Last Modified: April 30, 2013
Contents: Basics; My Remote Antenna Controller Won't work; Sluggish Movement; Stubby Antennas; Too Many Ground Straps; It Just Won't Match; Short Leads; Shorted Leads; Secure Connections; Loose Coax Connections; Big RFI Sources; End of Travel Issues; SWR Detect Issues; Odds & Ends;
Antenna problems can be always be solved, but may require a complete change in the architecture.
The most common antenna problem is the result of improper mounting. Lip mounts are the prime example! Designed to attach to trunk lids, hatches, or doors, every time they're opened or shut, stress is put on both the antenna and mount as shown in the photo at right. Sooner or later, the mount loosens. Adding insult, users often sand the paint and zinc undercoat to bare metal which promotes rust and accelerates the failure rate. The hidden factor here is the loss of an adequate shield (RF) return for the coax cable. High SWR, common mode issues, and intermittent receive are some of the indicators of a loose mount.
Another common factor is the failure to follow the instructions which came with the product. This is especially true of automatic antenna controllers, and their requisite chokes. If you use a remotely-tuned (screwdriver) antenna, pay attention to the next section.
All mounts which uses set screws to hold them on, it will fail in time (typically short). Ground straps are not a cure! Even the lightest of HF antennas will stress their mounts when attached to trunks, hatches, or doors. Abbreviated mounts always add to ground losses, which increases the likelihood of common mode currents, which in-turn increases RFI.
My Remote Antenna Controller Won't Work
The one single issue which causes remote antenna controllers not to function properly is RF getting into the logic circuitry. There are two main avenues; The motor leads (and reed switch leads if used), and the coax cable (common mode).
The motor (and the reed switch) leads of remotely controlled HF antennas operate above RF ground. Therefore, they must be choked! With two exceptions, the manufacturers' supplied motor lead chokes (usually in the form of bare ferrite beads) are wholly inadequate for the purpose.
The requisite choke must have an impedance at least two magnitudes higher than the antenna's impedance, and be primarily resistive at the frequency of operation. In other words, greater than 5 kΩ. If you're using a stubby antenna and/or a poor mounting technique (a lip mount for example), the choke may need to be two, perhaps three times as large (10 kΩ to 15 kΩ!).
These chokes must be as close to the antenna as possible. This negates mounting them inside the vehicle, or next to the controller. Remember! All of the wire before the choke is part of the antenna, and must be kept outside the vehicle! If you do not, RF can be induced into the vehicle's wiring, causing great grief and frustration. Again, this article explains how to properly wind the chokes. There are no substitutes, or work arounds for this choke. None! And no, 4 or 5 turns is not enough! And, proper winding will absolutely require removing any outer sheath or covering on the antenna control cabling.
There will always be some common mode current flowing on the outside of the coax cable in any HF mobile installation. The larger the ground losses are, the greater the common mode currents will be. Clip mounts, mag mounts, poor mounting positions, ineffective bonding, and inadequate coax shield returns, all add their part. Again, a ground strap to the nearest hard point is not a cure for common mode current! It must be choked off!
These issues are covered in more detail below, and in the linked-to articles therein. If you take nothing from this article, it should be this; poor mounting techniques are the single largest contributor to RFI issues, poor antenna efficiency, and continuing problems with RFI, both ingress and egress. Again, the use of short, stubby antennas greatly exacerbates this issue.
If your antenna came with a built in reed switch, and your controller doesn't require one, here's some sage advice. Do not DC ground the reed switch leads! Instead, either tuck them back inside the antenna mast, or solder on a wire lug, and connect them to the antenna's mast. And don't leave them dangling in the air!
Installing properly located chokes may require that the factory power connector be cut off, and replaced. If you're reluctant to do this up front during installation, remember to leave yourself enough cable to rework the installation later when you discover you should have cut the connector off in the first place. Further, factory cables are usually covered with an outer vinyl sheath which will need to be removed. Contrary to popular belief, modifying factory cables, and connectors is not the end of the world!
Some factory-supplied cables contain both a coax run, and a pair of #18 gauge wires for the motor within the same sheath. The coax is RG58B, which has a solid, copper plated, steel conductor. Solid conductor wire should never be used in a mobile installation. The typical outcome is, the soldered connections corrode, or the wire breaks due to vibration. This results in a open circuit between the transceiver, and the antenna, all too often resulting in blown finals. If this is what you're using, replace it before you have a problem!
