A High Efficiency "Screwdriver" Remote Tuned Mobile Antenna


May 22, 2013

Documenting my installation of the GS3 HF Mobile antenna custom built for me by Gary Stookey, KI7SD

"It's old enough to vote, so what do a few more holes matter?"

"Why wash it? A good coat of dirt protects the paint."

GS3 antenna and 1/4 wavelength 6-meter antennas on van.

After using various HF antennas on my old but reliable Chevy Van, including several of the HamStick design (rather poor efficiency,) my original High Efficiency Mobile Antenna (works great, but inconvenient to change coils,) and my old but reliable Webster Bandspanner antenna (somewhat easier to use,) I came to the realization that at my age, it was not only inconvenient to stop the vehicle and climb to the roof to change bands, but dangerous as well. After considering the various factors, I finally gave in and decided the time had come to install a remotely adjustable "screwdriver" antenna. But which one?

Having done a lot of antenna modeling and actual field testing, I knew that a short-length inductive loaded vertical antenna mounted on a vehicle would be a poor radiator, no matter what the design. The worst case would be a combination of a small diameter coil with small wire and a short mast section beneath the loading coil. Unfortunately, that eliminated most of the commercially available screwdriver-style antennas. Reading the various reviews of screwdriver antennas provided wildly varying reports, but little solid data on which to base a decision on what antenna to use.

After a lot of reading and some number crunching, I decided that the antenna would have to have a loading coil with of three-inch or greater diameter and wound with 14 gauge or larger wire. Although I would have preferred the antenna to be able to operate between 160 meters and 6 meters, I knew that was not going to happen due to conflicting requirements needed for a wide frequency range versus high radiation efficiency. I decided on a 75 to 10 meter operating frequency range for a workable antenna.

Another major factor was the available clearance height of the installed antenna. Since the antenna would be mounted on top of the van for good radiation efficiency on 75 and 40 meters, I knew that I would be limited in overall antenna length. The antenna would have to be shorter than it could be if it were mounted on a smaller and lower vehicle, such as a pickup truck or a sedan. I knew I could use a bumper mount which would place the antenna closer to the ground and let me use a longer top whip, but I also knew that would seriously degrade the radiation efficiency of the antenna, so I chose not to do that.

I live at the end of a rather bumpy dead-end country road that has many low hanging tree branches, many of which I have "trimmed" with my previous mobile antennas. (And some of the antennas have been trimmed by the tree branches!) Even more hazardous to the health of the antenna (and me!) is that along one stretch of the road, the local 7,200 volt power line is rather close to the roadway because the road rises a bit right where the line dips the lowest between power poles. On occasion, when it was adjusted in length for 75 meters, the Bandspanner antenna came in contact with the lower of the power lines. That is NOT good! Luckily for me, the lower wire that the antenna touched was the neutral / earth return wire. The actual high tension line is strung about 4 feet above that wire.

After careful consideration, I decided that the best antenna for me would be the GS3 antenna, manufactured by KI7SD, Gary Stookey (GS-MFG.COM.) I liked Gary's construction techniques and antenna design - he builds his antennas as I would. Gary said that he would build the antenna for my particular setup and requirements. The finished antenna would be short enough so that the top of the loading coil would be lower than most of the heavy-duty tree branches that it would encounter on the road to and from my house. With a strong top whip, I figured it could handle the impact loads from the rest of the tree branches pretty well.

Another advantage of the GS3 compared to other antennas using similar sized loading coils is that the coil is permanently affixed to the lower mast. This is possible since the slider that runs over the coil windings for tuning moves up and down inside the coil instead of the more commonly used arrangement of moving the coil up and down past a fixed contact. That also means that the overall length of the antenna remains the same no matter where it is tuned. Because the loading coil is a permanently attached part of the antenna and does not move up and down with antenna tuning, there is much less chance of damaging the coil and tuning mechanism should the antenna happen to encounter a major impact.

The GS3 antenna installation from the side of the van.

The lower base tube of the GS-series of antennas is made of a heavy gauge 2" diameter aluminum tube for strength and low RF resistance. (All the specifications for the antennas are on Gary's web site.) The loading coil is permanently and securely attached to a metal strengthening ring that is located at the bottom of the loading coil. The top whip that I chose to use with the GS3 is only 55 inches long. I needed to use a relatively short top whip to be able to have enough vertical clearance for obstructions. Ideally, the top whip should be at least a foot longer for better radiation efficiency and lower loading coil losses.

With a top whip this short, the lowest frequency the antenna will tune to is about 3700 kHz, but that is low enough for my use. (I do have a longer top whip that I can install that will allow the antenna to tune down below the bottom of the 80 Meter band.) With the 55 inch whip, the antenna requires about 1 to 2 turns of the loading coil to resonate on 10 meters. But at that frequency, the overall loss is quite low, and the antenna efficiency is fairly high. All things considered, the short top whip is a reasonable compromise for my use. Since the loss resistance of the loading coil is so much lower than the Bandspanner antenna, the overall results are better, plus I have the advantage of an antenna that I can retune on the fly from the drivers seat of the van.

The loading coil of the GS3 antenna.

The GS3 loading coil is a real work of art. The antenna is strong enough to withstand almost any impact. The coil windings are inside the sturdy fiberglass tube, which Gary molds around the loading coil. Plastic end supports are used to eliminate the excessive RF losses that are common to many other screwdriver antennas that use metal end caps on their loading coils. Most of the RF loss in the loading coil will be at the top end of the loading coil, and Gary uses only plastic end caps at that point to minimize the losses. Gary builds his antennas as I build mine - engineering considerations first, for a better signal.

The top of the GS3 loading coil.

The topmost few turns of the 14 gauge wire loading coil can be seen in this picture. The wire is firmly and permanently embedded into the inside of the fiberglass tube. This construction protects the coil and the sliding contacts from deterioration due to the weather. A sturdy 3/8-24 thread brass fitting to accept the top whip is mounted in the center of the upper plastic coil cover. The whip seen here is one is one that I have used with various mobile installations since I had my first 1961 VW bug. It is cut for the 6-meter band, and it has survived at least five different vehicles. The base stud was originally chrome plated brass, while the whip itself is made from what is apparently indestructible 1/4" diameter stainless steel.

The rooftop magnet mount system as seen from the rear of the van.

In this picture, you are standing on the roof of the van looking down at the wide spaced magnet mount I constructed for this antenna. Engineering-wise, a magnet mount is not what you want to use with a mobile HF antenna. But since I have had very good results using magnet mounts on this van (probably because of the large mass of metal under the antenna) I decided to try it with the GS3 and see how it worked. I figured that if it failed to perform well enough for me, I would just drill a few more holes through the roof and install a permanent mount.

This magnet mount system was modeled after a magnet mount that I originally purchased from the Lakeview antenna company some years ago. The original design used four magnets arranged in a square pattern similar to this one but with a considerably smaller footprint. It always worked very well, and never once in over 50,000 miles of driving did it come loose from the roof. That includes the time when I had a HamStick antenna ripped from the mount when it became snagged in an overhead obstruction.

This magnet mount system uses four magnets, each one measuring 5 inches in diameter. Because the steel sheet roof of the van is fairly thick, the magnets stick extremely well. The metal bars connecting the magnets are each made of two lengths of hard aluminum stock. Each section measures 1 inch wide by 1/4 inch thick by 36 inches long. Because there are two of these strips in each bar, this gives a total thickness of 1/2 inch per bar. Making each bar from two sections of metal instead of one solid bar allows for slightly more flexibility so that as the antenna moves on the mount there is less tendency to detach the magnets from the roof of the vehicle. The horizontal plate that holds the antenna is made from 1/4 inch thick by 2 inch wide by 23 inch long hard aluminum stock. All the bolts and other hardware are stainless steel.

The rooftop magnet mount as seen from the front of the van.

