Hi Robert, WB4APR
Don't forget that if you have wonded the coil over a 1 Megohm 1/4 W resistor if it is a spiralized type resistor it has a small inductance that at the end of the job is connected in parallel to the your copper wind affecting the final value of the total inductance.
To tune exactly the inductance to 28 MHz is suffice to cover the inductance with a small soft copper foil and solder it only at one end of the resistor and cut and try with it's lenght.
Best 73" de i8CVS Domenico
----- Original Message ----- From: "Robert Bruninga" [email protected] To: [email protected] Sent: Thursday, October 09, 2014 4:04 PM Subject: [amsat-bb] How you wind it
How you wind a coil makes a BIG difference.
Ever wonder why your projects don’t work like the authors?
Im winding 42 turns onto a 1 Megohm 1/4W resistor to make a loading coil
to
make a 5’ whip antenna on a cubesat tune to 28 MHz.
I was getting nowhere trying to add and subtract turns. With a bandwidth of only 200 kHz it was inconsistent getting to 28.120 center freq.
Then I simply took the same length of wire and tried 3 methods all with
the
same number of turns.
1, down-and-back-and forward for 3 layers = 24.5 MHz
2, left-to-right but scrambled 3 layers along the way = 30.0 MHz
3, just a little squeezing of turns that had loosend = 28.8 MHz
Wow, just changing the winding method moves the 0.2 MHz bandwidth as far
as
30-to-1!
The difference of course is the stray capacitance between the end turns.
So winding method is important!
Just thought I’d share the results…
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