With one exception, remotely-controlled HF mobile antennas incorporate a 1/4x20 threaded rod which is turned by a reversible DC motor. This means the motor's output shaft must turn 20 times to move the coil assembly (or shorting bar) 1 inch. The motor's nominal RPM at its rated voltage (not necessarily 12 vdc), the applied voltage, the ratio of the reduction gearing, and the overall length of the coil assembly, all effect the length of time it takes the antenna's coil to transverse its travel length. Even with a good, stiff 14 vdc (engine running), tuning from 80 to 10 meters may take as long as 60 seconds or a bit more. Models with 160 meter coverage might take as long as 2 minutes. It should also be noted, that cold weather can stiffen up the works, which relates to longer tuning times. This raises the question; how long it too long? With some fear of being evasive, that depends. To answer the question, let's look at a few scenarios which can cause extended transverse times.
No doubt, the worse thing you can do is lubricate the antenna's coil! Any compound, oil, grease, spray, you name it, applied to the coil assembly will attract road debris (mainly metallic brake dust), dirt, moisture, and other contaminants, which will at a minimum exacerbate the problem and/or cause arcing between coil turns, with predictable results! While lubrication might speed up the movement process at first, sooner or later, the problem will become a sticky, non operable, mess!
All automatic antenna controllers (they're really not tuners) have some form of current detection. Typically the voltage drop across a low-value resistor is measured. If the supply voltage is low and/or the resistance in the motor lead wiring is excessive, the motor will run somewhat slower than it would if supply voltage were applied. In other words, they are dependent on the supply voltage being stable. To put this into perspective, if the supply voltage to the controller is less than ≈12 vdc (with the engine running), you need to rethink your wiring scenario.
Some amateurs feel that isolating the power to their transceiver and/or amplifier is a way to eliminate all matter of RFI, ground loops, common mode current, poor mounting techniques, and other mobile operating problems. It isn't! One thing it does do, is reduce the voltage applied to the antenna's motor! Remember, the nominal resting voltage of an SLI battery is about 12.2, but may be slightly higher in some AGM battery types. If you place a 10 amp load on the battery (≈30 watt tuning carrier), the voltage might drop below 11.5 at the antenna (voltage drop in the wire, and current detecting resistance). This will cause most antennas to slow down at lot! As a result, the setup procedure, and tuning should always be done with the engine running.
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). Their sturdiness is suspect at best, and when combined with a lip mount, operational problems abound. Even if you properly bond across the hinges, the level of ground loss will still higher than it would be if the same antenna were mounted atop the quarter panel. The hidden issue here is common mode current, which must be properly choked.
The left photo shows a mix 31, 3/4 inch ID split bead with 7 turns of RG8X. Note that the coax is not tightly wound, but has a diameter of about 3 inches. Any tighter and the center conductor could migrate and cause a short. This choke has an impedance (mostly resistive) of ≈2,500 ohms at 10 MHz.
Like motor lead chokes, coax chokes must be mounted outside the vehicle—an almost impossible task when using a lip mount. If you don't mount them outside, you'll have both ingress and egress RFI problems. Remember, common mode current can flow either way. That is to say, out of the coax when you transmit, and into the coax when you're receiving. If this is the case, the majority of the incoming RFI you experience will be a result of common mode currents. Yet another good reason to properly mount antennas!
Just for the record, short, stubby antennas typically don't require matching, because coil losses (low Q) raise the input impedance close enough to 50 ohms to provide a fair match to the feed line (low SWR).
There is no benefit in using coax larger than RG8X, even if you run high power. This is true because the difference in loss between 10 feet of RG8, and 10 feet of RG8x (an average mobile installation length), is a fraction of a dB. And, there is another good reason too. In order to duplicate the aforementioned common mode choke when using RG213-sized coax (one turn beads), would require 49 separate beads; about $250 worth! And do not use any type of coax with a solid center conductor, as sooner or later the center conductor will fail due to vibration stresses.
Too Many Ground Straps
The usual call-for-help conversation starts out with these words; "I grounded the antenna." If that were true, it wouldn't work! Obviously, they're referring to the antenna's mount. The second part of their conversation includes; "I grounded the radio." The radio is already grounded through its power cable. These statements point out an unfortunate truth; Far too many amateurs confuse the need for a DC ground with the need for a ground plane. As a result, they often end up creating a ground loop, which they assume is an RFI problem. Here's how to avoid this scenario.
