[TowerTalk] 40m 4el KLM - replacing linear loading with coils

[Brian Beezley]

The forward gain difference between my single-coil and two-coil models 
is 0.02 dB. This is with no coil losses, whose difference might easily 
overwhelm a small directivity difference. But I restored the coil losses 
and got the same result.

Input resistance is in the mid-20s in ohms for the single coil model and 
in the mid-40s for two coils. I had expected band-edge reactances to be 
about the same for both models and thus SWR to significantly increase 
for the single-coil model. Instead, the reactances are similar to the 
input resistances, with only a small difference in band-edge SWR for the 
two designs when each is matched at band center.

I neglected to say that the AWG 5 copper-clad aluminum wire weighs 37.42 
lbs/1000' while pure copper is 100.2 lbs/1000'. 3/16" OD refrigeration 
service soft copper (0.1875" instead of 0.1815") with a 0.03" wall 
weighs 57.5 lbs/1000'. 1/8" OD is 34.7 lbs/1000'. An optimized 7.7 uH 
coil using 1/8" copper is 5.6" by 3.3" with a Q of 985. (As wire 
diameter decreases, the proximity effect that limits coil Q decreases, 
which enables coils that are smaller, lighter, and cheaper.)

This describes an interesting trick for 40m Yagis:

http://ham-radio.com/k6sti/owa.htm

I had toyed with the idea of writing a little utility that generates 
wire segments for an element bent using dacron lines that run from 
somewhere near the half-element midpoints back to the boom. This would 
eliminate the need for a custom bent-element mount. You could even run 
the lines between driven element and reflector to bend both, which may 
improve gain even more. To do this right you need the taper schedule and 
tubing bending parameters. Even better would be to include the dacron 
line tension in the optimization parameters and then optimize everything 
simultaneously. I'll leave this for someone else.

If you think the 0.5 to 1 dB a conventional OWA costs in forward gain is 
inconsequential, check this out:

http://ham-radio.com/k6sti/pileup.htm

The analysis isn't definitive. But the next time it takes forever to 
bust a pileup with your OWA, you may not be able to get the probability 
curves out of your head.

COIL 3.90 is here:

http://ham-radio.com/k6sti/coil.zip

Brian