dBi is used in range calculations because a isotropic antenna has an even radiating field leaving only distance as a variable when calculating path loss. After determining the path loss for a given distance, the antenna gain, transmit power, receiver sensitivity, receiver noise, cable loss, and connector losses are factored into the equation to give a link budget. With the excess allocated to fade margin. ERP calculations are used by regulatory agency's to determine possible interference.
Art, KC6UQH
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Edward Cole Sent: Thursday, April 22, 2010 8:47 AM To: w3hf@arrl.net; amsat-bb@amsat.org Subject: [amsat-bb] Re: Arecibo on 432 MHz Moon Bounce (some calculations)
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
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@hotmail.com ======================================
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