First, if the antenna is properly mounted, no further grounding, strapping, scraping to bare metal, or connections are necessary. To be sure, there needs to be a return for the coax shield. However, that's covered under the words, properly mounted. Bonding is also an important undertaking, but it shouldn't be a work-around for improper mounting. Running ground strap hither and yon to the chassis and/or body of the vehicle shouldn't be required, and may actually cause a ground loop to occur.
Here's a good rule of thumb with respect to grounding antenna mounts, and radio chassis'. If a ground strap fixed, reduced, or eliminated an RFI problem, then something else in the installation was amiss. Fix it, don't patch it!
The shield of the coax needs to be consistent with the start of the ground plane. If you read this section of the Antenna Mounting article, you'll have a better idea of why this is such an important attribute.
It Just Won't Match
Everything you need to know to match an HF mobile antenna is in the Antenna Matching, and Antenna Coil Adjustment articles. Since you have to know the exact resonant frequency of the antenna while you're in the process of adjusting a shunt matching coil (you cannot use an SWR bridge), you'll need an antenna analyzer or VNA (Vector Network Analyzer). The articles point out that it is much preferred to use a home brew shunt matching coil, rather than a commercial one like the MFJ-908 shown. Changing the shunt coil's value isn't necessary once it is properly adjusted. However, antennas covering 160 meters may require a switched inductor as mentioned in the articles.
The resonant frequency is when the reactive element (X), equals zero! It is not the lowest SWR. The only time the two will coincide is when the characteristic impedance of the antenna equals that of the SWR bridge, and the coax feeding it (≈50 Ω). Consider this; An unmatched HF mobile antenna, of reasonable quality, will have a measured input impedance of ≈25 ohms at resonance, and will therefore exhibit a 2:1 SWR. However, it is possible to move the transmit frequency and get a lower reading, but the antenna is no longer resonant! This points out the need for an impedance measuring device such as an antenna analyzer.
Assuming you've read the aforementioned articles, and you still can't find a compromise, here are the most common reasons. First, the antenna analyzer should be connected to the antenna through a short piece of coax (≈12 inches), not at the radio end. The shunt matching coil should be mounted as close to the antenna as possible. If you try to use an MFJ-908 for example, and it is mounted near the radio, the chances are very good that you won't find a good compromise, try as you might!
With just a few exceptions, the motor lead chokes supplied by every antenna manufacturer are inadequate! An inadequate RF choke will affect the input impedance. As I point out in the articles, the motor leads should be disconnected at the antenna when adjusting the shunt matching coil. Once you've matched it, and reconnect the motor leads, the input impedance should not change. If it does, even by one ohm, the choke is inadequate!
Scorpion®, and Tarheel® (perhaps others) supply a base matching coil with their antennas. If you mount these antennas decently, and you use an automatic antenna controller, the chances are you'll have to adjust the supplied coil, usually by elongating it. Because of the mounting location of some factory coils, this is not always possible. The solution is to wind a replacement one as described in the Antenna Matching article which is much easier to adjust. At a minimum, the factory coil should be relocated away from the mounting bracket.
Lastly, antenna manufacturers often tell customers, to cut their coax feed lines to a specific length in order to achieve a good match. All this does is mask the problem by moving the SWR node to another part of the feed line. While this might work in some cases, it won't fool most of the automatic controllers on the market. The question remains, how long should the coax feed line be? The answer is, long enough to stretch from the antenna, to the radio in question, with enough left over to wind a 6 or 7 turn common mode choke. This means most preassembled coax cables will need to be shortened. If that is the case, read this coax article.
Unfortunately, antenna manufacturers don't provide enough lead length, or they have pre-attached connectors. As a result, you have to splice the leads in order to attached the RF choke at the base of the antenna where they must be located. Most owners are reluctant to do so for warranty issues. In some cases, you can remove the antenna's base plug and motor assembly, and replace the factory wiring. However, at least four manufacturers pop-rivet their motor/reed switch assemblies to the inside of the mast making this difficult if not impossible.
Here's what I suggest. Carefully cut the existing wires close to the base of the antenna leaving about 2 inches. Attach a male Molex connector (2 or 4 pin as required) as shown in the photo, or use Anderson Power Pole connectors. When you wind your choke(s), use mating connectors to extend the leads. This makes antenna removal easier too.
