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The question is as follows:

An interfering signal from a transmitter is found to have a frequency of 57 MHz (TV Channel 2 is 54 - 60 MHz). This signal could be the:

And the options are as follows:

A) crystal oscillator operating on its fundamental
B) second harmonic of a 10 metre transmission
C) seventh harmonic of an 80 metre transmission
D) third harmonic of a 15 metre transmission

Now in my mind options B and D are the same frequency as the second harmonic of a 10 meter transmission is 30 MHz*2=60 MHz and the third harmonic of a 15 meter transmission should be 20 MHz*3=60 MHz. I don’t see how one answer could be more correct than the other.

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    \$\begingroup\$ I'm voting to close this question as off-topic because it's not a general EE question, it might be better off at ham.stackexchange.com. \$\endgroup\$ – Marcus Müller Nov 8 '18 at 16:56
  • \$\begingroup\$ "harmonics" are sometimes counted differently; in some contexts (not really in this, but whatever, ham radio exam questions are full of incorrect and / or vague formulations in many countries of this earth), only the odd harmonics count; in some I could imagine you'd only count the even harmonics. I don't know – maybe this is one of these cases. \$\endgroup\$ – Marcus Müller Nov 8 '18 at 17:08
  • \$\begingroup\$ Also, let me do the quick math: $$ f=\frac c\lambda = \frac{3\cdot 10^8 \frac{\text m}{\text s}}{15\,\text m }= 20\,\text{MHz,} $$ so no matter which harmonics you count, 57 MHz is not a multiple of 20 MHz and B and D are both equally wrong. \$\endgroup\$ – Marcus Müller Nov 8 '18 at 17:13
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    \$\begingroup\$ I fully agree this is a stupid question, but if your frequency control is not reliably with 3 MHz (capitalization is important, as M = mega = 1 million and m = milli = 1/1000) of 57 MHz, that's a frequency error of more than 5%: If your oscillator is that unreliable, no matter what you do with it, turn it of, and repair or replace it. No discussions: that is a broken oscillator. \$\endgroup\$ – Marcus Müller Nov 8 '18 at 17:18
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    \$\begingroup\$ The 15 metre band is 21 - 21.45 MHz, so the third harmonic will be above 63 MHz - too high. 10 Metres is 28 - 29 MHz, so the second harmonic will be 56 - 58 MHz - looks like B is the right answer. (where did the 20 Hz and 30 Hz come from?) \$\endgroup\$ – Peter Bennett Nov 8 '18 at 18:47
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The 15 metre band is 21 - 21.45 MHz, so the third harmonic will be above 63 MHz - too high. 10 Metres is 28 - 29.7 MHz, so the second harmonic will be 56 - 59.4MHz - looks like B is the right answer.

You have to look at the defined band frequency limits, rather than the frequency calculated from the band name.

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    \$\begingroup\$ This is why this should be on the ham site! \$\endgroup\$ – mike65535 Nov 8 '18 at 19:10

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