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: amsat-bb-boun...@amsat.org [mailto:amsat-bb-boun...@amsat.org] 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" <zack.kd8...@hotmail.com> To: <amsat-bb@amsat.org> 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 AMSAT-BB@amsat.org. 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 AMSAT-BB@amsat.org. 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 AMSAT-BB@amsat.org. 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