Resistor color codes are a very good example of industry standards. Standards extend to DC motors too. When the polarity is observed (positive on + and negative on -), the motor rotation will be clockwise with respect to the shaft end. For small DC motors, the positive is always red, and the negative always black. However, not all motors come with leads, or have a specific color which may vary between manufacturers. At least 3 use what ever wire they have on hand, thus two otherwise identical antennas may be wired with different colors. So, before you mount your antenna, it is best to check it out on the bench to make sure it gets wired correctly. Just be very careful not to apply DC power to the reed switch!
I have been openly (and often) criticized for recommending replacing factory connections with Molex® or Power Pole® connectors. The usually basis is that the connectors are exposed to the weather. That's a fact, but they do dry out. Those rubber ones aren't as waterproof as you might think. Once the moisture seeps in, it stays! This eventually causes the connections to rust or corrode with obvious results. In the 40+ years I've operated mobile, I have never had a Molex® or Power Pole® connection fail. However, I know a sure-fire way to make it happen; just cover it with electrical tape! If you just have to cover the connection, use Rescue Tape (see below). If you use Power Pole® connectors, don't use black and red ones (should be obvious why), rather select two of the other 8 colors they come in.
The easiest way to check for shorts (or opens), whether they be internal, or external, is simply to measure the resistance at the controller end of the antenna motor feed line with a DVM or VOM. There should be no connection between either lead, and chassis ground. The resistance across the leads will typically be between 4Ω, and 15Ω, depending on the make, and model. Obviously, if they are open or shorted, you have a wiring problem which needs to be traced.
While the occurrence of an internal shorted antenna lead is rare, 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.
Referring to the photo of the SDA-100 above-right; at the very bottom of the photo are the connections for the coax feed. The supplied shunt coil (not shown) connects across these connections. The hole for the bottom connection (at right) is drilled and threaded clear through the base insulator. Supposedly, this ground connection provides a secure return for the shield of the coax. It doesn't. Since it is very important to provide a secure connection between the coax shield and the ground plane, a short, braided jumper should be installed between the screw and the mounting mast (plate).
The upper screw (barely shown in the photo) is also drilled and threaded clear through the base insulator. If you bottom this screw, you short out the coax via the mounting mast with obvious results. Although the owners manual mentions this, of the four examples I've seen, three had this problem. As I mentioned at the start of this article, it pays to read the instructions before installation begins!
These aforementioned problems are not unique to the SDA-100, as several other popular models are nearly identical, or at least have identical problems. That is to say, the connections between the antenna, the shunt matching coil, and the coax feeding them, can become loose. This can cause all sorts of problems, both minor and major.
Another problem area is the connection where the whip screws in. Even when short whips are used, this connection is the one that receives the most stress. If you're experiencing some intermittent receive, this is the first place to look. If you use a quick disconnect for the whip, you should use one that is not spring loaded. One of the best ones come from Breedlove Machine Shop. They also make a combination fold over/QD that is quite clever in design.
A few additional comments about the SDA-100. For the money, it is a fair performer. Except for the pop rivets holding in motor assembly, it appears well constructed. It does have a couple of drawbacks. The antenna's base plug accepts a one inch pipe thread, which is a better mounting method than the U-bracket some similar antennas use. However, this fact leads some folks to mount the antenna atop a long pipe stub which adds a lot of ground loss, and makes matching difficult. Lastly, as pointed out above, the methodology used to secure the matching coil isn't all that great. If you experience matching and tuning problems, this is the first place to look!
Loose Coax Connections
This article explains how to properly solder PL259s. It also points out the need to use decent quality connectors, and there is good reason to do so. Remember! Poor solderability equates to loose connections!
Belden's® RG8X (part number 9258) is decent coax, and it weathers well. However, some brands aren't worth the effort. If you run high power, and suddenly have an RFI or intermittent SWR problem, the first place to look is at the coax. Click on the photo at left, and look at the small cracks is the foam dielectric. Once the cracks were discovered, the burn spot, right photo, was found by pulling the coax through a soft cloth looking for a bump. Incidentally, the coax in question was in service less than 6 months. The burn occurred in an area protected from the elements, and was not subjected to stress or sunlight.
