# How does one (easily) measure "power spectral density"?

Discussion in 'Ham Radio Discussions' started by VK3CHI, Nov 3, 2019.

1. ### VK3CHIHam MemberQRZ Page

Thanks to everyone for all the replies! It seems unbelievably deceptive that the calculation is so trivial.

For some reason I was under the impression that the waveform shape or modulation would influence this calculation i.e. 8kHz FM has a different power spectral density to 8kHz voice, which is different to 8kHz SSTV, which is different to 8kHz "whizzbang"-mode, etc, etc. I suspect I'm missing part of the story or confusing myself with an irrelevant parameter. If someone knows what I'm referencing, I'd appreciate it if they could set me straight!

On a side note, this perhaps has more to do with the wording and language of our legislation, rather than technical matters.... but would the "1W per 100kHz" apply to the entire signal? or would it only apply to the signal outside 8kHz? I think this is what Frank G0GSR is alluding to in the post above, and I'm not sure what it means either. For example, if the signal was 10kHz wide, would the restriction apply only to the 1kHz excess on either side of the 8kHz? or the entire 10kHz?

Andrew
VK3CHI

P.S. In answer to AH7I's question, I had a quick look at legislation regarding our; CB radio, LIPD, radio-controlled model, and scientific class licences, and none appear to have a parameter like this one.

2. ### NE1UHam MemberQRZ Page

@VK3CHI ... assuming that there is some engineering understanding of power spectral density by some lawyer/ham in the legislature, I agree with your "impression that the waveform shape or modulation would influence this calculation" would be the correct assumption .. as it would be in most every engineering school. I would would stay with that until some clarification was issued. Therefore, power spectral density would be calculated via some integration of a spectrum display (aka cheap spectrum analyzer could probably accomplish this).

What is suggested to just average power over the bandwidth is *very* conservative and would certainly be compliant. It does nothing to promote technology and understanding in the hobby.

3. ### W4ZDPlatinum SubscriberPlatinum SubscriberQRZ Page

My wife's spectral density is huge. And she has the power to take me down. So I take power spectral density very seriously. Don't wanna mess with the XYL, for sure. I don't ever attempt to calculate its magnitude, that would be frightening. Can you say diapason? It's 2:30am and I ain't got nuthin' better to do than emit inane posts.

4. ### G0GSRHam MemberQRZ Page

I just think that they are just trying to discourage use of and minimise interference to other band users if you should choose to use a wide bandwidth mode on the permanently crowded lower bands whilst still allowing some experimentation.

Frank