I'm almost finished with my screwdriver design and have all the bit here to start making parts. I'd like an opinion on where the coax choke and shunt coils should be located. I have the top part of the antenna all sort and just have to finish the lower portion of the mast. A section view is shown below:

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I am going to bring coax in from the bottom of the antenna thru a bulkhead and attach the shield and center to the feed points as shown with ring terminals. The mast is 2.75" ID tubing. I can wind LMR-195 on a FT-240-31 torroid and have fit internally below the stepper motor, is that an acceptable location or will being inside the mast affect its operation?

I assume I need a relay selectable shunt coil, does it go ahead or behind the choke? Can it be in the mast or will it have capacitance issues? On a lot of other installations I see it mounted externally or remotely, I'd really like it either inside the mast or inside the body of the truck. If it's inside the truck is there any practical difference between adding a shunt coil or just activating the autotuner that's in the box with the radio?  

My motor driver could also be in the base of the antenna or in a box in the truck, currently I was thinking to mount it in the truck and run the 4 motor leads and 2 limit switch leads out to the antenna in some "tray cable" than has pairs of wires individually shielded. The other choice is to run +12v, gnd, enable, step and dir wires out. Then only one wire has pulses on it vs 4 but I need more conductors in total. I'm also not sure which way is easier to choke or if the stepper driver would freak out from the RF if it's inside the mast.

Can't help you.  But it looks like an impressive project.

Been working some on the antenna and thought I'd post an update here since the original thread is in the archives. The coil form is almost done.

I made a gundrill adapter for the tailstock to punch a .625 hole thru 26" of the coil form.

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I went to thread the OD 6tpi and realized that the top row on the threading chart needs a gear swapped out, opened the cover and found out that I don't have the required gear :(

So I went into work and setup the part and threaded it on the lathe in the engineering shop.

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Slid into the upper mast. I'll groove it for the finger stock and then start on the middle insulator thing.

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Love your idea and I'm jealous of your equipment. Can I be your friend?

Shunt coil will depend on the size of antenna. For a smaller size, you may not even need a shunt coil. The coils should be placed behind the coax choke (if I understand your description correctly). I'd place it outside if needed.
I wouldn't worry about a coax choke. A couple of snap-on ferrites may be enough. Install a choke for the motor control and the feedback wiring (assuming you'll have a feedback for the antenna position). A magnet on the rotating shaft and a vacuum Reed switch may do the trick unless you want to go fancy and install an encoder. A choke for these wires will be mandatory (if I understand your design correctly).

Also, why are you having the feedpoint in the middle of the base (based on what it looks like on the drawing)? Take advantage of the base and make it a center loaded antenna. The space inside the base will other ways be wasted since you can't place the worm gear rod inside the tuning coil. Connect the feedpoint at the bottom of the base. The base will have to be isolated, obviously. Center loaded design is a bit more efficient than a bottom loaded one.

Also, I'd look at either choosing a 2 speed motor or a variable speed system if possible. Most screwdrivers are too slow when changing from 80 (or 160) meters to 10 meters. It may complicate this a bit and may not be easily justified. You choose what's important to you.

Are you thinking about building your own, automatic controller for it? Just curious.

The feed point is about 8" from the mounting flange. It's going on a military antenna mount on the back corner of my M1009, same mount it getting added to my Suburban. I wanted the top of the coil even with the roofline.

I decided to put a FT240-31 inside the base and I'll put shuntcoils in a box with the swr bridge and antenna controller.

I'm using a RPi to control a nema 17 stepper driver and a fast pitch ballscrew (I had it from another project). It takes 13sec on the bench to transit the coil. Currently the software runs it like a single axis cnc stage. It homes on startup then drives to a target position. A manual override switch moves the target position.

After I get the build finished I'll work on the software more. I'm using a RPi so I can use rigpi to poll the icom radio for the frequency, then I can use a lookup table. I can also use rigpi to make the radio transmit a low power carrier and then use the swr readout along with the aforementioned table to position the antenna automatically. I also have an external swr bridge I might hook to an ADC to get it into the RPi. I'll keep a full manual mode for when an extension is clipped to the whip.

Awesome project and machining.

How exactly are you making the coil adjustable?

How is the coil connection made and what exactly does the motor move?

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Thats the basic screwdriver antenna. Named after the electric screwdrivers that the first designs used. I didn't want the motor above the insulator. The other designs rely on the finger stock friction to keep the coil from turning. If it slips in the finger stock it rotates and doesn't extend and the controller gets lost. I wanted something faster and I wanted an automatic antenna controller.

Other designs use one (Scorpion uses two) reed switches that count screw revolutions giving a count each ~1/16". One full step on my motor is .003".

