[R-390] Antenna Noise

[Bob Camp]

Hi

Simply put it's how much "extra" noise the antenna has.

A resistor at room temperature generates a certain amount of noise all 
by it's self. In a 50 ohm system this is about -174 dbm in a 1 hertz 
bandwidth.

Any antenna the size of a dipole or smaller *if* properly matched to 
it's load (and that's a big if in the case of antennas well below a 1/2 
wavelength) will pull a certain amount of power out of the air.

To keep things simple let's just say that we compare a normal 50 ohm 
resistor to a normal dipole and that the dipole is up high enough that 
it is working well.

The common way to look at it is "how much more noise did I get from the 
antenna than from a normal resistor?". It's a lot easier to understand 
1.9 db that way than in terms  something like -172.1 dbm in a 1 hertz 
bandwidth. The definition also fits into the noise figure concept 
fairly well. You can also look at it as the noise figure of your 
antenna. People sometimes carry it one step further and talk about the 
noise temperature of the antenna.

People have spend a lot of time looking at how much background noise 
you get at various frequencies, times of year, angles of polarization, 
and for all I know as a function of the phase of the moon. You also see 
data up at radar type frequencies that is correlated to how far above 
the horizon the antenna is pointed. If anything there is more data on 
this stuff than you can keep up with.

The measuring systems I am familiar with sit there on the antenna and 
average the output of a receiver over a period of several minutes and 
then record the result. When you do this there are a bunch of variables 
like antenna gain and the sensitivity of the radio. One of the most 
common ways to do this was with an R390 hooked to a chart recorder. An 
awful lot of universities have an R-390 or ten sitting around from 
these 1950's and 1960's projects.

I suspect that does not qualify  for the winner in the "simple 
explanation" category .....

	Take Care!

		Bob Camp
		KB8TQ





On Aug 20, 2004, at 12:20 PM, Roy Morgan wrote:

> At 11:32 AM 8/20/2004 -0400, Meir WF2U wrote:
>> ...  the ambient atmospheric noise in the HF band (up to 30 MHz) 
>> averages 8 dB.
>
> Can someone give a relatively simple explanation of what this means?
>
> I seem to remember that the "8dB" indicates that a certain (standard?) 
> antenna will deliver 8 dB more noise to the receiver than a resistor 
> (at room temperature?) equal to the antenna's impedance.  Do I have 
> that right?
>
> Maybe we could watch the RF Level meter and see such a change.
>
> Related musing: I wonder if anyone has measured the response of the RF 
> Level meter and receiver system to see if the indications are anywhere 
> near right.
>
> Roy
>
> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
> roy.morgan at nist.gov --
>
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/faq.htm
> Post: mailto:R-390 at mailman.qth.net
> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>