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, ...


14

The last mass-produced vacuum tube was the Cathode Ray Tube, Thankfully those are now going the way of the dodo, replaced with much more practical LCD or OLED displays. (Edit: it turns out the magnetron is still mass-produced, for microwave ovens. Although apparently semiconductor replacements already exist, and soon the magnetron will go the way of the CRT....


11

The short answer is that these are "standard" frequencies set aside for IF use and are reasonable free from interference. The longer answer follows if you are interested in the why. The choice of an IF frequency is one of those design tradeoffs. The lower the IF frequency used, the easier it is to achieve a narrow bandwidth to obtain good selectivity in ...


10

What is dBm? dBm stands for decibels relative to one milliwatt. Decibels represent multiplicative factors, or ratios; by establishing a specific reference level they can instead be used as absolute values: 0 dBm is 1 milliwatt, 3 dBm is approximately 2 milliwatts, etc. How do I convert it to watts? Convert the decibel value to a scale factor and ...


9

Signal strength and dynamic range Generally speaking, dynamic range is the ratio between the strongest and weakest signal that can be received. In a digital signal, the dynamic range is determined by the number of bits per sample: the strongest signal is one which uses the full range of the sample values, and the weakest signal is one which uses only two ...


9

As far as I know, vacuum tubes are used in newly-manufactured radio equipment (as opposed to still-in-use old equipment) for one purpose: high-power amplifiers. The advantages of vacuum tubes in this application are essentially from the fact that the tube can be built as a large and sturdy device. Semiconductor devices may fail due to overheating which ...


8

One way to tell is by its effects. Do you hear a garbled version of yourself in nearby speakers when you transmit SSB? Do GFCI outlets pop even though no one is being electrocuted? Does handling the transmission line give you RF burns when transmitting, or change what you hear when receiving? If you had these problems and now you don't, you must have been ...


8

There is a special adaptation of the Friis equation, called the Radar Equation, that describes the range of a radar system. Here is a basic version that calculates the maximum range of a radar system: $$R_{max}=\sqrt[4] {\frac {P_tG^2\lambda^2\sigma}{P{_{r(min)}}*(4\pi)^3*L}} \tag 1$$ where $R_{max}$ is the maximum range in meters, $P_t$ is the transmit ...


8

SMD chip inductors, compared to larger ones, will generally have: lower Q higher DC resistance higher SRF lower DC saturation current, if they're ferrite cored But read the datasheet and see if they'll work for your application. Often the higher SRF, smaller size and lower cost are the most important factors. For filter design trade-off, I'd suggest: ...


8

Vacuum tubes are neither outdated nor less efficient. See https://spectrum.ieee.org/semiconductors/devices/the-quest-for-the-ultimate-vacuum-tube The above IEEE Spectrum article talks about a vacuum tube amplifier that is more efficient than any solid state amplifier. This is just one example of active research on vacuum tubes.


7

QRP only means low power, often 1W or less. While you could technically have a 1W SSB transceiver and call it QRP, the practical range of such a radio would be limited. It is much easier to hear a single tone over the noise than it is to hear a voice which has its energy spread over a wide range of frequencies. Thus we say CW is a more sensitive mode: it ...


7

"Smart antennas" are antenna arrays with software-controlled phasing. Antenna arrays are commonly used in the amateur radio hobby, such as in Yagis, four-square antennas, etc. However, the phasing for these arrays is either fixed (as in the Yagi) or sometimes selectable between a few configurations via a switch (as in the four-square). The individual ...


6

If I use them as a dummy load for transmitters 144mhz and down, how bad would that be? It's just a mismatched load, and the consequences are the same as for a mismatched antenna. Check your transmitter's manual for the degree of mismatch it can handle and any applicable warnings. Modern transmitters typically have self-protection circuits and will not be ...


6

As Phil Genera says, the KX3 uses a different type of receiver that requires a different kind of filter. A traditional superheterodyne receiver, which is what you'd find in most older radios, uses a mixer to convert the desired signal to a fixed IF of say 10.7MHz and then uses a very narrow bandpass filter at that frequency to select only that signal. ...


6

Highest frequency stability is generally achieved when the oscillator is kept running while some following stage (or stages) is (are) keyed. However, the very high Q of a crystal-controlled oscillator permits direct keying. While the cathode, grid or plate of any stage can be keyed depending on design goals and available parts, an oscillator should be keyed ...


