tomnexus
  • Member for 7 years, 1 month
  • Last seen this week
  • Vancouver, BC, Canada
Eliminating RFI from raspberry pi
2 votes

Do the spikes still appear when you connect the RTLSDR to a laptop instead? Are they definitely caused by the Pi? I can see a few tones near 1420 MHz on my RTLSDRblog (with an antenna), they get ...

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How do you calculate peak voltage given VSWR caused by impedance mismatch in a 50-ohm system?
4 votes

For amateur use, breakdown from overvoltage is probably not something you need to worry about. The main limits to power handling of a cable come from resistive heating and dielectric heating. Arcing ...

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GS cards in NEC decks
2 votes

You're right the documents are a bit thin here. Going through the source code would be one way. I tried this out quickly in NEC, as it helpfully prints out the whole geometry in the OUT file. The ...

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Reduction in transceiver power output into high SWR
2 votes

My Icom T81 handheld has a detailed circuit diagram and description in the service manual which can be found online (well done Icom!). It shows how the handheld will fold back power when the SWR is ...

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Whip/stub antenna sorting with SA using VSWR
3 votes

I think you've just described the awful truth about handheld antennas. I've always worked on a figure of -3 to -6 dB in ideal conditions, just because the whip is short and lossy. Then many more dB ...

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How should I think about "extra" impedances inserted within/across a transmission line?
2 votes

To solve the first circuit: Divide the transmission line into two lines, one on the left, one on the right. Zoom in to the centre portion. When you are in a small ($\ll\lambda$) region, the normal ...

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Ferrite unun transformer to match antenna resistance loads of <50Ω?
Accepted answer
2 votes

The gold standard books on baluns are by Jerry Sevick. They all seem to be out of print and expensive on ebay, but you may find one, or someone willing to summarise the relevant pages of their copy. ...

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Antenna Size limits at 2-30 MHz (10 m to 150 m wavelength)
1 votes

You first need to define "efficient" :) For narrow band antennas the Chu Harrington limit gives the maximum possible efficiency and bandwidth of an antenna. See the wikipedia article and its ...

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Why so high values of noise température of a spectrum analyser?
2 votes

A noise figure of 22 dB is pretty normal for a spectrum analyser. Expressed as a noise temperature it looks like a large number, but there are many equations which yield unexpected and large numbers. ...

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How to correctly model a small helix antenna
Accepted answer
3 votes

In my experience this can't be modelled in NEC. It doesn't handle tight corners, close segments, short segments like this. Your geometry input file looks technically correct, but you're asking too ...

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Transmitting Morse Code as Tone vs. Carrier
5 votes

As a mode for communication, as you say, it's pretty redundant and inefficient. I've never heard of it being used seriously. Occasionally people might hold a morse code practice class and transmit the ...

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Will there be common-mode currents if a coax followed by a ladder line is terminated in a dummy load?
Accepted answer
6 votes

Yes it radiates. How much depends on the lengths of the wires. The point at which you connect the coax to the two-wire line is a bit like a dipole feedpoint, with about half the voltage applied to it. ...

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Impedance miss-match short vs long section?
Accepted answer
6 votes

Hams usually send just one frequency down a transmission line (as opposed to say high speed data like HDMI). So it's much simpler to analyse impedance down the line, than look at the reflections. ...

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Coaxial cable identification
Accepted answer
3 votes

That is Sucoform 141 or a clone. A semi-rigid coax with PTFE insulation, 71% velocity factor. It's useable up to 33 GHz, though the SMA connectors won't go so high. Loss is about the same as RG-58. ...

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How much attenuation (dB) from a balun is expected?
6 votes

If you do it right, the bead(s) shouldn't get hot at all, and shouldn't add much loss. The impedance of the bead(s) should be about 10 x the impedance of the dipole. The current on the feedline with ...

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Do these look like amateur radio antennas?
13 votes

It looks like a fibre or telephone cable strung between buildings. It sets a good precedent for setting up your antenna though - if you can get access to the other rooftop at night, and a catapault or ...

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Link Budget - Are there any restrictions in the formula?
Accepted answer
7 votes

The Antenna Gain in the formula is the far field gain - at an infinite distance, or at least far enough that the antenna is indistinguishable from an isotropic source or a dipole. When you're close to ...

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Is an antenna always matched to free space impedance?
3 votes

I don't think the "matching to free space" should be taken so literally. I've never seen an actual equivalent circuit in a textbook or used it to derive any property of an antenna. Sure an ...

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Transmitted audio coming out through my speaker
Accepted answer
6 votes

The manual, on page 56 says: TX monitor allows you to monitor the on-going transmission sound. This is convenient when you want to check the modulation sound quality of the transmission. In FSK mode, ...

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Mounting a vertical antenna on a fiberglass mast
1 votes

You're right to move it further from the building. But in spite of what the website says, the antenna will definitely perform better if there's an RF ground or counterpoise available at its base. ...

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Does an SWR higher than 1:1 necessarily cause additional heat in the transmitter?
1 votes

An SWR of 1.5:1 is a feature of the line and the load, it doesn't tell you much about the transmitter. If the source were 50 ohms, (a perfect voltage source and a 50 ohm resistor), efficiency would ...

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What is a "direct sampling superheterodyne" receiver?
1 votes

The receive chain might have a tunable Local Oscillator, Mixer and BPF, before reducing the sampling rate further for final processing, detection etc. Isn't this a superhetrodyne receiver? The only ...

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When is a balun a Guanella?
3 votes

A Guanella balun or transformer is constructed so that there is effectively no magnetic flux in the core(s). There is of course some, otherwise it wouldn't have any inductance, but the common-mode ...

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How does the material of a fishing-pole mast affect antenna performance?
0 votes

You should definitely wrap a wire, or perhaps three wires, on the outside of the carbon mast. These could be wound in a slow spiral to stop them flapping. The efficiency will not be compromised much ...

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Maximum frequency for discrete components bandpass filter
3 votes

Two datapoints that might help: A previous company we had a 20-6000 MHz diplexer with a changeover at about 900 MHz, using 0402 parts. Some trial and error to get it to match the simulated design. A ...

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Were there any 6m handhelds produced ever?
11 votes

The Yaesu VX-5R had/has 50-54 MHz FM, 5 watts. Nice compact little radio! The VX-7 and VX-8 can also transmit 5 watts FM on 6 m, and 1 watt AM. None of the 3 will transmit SSB. It had a little ...

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Choosing a Relay for HF Applications
3 votes

Antenna tuners use fairly standard 250 V relays, so this would be a good place to start. The SGC-230 is an example I know. This post has some nice photos of its relays. Smaller tuners use little black ...

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3D Printed yagi antenna
4 votes

Some general hints from my experience: Use wires not printed metal. You could stop the print for a while to drop in the wires, then finish it, or push them in later. For sanity's sake, design your ...

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What is proper procedure for asking for "distant" contacts on 2m FM on simplex?
Accepted answer
10 votes

One would usually call CQ DX, but on 2 m FM that would sound a bit odd. Also FM will be heard by many people close by, who will find CQ DX quite funny. You could simply call and explain what you're ...

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Are there any cons of running shack on battery?
5 votes

Possibly an issue with your HF CB rig is that the output power will be lower at 12.1 than at 13.8 or 14.0 volts. I've seen on 100 watt ham rigs that the final amplifier runs directly from the DC ...

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