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

Stephen Melachrinos melachri at verizon.net
Thu Apr 22 19:31:14 PDT 2010

Nigel -

There are lots of lots of reasons ...

1. All link budgets use a path loss calculation that's also referenced to an isotropic radiator. If you use antenna gain referenced to a dipole, you'll have to add back the gain of the dipole (referenced to the isotropic radiator).

2. It's the standard way of calculating gain for virtually all professionals in the satellite, radar, deep space, avionics, microwave and many other fields. 

3. Arguably, dBi is less ambiguous that dBd. By definition, an isotropic reference has the same gain (0 dB) in all directions. A dipole has a directional pattern, so dBd only makes sense if you also define the direction. I believe the assumption is gain broadside to the antenna, but that is still an assumption.

4. If you don't want to actually do link budgets, so you say reason #1 above is irrelevant, then your only real purpose is to compare two different antennas. And then the difference is always going to be dB, whether the two antennas are specified in dBi or dBd. So I still say to use the universal, unambiguous standard.

5. Finally, and most significantly, your statement "Why would I believe, or want to use, something I can neither have, use or measure? An isotropic antenna doesn't exist." is just as applicable to the ideal dipole that is your reference for dBd as it is to an isotropic radiator. You can't build a dipole that has zero resistance along its physical length, you can't build a dipole that has zero interaction with its feedline, and you certainly can't achieve any placement that perfectly represents the "dipole in free space" assumption upon which the dBd reference is based.  

In summary, there are plenty of reasons whay virtually anyone who builds or uses antennas for a living, does it in dBi


> Apr 22, 2010 09:15:04 AM, nigel at ngunn.net wrote:

> I guess because it's impossible to build an isotropic radiator and therefore just as impossible to measure it.
> Why would I believe, or want to use, something I can neither have, use or measure?
> An isotropic antenna doesn't exist.
> > In fact, the amateur community is the only place where there is a fascination with the dipole reference.

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