Friday, October 9, 2020

Harmonic content of mcHF's output


From time to time I've read comments on various forums that mcHF transceivers should not be used with amplifiers because of the high harmonic content on the output.

I almost always use an amp with mine when operating from home so I decided to measure my  mcHF. My amp, a Hardrock-50, contains its own lowpass filters on the output so I was never concerned with whether or not the mcHF was "dirty".

Here are the results, made with a recently (July 2020) calibrated HP-8563A spectrum analyzer.

Each horizontal line represents 10dB. So the top line, where the fundamental is placed, is the reference or 0 dB. The next line down from that is -10dB, then -20dB, etc.

I've put a delta marker on the first harmonic and it's easy enough to count down from the top of the display to see the attenuation of subsequent harmonics, keeping the 10dB per line in mind.

With the exception of 17 and 60 meters, I'm happy with the performance of my mcHF.

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10 meters

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12 meters

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15 meters

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17 meters

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20 meters

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30 meters

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40 meters

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60 meters

80 meters

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4 comments:

  1. David Ryeburn VE7EZM and AF7BZOctober 9, 2020 at 3:49 PM

    I thought garbage had to be down 43 dB. It isn't, on some bands.

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  2. David VE7EZM & AF7BZ wrote to inform me that FCC requires spurious emissions to be at least 43dB below the carrier. I had to look it up, thinking that 30dB was the threshold of legality. David is correct for radios with an output greater than 5 watts, which the mcHF is, therefore my mcHF is illegal on 17, 60 and 80 meters when I operate it at its 10 watt max output (and on 17m regardless).

    https://www.law.cornell.edu/cfr/text/47/97.307

    In the next couple of weeks, I'll measure the harmonic content of the Hardrock-50 amp while being driven with the mcHF. The amp has its own LP filters and I'm curious as to how effective they are.

    David also informed me of the difficulty of posting his comment here due to Google Blogger's implementation of Captcha. I've turned that beast off.

    Thanks David, for both sets of info. 73,

    John AE5X

    ReplyDelete
  3. what's the harmonic output levels at the more nominal 7W maximum output? (consider a table of results by band in addition to photos?)

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    Replies
    1. These examples were made at 4 watts out - I doubt there would be much difference at 7 watts.

      But more importantly, I should point out that these are for my mcHF. The mcHF is a kit and builders will differ in how they wind the coils used in the LP filters. I could tinker with the windings as well, spreading them out or adding/removing a turn here and there to achieve different results.

      With the nanoVNA now being available at a reasonable price, new builders of this and other kits can manipulate the windings of output filters and know with certainty its cut-off frequency, insertion loss and attenuation levels at various harmonic frequencies. I'm looking forward to the upcoming QCX-mini just so I can tinker with this aspect of its build.

      Whether or not these examples are typical of all mcHF's...I have no idea.

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