clue how a grounded cathode amplifier might be designed. Which means I don't remember that much. But one thing I do recall, which had ...
- 78.7k
4
votes
Tube transmitter issues
(Caveat -- I'm a toob fan-boy, but I don't have many actual tube circuits under my belt. They're just vacuum-mode FETs to me.).
The final amplifier stage is screen modulated. Either someone is ...
- 47.1k
4
votes
Accepted
What causes 100Hz spuriour emissions on AD9850 WSPR transmitter?
100Hz or 120Hz noise is often caused by power-supply ripples or rectifier glitches. those being twice the mains frequency in different locations.
- 36.4k
4
votes
Accepted
RF PCBs on 2 layer FR4
Instead of using a standard 1.6mm PCB (1/16"), use a 0.6mm board and then your trace width (per the rule-of-thumb that you want to use) will be 1.2mm. The Chinese prototype board houses like ...
- 4,116
4
votes
Getting started with Radio design
You'd not start by buying hardware – you'd start by
understanding how these devices work, then
implementing them in software, then
making your software fast enough to run in realtime, then
you ...
- 100k
4
votes
Accepted
What is the name of this antenna?
It does not appear to be a no-groundplane antenna, you can see a threaded hole where a ground plane radial would screw in. They come out at around a 45 degree angle and there is usually 3 or 4 of them....
- 27.9k
4
votes
Why would the power sent in the first place for it to be reflected on say a transmitter if antenna is open, or a higher impedance antenna connected?
What happens depends on the frequency of the signal you're transmitting. Let's consider the common case where the conductor (i.e. coax cable) is much longer than the wavelength of the signal. This ...
- 20.5k
4
votes
Accepted
Understanding FM Broadcast Receiver with RDA5807FP
I think the RDA5870FP is a programmable FM Radio chip can be controlled in a simple way with push buttons when pin6 is held High. Then the GPIO pins 1,7,8,15,16 are then: power, ch-, ch+, vol-, vol+ ...
- 56
4
votes
How accurate is the antenna reciprocity principle in practice?
Local background noise is hardly ever the same. Even not-so-local background noise is often different, since each station in a long-distance contact "sees" a different part of the ionosphere....
- 8,227
4
votes
Autotransformer Voltage Ratio Limit
There is no hard limit.
An autotransformer ratio close to unity is very much more efficient than an isolating transformer, because so much of the winding is shared.
As the ratio increases, the benefit ...
- 174k
3
votes
Accepted
Coaxial cable for the DC-10GHZ range
10GHz is very high frequency, and good cables for it won't come cheap. You will need to read the datasheets for specific cable products from specific suppliers.
The attenuation at 750MHz will not be ...
- 174k
3
votes
Wire size question
0,3 mm in English is 0.3 mm.
A Copper Wire Table tells me that #28 wire is 0.321 mm, and #29 is 0.286 mm.
- 60.8k
3
votes
Accepted
How to DIY BaoFeng radio-to-soundcard cable?
It looks like somebody solved the exact problem you are mentioning, and it requires an intermediate board between the sound card and the radio. Here is the suggested pinout:
The original article can ...
- 146
3
votes
How, exactly, does the 'input signal' wave(s) modify the carrier-wave of a radio signal?
The process of getting an input signal to modify the carrier-wave of a radio signal is known as modulation. The input signal is called the baseband signal or modulating signal. The combined signal is ...
- 4,433
3
votes
Why does this transistor oscillator not work at 100 MHz?
You've chosen a transistor that doesn't have much gain at \$100\mathrm{MHz}\$, and
Your tank circuit's characteristic impedance is much, much too low.
I don't have data for a 2N2222 handy, but the ...
- 47.1k
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