If you're compelled to weatherproof exposed coax connections, then Rescue Tape is the solution. Rescue Tape doesn't hold up to abrasion well, but it sure does seal out moisture. Ace Hardware stores carry Rescue Tape. It also comes in black and clear, besides the colors shown. It is truly amazing material. It is dielectric strength exceeds 8 kV per layer, and it is absolutely waterproof. When you remove it, there is no residue, period! This is not true of some of the copycats, so it pays to buy the real McCoy!
There are other problems relating to coax. I shudder every time I see an installation where the coax is routed through a door or trunk lid seal. Trust me, this is a failure point waiting to happen, especially if the coax is RG174 like that supplied with most trunk lip mounts. And speaking of RG174. The center conductor is #26, and it will barely handle 100 watts at a 1:1 SWR. Dump 200 watts from a Kenwood TS-480Hx, and poof! What's more, at 15 MHz (20 meters) the 6.5 foot length supplied with the K400 mount has almost .5 dB of loss at an SWR of 1:1. At 2 meters it is over 1.2 dB, and on 70 cm, it is almost 6 dB! Or about 3 times greater than RG8X.
The antenna end of the coax feed isn't always a PL259. If yours requires spade lugs, remember this. Crimping alone will not adequately ensure a lasting connection, so soldering is also a necessity. Use either adhesive heat shrink or Rescue Tape to weatherproof the connections. The use of star washers under screws and bolts is also recommended.
I should mention that Scorpion antennas now use weatherproof N connectors, which aren't readily available except on-line. The RF Connection is a good source for both solder and crimp on N connectors for RG8X. If you use the crimped ones, bite the bullet, and buy the crimp tool. Installed properly, they're absolutely waterproof!
Some brands of antennas use a tapered plug at the bottom of the mount which mates with a socket at the base of the antenna. While secure most of the time, the parts are made of brass which corrodes in some climates. NoOx® and other anti-corrosion products will help prevent this.
Several antenna manufacturers supply coax with a solid wire center conductor (typically, copper plated steel), rather than a stranded (copper) one. As a result, the center conductor is hard to solder, and breaks easily often resulting in fried finals.
Big RFI Sources
Every single HF mobile operator has at least one RFI issue, even if they don't realize it. It may be related to on-board electronics, ground loops, or their mounting methodology. What follows are a few causes (certainly not all) most operators don't think about.
Absolutely, the biggest single factor is mounting methodology. I'm not naive, and I know I'll never convince everyone to drill holes in their vehicles, leased or otherwise. Nor will I be able to convince anyone not to use their spouse as an excuse for using a mag mount or other poor mounting choice. This said, be advised: Low mounting (trailer hitch mounts are a very good example), mag mounts, trunk lip mounts, clamps, etc., all reduce efficiency, and increase RFI issues. Ground straps are not a cure, and may actually increase RFI if incorrectly applied.
I mentioned this above, and I'll mention it again. Coax chokes must be mounted outside the vehicle. If you don't, you'll have both ingress and egress RFI problems. To repeat; common mode current can flow either way. This is to say, out of the coax when you transmit, and into the coax when you're receiving. If this is the case, the majority of the RFI you experience will be as a result of incoming common mode reception.
Incorrect wiring practices are the second biggest cause. Aside from the direct RFI issues, the resulting ground loops are the hardest of maladies to find and cure. Do yourself a favor, and take your time to do your installation correctly. Not just to prevent RFI, but from a safety standpoint too!
I already mentioned stubby antennas, but because of their popularity they've become the third biggest cause of RFI issues. They're not cheaper than their bigger brothers, and their efficiency level is about one fourth as good, at best! These facts alone, should be food for thought. Speaking of which; the most popular HF mobile antenna model? The ATAS series from Yaesu. It just so happens, it is also the lossiest, remote controlled antenna money can buy, and it isn't a cheap purchase either!
In fourth place is the lack of understanding of what is, and what isn't, ground and or ground plane! In about a dozen places on this web site, I remind readers that it is the mass under the antenna that counts, not what's along side. Unfortunately, a lot of amateurs believe a ground strap is a replacement, or a substitute, for an adequate ground plane. It isn't! And just because you DC ground something, doesn't mean it is RF grounded too, or visa versa. Lastly, keep this thought in mind; the higher the ground losses are, the worse the RFI problems will be, no matter how many ground straps you use.