I'm not a software guy, I'm an ME/EE who designs heavy equipment structures and hydraulic systems for a living and put myself through school in tool and die shops. The software aspect is fun as I can do whatever I want to learn, I might add a screen or use the RPi to run the station and just vnc in vs using a usb cable for digital stuff. Lots of choices.  

I also didn't want to spent $1k for a kitted Scorpion, though I absolutely understand why it's that expensive. I already had the motion control bits. The mast tubing plus coil form material was more than a used Tarheel. But that isn't the point...

Coil form is complete and the 10awg tinned wire is wound. I also completed the upper mast tube and test fit it. I need to open up the finger stock groove just a bit more.

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The coil wire makes a loop under the brass bolt and thru the hole.

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To keep the coil form from spinning I designed a little truck that fits in a pocket and holds a pair of pipe cutter wheels. This should make a track up the inside of the mast and keep the form from turning.

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Have company with a little one here this week so progress in the evening has been limited to making drawings.

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OP has building skills...sadly I do not.

That looks awesome!

Had a couple hours to spin handles today. The part I call the isolator is almost done. One end holds the ballscrew bearing and the upper mast, the other mounts the stepper, limit switch, and lower mast. The feedpoint it also in the isolator.

The pic below is the 2nd milling setup, for the first the rotary table was vertical and I did the radial holes and the limit switch pocket. Next it's back into the lathe to trim 1" off the end and cut a bearing bore then back to the mill for 6x tapped holes and it'll be done.

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I have four flat parts needed to get profiled out of 1/8 and 3/16 6061-T6. For giggles I uploaded them to Sendcutsend.  

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Came out to ~$8ea if I used 3/16" thk mat'l for each and I decided it wasn't worth the time to program four separate parts in CAM then cut them out, oh and I also didn't have enough material. For just over the cost of getting the material I have finished parts. Since I would have cut them dry if the endmill loaded up I'd have been really underwater compared to just having them laser cut.

Just need to get the flange welded to the lower mast when it comes this week and finish a few other small parts for the inside.

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Next design task is making a box to hold the controller, rf relays / shunt coil(s) and the swr bridge, I'm thinking a 556 ammo can, possibly cut down.

Do I need fancy vacuum relays for the shunt coil?

Great job!

I wouldn't use a vacuum relay unless you plan to run a full legal limit and switch it often. Also, some vacuum relays don't like vibration that can be present on a mobile antenna. You should be ok with a 600V, 10A rated relay. Personally, I'd install a manual switch and experiment first. You may never need to disconnect it.

Great job on this build! You do have great skills!


BTW, I seriously thought about designing and building a prototype, screwdriver type antenna for applications where there are strict antenna restrictions. It would be a longer version of a Tarheel type antenna, hidden inside a fake, plastic Palm tree, cactus or a similar short tree. The palm leaves can also act as a capacitive hat on top of the antenna, to further improve efficiency. Unfortunately, my CAD skills are only limited to drawing simple 2D layouts and electrical schematics. I had 2 designers who did the rest of the CAD work for me. Right now, I don't even have CAD on my computer.

I use cad (solidworks) 4-8hrs a day for work. For personal use I have a EAA membership just to get access to SW. I use that for cad and cam (I have a Bridgeport BOSS with a controlectomy.)

Once I get basic functionality I'll install it on the truck and make some tables of extension vs the frequency that the reactance is zero. That should give me what I need for the shunt coil I think.

I use a Tarheel Model 75A on my vehicle. It works just fine without a coil. Their larger models do come with a coil.
Once you install the antenna, please post your results here.

Don't forget to bond the exhaust to the vehicle body in several places. If you drive a truck with a frame, bond it to the body as well. It does help quite a bit.

Also, don't forget to install an RF choke on the control cable. It will become a part of the antenna if you don't. Tarheel sells snap-on type chokes. They claim that one is enough but I added another one to make sure it cuts the RF sufficiently, on the lower bands.

Getting close, made the custom jaw coupling insulator over lunch today. I very much dislike the BOSS milling machine but sometimes it's easier than a bunch of rotary table setups on the manual mill. It was a bit of cut, check, remodel, repost and recut to get the dimensions right on the chinesium coupler.



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The laser parts from SendCutSend came and they look nice. Hole diameters around 50% of material thickness look nice. I wish they'd keep up with the consumables at work, over there laser cut plate edges look like they could pass for a rasp...

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Got the bracket moved to the rear corner and the flange welded to the lower mast. Not too bad for not welding aluminum in many years.

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Mocked up (motor isn't installed but lead screw is) in the down position, should be about right for 15m with the long whip.

