Sorry, I should have elaborated.... dB is a reference to power. ANd power is proportional to Voltage squared. So when the voltage of an RF field is down by 0.707 squared it is down by one half. Or - 3dB.
And when we use the cosine law we are referring to the angle off axis. So straight on is 0 angle and is 0 dB loss. 45 degrees is 3 dB down. But going another 22.5 degrees or 67.5 degrees off axis is 0.38 squared which is.144 which is -8.4 dB . Getting down to 90 degrees where the cosine is 0 might also seem confusing, but remember we are comparing "dB down" which is a ratio of the full power available, lets say 1, divided by the smaller power we would get at off angles. When we get to 90 degree off angle, and a cosine of 0 then we are comparing "1" straight on, divided by the "0" we get when we are cross polarized and so we get 1/0 which is infinitely "down"...
I guess there is a better way to explain it... but I have not had my coffee yet.
Bob
On Sat, Jan 9, 2016 at 12:20 PM, Steve Kristoff skristof@etczone.net wrote:
Well, the cosine of 45 degrees is actually 0.707. I suspect that the article is implying that the loss is linear. Since 45 degrees is half of 90 degrees, then you lose half the power, or -3dB. Following that logic, at 22.5 degrees you'd half one-fourth the power or -6dB, etc.
I have no idea if the article is correct, but the cosine of 45 degrees is 0.707
Steve AI9IN
----- Original Message ----- From: "Robert Bruninga" bruninga@usna.edu To: "amsat bb" amsat-bb@amsat.org Sent: Saturday, January 09, 2016 12:05 PM Subject: Re: [amsat-bb] Polarity
We all know that [matched polarity] = Zero db loss from cross
polarization.
And then in theory, the "Infinite loss" when cross polarized... [Pracically],it's more like 30 to 40 db or so [ or much less in the
presence of reflections]
Now does anyone know of a chart ... for every degree of something really
is?
Like 45 deg is 3 db down, 50 deg =? 55 deg =? etc.
The cosine table works for all angles.. The cosine of 45 degrees is 1/2 which is -3 dB etc...
Bob, Wb4APR _______________________________________________ Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
Steve Kristoff
skristof@etczone.com
"A few chords strummed on a ukulele, enough to please a few others beside yourself, does more good in this world than the combined efforts of all the financiers and politicians that ever lived." - Frank Littig, Littig's New Harmony Self Instructor Chords for Ukulele, Banjuke or Taro Patch Fiddle, Chart Music Publishing House, Chicago, Illinois, 1924 _______________________________________________ Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb