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

[Tim Hills <thills@xxxxxxxxxxxxx>]

I'll get some pics of it tonight, the rod is encased in PVC fittings and 
I'll grab some shots next time I open it

The spirals are mounted parallel to each other about 1" apart and 
grounded at the center tap. Symmetry is maintained since one coil turns 
inwards to the center where the ground/common is and the second winds 
outwards in the same direction. I've tapped the resonant loop directly 
both single sided and balanced for some time now. The twin spiral was 
built more out of curiosity and the original configuration was balanced 
with no secondary loop with J310s in the amplifier.
The varactors are connected back to back with anodes connected to each 
side of the loop and bias is fed through a 100K resistor. The upper and 
lower limits are set through pots connected to a precision voltage 
source and buffered through op amps. A 10 turn pot is used to set the 
tuning voltage and fed to the bias resistor through another op amp for 
buffering. A quad op amp handles upper and lower limit buffering, 
precision voltage from a TC Zener source and output buffering/filtering 
The loops plug in through stereo jacks so I can swap them out easily.

Another difference between the 2 is the rod uses a single pair of 
closely matched varactors while the twin spiral uses 8 random varactors 
out of the bag which is probably why the Q is lower than expected. The 
rod having such sharp null probably has to do with it being 11 inches 
long and the way it's wound.

One problem I'm having at the moment is waiting on a new head for my 
Weller soldering station, on order but not here yet. I'm NOT going to 
try working with the SMT FETs with a 40W Radio Shack ungrounded 
slobbering iron...

Any amplifier adds noise though atmospherics overwhelm the noise from a 
well laid out MOSFET input. The preamp on the ferrite is actually the 
front end of my homebrew receiver, also waiting for parts, that is 
balanced and uses 2 BF904WR MOSFETs on the inputs though I've grounded 
one side for now. Once I get some other things taken care of the rod 
will have a balanced amp in the head. I think the main advantage of the 
rod is the XYL irritation factor is lower. The  best reception in the 
house is in the living room and she's not fond of the idea of a "butt 
ugly gizmo" in her living room.

Tim Hills
Sioux Falls, SD
> 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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