Last Modified: December 20, 2013
Contents:Basics; Proper RF Bypassing; Other Caveats; SWR Considerations; How They Work, Switch Type; How They Work, SWR Type; Manual Controllers; Stepper Controllers; Built In Controllers; Odds & Ends;
All remotely controlled HF mobile antennas require some sort of device to power the motor. It may be as simple as a DPDT rocker switch, to a fully-automated system requiring minimal attention from the operator. Whatever system is used, there are several prerequisites which must be performed to assure smooth operation.
1). The motor leads must be properly RF choked, and most factory-supplied ones are inadequate for the purpose. How to properly choke them is covered below.
2). The reed switch (turns counter) leads, if used, must be seperately choked.
3). The antenna needs to be properly matched. The highlighted article explains how that is accomplished.
4). All mobile installations have some level of common mode current flowing on the coax which must be properly choked. The highlighted article explains how that is accomplished.
Short, stubby, HF mobile antennas are very popular due to their diminutive size, light weight, and apparent ease of mounting (they're not necessarily less expensive). They exhibit several unforeseen consequences because of their diminutive size, not the least of which is their miniscule motor, which is easily damaged under stall conditions. If you opt for an automatic controller, make sure the controller manufacturer's instructions are followed closely to avoid damage to the motor. Impedance matching isn't necessary due to the high resistive losses they exhibit.
Proper RF Choking
The motor and reed switch leads of all remotely controlled antennas operate above RF ground potential. The amount of RF present on the leads depends on several factors, especially where, and how the antenna is mounted. In any case, every single one of them require an adequate RF choke on these leads. Inadequate choking will destroy any type of automated remote control, and will cause an inordinate amount of RFI problems. And just because you didn't have any problems with your manual control, means nothing when it comes to computerized controllers.
The chokes must be installed outside the vehicle, and as near to the base of the antenna as possible. If you mount them inside, you will be plagued with RFI issues, including erratic controller operation. The reason? Everything before the choke is part of the antenna, and will radiate! What's more, if the RF can get out, it can get in too! Remember, the interior of a modern vehicle is almost as RF noisy as it is under the hood! If the leads are improperly choked, that high level of RFI will be picked up by the wiring.
The ferrite beads discussed below are indeed RF chokes. There is a complete explanation of how they work here. As noted, the amount of choking required varies with the installation, but it is always prudent to error on the excessive side. Just keep in mind, the poorer the mounting location (trunk lip mount for example), the more bypassing (higher choke impedance) is needed.
Most antenna manufacturers supply ferrite beads or toroids with their antennas for use as chokes. Unfortunately, most are inadequate for the purpose because they are physically to small, and/or of the incorrect ferrite material (mix). What follows is a best foot forward approach, and will suffice for all currently manufactured controllers.
As a rule of thumb, a choke should have an impedance of at least two magnitudes greater than the impedance of the circuit it is applied to. In other words, at least 5k ohms, and perhaps 2 or 3 times this in some cases (i.e.: short, stubby antennas and/or poor mounting schemes). Mix 31 split beads are ideal for this application, but it takes 8 turns to equal 5k ohms. Thus, you'll need to use the 3/4 inch ID ones. If your antenna (or controller) didn't come with these specific split beads, they're available from DX Engineering, and other amateur radio dealers. A package of five of the 3/4 inch ID units costs about $30. Don't assume you can get by with a lessor number of turns, or a different ferrite mix. You can't!
It is common to see motor lead chokes wound with existing wire, replete with the outer sheath intact. The resulting choke's impedance is always inadequate, even if you use additional beads to wind an equal number of turns as described above. If you want to know why this is so, read the Split Bead article.
The choke shown at right is comprised of 13 turns, of nylon insulated #18 stranded wire, wound on a 3/4 inch ID mix 31 split bead. Its impedance is ≈10k ohms at 10 MHz. The turns should not be overlapped or twisted, as doing so reduces the impedance of the choke. The wire shown is Carol Cable Division, E10325.18.10 or equivalent. It is commonly called security cable, and is covered with a vinyl sheath. In this application, the sheath is removed, and the wires straightened (untwisted).
