[amsat-bb] Polarity
Robert Bruninga
bruninga at usna.edu
Sat Jan 9 17:40:55 UTC 2016
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 at 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 at usna.edu>
> To: "amsat bb" <amsat-bb at 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
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>> Steve Kristoff
> skristof at 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 at amsat.org. AMSAT-NA makes this open forum available
> to all interested persons worldwide without requiring membership. Opinions
> expressed
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