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Easy Method for Cutting Grooves for the DK3 Screwdriver Antenna Coil Form

from Bill Pong on January 14, 2017
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An Easy Method for Cutting the Grooves for the DK3 Screwdriver Antenna Coil Form

Ever since I read Don Johnson’s article on the DK3 screwdriver antenna, I always wanted to build this fascinating mobile antenna. What always put me off was the fact I had no way of making the grooves for the coil form. -- Until now.

This short article will describe how to cut the grooves for the coil, using a drill press, a Dremel bit, and a jig to hold and manipulate the PVC pipe. If you do not own a drill press, perhaps you can rent or borrow one from a friend or hardware store.

You MUST have the following:

- Drill press, whether it is a bench or stand model, preferably 10" or larger
- Dremel engraving bit, part number# 113
- Simple jig to hold the PVC pipe and move it along a 3/4" 10 TPI threaded rod

The basic operation of cutting the grooves using the drill press and Dremel bit are the following:

- A jig is constructed that allows the PVC pipe to travel along a length of ¾” threaded rod. This threaded rod has 10 threads per inch (TPI), available at any reputable home renovation store. I used a 2-foot length. - The above jig is firmly clamped to the bed of a drill press. The drill press has the Dremel engraving bit part #113 mounted in its chuck. The drill press depth of travel is adjusted to allow only enough travel to cut a groove that is half the thickness of 16 gauge wire (.06mm or thereabouts, no need to be super accurate, my grooves are albeit deeper)
- The PVC pipe is held firmly in the jig using 2 Velcro straps, one on each side of the area where the drill cuts the pipe.
- The drill press is turned on; the drill chuck is lowered onto the PVC pipe using the right hand. The left hand slowly rotates the PVC pipe along the threaded rod. Both actions work to produce a traveling groove along the length of the PVC pipe. According to my calculations, I needed to cut approx. 15 1/2 inches or so of threads to accommodate 75 ft of 16 gauge wire. I ended up cutting 17 inches of threads, giving me a total of approx. 83 ½ feet coil wire.

At the end of several hours of cutting the PVC pipe (approx. 12 hours), I ended up with this coil form (diagram 1):

PVC pipe jig notes:

The jig basically looks like this (diagram 2):

You see the drill press in picture left. The PVC pipe is strapped down onto the jig with Velcro straps. The PVC pipe is attached to the threaded rod by a small threaded rod adaptor which has an extended ¾” nut mounted inside it.

Here is a close-up of the threaded rod adaptor:

This is a close-up of the threaded rod adaptor removed from the end of the pvc pipe:

The extended ¾” nut is held inside the 1 ½” adaptor using ¼” set screws which are 3/8” long. The extended nut itself also has a shallow dimple drilled into each face. The set screws are then screwed down into each respective dimple.
This modification locks in the extended ¾” nut so that it absolutely does not shift position within the adaptor. I found out the hard way when I tried to cut the threads using only tape to hold the extended nut in place -- WRONG.
I also forgot to mention that the ¾” extended nut has a tight friction fit inside the 1 ½” adaptor. If you are sizing the extended nut and the adaptor, try several extended nuts - I found that there are some that fit loosely, some that fit too tight, and some that fit just right. You will have to try several nuts to find the one that fits just right.

Here is a picture of the extended nut removed from the adaptor:

Here is a close up picture of the set screws in the adaptor, with the extended nut removed:

A close up of the extended nut loosely sitting inside the adaptor, without pressure from the set screws:

I am not posting exact measurements for making the jig because it is not really that important. The constants in the jig are the following:

- ¾” threaded rod length is 2 ft, available at any hardware store
- The diameter of the PVC pipe is important and must be measured and recorded. This is needed to figure out the exact distance the threaded rod is mounted offset from the 2 side strips of the jig.

Here is a close-up of the mount for the threaded rod:

It is important that the mount for the ¾” threaded rod is made as precise as possible. If the threaded rod mount is not that accurate, the rotating motion of the ¾” extended nut will not be smooth. It could also introduce some inaccuracy in the cutting of the grooves.

That’s about all there is to making the jig to hold the PVC pipe, and to rotate it as you cut the grooves with the drill press. Make sure you have a radio on that is tuned to your favorite station. It is deadly boring cutting these grooves.

It did not cost that much to build the jig, especially if you have any scrap boards lying around the workshop. And all the fasteners and screws came from the local hardware store.

I hope this article re-sparks interest in building the DK3 screwdriver antenna. The coil form is not that hard to make, especially if you use a jig to manipulate and feed the PVC pipe thru the cutting process. I found it quite satisfying being able to cut this part of the antenna without too much hardship.

Some important notes you may want to use:

When operating the drill press and rotating the PVC pipe to cut the grooves, please check every half hour or hourly, the tightness of the 2 nuts that hold the ¾” threaded rod in the mounting bracket. This is to ensure that ¾” threaded rod does not move at all.

Periodically check the tightness of the clamps holding the jig on the drill press table. They must always be tight.
It is important to take breaks every half hour or hour to reduce fatigue. Do not operate for prolonged periods – you risk an accident if you do so.