In this view you are standing on the roof of the vehicle looking towards the back. The common mode RF choke for the coaxial feedline cable to the antenna may be seen near the lower right magnet. This is closest to the driver side door. Slightly above and to the right of the antenna base spring mounted on the two parallel fiberglass support rods are the two common mode RF chokes for the motor power leads and the reed switch wires. Common mode RF is the biggest cause of troubles and tuning problems in HF mobile installations. (See K0BG's article.) You can also see the shunt coil that is mounted at the base of the antenna next to the base spring. This is required to compensate for the capacitive reactance of the short antenna at HF.

RF coax cable and control wires to inside of van and 6-meter antenna.

Near the front of the van is a quarter wave 6-meter antenna. Unfortunately the 6-meter antenna interacts somewhat with the GS3 antenna on 20 meters. The unwanted interaction occurs because the 6-meter antenna and it's coaxial cable are resonant slightly below the 20 meter band. This interaction affects the reactance of the GS3 antenna causing the SWR across of the 20 meter band to be about 3 to 1. However this isn't a real problem since the auto tuner in the TS-480SAT easily handles that much mismatch.

Below the 6-meter antenna (seen here positioned towards the rear of the vehicle) is another magnet mount. I can install a scanner antenna or a UHF antenna on this mount if needed.

The RF coax and control wires from the GS3 are fastened together with plastic ties. These lines come together with the coax feedlines from the other two antennas where they go over the edge of the roof and down into the passenger compartment of the vehicle. A small but powerful magnet is used to hold the wires for the GS3 antenna in place on the roof of the vehicle.

RF feed to base of antenna.

A small metal bracket holds an SO-239 connector for the RF feed going to the antenna. Installing a connector here makes it much easier to remove the antenna and the magnet mount from the roof of the vehicle should it be necessary. There is a small piece of 1/4 inch thick clear plastic placed between the metal bracket and the roof of the vehicle. This prevents the edge of the metal bracket from scraping the paint on the roof of the vehicle as the magnet mount flexes when the vehicle is bouncing up and down on our very bumpy country road.

When the GS3 antenna is used with a pickup truck or a sedan, it is usually attached to a fixed mount. However, I didn't think that was a good idea when the antenna was mounted on the roof of the van. I thought that mounting the antenna on a stiff spring would be a better and safer option. I had unpleasant visions of a firmly mounted antenna encountering a low hanging Oak tree branch! A base spring it would be.

The spring that I chose for mounting the GS3 is Cal-Av model MARK-5. This is a heavy spring, (Cal-Av makes much larger springs) and weighs slightly over 2 pounds. That's much heavier than your Radio Shack antenna spring. It takes a lot of force to bend it, and it works quite well with the rather heavy GS3 antenna. When I am driving, the antenna does not start to tilt back from the wind pressure until I reach a speed of between 45 and 50 m.p.h.. But the MARK-5 is flexible enough to still allow movement when the antenna encounters tree branches and other obstructions.

The shunt inductor at the RF feedpoint.

Gary supplies a shunt inductor to use with the GS3 antenna when you purchase his antenna, but Gary's inductor did not have quite enough inductance to properly tune the antenna on 75 meters when it was mounted on the roof of the van. I suspect that is because of the large amount of metal of the van under the antenna makes a better than usual (for a mobile system) ground plane. I made a new shunt inductor a little larger in diameter following the excellent suggestions on K0BG's web page about antenna matching. It worked perfectly, and the antenna tuned up nicely on all bands, 75 through 10 meters. I used my AIM-4170 to sweep the antenna when adjusting the shunt coil.

Common mode RF choke for RF coax.

To prevent RF feedback into the gear in the vehicle, it is necessary to use some common mode RF chokes on all the lines from the antenna. For the choke for the RF coax cable, I used two Mix 31 ferrite split beads and ran a total of 6 turns of RG-8X coax through the beads. That provided sufficient impedance to eliminate and problems. The ferrite beads are attached to the magnet mount support bars with plastic ties.

Common Mode RF chokes for motor power wires and reed switch.

I did the same thing with the motor power wires and the wires to the reed switch on the antenna. A single Mix 31 ferrite split bead was enough for each set of wires. There is one choke on the motor wires, and one choke on the reed switch wires. There are 16 bifilar turns of wire on each toroid. They were wound according to the directions on K0BG's web page. Because the wire in these choke coils covers the outside surface of the ferrite beads I did not want to mount these chokes against a metal surface. Instead, I mounted them on two fiberglass rods, one of which passes through the center of each choke. The fiberglass rods are fastened to the aluminum bars with stainless steel hardware. The rods themselves were cut from inexpensive electric fence support posts.

Common Mode RF chokes for motor power wires and reed switch.

The wires from the common mode RF chokes connect to the wires coming from the GS3 antenna through a Molex connector. The connector is sloped slightly downwards. The end of the Molex connector closest to the antenna is the highest end of the connector. That end of the connector is filled with silicone RTV sealer to prevent rain water and moisture from getting into the top of the connector. The bottom end of the Molex connector closest to the chokes is left open. That allows any moisture that does get into the connector to drain out. Although you can't easily see it in this picture, the end of the brown cable from the MFJ antenna controller inside the vehicle connecting to the RF chokes has the top open end of the insulation where the wires exit the cable bent over and pointed down. This is visible as a slight downward bend just past the plastic tie wrap to the left of the wire crimp splices by the RF chokes. This prevents any rainwater from getting into the cable and working its way downward and into the vehicle.

Magnetic reed switch mounting.

The reed switch that connects to the antenna controller is mounted under that mass of tape you see just below the plastic tuning position indicator cover below the loading coil.

Magnetic reed switch mounting.

It's rather a sloppy looking job, but it does work. Gary does not supply the GS3 antenna with an internal reed switch. However he did mount a magnet on the motor shaft for me, so I was able to attach a reed switch to the outside of the antenna. If nothing else, having the switch on the outside of the antenna makes it easier to change the switch when it eventually fails. There's a lot of extra tape visible on the antenna where the switch is mounted. That's because there are several wire splices and some extra wire under the tape. I tried three different reed switches before I finally found one that worked satisfactorily. The reed switch that I ended up using is a Hamlin reed switch. The manufacturers part number is 5930011766643. Eventually I'll get around to removing the extra tape and straighten out the wiring to improve the appearance.

MARK-5 base spring.

This is one tough spring! But it's what this antenna needs because the weight of GS3 without a top whip is about 9 pounds. That's a lot of antenna to have mounted on top of a spring, but the MARK-5 handles it just fine.

I left enough slack in the motor and control wires from the antenna so that no matter what the position or tilt of the antenna is as it moves around on the base spring, the wire will not be flexed excessively. I also covered the wire with some convoluted tubing for additional protection from sun, wind, and weather.

Yeah; I know - it sure needs a good wash job! Well, the van always gets parked in the same direction, and as you know, moss grows on the North side of the tree...

The coax cables from the antennas on the roof of the vehicle and the brown control wire from the GS3 antenna come down from the top of the roof over the rain gutter and into the vehicle through the door seal. What looks like a cut out in the rain gutter is actually placed there by the factory to allow rain water to drain at that spot instead of running down over the door entrance. It made a convenient place to bring the cables over the edge of the roof.

I decided the simplest way to keep the cables in place for the door seal as it closed against the frame was to use some cable ties and self tapping sheet-metal screws. This method keeps a cables firmly in place so that once the door seal has taken a set around the cables I don't get any water leakage into the vehicle.

I use another cable clamp to keep the wires tightly against the door frame.

I use more cable clamps to bring all the wires straight down by the edge of the door frame.

The electronics package is a Kenwood TS-480SAT. It is anchored to the floor behind the driver seat. I used two wooden blocks to elevate the radio slightly from the carpet to allow for plenty of ventilation space. The extra cable length is coiled up and stuffed under the bottom of the radio. A terminal strip is mounted on the wall of the vehicle to provide connections for primary DC power, MFJ controller power, GS3 antenna motor leads, and the antenna reed switch.

