[HBR] 160 Meter Coil Data

[Walt Hutchens]

Dan said:
> Not sure if you read my earlier post, since you didn't address it.  But, I 
> spent a few hours doing some calculations to design some 160 M coils yesterday.
> 
> It is not necessary to use 3 point tracking to cover just a ham band.  
> Seventy cps tracking is kind of ridiculous when the bandwidth of the front end 
> circuits is probably more than 1000 times that great.  The coil design that I show 
> below (and posted earlier) has a tracking error of 4.5 KHz.  This is called 2 
> point tracking.  It is usually just set at each end of the band, like Ted's 
> alignment procedure.

Dan, I'm sorry -- I wasn't responding to your post and certainly didn't
intend to sound critical -- I've been meaning to blather about the
general subject for several days and just got around to it.  I agree
with you that tracking better than a few kcs is of no great value --
even with a Q of 200 -- higher than a practical 160M coils would have --
you've got a 10 kcs half power bandwidth.

With three parts you can adjust (coil, trimmer, padder) you can take
your choice of a three point fit (name your three points) or any of an
infinite number of two point fits and there are MANY that will be
satisfactory when the tuning range is only ~10%.

That's actually a good way to understand what's going on.  Over a broad
range you can take ANY inductance and by using the right trimmer and
padder values get a fit at the low and high ends of the range.  The
middle might either be high or low -- or, with the right inductance,
spot on.  So by varying the inductance and compensating the ends with
the trimmer and padder you can get an error curve that varies from a
hump in the middle to a valley in the middle, and passes through a
condition where it looks like an 'S' lying on its side -- zero at both
ends AND in the middle.

(Command receiver calibration and sensitivity can be significantly
improved if you have the patience to tweek the coils as well as the
trimmers, thus getting a three-point fit between the oscillator
frequency and the dial calibration and two points between the antenna
and mixer tuned circuits and the osc.  But these receivers have a much
greater tuning range per band.)

> I used 1.79 to 2.01 MHz and a cap range of 6.6 to 23 pF (Miller Catalog No. 
> 1461-BS).  Try these values in the tracking program and see if the L's and C's 
> that I derived aren't pretty close.
> 
> Here is a repeat of my results:
> 
> 
> L1 and L2 (Antenna and RF):
> 99.74 uH with parallel C of 56.26 pF.

> L3 (Oscillator):
> 15.74 uH with parallel C of 116.2 pF.


For those numbers the program spits out:

Aerial circuit values       Oscillator values

Inductor        99.75 uH    Inductor        18.35 uH
Trimmer         56.26 pF    Trimmer         99.56 pF
                             Padder         195.83 pF

That is claimed to yield a maximum tracking error of 102 cps.  Pretty
close to a three point fit, although it's really just a two point.  In
practice a 200 mmf fixed padder would be plenty close enough if the
inductor and trimmer were variable.

What the program has done is stick in that series padder
(counterintuitive word if you ask me) -- it will do that with all of
these tracking calculations.  The oscillator values you calculated are
those that would give the right end points if the padder was shorted out.

Loosely speaking when you have all three parts in the circuit you adjust
the mid point with the inductance -- it moves everything in the same
proportion.  Then you adjust the low end with the padder and the high
end with the trimmer.  Repeat until satisfied.

But ANY of these answers will work fine, including your no-padder ones.

Walt
KJ4KV