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And fully raised:

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I need to massage the antenna bracket or mount it standing on some studs and nuts, I marked the holes carefully when I moved the bracket but it sits out of level side to side. Might need a heavier spring too.

You could let air out of the left side tires until it sits level.
That is very nice work, Looking forward to seeing how well it works for you.

I was going to say, just pull out the 10 lb. knuckle-adjuster and tweak the level on that mount bracket a little bit.

Next time I post a picture of it, it will be straight.

Worked a bit today to assemble the motor mount and limit switch. Tomorrow I hope to finish the last of the machined parts and then I can assemble it and hopefully not take it apart again.

The bonding components came today. That will be Monday's project.

that is a sweet antenna.

The slight lean isn't going to effect anything.

Hint, while your cruising down the highway QSOing of the radio, the antenna in the wind is bend, it isn't vertical.

very cool

if I had your shop and skills,....I’d get 20% done, and end up with a STEN GUN

( j/k atf moles !)

PS - I love my TARHEEL II

it’s wonderful to change from 40m to 10m with perfect SWR ....while mobile

I got the bonding system installed last week. I bonded the hood to the chassis ground and down to the frame. I bonded both tailpipes to the frame. I bonded the body to the frame at the taillight and bonded the antenna mount to the same place. The doors and tailgate are bonded to the battery.

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Here is the completed antenna installed (less shunt coil):

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To test minimum functionality I made a quick hand controller with up/dm, rapid, and a program on/off selector. I used RJ45 bulkheads on both ends so I can use CAT7 cable for the extension, then I can leave that cable in the vehicles and no have to move it when I move the radio back and forth from the CUCV to our Suburban.

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I put the RPi in a metal case that has screw terminals for the header pins. I'm using a relay that powers the RPi and driver, it is kicked by the tuner port turning on when the radio is powered on.

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I found an issue with the bulkhead jumpers I'm using for USB and Ethernet. Both ends have shielded plugs but the shield on the 1ft cable isn't connected to either plug! The wires inside also aren't twisted pairs. Lame. I'm going to have to figure something out as that was a pretty integral part of how I have the control and motor wires shielded.

I also need to find a shorter whip, the big one was really intended for fixed operations on 80m, it resonates on 20m when it is raised about 3/4". I started having VNA issues last night when fooling with the antenna and don't have plots. It is 50ohm on 20m and down at 80 it is around 10ohm. I'll wind a shunt coil between meetings today and fool it it more later.

Currently the antenna controller stuff is just plopped onto of the radio in its box and I don't have all the little wires choked. It makes broadband noise on the antenna when the motor runs. Not good for automatic tubing however it's easy to tune by ear. Run the antenna up until the S-meter peaks and it's pretty close. I'll button up everything better once I have the shunt installed and have tested a different whip length.

After fighting much rfi, which to be fair I had issues with before where my 7100 would turn itself off when transmitting (homemade cables oi...) I finally got enough chokes in the right places and made a contact with a net on 40m :)

I used 8 turns of 14ga bare wire on a 1" dia form for the shunt coil. Moved the 14ohm I had on 80 to 48ohm. A little higher than I wanted but it's good enough. The bandwidth on 40 is pretty wide. On 80 it's about 15khz before it's over 2:1.

Its been raining all evening, it stopped and I went back out to try and do mobile check in on the arfcom net. On 40m I couldn't hear Gyprat (typical as we are pretty close) so I rolled down to 75m and thought I'd tune up near where he usually calls. Fail. SWR was 3+ everywhere. Hooked the VNA up and it was 25ohm now vs the 48ohm it was before. I took the shunt off and the SWR went down... I made sure the shunt was dry and put it back, no change. I took the antenna half apart to look for water ingress and nothing.

No idea what is wrong with it. Maybe tomorrow it will dry off and be back how it was...

I do think I need to add a vent line so the bottom gasket on the cover stops sucking in water when it extends and the outside is wet. Everything is fit pretty tight and there are orings everywhere to keep the water out but it acts like a syringe and sucks water through the cover seal.

Update: the above issue was a result of hamfistedly breaking the feedpoint and the coax center conductor barely making contact. Fixed that and it's been gtg.  

Been using it daily on my commute and it's tons of fun. Used it for field day some and it's been a blast.

Major goal accomplished this week: checked to the arfcom ssb net on both 4 and 80m.

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The controller is working well, still might port it over from python to micro python and use a pico instead of the 3b+ so I don't have to shut it down each time. Takes about 10sec to traverse the antenna range, when you flip the power switch it retracts all the way down and then gives you 5sec to change your mind before it shuts off the pi.

Nice work.