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Re: preamps--mast mount vs. in shack

on 11/29/00 9:57 PM, John Stephensen, KD6OZH at kd6ozh@gte.net wrote:

> Your method is correct for calculating the noise figure of the amplifier and
> the noise figure at the antenna feed does increase in proportion to the loss
> in the coax preceeding the amplifier. However the noise figure numbers
> assume that all components are at 290K and the antenna temperature at radio
> frequencies is much different. The antenna temperature depends on noise
> picked up from the sky at which the antenna is pointed. SInce all components
> are not at the same temperature, the noise figure must be converted to noise
> temperature referenced to the output of the antenna and then all noise
> contributions must be added to determine the total noise level.

I agree with all that you said.  I see now how you calculated your noise
temperatures.  When you talk about antenna temperature above, I agree.  That
will affect the noise in your system.  However, the original question was
what the difference is between preamps being in the shack or on the pole.
It matters not what the noise coming into the antenna is in this case other
than it may be the greatest contributor to SNR as you pointed out.  However,
the longer a piece of coax is, the more loss and the more it contributes to
the noise.  It still stands that even at varying temperatures, placing the
preamp BEFORE the length of coax is far better than after it.  However, the
question is does it matter significantly on 2m in a practical way (ie: can
you tell a difference)?  From what you have shown it really does not.



Jon Ogden
NA9D (ex: KE9NA)



My President is George W. Bush -> The legal winner in Florida

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