TopBand: RE: loops and preamps

[w8jitom@postoffice.worldnet.att.net]

> From:          Eric Gustafson <n7cl@sparx.mmsi.com>

> My initial experience has shown me that I need to get rid of the
> shield if I want the useful bandwidth to extend up above 14 MHz.

The shield is unnecessary anyway, if the antenna is properly coupled 
to the amplifier. It's a "wives tale" that noise is electric field 
dominant, and an even bigger "wive's tale" that the shield stops 
electric field response. It is impossible to have a time varying 
magnetic field without a time varying electric field. Eliminate one, 
and you eliminate the other. 

The real effect of the shield is the shield itself becomes the 
antenna, and the coil inside simple extracts energy from the 
shield (the "real" antenna)! Of course the shield is exposed to 
electric fields just as well as magnetic fields. If it wasn't, it 
would be stone dead for receiving....or Maxwell's and Faraday's 
equations would be incorrect. 

Somehow we have lost track of that in our amateur radio loop 
articles! I'm sure you know all that with your background, but I 
thought it was worth mentioning to anyone else building loops. 

Unbalance the "shield" by mounting it or building it 
non-symmetrically, and you have an unbalanced antenna sensitive to 
common mode problems! That's because the shield really isn't a shield 
at all, it's the actual antenna!

Rather than tie up the reflector, I'll e-mail you on the rest since 
it is not of general interest for 160.

> I am now running the loop unshielded.  To preserve balance and
> minimize common mode amplitudes, the physical construction is
> carefully done and the loop is center tapped and referenced to
> ground at the center tap.  Now, with 6 turns in total, into a
> balanced preamp, the low end is down near the desired 100 KHz and
> the high end is well above 21 MHz. 

Bingo.

All the "shield" really does anyway is force or help the builder
to "balance" the antenna construction, since the single turn shield 
becomes the antenna.

73, Tom W8JI
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