Decent quality #18 stranded hookup wire (never use solid), whether it is insulated with Nylon®, or Teflon®, will have an outer diameter of .068 inches. Cheap automotive wire is typically .108 to .115, and is too thick for this application. It really doesn't matter where you buy the wire, or what color it is, or what the insulation material is. As long as the OD is between .068 to .070, it will work fine. You'll need about 60 inches of wire. If you are careful, and place the turns parallel to one another, you can get 13 turns. There should be three loops of wire on each face of the core (see right photo, double click to enlarge). Again, overlapping or twisting the turns will reduce the impedance of the choke, perhaps as much as 50%. If you need instructions, here it is: How To Wind A Choke.
If you're still plagued with RFI on the motor leads, and common mode currents on the coax, it is a sure sign you do not have an adequate ground plane under the antenna. While it sounds like a broken record, it is the mass directly under the antenna, not along side, that counts! In extreme cases of poor mounting, shielded cable, and liberal use of bypass capacitors (paralleled .1 uF and .01 uF ceramic caps) on the output side of the chokes may be required.
For some folks, it is difficult to understand why there is a need for chokes in the first place. The simple answer is in the first paragraph of this section, but lets look a little deeper. Assuming the base of our antenna is properly matched and at resonance (50 ohms resistive), and the RF power is 100 watts, then the RF voltage at the base of the antenna will be ≈70 volts. However, the motor isn't mounted at the base, it is mounted up inside the antenna. How far depends on the antenna model. In any case, the minimum RF level will be the same as at the base. The further away from the base the motor is mounted, and the higher the ground losses are, the higher the RF voltage will be. It is not uncommon for the RF voltage to be as high as 250 volts, and in a couple of (known) cases, nearly 1,000 volts! That's high enough to zap any solid state controller! For additional information, read the Common Mode Current article.
The average remotely-controlled HF antenna draws about 250 to 300 mils while running, and somewhat over 1 amp when stalled. There are exceptions, of course. One model draws 2.5 amps while running, and nearly 8 amps at stall! In any case, accessory sockets and data ports should not be used to power any load over the maximum specified by the manufacturer. For example, Icom specifies a maximum of one amp (total) for its various ports, as does Yaesu. Although these ports are fused, in most cases a circuit board trace or switching transistor will fail before the fuse blows, resulting in an expensive fix.
To avoid this possibility, it is good advice to use a RigRunner or similar device for the power feed to the controller, and any other ancillary device(s) in use. In most cases, the circuit should be protected with a fuse no larger than 3 amps, and no larger than .5 amps if you're using a Lil Tarheel. This protects the controller, and the antenna's motor from damage.
There is another equally important caveat to remember if you're thinking about upgrading from an Icom IC-706 to a new IC-7000. Most devices which work flawlessly with a 706, will damage a 7000 beyond repair! Here's why.
The 706, and the 7000 differ in the way their tuner port is configured. What works okay with the 706, may not work okay with the 7000. The reason is, a lot of the 706 devices place a resistor and a cap between Pins 2 (13.8 vdc) and Pin 3 (Start) as a time constant. Doing this on a 7000 causes the fan to run fast or slow depending on the value of the resistor. Further, Pin 1 (TKEY) is also shared internally with the temperature control circuitry, and must be left floating. If either condition exists, the fan may not operate correctly, which can lead to final failures!
To reiterate, non-computerized devices used to mimic the AH-4, and trick the IC-706 into transmitting 10 watts of carrier are not compatible with the IC-7000. This includes the suggested circuitry from SGC, at least three of the older model screwdriver controllers, and most small tuner modules. The Better RF TCSC (no longer available) is specifically designed to work with the IC-7000. Their older units made for the IC-706 model are not compatible! Please do yourself a favor; even if your old 706 unit appears to work correctly with your IC-7000, don't use it!
There are at least two screwdriver antenna brands which use a slip clutch to protect the motor when the antenna reaches one end of its travel. Thus the difference between run and stall current, is less than what can be detected by the circuitry built into most automatic controllers. In at least one model, the turns-counting reed switch continues to count turns, even when the antenna is at one end or the other. This limits the choice of antenna controllers to manual ones, which are less than ideal.
The occurrence of shorted leads is rare, but it occurs often enough to be of concern. When you first unpack your antenna, carefully look over the leads where they exit the antenna base structure. Look for any breaks, cuts, or abrasions in the leads. Check for any continuity between each lead and the mast of the antenna; there shouldn't be any! If there is, call your supplier before installation begins.
