[TowerTalk] Coax cable economics. (was Mosley

[Jim Brown]

On Thu,2/19/2015 5:15 AM, Patrick Greenlee wrote:
> Will someone please explain why what I'm about to write is not true 
> (if that is the case.)  I'm trying to follow the "debate" regarding 
> coax losses and their effects on transmit and receive.
>
> The receiver has an inherent internal noise floor which can be 
> qualitatively observed by shorting the input coax fitting and cranking 
> the RF and AF gain up.

As someone else noted, a shielded 50 ohm termination should be used 
rather than a short. Also, to make a valid test, you would need to 
disable the AGC and measure the difference in the audio output with and 
without the antenna connected. That capability is built into the 
Elecraft K3, and can also be observed on the P3 panadapter.

But the simple answer to your question is that the noise coming in on 
the antenna on the HF bands is usually much greater than the circuit 
noise within the radio. That noise comes from local noise sources -- 
power systems, switch-mode power supplies, digital circuitry of all 
sorts -- and noise propagated from distant sources, just like any other 
radio signal -- lightning plus all those other local noise sources 1,000 
miles away.

Here's an experiment you can do if you have an audio voltmeter to hook 
up to your receiver. Measure the noise on bands like 160M, 80M, and 40M 
during the day, then measure them again at night. The difference is 
noise that is propagated from distant sources. Try the same thing on 20M 
when they're dead (usually at night), and again when it's open.
A difference of 10 dB or morebetween open and dead is common.

73, Jim K9YC