Re: [IRCA] New logbook started and new antennas tested

[Rick Kunath <k9ao@xxxxxxxxxxx>]

On 09/03/2011 12:32 PM, Tim Hills wrote:

> One is a dual spiral, 10 turns total and 30" diameter in 2 layers 1"
> apart with varactor tuning and a single turn pickup loop.

I'd be interested in seeing a photo of that loop, and a schematic as to 
your connections.

I've built a lot of loops over the years and have some ideas that you 
might find interesting.

I built a lot of spiral and solenoidal loops, as well as ferrite loops. 
The air-core loops always outperformed the ferrite loops. One of the 
things that I found important as far as loop pattern and null depth is 
symmetry, something yo cannot get right with a spiral loop because the 
windings are different diameters. Traditional split-stator air 
dielectric tuning capacitors always gave me a higher Q than varactors 
did. And this is important as you referred to off-channel rejection. 
With the frame of the split-stator capacitor connected to the loop 
center-tap I was able to get some good pattern symmetry and good null 
depth.

I either used a center-tapped secondary winding, or later on did away 
with the secondary winding entirely and used a high-impedance FET preamp 
across the primary that did the preamplification and conversion to 
unbalanced low-impedance output.

Rick Kunath
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