[600MRG] 630 meters receive antenna question

[Eric NO3M]

Length will affect external noise (ie. propagated noise, "signal", etc.) 
to some extent.  See this writeup with diagrams:

http://dl1dbc.net/SAQ/miniwhip.html

System noise, however, is not affected by the whip length, but rather 
the amplifier's NF, common-mode, and other "system" factors.   As long 
as system noise is below propagated noise, performance should be similar 
to a standard vertical in terms of SNR, which is what matters when 
trying to decipher intelligible "signals".  On LF/MF, with the 
considerably higher average noise temperature, it's relatively easy to 
accomplish that with care in the amplifier design and adequate 
common-mode choking.  Locally generated noise/interference is also an 
issue and why these types of antennas are recommended to be mounted well 
away from any local noise sources.

A shorted whip, E-probe, or high-impedance antenna (whatever you want to 
call it) is a convenience antenna, but not a compromise antenna if 
properly designed and mounted, as compared to a large vertical.  By 
nature of what it is, it will not provide any directivity in relation to 
propagated noise / signal nor nulling of local noise.  Observed 
performance differences with say a transmit vertical are probably more 
incidental in nature than by design, and in most cases, related to local 
noise infiltration, where the mounting of the short, amplified vertical 
just happens to pick up less locally generated noise or visa versa.

73 Eric NO3M / WG2XJM

On 05/19/2016 01:41 PM, William E. Isakson wrote:
> Yes Jim, I am using a 5 foot vertical up on a mast with a active preamp for
> receive.  It works pretty good.   Comparison is hard for me to give because
> when I was using a top loaded transmit antenna for receive that worked
> considerably better,  but I do know that my ultra weak transmit signal was
> received by a small mobile antenna used with a preamp from the San
> Francisco Bay Area to Oklahoma.   Still I can say with certainty that my
> small antenna with preamp is no match for Rudy's set 45 miles north of my
> current location in Oregon.   Does generally work though.  The thing about
> active antennae is that they are a voltage probe antenna, not a current
> antenna, so the length is not a big factor.  As I understand it you might
> get less noise with shorter ones, but not less signal.  That is, 4 feet
> might be better than 8 feet.
>
> Bill Isakson WD2XSH/44
>
>