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Re: preamps--mast mount vs. in shack



At 145 MHz galactic noise is about 3 dB above thermal noise so the location
of the preamp will make about a 0.6 dB difference in the signal to noise
ratio.

580 K  Galactic noise
  35 K  0.5 NF Preamp
---------
615 K  total noise temperature

580 K  Galactic noise
  92 K  noise from coax.
  35 K  0.5 NF Preamp
---------
707 K  total noise temperature

707/615 = 1.15 = +0.6 dB

At 435 MHz mounting a similar preamp at the antenna will make a bigger
difference because the noise level is about 9 times lower and the noise from
the coax is about 1.7 times higher.

  64 K  Galactic noise
  35 K  0.5 NF Preamp
---------
  99 K  total noise temperature

  64 K  Galactic noise
160 K  noise from coax.
  35 K  0.5 NF Preamp
---------
259 K  total noise temperature

259/99 = 2.62 = +4.2 dB

73,

John
KD6OZH

----- Original Message -----
From: "Mark L. Hammond" <hammond@surrealnet.net>
To: "AMSAT-BB" <AMSAT-BB@AMSAT.Org>
Sent: Tuesday, 28 November 2000 22:54 UTC
Subject: [amsat-bb] preamps--mast mount vs. in shack


> Question for the list---
>
> Assume I put a preamp with the following specs in the shack vs. on the
mast
> (at the antenna).
>
> Frequency 144-148 MHz
> Noise factor(dB) 0.5
> Gain(dB) 24
> 1 dB Compression (dBm) +12
> 1 dB Bandwidth (MHz) 7.0
>
>
> Assume the coax is 75' of 9913, which claims 1.6 dB attenuation in a 100'
> length at 150 MHz.
>
> How do I calculate the benefit of having the preamp at the antenna, rather
> than in the shack?  I've browsed the web a good bit, but not found the
> right place yet (and a Handbook on CD is on my Xmas list!)
>
> I guess the 75' length has 75/100 x 1.6 = 1.2 dB loss.  So, I can get that
> far, if I did that math right.  It has something to do with signal to
noise
> ratio...but not sure how to calculate it.  I understand the 1.2 dB loss,
> but I think there is more to it than just that loss...
>
> Any pointers to a site that discusses this?
>
> Thanks in advance,
>
> Mark L. Hammond  [N8MH]
>
> ----
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