[amsat-bb] Cables loss

[Zach Metzinger]

On 07/19/18 12:17, Dave . wrote:
> While I'm still new to the sats I have been doing VHF/UHF weak signal work
> since the 80's and I've always preferred to spend the money on feedline
> rather than pre-amps. Given that pre-amps amplify the rx noise and signal
> you really don't improve signal to noise unless the noise figure of the
> pre-amp is better than your receiver. Usually your just amplifying the
> signal to make up for the loss that's going to happen as the signal makes
> it's way down the coax.

Hello Dave,

With respect, your understanding of the problem is quite incorrect.

You should definitely "set" the noise figure of your receive system as
early as possible with a LNA. Any decent LNA will have a noise figure
far lower than your receiver, not to mention to the coax attenuation
adding _directly_ to the NF of your system.

I used the data here:

http://www.df9ic.de/tech/trxtest/trxtest.html

Let's pick the IC910H, with a 3.7dB NF. Now, add 100' of LMR400 for a
loss of 1.5dB at 2m. You end up with a NF of 5.2dB.

Next, let's try a LNA before that coax with a measly 1dB NF and 15dB of
gain: NF of 1.325dB at the receiver.

https://www.pasternack.com/t-calculator-noise-figure.aspx

This can easily be seen with the Friis noise equation. Here's a good
explanation with the (simple) math:

https://www.microwaves101.com/encyclopedias/noise-figure


I'd invest in a good LNA at the antenna and a PA in the shack before I'd
ever think about upgrading my coax.

--- Zach
N0ZGO