Not all leads are as well protected with heat shrink or other insulating material. On many popular brands, the antenna structure where the leads exit is also very sharp. Note the nick in the white wire in the right photo (click for larger view). This nick was caused by the sharp edge at the bottom of the mast (it was pulled further from the base after the short was discovered). Further note, there is no protective sleeve around the leads coming out of the Ameritron SDA-100 shown. If you have the wherewithal, add a sleeve as a preventive measure.
After you install your antenna, you should repeat the continuity check. It pays to remember an important point. If any of the leads are shorted to the mast of the antenna, properly RF choked or not, transmitting will destroy what ever controller is attached.
As mentioned above, proper antenna impedance matching is a absolute prerequisite when using an SWR detecting controller. While this is covered well in my Antenna Coil Adjustment article, it bears repeating here. The use of UNUNs, switchable capacitance boxes, and autotransformers have their places, but if you're using an SWR detecting antenna controller, a fixed shunt coil is the only correct solution assuming you want true, automatic operation.
Shunt matching coils are easy to make, but require some setup time for best results. If you follow the instructions in the aforementioned article, and take your time adjusting the coil's spacing (inductance), you'll find a setting that will provide an adequately low SWR (<1.6:1) from 80 through 10meters. And do yourself a favor; buy an antenna analyzer. Nowadays, they're as important as SWR bridges were during the 50s, 60s, and 70s.
If you're using one of the shortened versions of the Tarheel (Lil Tarheel) or High Sierra Sidekick (now a silent key), SWR matching is seldom necessary due to the additional resistive losses these antennas exhibit.
How They Work, Switch Type
There are two basic scenarios used to automatically reposition the coil in all remotely tuned antennas. One, like the MFJ-1924 (Ameritron SDC-102B) shown at right, is to keep track of the number of turns the adjustment shaft makes. This is accomplished by using a magnet attached to the motor output shaft to open and close a reed switch (almost all late-model, remotely controlled antennas have one built in). During set up, the antenna is parked at one end or the other. Then the resonant points are found (you have to do this yourself), they are stored in multiple memory locations. As long as power remains applied to the controller, a simple button push will move the antenna to a specific preset point. Some controllers, utilize the radio's data port (CI-V on an Icom for example), and reset the antenna to the nearest preset.
There are a few drawbacks to them. If power is lost to the controller, all of the memory positions go away. Obviously, you have to go through the set up process once again, so keep track of the readout numbers for each band location in case future reprogramming becomes necessary. This means you have to power the devices separately, not through the radio's accessory socket. Also, in cold climates, it is possible to draw enough starting current to lower the battery voltage down to a level which will cause some units to loose their memories.
Another basic problem with the MFJ units, is their sequential programming. In other words, to reset the position (frequency) on just one band, you have to go through all of the bands. Another good reason to write down the readout numbers.
Making matters worse, the MFJ SDC-103 (a sister product to the aforementioned) share a manual. The manual clearly states that all of the band memory positions are preset to on, when in fact they are preset to off! Once you get past the omissions and pigeon English, they work so-so. Their biggest drawback are the cheap push buttons. They tend to double up, making programming even more difficult.
A lot of folks end up using a manual controller like the Ameritron SDC-100. Just like its automatic stable mates, proper RF bypassing is a must. Even then, the SDC-100 seems to miscount more than it should. This fact requires parking the antenna and resetting the counter.
The term parking refers to collapsing the coil of a screwdriver antenna into the mast (highest frequency position). Depending on the brand and model of the controller, and the radio is it connected to, some park the antenna when the controller is switched off. This is not an ideal situation, as it adds wear and tear to the motor assembly.
If you use an SDC-100, and it is operating erratically, it could be the stall current isn't set correctly. The procedure for doing so is not in the manual. However, you can down load the instructions here.
Speaking of parking. MFJ has redesigned their MFJ-1924 (Ameritron SDC-102B). It now automatically parks the antenna (assumedly when you turn off your radio). In their words; automatically bottoms your antenna for parking in your garage and resets and calibrates your counter each time to eliminate antenna slippage (?) and turns counter errors. This run-on sentence (statement) has a hidden factor. Aside from the wear and tear on the antenna's mechanical parts, it is an admission that counting errors occur. These may be caused by improper choking of the control leads, or common mode current, and even poor controller design. If you're still experiencing counter errors, the Home Brew Things article contains a simple circuit which minimizes counting errors. Incidentally, MFJ sells add-on reed switch kits for antennas which do not have them built in. About the only brand that didn't was High Sierra, and they're no longer being made.
