I would like to use SDR to receive and transmit on CB, UHF and VHF. Is this possible and if so what type of equipment will I need (antennae, dongle etc)? I currently have both UHF/VHF and CB but atm have separate antennae for each rig. Reading about SDR I think I can have one single antenna and use the computer magic to allow me to work with all frequencies that antennae receives, am I mistaken?

  • $\begingroup$ You don't mention whether or not you have an amateur radio or other license. You need to check whether you are allowed to transmit on the frequencies you have in mind. Can you please provide more information? $\endgroup$ Dec 21, 2017 at 3:32
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    $\begingroup$ In Ireland CB on the frequency I intend to use is exempt and the UHF vhf is on freq that are used in kids walkie talkies so as far as I know they are also unrestricted. Does my legal status effect the answering of a technical/ theoretical question @Mike Waters? $\endgroup$
    – welfare
    Dec 21, 2017 at 7:02
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    $\begingroup$ One's legal status is very relevant. We should not be seen to be encouraging illegal activities. If someone were asking questions about how to commit a serious crime (e.g. asking questions about poisons in the chemistry SE), they would not get many answers for the same reason. And any 'helpful' answers that they got may get deleted. $\endgroup$
    – Scott Earle
    Dec 22, 2017 at 1:14

3 Answers 3


You didn't mention which country you're in, but in the US, it is only legal to transmit on CB with a "type-accepted" radio. In other words, the manufacturer of the radio must apply to the FCC for permission to sell the model. It's not legal to build your own CB transmitter.

Regarding using a single antenna for multiple bands, a transmitter typically expects the antenna to have a certain impedance. It is possible to use a single antenna for multiple bands, but in that case a simple antenna will have quite a wide variety of impedances across the multiple bands. For a very low-powered transmitter this may not pose a problem. For higher-powered transmitters, a mismatched load can cause very high voltages that can easily damage the transmitter. (Modern high-powered transmitters often include a circuit to automatically reduce power when a mismatch is detected, to protect the electronics.) Another problem is that the efficiency of the transmitter and the feedline can vary considerably if the frequency strays far from the optimal range.

How to design an antenna and feedline system for a wide range of frequencies deserves to be a separate question here, but let me say that the most economical solution to the problem is often to have separate antennas and a coaxial switch to choose between them.

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    $\begingroup$ I'm in Ireland, cb on the bands I'll use is unrestricted $\endgroup$
    – welfare
    Dec 21, 2017 at 7:05
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    $\begingroup$ Right, I'll leave the language about CB in the answer though to help future readers who might wonder the same thing. If you feel that an answer has satisfactorily answered your question, please accept it by clicking on the check mark. Welcome to ham.stackexchange.com, and happy holidays! $\endgroup$
    – rclocher3
    Dec 21, 2017 at 14:59

Reading about SDR I think I can have one single antenna and use the computer magic to allow me to work with all frequencies that antennae receives, am I mistaken?

Well, I always explain it that way:

An SDR is like a soundcard on speed. Usually, with a mixer.

So, what the SDR does (in Receive direction) is take a spectrum of bandwidth $B$ (for example, 10 MHz) around a center frequency $f_c$. So, all signals between $f_c - \frac B2$ and $f_c + \frac B2$. It then mixes them down, usually to baseband, which means that these frequencies all get shifted by $f_c$, so that the same $B$ wide swath of spectrum ends up around 0 Hz, that means in $-\frac B2$ to $+\frac B2$.

Then, it uses, just like a sound card, a fast (dual) ADC to digitize the full $B$.

Now, flexibility comes from two things:

  1. You now got a digital signal. FM? AM? SSB? PSK? APRS? Doesn't matter, it's in your computer, you just pick the right software to demodulate that. Also, you can have a digital filter in software that is much sharper than any physical filter and select exactly one channel.
  2. The $f_c$ is usually pretty freely choosable. So, you can tune your SDR to receive e.g. 20 MHz to 30 MHz, or you can tune it to 140 to 150 MHz. In the end, the original center frequency doesn't matter for the processing of the signal.

So much for theory. In practice, of course, a couple of things must still work out:

  • your SDR must actually be able to tune to all frequencies you want. If it doesn't, this can often be compensated with an external up- or downconverter, but that'd basically not achieve your single device goal
  • As you might know, mixers aren't inherently perfect. They tend to not only mix down the intended frequency, but also harmonics of that (amongst doing other interesting things). Hence, an SDR must include some sort of selection filter that avoids unwanted intermodulation products. Now, the more flexible a device is, the less fixed and the less great these filters can be. Not a problem if your antenna only works well for the frequency band you're interested in. Not as cool if your antenna actually works for multiple bands. So, you might need preselection filters. Details can really only be discussed if we know what SDR you're considering.
    • A commonly overrated problem is deafening by stronger transmitters in the same band. Whilst this might be a problem in extreme situations (and you can only remedy that with analog filtering), it's astonishingly easy to avoid (offset tuning, oversampling, predistortion).

Para experimentacion y usando cargas ficticias, sin antena, puedes hacer un transceptor usando por ejemplo limesdr (transceptor fullduplex) o tambien hackrf one (halfduplex) El hackrf one se adapta muy bien a la ideologia de transceptor de radio aficionado y puede funcionar desde 1mhz a 6ghz aunque es de 8bit y eso lo hace mas vulnerable al ruido.

A efectos de experimentacion y poniendo en lugar de antena una carga de 50 ohmios, no tienes ningun problema.

Seria interesante hacer una comunidad de radioaficionados programadores y hacerle una gui web en java script.

Per Google translate: For experimentation and using dummy loads, without an antenna, you can make a transceiver using for example limesdr (fullduplex transceiver) or hackrf one (halfduplex). The hackrf one adapts very well to the ideology of amateur radio transceiver and can operate from 1mhz to 6ghz although it is 8bit and that makes it more vulnerable to noise.

For experimentation purposes and putting a 50 ohm load in place of antenna, you have no problem.

It would be interesting to make a community of amateur radio programmers and make a web guide in java script.


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