I made a slip over wooden cover and painted it flat black for low visibility. When the cover is placed over the radio, someone looking into the vehicle from outside can't see the radio. The case has plenty of clearance space around the radio for ventilating air to get in and out. The back of the wooden enclosure is open so the cooling fans and the TS-480SAT can exhaust the hot air with no restrictions.

This is the view of the radio installation as seen from the rear passenger seat. (Note that the dog is an optional but self-installing feature of this vehicle.) The enclosure is sturdy enough that passengers can put their feet on the box with no damage.

Terminal block connections (by pairs) from left to right: GS3 antenna reed switch, GS3 antenna motor, DC power to the MFJ-1924 antenna controller, DC power to TS-480SAT, fused DC power from vehicle battery. The fuse protects the +12 V DC power line to the antenna controller.

Modified MFJ-1924 Antenna Controller

One of the advantages of a screwdriver antenna is that you can control it from the operators position. The simplest control is simply a double pole double throw switch. Flip the switch one way, the antenna tunes up in frequency, flip the switch the other way, and the antenna tunes down in frequency. This is a simple and reliable method of controlling the antenna, but it has the disadvantage that you don't have any way to tell where the antenna is tuned at any given time. You need some sort of antenna controller for that.

The MFJ-1924 antenna controller is designed to remember 10 specific tuning settings. It does this by counting the number of revolutions that the shaft of the tuning motor makes. In order to do this, the MFJ assumes that there are two things available from the antenna.

The first thing the controller needs is a contact closure of some sort, usually from a reed switch mounted inside the antenna. This tells the controller how many turns the tuning system has made. Note that this does not directly correspond to the number of turns in the coil, but rather how many rotations the tuning shaft has made. It doesn't matter though because the tuning system will make only X number of turns from one end of the tuning range to the other.

The second thing the controller needs is some way to tell when the tuning system has reached the end of its travel range. Most screwdriver antennas simply run the tuning mechanism into mechanical end stops and jams the motor when the end of the tuning range is reached. This stalls the motor and causes the motor drive current to increase dramatically. The MFJ-1924 controller is designed to sense this increase in current. At that point, the controller shuts off power to the antenna tuning motor.

Sometimes, for reasons known only to Murphy, the controller will lose track of how many counts have gone by. For this reason, the MFJ controller has a function which enables you to "bottom" the antenna. This function tunes the antenna to the lowest frequency, stops the drive motor, and then zeroes the turns counter. So far, so good. It looks like the MFJ-1924 antenna controller is just what I need to control the GS3 antenna. Well, it was, almost.

First problem - no reed switch in the GS3 antenna. That one was fairly easily taken care of. Gary had already installed the magnet inside the antenna, so all I had to do was to mount a reed switch on the outside of the antenna, run the wires down the lower mast of the antenna, and then run them through a common mode RF choke.

Second problem - no stalled motor current for the controller to detect. In order to protect the tuning motor in the GS3 antenna Gary builds in a slip clutch arrangement. That means that when the antenna tuning mechanism reaches the end of its travel, the motor will continue to revolve but the tuning mechanism stops moving. Unfortunately for the MFJ antenna controller, this has the disadvantage of not allowing the tuning motor drive current to increase very much above the normal running current. The advantage is that it is impossible to damage the tuning mechanism or burn out the drive motor by running it into the end stops. I found that I was unable to get the MFJ controller to sense the end of travel of the tuning mechanism. I even went as far as to dig into the MFJ's internal circuitry and increase the sensitivity of the drive current detection circuit, but in the end, I found that there just wasn't enough difference in normal tuning and end of travel motor current to make the controller work reliably.

The solution ended up being a combination of electronics and brain power. I made a slight modification to the controller. Now, when I need to reset the controller and "bottom" the antenna and zero the counter, I place the MFJ controller in its counter reset mode. This starts tuning the antenna to the lowest frequency position. Meanwhile, I sit quietly and listen until I hear the "thunk-thunk-thunk" of the drive mechanism slipping when it reaches the lower end of the tuning range. Since the antenna is mounted on the metal roof, the sound is quite audible. At that point I press the little Magic Button on the bottom of the controller.

