# [amsat-bb] Re: Preamp Distance from Antenna

i8cvs domenico.i8cvs at tin.it
Sat Apr 2 07:37:45 PDT 2011

```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" <matthias.bopp at gmx.de>
To: <AMSAT-BB at amsat.org>
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.

Best regards

Matthias

www.dd1us.de

-----Ursprüngliche Nachricht-----
Von: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at 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

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.kd8ksn at hotmail.com>
To: <amsat-bb at 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?
>
>
> Zack
> KD8KSN
> _______________________________________________
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```