How They Work, SWR Type
The other method is to (automatically) detect the SWR. Although different makes use different strategies, they all use the SWR information read from the radio's data port, or an SWR detector like the TurboTuner-2 does, shown right, or a remote SWR detector like the new TargetTuner shown below left. Depending on the make, a push of the radio's tuner button (or one on the controller), causes the radio to transmit at a reduced power setting. The controller then powers the antenna's tuning motor. When the preset SWR limit is reached, the controller stops the transmission and shuts off the motor.
In some cases, the antenna motor will be going in the wrong direction. Once it hits its travel limit (and the stall current preset is adjusted correctly), it reverses and goes back the other way. If the SWR limit is set incorrectly (especially on 80 meters), it will cycle through its opposite travel limit. Some units have a built in limit for the number of cycles, but all of them can cause excessive motor wear if not setup correctly. That is to say, the SWR detection level, motor stall and operating current, and motor direction settings must be correct for your installation. Emphasize your, because variations in manufacturing tolerances, and the length and wire size used make each installation unique.
It should be noted that some models designed to be used with Icom transceivers use the CI-V port. Some control and mail software programs which share the CI-V port, can cause compatibility issues if they're not set up correctly.
Lastly, some automatic controllers draw antenna motor current directly from the radio (mainly Icom models) which is not an ideal situation. If your controller of choice draws power from the transceiver, it should be modified to prevent possible damage to the transceiver.
The TargetTuner has both automatic and manual tune modes which is convenient. However, it does not interconnect to the transceiver in use. Therefore, using either mode requires the user to transmit a dead carrier (AM or RTTY mode) before tuning can commence. While there are ways to fool some late-model transceivers to transmit a carrier by pushing their Tune Buttons, most of the on-line circuitry to do so is questionable. There is no amplifier bypass either, so if you need one, here's the solution for that issue. Incidentally, its rumored that West Mountain is redesigning the TargetTuner (December 2014) for smoother operation.
There are so many different manual controllers on the market, it is difficult to lump them all together. Most aren't much more than a DPDT, center-off switch. Reading the SWR is left to the user.
The High Sierra i-Box isn't much more than a manual control. That said, some models of the i-Box interconnect with the transceivers tuner port (Icom IC-706 for example). When the antenna switch is operated, the radio transmits at a reduced power setting. In the case of the Icom IC-7000, you have to manually switch the radio to RTTY mode to get the requisite carrier. Here too, you need some way of reading the SWR.
If you want to make a manual controller, with an end of travel indication, look in the Home Brew Things article.
Most manual controllers which work perfectly with an Icom IC-706, only appear to work correctly with an Icom IC-7000. The problem is, manual controls made for the 706, usually pull the TKEY (pin 1 of the tuner port) line high to 13 volts. If you do this to a 7000, the cooling fan will not work correctly which may result in failure of the finals and/or the CPU! Forewarned, forearmed!
There are at least two antenna manufacturers who offer a stepper motor configuration rather than the typical DC gear motor. This allows precise repositioning of the resonant point, and speeds up the retuning (≈<20 seconds). Since they're proprietary controllers, their major drawback is cost. In one case, the controller sells for twice the cost of the antenna it controls! While the design has merit, spending $1,200+ dollars to gain a few seconds of retuning time is difficult to justify.
Built In Controllers
Codan® and their successors offer a base-loaded HF mobile antenna with a built in controller. They have several drawbacks, not the least of which is their maximum power rating. Especially if installed as suggested (low bumper mounting), there is excessive RF imposed on the control leads, and choking the leads requires multiple ferrite beads. Although designed for military use, the control mechanism is not robust. The fact their base loaded, places them at the bottom of the efficiency curve.
Odds & Ends
If you use a separate SWR bridge with your automatic controller, be advised their readings may or may not agree. This is a direct result of measuring the SWR at different points along the feed line, and is not an indication of a problem per se.
Fully automatic antenna controllers are the wave of the future. In view of the fact that many municipalities are enacting so-called cellphone laws limiting the use of on-board telemetrics, we need to employ every device we can find that will make our mobile operation less distracting, and thus safer.