Hi Matthias, DD1US
I know that the negative effect of the added losses in front of a preamplifier and thus reduced sensitivity is also a function of the noise level due to the Antenna Temperature.
In a terrestrial system for 70 cm the antenna at zero degrees elevation sees at best a Noise Temperature equal to the ambient temperature of 290 kelvin if you are fortunate enough to live in a rural area.
In a satellite system a good antenna with low side lobes when aimed at high elevation sees a "cold sky" and thus the " Antenna Temperature" is around 50 kelvin i.e. much lower than the ambient temperature of 290 kelvin.
Now suppose that the loss of the coax cable betwen the antenna and the preamplifier input is 0.7 dB and the Noise Figure of the preamplifier is 0.55 dB and the antenna is used for terrestrial use at zero degrees elevation so that the antenna sees at best 290 kelvin
Coax cable 0.7 dB = 51 kelvin Preamp NF 0.55 dB = 39 kelvin Antenna temperature = 290 kelvin -------------- Total system Temp. = 380 kelvin ( for terrestrial use)
Now suppose that the antenna is aimed at the satellite and sees a "cold sky " at 50 kelvin so that the total system Temperature is reduced to 51+39+50 = 140 kelvin ( for satellite use )
The improvement of sensitivity of the same receiving system looking at the satellite against terrestrial use is 10 log (380 / 140 ) = 4.3 dB 10 in Signal to Noise Ratio.
By the way now suppose that the preamplifier is NOT antenna mounted and the line losses between the antenna and the shack where the preamplifier has been mounted is tipically 3 dB = 290 kelvin
For terrestrial use at zero degrees elevation the total system Temperature is 290 + 39 + 290 = 619 kelvin
For satellite use with high antenna elevation the total system Temperature is 290 + 39 + 50 = 379 kelvin
With 3 dB of loss of coax cable between the antenna and preamplifier the improvement of sensitivity of the same system looking at the satellite against terrestrial use is reduced to 10 log (619 / 379 ) = 2.1 dB ONLY 10 in Signal to Noise Ratio
This is why prescinding from where the antenna is aimed or towards the horizon or towards the sky in order to gain in sensitivity it is necessary to keep the losses between the antenna and input of preamplifier as low as possible.
Best 73" de
i8CVS Domenico
----- Original Message ----- From: "Matthias Bopp" [email protected] To: [email protected] Sent: Saturday, April 02, 2011 12:22 PM Subject: [amsat-bb] Re: Preamp Distance from Antenna
Hi Zack and Domenico,
I would like to add, that the negative effect of the added losses in front of the preamplifier and thus reduced sensitivity of your system is also a function of the noise level: in terrestrial systems where your antenna typically sees a noise temperature equal to the ambient temperature all written below is reflecting the negative effect quite well.
However in a satellite system where your antenna sees the noise of the sky the "Noise temperature" is a function of frequency and at VHF/UHF/SHF the sky is "quite cold" and thus the "noise temperature" is much lower than the ambient temperature.
Therefore the negative effect of the degraded noise figure is much higher. Please keep this in mind.
Best regards
Matthias
www.dd1us.de
-----Ursprüngliche Nachricht----- Von: [email protected] [mailto:[email protected]] Im Auftrag von i8cvs Gesendet: Donnerstag, 31. März 2011 00:03 An: Zachary Beougher; Amsat - BBs Betreff: [amsat-bb] Re: Preamp Distance from Antenna
Hi Zack, KD8KSN
The loss in dB of the coax cable from the antenna to your preamplifier adds directly to the Noise Figure NF of your preamplifier. As an example if the loss of the coax cable is say 0.7 dB and the Noise Figure of the preamplifier is 0.55 dB as specified by the manufacturer then the overall Noise Figure of the system antenna plus preamplifier is 0.7+0.55=1.25 dB
This is why to reduce the overall Noise Figure of any receiving system it is necessary to install the preamplifier as close as possible to the antenna connector.
By the way the gain of a ARR SP432VDG preamplifier is 16 dB and as connected in front of your 70 cm receiver you will see an increase of noise level of about 3 S-units in your S meter but the overall Noise Figure of your receiving system will be much lover than the Noise Figure of the 70 cm receiver alone.
In this condition using a preamplifier the 70 cm receiving system will be more sensitive but it will be more susceptible to intermodulation and overloading when you transmit to a satellite in the 2 meters band.
If you suffer with intermod the only cure is to add a 6 dB attenuator from the preamplifier output to the 70 cm receiver input but if you use a 70 cm tranceiver don't forget to remove the attenuator before to transmit in 70 cm
I hope this helps.
73" de
i8CVS Domenico
----- Original Message ----- From: "Zachary Beougher" [email protected] To: [email protected] Sent: Wednesday, March 30, 2011 4:24 PM Subject: [amsat-bb] Preamp Distance from Antenna
Hi,
I am in the process of getting the ARR SP432VDG preamp for my mobile setup (and just to have around), and I have read that the closer to antenna you mount the preamp, the lower noise figure you will have. My question is, what is close to the antenna - 1ft., 5ft., 10ft.? Ideally, I would probably mount it about 5-6ft. from the antenna (1/4 wave) - would this be acceptable?
Any additional advice would be appreciated!
Zack KD8KSN _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
_______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
_______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb