TopBand: RE: loops and preamps

[Eric Gustafson]

>From: w8jitom@postoffice.worldnet.att.net
>Date: Tue, 7 Oct 1997 08:23:21 +0000
>
>Hi Peter,
>
>> Eric N7CL has a point. It all depends on how much or how little
>> background noise there is. I use a 2 turn loop about 1 metre on a side,
>> with a push pull cathode follower, and get more than enough noise - it's
>> better than the vertical, but still not good enough!
>
>How do you avoid unwanted common mode excitation of the push-pull 
>follower? High input Z amps, especially those with ground referenced 
>supply and signal leads (generally the case with tubes), require 
special precautions.
>
>If you are talking about Brian Besley's QST antenna, it is a good 
>system. But like all small balanced systems it requires very low 
>common mode response in the transmission line / amplifier system.  
>Otherwise, it would just act like a mess of wire random wire instead 
>of the well designed antenna it really is.
>
>73, Tom W8JI


Hi Tom,

Actually, I should have defined what I meant by "small".  I have
been playing with shielded untuned multiturn loops of
approximately 0.5 meter diameter.  I have my reasons for wanting
the physical size of the loop to be small.  I won't go into them
here.

I have not yet tried ferrite loading since it appears that I can
get the bandwidth I want without having to deal with any
variables introduced by a core.

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 goal is an antenna that appears "external noise flat" to a
.25 uV receiver from 100 KHz to at least 20 MHz and hopefully 30
MHz.  Currently, with about 5 turns (all in the same shield
tube), the low end seems to be down around 300 KHz.  I have
noticed improvement in SNR from running the loop into a balanced
preamp even though the split shield should be killing the
unbalanced electrostatic noise energy without significantly
reducing the loops response to the desired radiated signals.
With the shield in place, the high end is at or just above 14
MHz.

The loop must be terminated in a very low Z to preserve the
bandwidth and recover useful signal levels.  So even though the
preamp input is differential, the input Z is not high.  Although
it _is_ significantly higher in common mode than it it is
differentially.

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.  The capture area is small
enough at the lower frequencies that I am running into NF
problems below about 2 MHz with a 10ish dB NF preamp.  So I am
interested in low NF, high dynamic range designs for use on
topband and lower frequencies.

I have had similar NF problems at topband frequencies with a very
small version of the EWE.  Fortunately, the EWE does not need a
differential amp for the antenna to function correctly.

73,  Eric  N7CL

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