9 votes

Why are germanium diodes specified in amateur radio designs?

A lot of designs that include germanium diodes are older, or based on older designs, when germanium diodes were commonly available. On the page for Germanium, Wikipedia says, "From 1950 through the ...
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  • 1,647
9 votes

How to reduce voltage ripple of a linear power supply?

Unless you haven't mentioned all the parts, you have built an unregulated power supply. The output voltage of a transformer varies depending on load, and the other components will just follow that ...
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8 votes
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Series resonant circuit, how to calculate voltage across the capacitor?

I encourage students to draw a simple schematic when it comes to word problems such as these. Here is an ideal series resonant circuit: simulate this circuit – Schematic created using ...
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  • 18.2k
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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Non-inductive resistors

For measurements the best resistor is a surface mount one - an 0805 resistor is non-inductive to a few GHz. Even better is to use two reststors, usually 100 ohms each, one each side of the coax. A 1/8 ...
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  • 8,235
7 votes
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Why are germanium diodes specified in amateur radio designs?

The lower forward voltage drop (0.3 volts or even less), compared to silicon diodes (0.7 volts or more). Having said that, there are some schottky diodes with a low forward voltage drop, too.
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  • 7,544
7 votes
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Is there a best ratio of L to C in a resonant circuit?

There is no hard and fast rule. Consider that in some LC circuit applications, a lower Q may be desirable in order to achieve a wider bandwidth. In other cases, a very high Q may be desirable for ...
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  • 18.2k
7 votes

Water cooling a GS-35B in lieu of air cooling?

The GS-35A tube is the liquid cooled version of the same tube. Some hams have fabricated their own version of a liquid cooling jacket to replace the fins on the GS-35B: The designer of this jacket ...
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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

Is double amplitude modulation useful?

One case where this might happen would be in the world of RF over fiber. In this technique, a radio-frequency signal is used to modulate an optical (usually infrared) signal for transmission over ...
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  • 9,275
7 votes
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What are the advantages for a receiver that uses an IF compared to direct conversion?

A superhet has two distinct properties, which may or may not be advantages: The image frequency is far away, and The IF is not DC In a typical superhet design, the LO and signal frequencies will be ...
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Unknown kit from DK0TV

This kit is a FM receiver built in 1977 by DK0TV. The receiver works on a single fixed frequency 30.000 MHz FM and has a very good sensitivity. It is used with a 10 GHz Gunnplexer to listen to the 3 ...
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  • 86
7 votes
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How to measure quartz crystal motional parameters using a VNA?

I don't know if this counts as "easy", but: Find the series resonant frequency. Multiply this frequency by $2\pi$ to convert it to an angular frequency, and call that $\omega_s$. Note the ...
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7 votes
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How to reduce VFO sensitivity to the hand capacitance?

As mentioned in the comment by henryflower, make sure that the shaft (and thus the frame) of the variable capacitor are grounded. You need to use rubber mounting grommets around the screws which bolt ...
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  • 3,512
6 votes

what are the alternatives to LM387 and SA602AN?

LM387 is just a really old, ordinary op-amp as far as I can tell. It has a funny pin configuration, so if you have a PCB already made you need to make modifications. I suggest cutting the traces, ...
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6 votes
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Easy way for an uncertified hiker to broadcast a distress signal

If you are in mortal peril, use any means necessary to make your situation known and get help. It doesn't matter if you are licensed or not. Consider the alternative. But a shortwave transmitter is ...
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6 votes
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How does this Pixie circuit produce a beat frequency?

Q1 forms a Colpitts crystal oscillator. It uses the reverse biased D2 as a varicap (variable capacitor) to pull the crystal frequency by 700 to 800 Hz, This frequency beats with the received frequency ...
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6 votes
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How to determine number of turns for a 1:1 balun?

The ideal number of turns depends on core material, geometry, and frequency. This is why you find such variance in how many turns should be used. More turns increases the choking impedance up to a ...
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6 votes
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Is double amplitude modulation useful?

Sure, it's technically possible. But I can't think of a reason it would be useful. The resulting modulation would be more complicated to implement, and occupy more bandwidth, than ordinary AM. ...
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6 votes

Significance of 1/4 wavelength with respect to antennas

The first part of why a quarter wavelength is special is actually understanding that it's not a quarter wavelength, but a half wavelength. Consider a quarter-wavelength monopole. If a wavefront ...
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6 votes

Design of oscillator

I would like to applaud your interest not only for homemade transmitters but also a theory! I started to build my own receivers and transmitters not a long time ago and completely understand how ...
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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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5 votes
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Why does "high SWR" damage transmitters, instead of "impedance mismatch"?

The VSWR isn't the problem per se, it's just the impedance that appears at the transmitter's terminals. Take the load at the end of the transmission line, transform it according to the electrical ...
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5 votes
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Why doesn't adding a resistor inline to a 12 V car charger change output voltage

You can't use a fixed resistor to control the input the voltage for an arbitrary electronic device, only one which is a passive load, such as a resistor, or an LED. (Note that even a simple LED ...
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  • 23.1k
5 votes

Are there an MTTF/MTBF/MTTR data on the typical tubes used in the FT-101/TS-520?

The original RCA specs for 12BY7 and 6146 did not disclose estimated lifetime of the tubes. An average tube has a lifetime of 2,000-4,000 hours, sometimes much-much more depending on usage. Given a ...
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5 votes
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What's the difference between a Return Loss Bridge and a Circulator?

For an ideal circulator, all the power entering one port exits the next port in the rotation and no other. So all the power in port 1 exits port 2, all into port 2 exits port 3, and all into port 3 ...
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5 votes

Feed-through Capacitors

Feed-through capacitors are still commercially available but their use in commercial applications has fallen off so prices have risen over the last few years. The concept with the feed-through ...
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  • 18.2k
5 votes

Is it practical to use a slug-tuned variable inductor in an antenna matcher (aka "antenna tuner")?

...variable caps are ... no longer manufactured in the old "interleaved plates, air spaced" form, and the tiny plastic dielectric ones that are still available can't take much voltage (and are ...
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  • 7,693
5 votes

Is it practical to use a slug-tuned variable inductor in an antenna matcher (aka "antenna tuner")?

Fair-Rite makes ferrite rods which would be suitable for HF applications. It should be possible to create or repurpose a screw-operated mechanism to move the rod into and out of a cylindrical coil. ...
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  • 7,693
5 votes
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Voltage vs. Current Balun for Dipole

You could rewind it, but the result may not necessarily be better. This is because the core must be optimized for different needs in each balun design. In what you're calling a "current balun", the ...
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