Topband: loop

[Tom Rauch]

Hi Bob,

> playing with 20 ft shielded loop for receiving. 

All the shield does is change the balance of the loop. The shield 
simply becomes the antenna, so it really doesn't shield anything. It 
is VERY important to ground the shield only at the exact electrical 
center, ideally to an independent ground that is isolated from noise 
sources like power line grounds.

It is folklore that noise is an electric field and a magnetic loop 
doesn't respond to noise or can be "shielded" from noise. At 1/10 
wl from a small magnetic loop, the response is actually electric 
field dominant (field impedance >377 ohms), shielded or not!     
 
> The loop is horiz. polarized, would another loop set fer vert.
> polarization and the MFJ box allow the combining/phasing of both
> polarization for better signal capture and worth the effort

I can mix V and H polarizations here. I find, as a general rule, it is 
useless. It aggravates fading unless I constantly ride phase and 
level controls. There are very rare periods of propagation when the 
ionosphere is stable enough to allow a period of successful 
phasing, but for the most part it is useless.

It is my experience, based on constantly using phased receiving 
arrays, that two like-polarized antennas with similar patterns can 
be successfully combined, like a Beverage and a four-square or 
even two small verticals, but mixing greatly dissimilar pattern 
antennas almost always isn't worth the effort.

There might be isolated cases where a dissimilar pattern 
combination "works", but my own experiments indicate it is nearly 
a total waste of time...and that agrees with conventional theory. 
 
> With limited space the size of the 20 ft loop fits well on the property
> and really allows me to null out power line influence and I find it an
> overall aid. 

Right on target. It is the loop's easily controlled null and directional 
response that improves S/N in some cases.

My suggestion would be adding a second identical loop 1/8 to 5/8 
wl away, depending on the directional pattern you want.
73, Tom W8JI
w8ji@contesting.com



--
FAQ on WWW:               http://www.contesting.com/topband.html
Submissions:              topband@contesting.com
Administrative requests:  topband-REQUEST@contesting.com
Problems:                 owner-topband@contesting.com