There is one ground strap which isn't needed, but will nonetheless act as a telltale indicator. To wit; If you ground the main chassis of your transceiver, and it cures or lessens and RFI issue, then something else in your installation is amiss. The three biggies, in order, are; RF flowing on the antenna's control leads (inadequate motor lead choke), common mode currents flowing on the coax cable, and/or a poor coaxial connection.
If you are using an Icom IC-706 or IC-7000, along with the remote kit, there is one very important point to be made. The 2 mm x 6 mm, phillips head screw which secures the separation cable's main body connection also grounds the shield of the cable. Leave the screw out, and you're going to have RFI issues, some of which you might not even know about. The data is in their manuals on page 10 and 16, respectively.
End of Travel Issues
Some manufacturers incorporate a Polyswitch® or Polyfuse® to limit the maximum current which can be drawn by the motor. This protects the motor from burnout. When the antenna is moved to one end or the other, the typical run current (250 to 350 mils but may be higher). At stall the current more than doubles, and maybe as high as 2 amps (2,000 mils)! In at least one case, stall current exceeds 8 amps! To the uninitiated, this fact can cause a variety of hard-to-solve (and fix) problems.
Automatic antenna controllers often have a park feature, or require the antenna to be parked during the setup procedure. Depending on the make and model, the motor current required to move the antenna to its end of travel may be more than its running current, but less than stall current. As a result, once you move to one end, you can't get it to move in the opposite direction. If your antenna incorporates a Polyswitch®, and this happens to you, just wait a minute for the Polyswitch® to cool, and try again. In rare cases it may become necessary to use a bench supply to unstick the antenna.
Another end of travel problem is hidden in the first paragraph, and you should take heed with the following. Literally dozens of manual controllers, and several of the automatic ones, draw power from the radio's accessory socket. With one known exception, the maximum total current draw from these ports is 1 amp! What's more, the voltage drop through the radio may exacerbate the end of travel problem, but that's not the worst that can happen.
Most miniaturized radios have an internal fuse. The Icom IC-706 has a 4 amp fuse, and the IC-7000 has a 5 amp one. The 3.5 amp ones in the various Yaesu models are surface mounted fuses, rather than plug in ones like Icom uses. In every case, a circuit trace feeds the accessory port(s), and the trace usually fails before the fuse opens! The best way to protect your investment, is to use a controller that uses a separate circuit to feed your antenna's motor, or rewire your unit so it does.
Internally, antenna controllers usually use a sub-ohm resistor in series with the motor leads. The voltage drop across this resistor is used to detect the motor run and/or the stall current. At least one commercial controller uses a jumper to adjust the resistance value, rather than change a CPU parameter. As a result, controllers designed this way don't work well with with antennas drawing less than 600 to 700 mils at stall, as they further exacerbate the end of travel stick problem.
SWR Detect Issues
Here's a really esoteric problem you will not see often, but one you need to be aware of. Let's assume you're operating on 75 meters, and decide to QSY to 17 meters. At some position of the coil below 20 meter resonance, there will be a dip in the SWR (a harmonic point) which will tell your controller you've reached 17 meters. Most of the time, the SWR will be about the same as the true resonant point. Then let's say you QSY down to 20 meters. If you're using a smart controller, it will correctly sense the correct direction, but in this case it is toward 40 meters, not 20 meters. This is not a fault of the controller, but simply a fact of life. The best way to avoid it is to QSY one band at a time. By the way, this only happens when moving to a higher frequency, not to a lower one, and is more prevalent in low-Q antennas with large end caps.
The input impedance of any HF mobile antenna is partially dependent (largely in some cases) on the ground losses which can vary from location to location. If you set the SWR threshold too low at one location (like your steel re-enforced driveway), your controller might not detect a low enough dip, and will continue to run past the lowest SWR point. This can occur even though the controller might incorporate automatic SWR threshold settings. The solution is to set the SWR threshold no lower than 1.5:1, and occasionally slightly higher. Again, this assumes the antenna is correctly matched!
Automatic antenna controllers are the wave of the future, and for all practical purposes, they eliminate the need for any additional metering. Yet a large portion of the amateur community just can't help themselves, and just have to use an external meter. If you do, don't be surprised if the readings differ, especially if your antenna isn't well matched. The reason is simply this. You're measuring the SWR (or power) at different spots along the length of the feed line.
Odds & Ends
If you're into V/UHF operation, then read the VHF Options article.