I drilled a small hole in the bottom of the controller case and installed a normally open push-button switch. I have the switch button protruding just slightly out of the hole so it can't be pressed accidentally. The switch is connected in series with a 50 ohm resistor. The resistor and switch were then wired directly across the tuning motor wires coming from the antenna controller. When the button is pushed, current flows through the resistor from the output of the MFJ controller. This extra current is added to the current being drawn by the tuning motor. This makes the MFJ controller "think" the motor has reached the end stop and has stalled. This tells the MFJ controller to turn off the power to the tuning motor and reset the counter to zero.

I knew I would have to take the MFJ antenna controller out of the vehicle from time to time, so I installed a Molex connector on the wires coming from the controller.

This is the mobile installation as seen the driver seat. Everything can be easily reached with one hand without stretching.

I used several cable clamps to secure the wires from the MFJ antenna controller. The controller itself is mounted to the vehicle with some adhesive backed high-strength hook and loop fastener material. The open spot in the dashboard is for the vehicle entertainment radio. I pulled the old radio out while I was doing the installation of the ham rig. I have a new entertainment radio ready to install. It will replace the old one which was killed by a nearby lightning strike. <!>

Here is a bottom view of the modified MFJ-1924 antenna controller. The Magic Button is at the far right of the controller. The switch was salvaged from a computer, and it's held inside the case with a blob of hot melt glue. Not fancy, but it works.

The silver switch is a single pole double throw switch. It inserts a 50 ohm 1 Watt resistor in series with the antenna tuning motor power wires. This cuts the speed of the drive motor by approximately half. If I need to tune the antenna manually, I can slow it down by using this switch. That makes it much easier to "tune by ear" if I need to tune to an odd frequency that is not programmed into the controller. It's also very useful if I am driving and want to manually change frequencies. I don't have to take my eyes off the road to adjust the antenna.

Here is the MFJ-1924 antenna controller after my modifications to it. After installing the Molex connector on the end of the wires, I use some convoluted tubing for protection. Hmm... I see a Blue Box inside the controller. That's not standard; I wonder what it's for?

At the top left of the picture is the Magic Switch. It is pretty well buried under the blob of hot melt glue I used to hold it in place. Above the two black motor control relays you can see the extra toggle switch I installed to change the motor speed.

Oh, the blue box? That's a relay that I had to install to make the controller happy. It turned out that the MFJ controller occasionally missed counts from the reed switch. This caused the tuning to be off by a turn or two very often, resulting in a higher than desired VSWR reading. I spent some time looking at the reed switch pulses with an oscilloscope, and although I did not see anything wrong with them, the MFJ consistently miscounted. Installation of various types of RFI filters and RC networks failed to resolve the problem, which happened whether or not I was transmitting..Finally I decided that the CPU in the controller must be very sensitive to the rising and falling edges of the switch pulses. Perhaps adding a relay would fix the problem. Taking another page from K0BG (scroll down to the "Debouncer") I tried a small 12 volt relay and found that it did help resolve the problem. It was almost - but not quite - perfect.

Digging into my NOS parts junkbox, I found a Mercury wetted reed relay. (Sorry OSHA & RHOS) I managed to stuff it inside the MFJ controller. I took the relay coil DC power from the switched +12 volts DC, so as not to have a constant drain on the vehicle battery should the reed switch stay in the closed position at the end of the tune cycle. After installing the relay, the problem of miscounts went away. Perfect! I just have to remember that the controller - and the relay - must be operated in an upright position or the Mercury will roll around inside the relay and short out the relay contacts, resulting in no count pulses. I figure the reason this relay works so well is that unlike a normal relay, a Mercury wetted relay has virtually no contact bounce. Once the contacts close, they stay closed, and when they open, they open cleanly.

Because I didn't have a 50 ohm 1 Watt resistor on hand, I made one by connecting a total of 4 - 50 ohm 1/4 Watt resistors in a series parallel combination.

Happy Mobileing de W5JGV

73, Ralph W5JGV


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