Re: [IRCA] IRCA Loop update

[Russ Edmunds <wb2bjh@xxxxxxxxx>]

--- Stephen Hawkins <ng0g@xxxxxxxxx> wrote:

 
> I am very puzzled by this.  It seems to me that a copper tubing loop
> would 
> just look like a very thick piece of wire about 8 feet long, or in
> the case 
> of the larger loop you built about 40 feet long.  I learned to make
> loops 
> from Ralph, and the wire loops I built have between 95 and 120 feet
> of wire.  
> It seems to me that all of that extra wire, especially when compared
> to the 
> smaller of the loops you built, would have a much larger "capture
> area" so to 
> speak, when compared to 8 feet of thick wire.

*** With wire loops, you may recall seeing it written that a
larger-diameter wire works better up to a point. This is largely
because of distibutive capacitance. Since the loop is magnetic, it
shouldn't be expected to behave entirely along electrical theory. The
greater the surface or skin area presented to receive the signal, the
better the loop's pickup will be. Back in the 60's and 70's people were
experiemnting with loops made from Litz wire, which is considerably
thickewr than what was usually used at the time.

The copper loop 'looks' to the incoming signal like a very big wire.

Equally important, however, is that unlike the wire loops we're all
used to, the copper loop is a broadband loop. Therefore there is no
fine tuning of the loop, which results in less ability to use the loop
to reduce adjacent channel interference. But with a BBL, there is
overall less gain, and less interference, and less electrical noise
than with a high-Q wire loop. Used with a receiver with IF filters with
deeper skirts, the business of dropping out the adjacent channel is
moved from the loop to the receiver.

At our Long Beach Island DXpedition in NJ this past weekend, we had
several Drake receives and also some Sony 2010's with narrow filters,
and in most cases, the Sonys could hear what the Drakes could in terms
of close-in splits, the differences being mostly those with the most
powerful adjacnet channels. These results were with an amplified wire
BBL, although we also used two BOGs. 

Once you get past the idea that you don't have the same kind of gain as
with a high-Q amplified wire loop, you suddenly realize that the reason
you needed all of that gain in the first place was to overcome the
noise and the adjacnet interference, and if you can reduce those with
the antenna, then you don't have as much need for antenna gain.


> 
> In an electrical generator the current is induced to flow in the wire
> by 
> passing a magnetic field across a coil of wire.  The larger the coil,
> or the 
> stronger the magnetic field, the more current you can generate.  In
> an 
> antenna the magnetic field is replaced by the radio wave from the
> station.  
> The stronger the wave, or the larger the coil of wire, the more
> current 
> should be induced to flow.  So it puzzles me that the 8 foot copper
> tube 
> works as well for you as 90+ feet of wire.  
> 
> In some kinds of antennas, like an ordinary dipole, for instance, a
> wire that 
> is very thick can add some usable band width to the antenna.  That is
> why 
> they used to build those "cage" dipoles that were several parallel
> wires, 
> held apart in a kind of circle by spreaders, to form an artificial
> "thick" 
> wire.
> 
> Does anyone have any thoughts that might shed some light on this.
> 
> Steve
> -- 
> 73 49 111 01001001
> Stephen Hawkins NG0G
> ng0g@xxxxxxxx
> 
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Russ Edmunds
Blue Bell, PA  ( 360' ASL )
[15 mi NNW of Philadelphia]
40:08:45N; 75:16:04W, Grid FN20ID
<wb2bjh@xxxxxxxxx>
FM: Yamaha T-80 & APS9B @15'
AM: Hammarlund HQ-150 & 4' FET air core loop


		
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