[IRCA] Working on the problem

[Tim Hills <thills@xxxxxxxxxxxxx>]

Thunderstorms moving through tonight. Thought I'd jot down what I've 
learned and see if anyone has any thoughts on improvements or something 
else I might try, other than moving....
I've been experimenting a bit with pulling usable signals out of the RF 
soup I live in and thought I'd share some of what I have found works so 
far. If you haven't seen my earlier posts I have 3 5KW+ MW stations 
beaming directional converging in my back yard and hash from high 
voltage lines 60' from the house. Plus the usual semi-urban noise, 
routers, street lights, etc. Also, my "shack" is in the basement in 
conductive ground and the house has the chickenwire reinforced plaster. 
Small lot and I want to be as invisible as possible, everything in the 
low attic preferably. It's a challenge but it's keeping me out of 
mischief and honing skills I haven't used in a couple of decades. ;)
Changing both the preamp and loop from an unbalanced to balanced design 
really cut down the power line hash and going from JFETs (J310s)  to 
MOSFETs (3N200s) with appropriate changes in biasing on the inputs 
pretty much cured the intermod problems and gives a much wider gain 
control. The BF199 bipolars are the same in the output stage. 10mA 
quiescent collector current gives best results.
Air core and spiral windings seem more immune to the common mode noise 
from the power lines than a solenoid and with a tighter null. The lower 
common mode noise was counter intuitive to me but I think the lower 
distributed capacitance. It seems that in high ambient environments 
ferrite rods have their own quirks that I plan on investigating later.
Right now I have 2 loops on the same wooden frame. 14 turns CT covers 
the MW band, 4 turns CT covers from 3.5MHz to a bit over 10MHz. 4 
MVAM109s in series/parallel for remote tuning, no pickup loop the signal 
is taken off the tuned loop directly through the very high Z inputs of 
the preamp. It's hung on a covered porch when in use with power, tuning 
and gain controls running through Cat. 6 cable.
Still working on the phased loop array. Better than 20 dB front/back 
ratio so far at 1000 KHz.
 No formal logs kept so far but I'm having more fun building than 
listening. Catch a station, go upstairs, turn the loop, is it better or 
worse? I need to get a rotator....
When I get something workable by the average homebrewer I'll post 
schematics and pics. When I work out the details of the preamp if 
there's enough interest I'll make kits available at cost/actual shipping.
Now, about that twin coax the previous owners had hooked to that dish....
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