[amsat-bb] Re: Arecibo on 432 MHz Moon Bounce (some calculations)

Edward Cole kl7uw at acsalaska.net
Thu Apr 22 08:46:33 PDT 2010

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
>(Professional) Satcom Engineer since 1979
> > "ERP is about 243 MW" and
> > that comes from the conversion from dBi to dBd.
>Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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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

73, Ed - KL7UW, WD2XSH/45
BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
EME: 144-600w, 432-100w, 1296-60w, 3400-fall 2010
DUBUS Magazine USA Rep dubususa at hotmail.com

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