TopBand: JT1X - Topband experiences

[Tom Rauch]

> Date:          Mon, 22 Jun 1998 15:54:27 -0400 (EDT)
> From:          W2pm@aol.com
> Subject:       Re: TopBand: JT1X - Topband experiences
> To:            ha1ag@usa.net, topband@contesting.com

> With that type of QRN I would have tried a small loop - Im thinking that the
> sig to noise level improb would have been far more drastic and effective
> albeit I'm sure the sig levels would have been way down but it seems to me
> most of the 160 serious dX'ers here are not small stations...

Hi Pete,

There seems to be a popular perception noise is an electric 
field and signals are a magnetic field. Actually they both have 
the same characteristics.

Small loops can help if  local nearfield noise "just happens" to be 
E-field dominant, especially if the loop is well balanced 
electrically (which is all the "shield" on a shielded loop really 
does!). But noise can also be magnetically coupled to the system in 
the nearfield. Nearfield coupling mode is really a matter of luck 
more than a  rule.

That's why it pays to try many different things, and why things that 
some of us find sucessful others find totally useless.

Once the noise is more than a large fraction of a wavelength from the 
antenna (distance actually depends on the characteristics of the 
source and receiving anatennas) the near field performance of the 
antenna doesn't matter since all field ratios are set by the 
"impedance" of the  environment around the antennas.  

If you guys had some idea how far away the noise is and what the 
source actually is, there might be a solution. I'd bet each  case is 
a specific problem though!

For example, since the dipole helped in JT1 a better  solution might 
be TWO phased small loops mounted horizontally or two dipoles phased. 

The fact S/N improved with a horizontal antenna likely indicates 
noise was local, and propagated via groundwave. If that was the case, 
a loop might not be better than anything else when mounted vertically 
unless carefully rotated and unless the noise is ONLY from one source 
in one direction. A short electric field probe mounted horizontally 
actually could be much better than the loop or dipole! 

The best receiving antenna I ever used, with the exception of a long 
aperiodic array of loops I used to work JA's day after day  through 
the 60 over nine LORAN of the 70's, was actually four short verticals 
spaced 300 ft broadside and 70 ft endfire. The verticals were 
essentially voltage probes, since they were high impedance low 
current devices (driving FET source followers at each element).

Unless the noise is from one general direction, polarity, and wave 
angle, solutions sometimes require lots of real estate.

For DXpeditions, it's a tough problem that may not have a practical 
solution that works for every case.

73, Tom W8JI
w8ji.tom@MCIONE.com

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