5

The problem you could run into is RF feedback into the sensitive mic input. This is particularly a problem if you run "QRO" - high power. It is best to use shielded multi-conductor cable, with the shield being used as the return for the PTT control. The mic output (signal and ground) are separately carried to the mic input connector, where the mic "ground" ...


5

There are many articles which describe how to use a R-Pi to emit (=transmit) a signal. However what most, if not all, articles fail to describe is that this signal is a very rough signal, usually not even a sinus, and has harmonics. It is these harmonics (=spurious emissions) which can cause interference to a whole range of services and devices when not ...


5

You are observing a practical example of harmonics. Harmonics of a Square Wave You do not mention how you are generating the signal with your Pi but it probably uses a simple square wave generator. Square waves have the characteristic that in addition to the fundamental frequency (the frequency that is being directly generated by the program in your Pi) ...


5

The problem is most likely the type 43 core material. It is quite lossy at 21 MHz and is a poor choice for the auto-transformer. Unlike a transmission line transformer, the auto-transformer relies on the coupling of the two coils to partially occur by passing flux through the core. The complex permeability of type 43 material indicates high losses for this ...


4

The reason that this is done is the difficulty of obtaining sufficient adjacent channel selectivity in the front-end tuning while still achieving high levels of image rejection across a range of frequencies as wide as the HF bands. The first intermediate frequency is higher, often in the range of 10MHz. This is used for adequate image rejection, while ...


4

Is there a particular fluid that is better than other in both performance and cost? Water is hard to beat in both respects. The trouble is it has this tendency to become a conductor as it dissolves salts, so it must be insulated, or somehow kept very pure. Sometimes the trouble of this is too much, so some sort of oil is easier. What sort of volume of ...


4

The KX3 receiver is a software defined radio SDR. The IF filters are made in the software. Since software IF filters are steeper than crystal filters the crystal filters are not necessary anymore. Another advantage is that SDR IF filter width is continuous adjustable. But band filters and audio filters are still LC filters like in the past. That has not ...


4

Your radio, as most modern radios, is a superheterodyne receiver. These receivers work by first converting the intended signal to a fixed intermediate frequency (IF, 445kHz in your case) then demodulating that. This is in contrast to a direct conversion receiver, which demodulates the signal directly, without first converting it to an IF. The ...


4

There are a number of commercially available RF data modems that operate within the amateur radio spectrum for hobby use particularly with RC (remote controlled) hobby aircraft. However, I believe your specific ITU Region imposes additional output power limitations which will make this challenging at 4 km distance. The RFD900 developed by an Australian ...


4

I've done some tests with what I have available (now including the CT-44 adapter). With the MH-37A4B Earpiece/Microphone connected, the internal microphone can still be used: whichever mic has its PTT switch pressed will provide transmit audio and the other will not. Furthermore, the external microphone is open-circuit when its PTT is not pressed. Therefore,...


4

Sending data is not so different from sending audio. You just have to come up with some kind of symbol which represents one or more bits of data. For example, you could encode your data by speaking "1" or "0" into a microphone. Someone at the other end listens to the audio and types in the numbers, and boom, you are sending data. Of course you probably ...


4

I'm not experienced with RC systems, but on general antenna design principles: The bend is fine. It's too small a feature to have much effect. The “ground” wire (really better described as a second antenna element) is fine. The antenna will radiate better in the direction that wire runs than others. The biggest problem with this layout is that your antenna ...


4

I'd use copper clad (30%) steel wire. It's stronger, less stretchy, and less costly, and the current only flows on the outside of the wire so the steel is irrelevant to the resistance of the conductor. You can also get a plastic covered version of this wire. This wire with a dark green plastic coating is very stealthy.


4

A crystal radio generally consists of a simple LC network and a diode detector. So in theory, altering the LC network would allow you to tune higher frequencies. The real issue is that signals you will find in the higher bands, simply do not have enough power to be detected without additional amplification. You have an even larger obstacle, however. A CW ...


4

Since this platform considers questions asking for products as off-topic, I'll interpret your question as: I have an IF receiver, but there's no good filters for the rather low IF it uses (455 kHz). What to do? If you're doing a new design, simply pick an IF that works with the filters you can get. That, and pretty much nothing else, is what defines ...


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