[amsat-bb] Re: Arecibo on 432 MHz Moon Bounce (some calculations)
Edward Cole
kl7uw at acsalaska.net
Thu Apr 22 09:08:21 PDT 2010
At 07:46 AM 4/22/2010, Edward Cole wrote:
>At 02:20 AM 4/22/2010, Stephen Melachrinos wrote:
> >Ah, but this focuses on my question: Why is ERP referenced to a
> >dipole? Why did someone assume that Arecibo's stated gain of 60 dB
> >was dBd and not dBi? I've never seen the gain of a dish antenna used
> >in satellite work quoted in dBd. All of the references for
> >calculating gain are based on the isotropic reference. And all of
> >the usages I have seen (in professional satellite work) use ERP and
> >EiRP interchangeably, and the i in EiRP is used to explicitly state
> >"referenced to isotropic."
> >
> >In fact, the amateur community is the only place where there is a
> >fascination with the dipole reference.
> >
> >The dBd specs are useless for any real calculation purposes. Satcom
> >engineering is much simpler if everyone quotes isotropic, and all
> >commercial/government/military satellite link budgets are based on
> >isotropic references.
> >
> >Steve Melachrinos
> >W3HF
> >(Professional) Satcom Engineer since 1979
> >
> >
> > > "ERP is about 243 MW" and
> > > that comes from the conversion from dBi to dBd.
> >_______________________________________________
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>
>In fact the first gain number published over a month ago was 58
>dBi. Then I suppose a bunch of hams complained that they didn't
>understand isotropic gain so the Arecibo folks kindly converted the
>number to 60 dBd. (i.e. unity isotropic gain, dBi=0, is what a true
>omni-directional antenna produces in free space)
>
>Does anyone on this reflector know the formula for calculating gain
>of a parabolic dish (Yes, I know-I'm asking if you know)? Did you
>know that Arecibo dish is spherical and not parabolic? So we can
>only use the gain number they provide (BTW the UHF line-feed corrects
>for spherical aberration of the dish surface at Arecibo). Arecibo
>can track a small amount of angle "because" the dish is
>spherical. It is my understanding (might be wrong on this) the
>line-feed can adjust for the amount of surface irradiated (which will
>change the gain).
>
>The formula normally used in radio astronomy and mw engineering is in
>terms of dBi. Most (not all) eme hams use dBi vs dBd.
>
>I am really amazed at this thread on amsat-bb. I thought the
>satellite community was more globally oriented (International). The
>different convention in expressing decimal numbers (aka using comma
>or period) is pretty well known (I thought). US/UK use period and
>most EU use comma.
>
>Most antenna analysis sw express gain in dBi
hmm dBi = dBd +2.15. Gain of dipole = 1.64 10Log(1.64) = 2.15 dB
so what gives here? is it 58 dBd and 60 dBi? Sorry if I wrote that
backwards. Or we just playing around with significant numbers and
gain is approx 58 to 60 dB (somethings).
Pat, Joe? can you please clear up this mess and state for everyone
what the gain is for Arecibo on 432-MHz?
73, Ed - KL7UW, WD2XSH/45
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