[HBR] 160 Meter Coil Data

[Walt Hutchens]

160 meter coils can certainly be wound for the HBR series: the 
percentage coverage is less than for 80 -- that is, 200 kcs from 
1800-2000 kcs vs. 500 from 3500-4000.

With stray capacitance typically running ~10-15 mmf I'd guess you could 
get a tuning range as great as 50% in the HF range with some sacrifice 
of performance.    Not the BC band and not even the range of 800-1500 
Kcs, because the stray capacitance of a coil that would cover such a 
range with such a small cap would further restrict the tuning range.

There are superhet tracking calculators on the web.  Google 
'superheterodyne tracking.' One that seems to work okay is at:

http://ironbark.bendigo.latrobe.edu.au/~rice/superhet.js/superhet.html

For the case in question -- assuming tuning 1.8-2.0 Mcs with a cap range 
of 27 to 3 mmf and NO coil taps -- the program yields the answers:

Aerial circuit values       Oscillator values

Inductor        61.89 uH    Inductor        10.82 uH
Trimmer         99.32 pF    Trimmer        177.65 pF
                             Padder         311.35 pF

These values appear to be a correct solution.

(There are an infinite number of correct solutions and you get to pick 
one depending on your view of what is best -- generally how you look at 
minimizing the tracking error.  This program appears to pick the 
solution that minimizes the maximum error, which for this tuning cap and 
frequency range is claimed to be under 70 cps.)

The term 'padder' is conventionally used to refer to a cap connected in 
series between the tuning cap and the coil, thus serving to reduce the 
effective maximum capacitance and set the minimum frequency for that 
circuit.  The 'trimmer' is the shunt cap. and will set the maximum 
frequency.  With the oscillator above the signal frequency and a tuning 
cap having identical sections, a padder would be required only for the 
oscillator circuit -- remember that trimmer hanging on the aluminum 
plate screwed to the right hand side of the command receiver tuning cap? 
That's the padder, used to calibrate the dial somewhere near the bottom 
of the tuning range.

Ted's use of a tap for the tuning cap eliminated the padder, meaning one 
less part for which ever circuit was tuned to the higher frequency -- 
the oscillator on the lower bands, the antenna and mixer on the higher 
ones.  This was a useful cost savings since these would have had to be 
rather precise silver micas or even a silver mica paralleled with a 
trimmer.  The cost came back in a bit of extra labor to make the tap but 
builders, presumably, had more time than money.

Use of taps on the coil or coils tuned to the lower frequency isn't as 
obvious.  Had he not done so he'd have had to fix the trimmer (across 
the tuning cap) to set the percentage tuning range and would have had 
only one possible inductance solution.  Use of a tapped coil allowed a 
smaller trimmer and a larger inductance, giving a higher Q (more gain, 
less bandwidth) again, without using another expensive part.

Walt Hutchens
KJ4KV