Ebay 45W HF Amp build and mods.

Discussion in 'Homebrew and Kit Projects' started by ANCELB, Aug 28, 2017.

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  1. K8JD

    K8JD Ham Member QRZ Page

    I'm trying to recall when antenna tuners went from a Pi network that is a low pass filter when tuned right, to a T network where it is basically a high pass filter.
    I think it was around the time when tube finals gave way to all solid state rigs.
    AF7XT likes this.
  2. G3YRO

    G3YRO Ham Member QRZ Page

    Because my mobile antenna has about 5 kHz bandwidth ! (so would be far better better at getting rid of any spurious radiation than any kind of filter)

    What makes me laugh about the people on this thread who are criticising this basic Amplifier kit is the fact that some people are talking as if we are only allowed to use Type Approved commercial equipment . . .

    No . . . we are licensed Radio Amateurs . . . we are allowed to use ANY equipment we like, whatsover . . . whether we have designed and built it ourselves, modified miltary or PMR equipment, or modified commercial Amateur equipment. That's because we have passed exams to demonstrate that we have the knowledge to make sure we are using it properly.

    Roger G3YRO
    Last edited: Sep 13, 2017
  3. AF7XT

    AF7XT Ham Member QRZ Page

    I prefer the Drake MN-2700B (with balun) if curious enough , look it up.
  4. N7EKU

    N7EKU Ham Member QRZ Page

    Have you measured this or are you just assuming? I am asking because I would think that antenna bandwidth does not necessarily mean the same as filtering; e.g. signal may still get out even if the SWR is high. A filter shunts frequencies beyond its cutoff to ground. Does an antenna do the same thing?

    Also, it could be that an antenna resonant at a design frequency, may also work on the third harmonic of that which is also the main problem harmonic of a push-pull amplifier.

    I don't recall anyone saying, or implying, that radio amateurs not buy kits and only use type-accepted equipment. Criticism of this particular kit was mainly in its biasing design. There were posts, or course, about filtering, because this one doesn't have any and many amateurs get conflicting information about the need for them due to manufacturers advertising, and lack of testing on this in amateur exams. I'd have to say also from posts like yours where you don't give enough information in the post to determine what you are doing.

    Also there seems to be confusion because some are advocating a Pi antenna tuner as being equivalent to an appropriately designed LPF. This is not true as a Pi antenna tuner would need to be designed to give the needed harmonic attenuation and not just design for giving low SWR. Most aren't designed this way.


    KU3X likes this.
  5. K1ZJH

    K1ZJH Ham Member QRZ Page

    Two variable inductors are more costly, cumbersome and lossier than variable caps.
  6. G3YRO

    G3YRO Ham Member QRZ Page


    On my 160m mobile antenna signals on receive are 40dB down if you go even 100kHz off the resonant frequency.

    And on higher bands there is nothing . . . the loading coil acts as a choke.

    So you could use an amplifier without any bandpass filter on the output without any worries of causing interference.

    And when I was talking about using an ATU to supress harmonics, of course I meant the right kind of ATU! One with a proper tuned circuit on the signal frequency, like you'd use to feed a high impedance antenna. (not one that just adds a small amount of L or C to correct a slight mismatch on a coax feeder)

    Sure, I didn't go into details . . . but the point I was making earlier was that you don't ALWAYS need the classic bandpass filter on the output of a transistor amplifier.

    Roger G3YRO
  7. KU3X

    KU3X Ham Member QRZ Page

    That practice is called, "Poor engineering !"

    You need to properly filter !
    I made the first comment about filtering. I did not fault the design of the amp. I just suggested that the next project be a BOM (bill of material) list for adding filters to the project. That is not faulting the design !
    Just because hams can build their own equipment does not mean that they can use dirty equipment ! Home brew equipment MUST COMPLY !
    Do you actually think that you can build your own amp with no filtering whatsoever, generate third order harmonics that are only 10 DB down from
    the fundamental frequency and use this equipment on a multi band antenna?

    I had an RFI complaint many years ago while using my Home Brew 4-1000 amp. The FCC wanted details of my complete station including
    how the amp was built. Why would they ask these questions if I could build my own amp and feel I did not need to address spurious radiation?
    Not that this applies but a bad TV cable installation was the cause of the problem.

    I find it hard to believe that the spurious radiation rules do not apply in the UK like they do in the USA?

    Barry, KU3X
  8. G3YRO

    G3YRO Ham Member QRZ Page

    No it isn't ! I think you're missing the point Barry . . .

    If I built an amp like that and wanted it to be capable of being used in any situation, then of course, I would add a set of bandpass filters for each band.

    BUT - and the point I was making - is that in certain situations (like I described) you WOULDN'T need bandpass filters, and there would be no harmonics radiated. So for example, if I wanted to only ever use it on my 160m mobile whip, it would be unecessary. (and so there would be no point)

    Of course we have similar rules over here !

    But it's down to each Amateur to make sure he uses his equipment in such a way that he doesn't radiate any - there is no actual specific requirement about the equipment, whether home built or commercial.

    So - for example - a taxi driver or a police force has to buy and use equipment that is "type approved" - that's all they are allowed to use, because they don't know what they're doing.

    Whereas as Radio Amateurs we are licensed to use ANY equipment we like . . . because we are supposed to know what we're doing.

    Roger G3YRO
    Last edited: Sep 14, 2017
  9. K1ZJH

    K1ZJH Ham Member QRZ Page

    You are deluding yourself if you think that antenna is not working as a radiator on odd order harmonics.

    KU3X likes this.
  10. G3YRO

    G3YRO Ham Member QRZ Page

    No I'm not !

    Sure, if it was a quarter wave long . . . but my mobile antenna couldn't possibly do that.

    Roger G3YRO

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