Custom air core loading coils?

[AD9CV]

I'm working on improving my 80 meter stealth "upright L" coil-loaded antenna. I've now
successfully constructed a large, low-loss 1:4 (1.5 [antenna] to 50 [feedline]) ohm unun and a low-loss,
high common mode impedance choke balun. I've gone from a single loading coil on one side to
two coils, one on each side. This makes the job of the choke easier.

I made a large loading coil, 77 uH, for the orginal; its 7 inches long and 6.5 inches diameter,
air core on three teflon ribs. The extreme ugliness of it as built ... I cound never get it nice and perfectly
neatly wound ... makes we want to get commercial air core coils. These need to be extremely high Q, like
500 to 700. I'm thinking about 4" diameter, 6 inch long, #8 wire, for each coil, 33 uH.

Does anybody know where I can get such things?
I no longer have access to a machine shop.

Even with my current coils wound on smaller ceramic cores, Q about 275, I'm getting good "skip"
results. NVIS type propagation is near zero, as its essentially a vertical.

Doug McDonald AD9CV

 

[RFI-EMI-GUY]

I'm working on improving my 80 meter stealth "upright L" coil-loaded antenna. I've now
successfully constructed a large, low-loss 1:4 (1.5 [antenna] to 50 [feedline]) ohm unun and a low-loss,
high common mode impedance choke balun. I've gone from a single loading coil on one side to
two coils, one on each side. This makes the job of the choke easier.

I made a large loading coil, 77 uH, for the orginal; its 7 inches long and 6.5 inches diameter,
air core on three teflon ribs. The extreme ugliness of it as built ... I cound never get it nice and perfectly
neatly wound ... makes we want to get commercial air core coils. These need to be extremely high Q, like
500 to 700. I'm thinking about 4" diameter, 6 inch long, #8 wire, for each coil, 33 uH.

Does anybody know where I can get such things?
I no longer have access to a machine shop.

Even with my current coils wound on smaller ceramic cores, Q about 275, I'm getting good "skip"
results. NVIS type propagation is near zero, as its essentially a vertical.

Doug McDonald AD9CV

You should have come to the Orlando Hamcation. Plenty of boat anchors with big coils. Dacades ago you could buy those ready made, Millen , Barker and Williamson, Airdux....

Coils and Coil Form

Coils and Coil Form

www.rfparts.com

Barker & Williamson Coil Stock | Surplus Sales of Nebraska

B & W, Barker & Williamson coils, vintage Pi-Dux coils are in stock at Surplus Sales at bargain prices. Open wound rf coils are hard-to-find.

www.surplussales.com

 

[AD9CV]

B&W never made coils big enough to be optimum.
I did find an old coil for sale that would be a bit better then what I have, but its ribs seem to have gotten fried. Fried
B&W ribs are lossy.

I did tests on my current setup, checking every part to see if it got warm with 100 watts fed in for 8 minutes.
In the daytime of course on 80 meters this bothers nobody. I could not measure or feel and warmth,
but on a windy day that's not "quantitative". Nevertheless, I get no noticeable heating as the answer.
The previouys setup had a choke balun that got hot.

Then at night I promptly achieved the whole ultimate goal of all this, which was to get confimed-by-LOTW
WAS on 80 meters by working Hawaii!

This antenna is stupefyingly narrow band, 10 kHz for a SWR of 2:1. I worked out a
convenient tuning method at the antenna, which needs a 1-40 pF variable cap that can take 2.5 kilovolts.
These one can get! Alternatively, I can just coil up the lose end that hangs down.

 

[RFI-EMI-GUY]

Buy a Delrin plastic cutting board and cut it for spacer(s). Use thin copper tubing like used for HVAC capillary lines for conductor. They are soft and you can wind on a form and thread into your spacers.

Maybe you can motorize your tuning some how. I have in storage an MFJ1786 HI-Q loop that has a nifty control box for tuning its capacitor remotely with DC imposed on coax. You could also do same with unbalanced line and some RF chokes.

 

[AD9CV]

I did sort of like your spacer idea for the old single loading coil. It worked but I never got it to
look nice and neat. I just can't do neat without a machine shop, which I no longer have since I retired.

Since this is 80 meters, and only works at night, I can hook my nanovna to the antenna feed point
and see it from the place where "tuning by crumpling up the end" takes place. This whole antenna is
on my patio and cantilevered out over it.
Its hard to see the nanovna screen in the daytime. The SWR at resonance varies across the band
from 1:1 to 1.4:1.