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Do you think it would work to combine a HackRF One and an amp like the following to create a QRP SDR?

Link to eBay

Output of HackRF is approximately 10dBm, which fit's this amp nicely...

What is the disadvantage of a wide frequency range amp? Or in other words, why is not everybody doing it that sells HF, VHF and UHF amps separately? Are there any frequency specific components in it? Or is it just the case that the higher the frequency gets, the higher the requirements for hardware components gets?

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Just to answer the second part of the question, you do not want an amplifier that has too wide a bandwidth, mainly to lower the complexity (and hence also the cost) of the amplifier.

All users of radio spectrum (including radio amateurs) must not cause any interference to other users of the radio spectrum, and that means that out-of-band transmissions must be kept to an absolute minimum.

Since all radios generate some out-of-band signals, there is usually some degree of filtering to make sure that those signals are kept to a minimum. More expensive radios usually have much more filtering than cheaper models.

An amplifier will amplify all signals within its passband, and so if you had a wideband amplifier that amplifies all signals from Top Band (around 1.8MHz) to the 70cm band (around 430MHz), it would also amplify any spurious signals coming out of the transmitter. (And this is before we talk about parasitic oscillations within such a wideband amplifier.)

For this reason, all good amplifiers that support multiple frequencies have a switched passband, where each band is selected individually (usually manually, but solid-state amplifiers have started to include automatic band switching) and there are good filters on the input and output to keep out-of-band signals to a minimum.

This obviously adds to the cost and complexity of an amplifier, and to cover a very large number of bands would at some point become prohibitively complicated or expensive.

A better alternative is to use less expensive components, and design an amplifier for a specific band or set of bands (e.g. HF, VHF or UHF, or perhaps a VHF/UHF amplifier that does not cover HF).

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  • $\begingroup$ My newest Idea is that I'm using a HackRF + AMP from 10mW to 3W (+ a lot of harmonics) and then another commercial AMP from 3W to 100W (getting rid of out of band emmisions through the filters in the commercial AMP) - do you think this will work? $\endgroup$
    – Romeo Kienzler
    Jul 5 '18 at 5:59
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    $\begingroup$ What specific frequencies are you thinking of - are you thinking of making a wide-band amplifier from 10mW to 3W for HF to UHF, and then using individual power amps to give you real power at the necessary HF, VHF or UHF frequencies? $\endgroup$
    – Scott Earle ♦
    Jul 5 '18 at 6:01
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    $\begingroup$ I am asking because making a fully wideband amplifier is hard, with problems (as mentioned above) like parasitic oscillation and the like. en.wikipedia.org/wiki/Parasitic_oscillation $\endgroup$
    – Scott Earle ♦
    Jul 5 '18 at 6:02
  • $\begingroup$ yes, that's the plan - buying (not making) a wide band amp from 10mW to 3W HF to UHF and then use individual HF and VHF/UHF AMPs from 3W to 100W or so...so do you think I'm getting rid of those parasitic oscillations from the filter of the individual high power amps? $\endgroup$
    – Romeo Kienzler
    Jul 6 '18 at 7:21
  • $\begingroup$ A wideband amp from HF to UHF might be hard to find. Using a lower-bandwidth high-powered final amp would be more practical, so that might work for you. Bear in mind that most radios that work from HF to UHF (and even higher sometimes) have multiple antenna outputs, so you could use a different amp on each set of bands, as well as an antenna suited for those bands too. You won't get a good, efficient antenna that will work from 160m to 70cms, for example - so I am now wondering what antenna you will plug all this into $\endgroup$
    – Scott Earle ♦
    Jul 6 '18 at 7:27
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If you are in the US, there is a limit on the number of dB increase a RF amplifier can have, and I'm certain that one is way past it.

It also says it is a FM amplifier, which limits it pretty good to a few bands.

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  • $\begingroup$ I'm in Switzerland, didn't check the laws so far. Also FM only doesn't make sense. I'm a SSB guy $\endgroup$
    – Romeo Kienzler
    Jul 4 '18 at 5:58
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    $\begingroup$ I'm not aware on any limitation on amplifier gain in part 97, only limits on power output. $\endgroup$
    – Phil Frost - W8II
    Jul 4 '18 at 11:55

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