How to calculate the number of inches of cut grooves in your PVC pipe.
The DK3 instructions basically point out that the builder will be winding a total of 75ft of 16 gauge wire onto the coil form. You will need to measure the diameter of the PVC pipe you will be using. Once you have that measurement, the following calculations are performed:

i. Calculate the circumference of the PVC pipe:
circumference of PVC pipe = diameter of the PVC pipe multiplied by 3.14159 (value of Pi)
Example: My PVC pipe is approx. 1.875 inches (1 7/8”), so 1.875 x 3.14159 equals approx. 5.89 inches circumference

ii. Calculate the number of feet of wire per 10 turns or per 1 inch of winding:
Total number of feet of wire per 10 turns or per 1 inch of winding = circumference X 10 / 12 inches per foot example: my circumference 5.89 inches x 10 = 58.9 inches / 12 inches per foot = 4.91 feet wire per 10 turns or 1 inch of winding

iii. Calculate number of inches of grooves to make 75 ft total wire wound on coil form:
Total number of inches of winding = total number of feet of wound wire / 4.91 feet per 10 turns or 1 inch of winding example: 75 ft total / 4.91 feet per inch or 1 inch of winding = 15.3 inches of grooves required to be cut in PVC pipe

If you have any more questions, feel free to contact me at the following email address:

Member Comments:
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Easy Method for Cutting Grooves for the DK3 Screwdriver Ante Reply
by W4XKE on January 14, 2017 Mail this to a friend!
Very nicely done. There was a QST article a few years ago to make coil forms by slicing PVC tubing lengthwise on a table saw and then notching the raw edges for the windings. The pieces were then glued back-to-back to complete the form.

My attempt proved rather dicey in holding the pipe safely or accurately for the lengthwise cut. My groove cutting process was inaccurate and time consuming.

So I ended up taking a piece of 2 1/4" PVC to the building maintenance instructor at the local trade school for him to thread it in his pipe threading machine. Not pretty but it did work - kind'a.

Your method looks a lot better and appears to produce a very nice product. Thanks.
Easy Method for Cutting Grooves for the DK3 Screwdriver Ante Reply
by N6JSX on January 14, 2017 Mail this to a friend!
Outstanding, great ingenuity and this same method could be used to make other HF wire antenna coils. I still hold out hope that HAMdom has builders, not just button-pushing buyers. With antennas becoming the last frontier of original design/build.

Only miss to this article is a pic of the end product with the wire showing it on the DK3.

Drill-presses can do lots of stuff - I made mine into a poor-mas mill. See my 18AVT coil refurbishment article(s) in eHAM achieves.
Easy Method for Cutting Grooves for the DK3 Screwdriver Ante Reply
by K7QQH on January 14, 2017 Mail this to a friend!
Well Done, and glad to see "hands on" hams are still creating new ideas and hardware - not just buying "radios" and operating them as "appliances".

I built my own mobile antenna from scratch (equation solutions and all) from excellent material in the ARRL Antenna Handbook and internet articles by the venerable K0BG and others. It CAN be done using the most important tools of all: Your brain and hands.

Best 73's

Roger C., K7QQH
RE: Easy Method for Cutting Grooves for the DK3 Screwdriver Reply
by KI7AAR on January 14, 2017 Mail this to a friend!
I'll give you credit for the innovation. However, considering 12 hours of cutting plus the time and materials of the fixture I would just found someone with a lathe. You would have cleaner grooves and 12 hours of free time to do something else.

I don't know if this would work for a screwdriver antenna application but, McMaster Carr has plastic edge trim that is U-shaped with toothed sides. It works great for coil forms and it's cheap. Four offset rows of this on a slightly smaller PVC pipe might do the job.

Another thought is to just use a pipe threading machine to cut a long section of threads. This is 11.5 tpi for the 1.5" pipe that you used and 8 tpi on larger 2.5" pipe.

Good luck with your project.
The DK3 Screwdriver Reply
by W4KVW on January 15, 2017 Mail this to a friend!
If it works as well as the real deal DK3 it will do an Awesome job.I owned a DK3 & chatted many times on the web with Don Johnson about the great antenna that it was.I used it with an ICOM 706 MKII & I worked the world on it & with ease.By far the best HF Mobile antenna I've ever used.That includes TARHEEL II,YAESU ATAS 120A(Worst of them all),Hustler,& Hamstick.It was daylight & dark better than any others I listed.It tuned quickly & stayed tuned.Only drawback was that it was a very long antenna & with the long stinger it hit a lot of stuff along the roads here in Northeast Florida.

Easy Method for Cutting Grooves for the DK3 Screwdriver Ante Reply
by VA7CPC on January 15, 2017 Mail this to a friend!
Thank you! The pipe-fitting adapter, with set screws, is the "magic part" -- very nicely done.

. Charles

Easy Method for Cutting Grooves for the DK3 Screwdriver Ante Reply
by K5WKS on January 17, 2017 Mail this to a friend!
Only suggestion I would make is to use the PVC pipe for Hot water. It is more rigid and doesn't bend when you transmit. No real cost difference.
RE: Easy Method for Cutting Grooves for the DK3 Screwdriver Reply
by K1YTG on January 21, 2017 Mail this to a friend!
At my shop I have a lathe and a milling machine.
The lathe has a gearbox that can be set as to threads per inch.
I am in Berkeley, CA
If a ham wants to build an antenna and needs use of these, he (or she) can use my equipment.
Perhaps a better job can be done on tools made for cutting threads.
Easy Method for Cutting Grooves for the DK3 Screwdriver Ante Reply
by W5QM on January 22, 2017 Mail this to a friend!
You can forego the drill press and use a Dremel tool, die grinder, or trim router (my choice) mounted to the fixture to cut the grooves. This is what you really need with the bit you are using as the drill press speed is not high enough. This will substantially reduce the time need to cut the grooves.

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