Auto-Couplers
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Contents: Caveat; Basics; Single Ended Auto-Couplers; Important Considerations; Addendum; A Word About Internal Couplers; Odds & Ends;
Here is an important caveat about the IC-7000. The port on the rear apron of the IC-7000 designed to interface with the AH-4 auto coupler is configured differently than the IC-706 series. Pin 1 (TKEY) input is shared internally with the temperature control circuitry. This pin MUST be left floating. If it isn't, the fan may not come on as required, which can lead to failure of the final transistors!
Devices used to mimic the AH-4, and trick the IC-706 into transmitting 10 watts of carrier are not necessarily compatible with the IC-7000. This includes the suggested circuitry from SGC, at least one of the older model screwdriver controllers, and most small tuner modules. The only US made one known to work correctly is from Better RF, and designed specifically for the IC-7000. Their older IC-706 model is not compatible. There is also an Australian one designed by Owen Duffy, VK1OD, which is compatible. If you are using one of Bob Lewis' (AA4PB) Ham-Kit interfaces, remove R1 (10k) entirely, as it is not needed. For more information on these devices, read the Antenna Controllers article.
Automatic antenna couplers, erroneously called antenna tuners, seem to be all the rage these days. They have a lot of good features; near instant band changes, no fuss, no bother operation, and for the most part they're worry free. The most popular ones are the Icom AH-4, the various SGC models, the various LDG models (that's an LDG AT-7000 in the left photo), the Yaesu FT-40, the Alinco EDX-2, and the various MFJ models. There are at least five other after-market manufacturers and/or resellers advertising in the pages of QST.
Speaking of QST, page 71 of the May 2004 is a review of five different models from three manufacturers. The August 2006 issue, page 56, reviews three more including Palstar's AT-Auto shown at right. The AT-Auto is one of the few commercial units which uses a motor-driven roller inductor and capacitor, instead of the usual relay-switched, fixed-value components. It's a little big for mobile use, even though it does operate from a nominal 13.8VDC.
A little closer to home, there are two major differences which need to be mentioned. Some units are designed to match coax loads and thus have a limited SWR matching range, typically 10:1 or less. An example is the MFJ-998 (rated 12 to 1,600 ohms) shown in the left photo. Incidentally, the MFJ-998 is rated at 1,500 watts once it is tuned. To keep full power off the coupler while it's tuning, it incorporates an amplifier bypass circuit. They claim to be the only auto coupler that has this feature, but the SGC 235 has had one for many years.
It should be further noted that the auto-couplers built into modern transceivers are only good for mismatched loads up to a 3:1 SWR, thus they are only suitable for extending the bandwidth of a mobile antenna, not matching it to another band.
The other type is designed for longwire and other single-ended loads, and has a much greater SWR matching range. An example is the Icom AH-4 shown lower left, and the FC-40 Yaesu shown lower right. Either can match an 8 foot whip from 40 meters through 6 meters. Since my focus is on HF mobile operation, it is these wide range types I'll cover here.
Matching a 102" whip with any coupler, no matter who made it, is a tough nut to crack. That is why most units on the market will only perform the match from 40 meters through 10 (sometimes 6 meters as well). There are a couple of reasons for this.
A 102" whip looks like -j8,000 ohms on 160 meters and the requisite series L would be as large as the rest of the coupler! It's tough enough on 40 meters where the reactance is a few hundred ohms. There's another reason which is more significant. Depending on the frequency of operation and length of the radiator, the reactive and the resistive values, the RF voltage present at the output of the auto-coupler may exceed 10 kv! This leads to all sorts of high-voltage design problems. Some manufacturers address this by using several relays in series. At the other end of the spectrum antenna currents can exceed the relay's current capability, but this is rare and much easier to deal with.
When couplers are used in a mobile environment with electrically short antennas, this high level of RF voltage can and does cause problems. Not so much because someone might grab the antenna when you are transmitting, although this is a concern, but because the antenna components readily available are not designed to handle this much electromotive stress. I cover this in a little more detail later on.
SGC who made two of the units reviewed by QST, has always referred to these units as auto couplers because that is what they do. They don't tune the antenna per sé, they simply perform a conjugate match between the impedance of the antenna and the source (transmitter). In simple terms conjugate means to join together or couple, hence the name coupler. It accomplishes this by using a series of inductors and capacitors, and may also incorporate a transmission line transformer such a balun or UNUN. The left photo shows the inner works of an SGC coupler. There are 38 relays, 12 inductors, and about 100 capacitors, all controlled by a Motorola CPU and proprietary algorithm.
With two exceptions, auto-couplers are all LC or Pi configuration and are configured as low-pass networks (series L, shunt C). Relays are used to cascade switch the inductors and capacitors in and out of the circuit, and can place the C (shunt reactance) at the output or input (both in a Pi configuration) to match higher and lower than line Z impedances. If you don't understand how all of this gets accomplished, Chapter 17 of the ARRL Handbook is a good resource. If you want a more detailed explanation, Dr. Walt Maxwell's "Another Look at Reflections" (available to ARRL members on-line) is a must-read.
Again is simple terms, a coupler is a matching network which transforms the antenna resistive value to that of the transmission line, and at the same time cancels any reactance it may have. Due in part to the large reactances we're dealing with, auto-couplers used in a mobile installation must be mounted as close to the antenna as possible. That is to say, inches, not feet!
You cannot use coax for this connection either, as even a one foot piece of coax will reduce the efficiency by 30% or more. The reason? Coax has about 25 pf of capacitance per foot. The capacitance of a typical HF antenna ranges from 20 pf to about 45 pf depending on its length and frequency of operation. Since our auto-coupled antenna is essentially a base loaded vertical, placing 25 pf to ground will shunt a large portion of the RF to ground. This interaction should not be confused with using shunt reactances to match a low Z HF antennas to a 50 ohm feed. That is a different animal altogether.
