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Re: HRD and two radios: was dual IC-706 station



At 08:53 PM 12/2/2005 +0100, i8cvs wrote:
>----- Original Message -----
>From: "Wayne Estes" <w9ae@charter.net>
>To: <dmg@bossig.com>
>Cc: <amsat-bb@amsat.org>
>Sent: Thursday, December 01, 2005 11:33 PM
>Subject: Re: [amsat-bb] HRD and two radios: was dual IC-706 station
>
>> Dave G. wrote:
>> > Never, ever, generalize... not no-how, not never !!!
>> >
>> > Pre-amps are primarily needed to overcome losses in
>> > transmission lines with he inherent increase in noise
>> > factors!!
>> >
>> > A good short read is in the  discourse in the HAMSAT journal
>> > on the wideband homebrew 432MHz pre-amp
>>
>> Wayne replies:
>>
>> I'll generalize a little more specifically this time.  Regardless of
>> cable loss, a <1 dB NF preamp allows one to hear weak signals better
>> than a transceiver whose internal preamp has a noise figure 4 or 5 dB.
>>
>> Wayne Estes W9AE
>> Oakland, Oregon, USA
>> ----
>
>Hi Wayne, W9AE
>
>I agree with you.
>
>Regardless of cable loss if you connect to the antenna a receiver
>with a noise figure of 5 dB and  adding a preamplifier the overall noise
>figure of the system decreases to 1dB than your S/N ratio in dB
>improves and the improvement can be much more than the difference
>between the above noise figures particularly if  the noise contribution
>of the antenna is low.
>
>As an example:
>
>In 70 cm a good antenna system has an equivalent noise temperature
>of about 50 kelvin when the antenna is looking at the sky in a quite 
>rural area far away from the city.
>
>A noise figure NF= 5 dB is equivalent to a noise factor F = 10 E 0.5 =3.16
>and the equivalent noise temperature T1= (3.16-1) x 290 = 627 kelvin
>
>A noise figure NF= 1 dB is equivalent to a noise factor F= 10 E 0.1= 1.26
>and the equivalent noise temperature T2 = (1.26-1) x 290 = 75 kelvin
>
>If we add to T1 and T2 the equivalent antenna temperature of 50 kelvin we
>get the  equivalent noise temperature Tsys of the 70 cm system connected to
>the antenna without and with the preamplifier.
>
>Tsys1 = 627 + 50 = 677 kelvin  without preamplifier
>
>Tsys2 =   75 + 50 = 125 kelvin   with preamplifier
>
>The improvement of the S/N ratio is thus 677 / 125 = 5.42 in factor
>
>or 10 log      5.42 = 7.34 dB   
>               10
>
>The above improvement is very high because if  we like to get it without 
>using the preamplifier we must increase the antenna gain by 7.34 dB
>
>This improvement in dB is greater than the difference 5-1 in dB between
>the noise figures because the noise contribution of the antenna in 70 cm is
>very low and is only about 50 kelvin
>
>Of coarse in 2 meters the equivalent noise temperature of the antenna
>is at best a litle bit less than 290 kelvin in a very good antenna system
>having small side lobes and located in a rural area.
>
>If we repeat the calculation for 2 meters using 290 kelvin instead of
>50 kelvin for the noise contribution of the antenna than the improvement
>using the preamplifier will be only 4 dB
>
>Considering to use a coax cable between the above preamplifier antenna 
>mounted and the above receiver in the shack than the coax cable must be 
>considered as a cascade stage with a noise figure equal to the loss in dB 
>and with a negative gain equal to 1 - F where F is the noise factor of the 
>cable.
>
>Best 73" de
>
>i8CVS Domenico
>----

Dom,

Very good illustration of the improvement that a preamp can make.  I have
had a spreadsheet program available on my website for some time that will
permit you to determine the effects of adding a preamp or two, or a preamp
convertor and receiver.  There are fields for inputing cable losses between
each component, and input fields for your antenna temp and sky temp.  The
program is taylored for determining eme or moonbounce S/N but one can just
use Part-I for seeing how NF and Noise Temp, cable losses, and placement of
equipment will affect total sensitivity.

http://www.qsl.net/al7eb/emelink.xls
or,
http://www.qsl.net/al7eb/emeOVROcalc.htm 

The www.qsl.net site has been down today so it may not work until later.
The first link will allow you to download the excel program to save in yoru
HD if you desire.  There is are links on the emeOVROcalc webpage as well.

enjoy!
73's,
Ed - KL7UW 
===================================
BP40iq,  Nikiski, AK      http://www.qsl.net/al7eb
Amsat #3212
Modes: V - U - L - S
===================================
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