The robustness of the RF ground is a major consideration. Classic examples of poor RF grounding are when the coupler cannot find a match, and/or is resetting itself during transmissions. One inch braid may work if the ground lead is short (less than six inches or so). In my installations I used 4 inch wide pieces of copper flashing material for the ground side lead to the chassis. It is important to remember that the ground side impedance must be much lower than the radiating element side. If not, the coupler will have a hard time deciding whether to match the radiating element or the body of the vehicle. This is especially important on the higher HF bands, and particularly on 6 meters.
The connections need to be very robust too. I used stainless steel fender washers on both sides of the strap with I/O star washers in between. Most importantly, the ground for the coupler cannot be coincident with any other ground! Failure to adhere to this will cause you serious RFI problems, and may indeed cause damage to your auto-coupler.
As I said above, high RF voltages are the major drawback to auto couplers. It requires the mounting hardware, especially the base insulators, to be able to withstand 10 to 15 kilovolts. Therefore, standard ballmounts and nylon washers are only good for 100 watts or so, and less if moisture is present.
In the right photo is the insulator off of an old GE Master ballmount. You can see the burned-out groove between the edge of the (removed) ball and one of the mounting bolts. By the way, coating the insulator with car wax exacerbates the problem. If it needs weatherproofing, use a good quality, high voltage lacquer, which you can buy it at most hardware stores for about $4 for a 12 ounce spray can. As an alternative, use Rain-X.
In the March 2004 issue of QST on page 33 is a fine article by Bob Lewis, AA4PB, a frequent contributor to eham.net. He describes how he conquered his high voltage problem. He also has a web site (Ham-Kits) where he sells the interface described in the article. Users of both an Icom IC-706 series (or Yaesu) and one of SGC's auto-couplers should look into this interface device. It allows the 
Icom to control the SGC coupler as if it were an Icom AH-4 including the PTT retune function built in to the transceiver; a great convenience for any mobile operator. Incidentally, Bob's site has a lot of good information on auto-couplers, and links to a site which sells the high-voltage insulator he describes in his article.
If you use a center load to increase efficiency as I have done in the past, it too must be able to withstand the high voltages present, and its self-resonant point must be above the highest band you intend to use. If you don't adhere to these requirements, efficiency will go down, not up, and you run the chance of arcing nearly every element of the antenna. As you can see in the left photo this is not an easy task.There is another potential problem readers should be aware of if you choose the auto-coupler route. Some auto-couplers have problems matching antennas which are inductively reactive (operating above resonance), and have a lumped constant (read that as a loading coil) in the middle. There are several reasons for this, not the least of which is the high RF voltage present, and the methodology used to detect the phase angles which varies with the manufacturer. The solution is to remove the loading coil and increase the over all length of the antenna. If you can't increase the length, the use of a cap hat is an alternative.
Since they're all well sealed with respect to moisture and dust (except for the aforementioned), this begs the questions; can an auto-coupler be mounted out in the weather like in the bed of a pickup? Can it be mounted under a vehicle? If so, what are the precautions? The short answer is yes, but the long answer is maybe if some environmental considerations are not followed. In some respects, it depends on the brand of coupler. For example, the LDG series of couplers are primarily designed to match coax loads, and are not suitable for outdoor mounting. Their discontinued RT11 (left photo) is an exception, and in fact is the model of choice even if it is trunk mounted.
To reiterate, coax type couplers like the RT11 are good choices to extend the bandwidth of a monoband antenna, but do not have the matching range of an AH4, SG237, or FC-40. Don't expect them to match an 8 foot whip on 40 meters, with or without their optional baluns or ununs.
There seems to be as much confusion about internal couplers as there are external ones. As I stated above; auto-couplers built into modern transceivers are only good for mismatched loads up to a 3:1 SWR, thus they are only suitable for extending the bandwidth of a mobile antenna, not matching it to another band. This brings up a major consideration; should they be used in conjunction with other matching techniques? The simple answer is, maybe.
The average mobile antenna varies between 14 and 35 ohms or so (3.5:1 SWR to about 1.5:1 SWR). If you use the correct matching method, no additional matching should be necessary. This said, far too many amateurs think a low SWR equates to high efficiency; nothing could be further from the truth!
As indicated in the aforementioned article, inductive matching has a bigger benefit than any other form. As examples, it provides a frequency independent match, and it DC grounds the antenna which is an aid to static control and safe operation. That is not to say, that built-in couplers can't be of benefit, they can be, but only if you're using a fixed antenna like a Hustler or Hamstick. In other words, they allow for a wider bandwidth than you would otherwise have (on the same band). Using one to operate on a different band other than the antenna is resonant on, produces a myriad of problems, and is almost guaranteed not to work, inefficient operation notwithstanding.
In short, inductive matching of a remotely tunable antenna is adequate for any installation, and will result in an SWR lower than 1.6:1 in every case. If it doesn't, you've done something wrong!
My Installations article has a few examples of what others have done with auto couplers (and other antennas) in a mobile environment, and some are quite clever.
Reiterating an important point; the combination of an auto coupler and an unloaded whip is the equivalent of a base loaded antenna. As such, their radiation resistance is approximately half that of an equivalent center loaded antenna. Thus ground losses must be minimized to maintain efficiency. This requires proper bonding and proper mounting. If you haven't done so already you need to read my article on Bonding.
Auto couplers can be of benefit, no matter the type. However, manual tuners in a mobile installation should be avoided, as adjusting them is a major nuisance, to say nothing of the distraction they cause.
If you wish to know what I use, and why I use it, read this.