In Search of 'The Perfect Mobile Antenna':

Because of the length and breadth of this article, I have chosen to break it into four parts. The first part covers the subject of resonator coils if you please and their size versus their Q, and their practical applications in mobile antennas. The second section deals with selecting an antenna to meet your needs, and the third addresses the requisite mounting of HF mobile antennas. The last is a few odds and ends, which might make your life easier.

As with my previous articles, I have not included any complex mathematics in an effort to address the newcomers. Those who wish a more complex explanation have many choices in the virtual plethora of ARRL publications available at low cost to any reader.

Before I start, a few words about abbreviations. In my title I used HF rather than High Frequency as most amateurs know what HF is. From here on, if I use an uncommon abbreviation I'll use parentheses to explain, such as HF (high frequency).

The most common vernacular for an HF mobile antenna resonator is coil. In some cases resonator and inductor are better choices, and for the most part they're interchangeable. While it is in the form of a spring its prime purpose it to cancel out the capacitive reactance inherent in antennas shorter than 1/4 wave length (less than 90 electrical degrees). So in reality it is an inductor with the same reactance, but of opposite sign (+j) with respect to the capacitance (-j).

While not practical, it is possible to have a full quarter wave mobile antenna even on 20 meters. In these cases there isn't any resonator. So from a practical standpoint, we're taking about HF antennas, which are short enough (less than 90 electrical degrees) to need a resonator, and those that don't still have some similar properties, but we're essentially ignoring them in the section.

Like springs, coils come is all sizes and shapes, and if you can think of a shape I suspect it's been made at some time or another. The most common HF antenna coil form is a lumped reactance. -- That is to say all in one small mass. This is in difference to a distributed reactance such as a helically wound coil i.e.: Hamstick®. Both assumed to have their specific attributes favoring them over their counterparts.

Of late, there seems to be a push for the bigger is better category of mobile antennas where the resonator takes on the proportions of a 5 gallon bucket or a spare tire! But are they really that much better? Are they practical from any standpoint, especially mounting? Is their advertised Q (Quality Factor) as good as the hype says they are? Is one form factor better than another? Are screwdriver style antennas really better than the fixed kind? Does short-tapping resonators reduce their Q? Are automatic antennas (tuner and whip setups) all that good? Could Joe Lewis really beat Mohammed Ali? The answers are obvious; it depends upon whom you ask. But, by the end of this section perhaps you will be better armed to ask the right questions and make the right choice for your dream HF mobile station.

All coils have a self-resonance point where the capacitance between the individual coils and the lumped inductive reactance self-resonant. The closer you get to the self-resonant frequency the higher the loss. The factors involved include the length of the wire comprising the inductor, the size of the wire, the type of wire and any plating or coating it may have, the form factor (length versus diameter), the insulating medium, and in the case of the HF mobile antenna, the presents of stray capacitance. The latter takes the form of end caps, mast and whip size, mast and whip length, body proximity, and ground proximity.

Oddly enough, these are the same factors, which control the Q. The Q of an inductor is the ratio of the reactance versus the AC resistance at the frequency of operation, not the DC resistance. While it is possible to measure Q with laboratory instrumentation, in the real world when it is mounted as part of the antenna, it is much more difficult. The point I'm trying to make here is this; while static Q (laboratory measurement) is important, it isn't the whole story.

Let's look at a popular large coil of the bug catcher style. It is 6 in diameter, 8 inches in length, and wound 6 turns per inch with 12 AWG silver plated wire. It's advertised as having a Q well over 1,000 at 4 MHz. While this high Q is commonplace for capacitors, it's nearly impossible to reach this high of a Q in an inductor of any form factor. The practical bench limit is closer to 800, and somewhat lower mounted as part of the antenna. While this coil is advertised to operate 40 through 10 meters, its use on the upper bands requires that the coil be short tapped. In other words, parts of its windings are shorted out. This does lower its Q further, and I'll cover this shortly.

This appears to be an ideal resonator; high power capability, rugged, bold look, etc. But is it? It does have a drawback not readily apparent. Its self-resonance frequency is right in the middle of the 20-meter band! While I agree you wouldn't be using this coil on 10 meters, and maybe not on 12 meters either, the fact that is self-resonant in the middle of the 20-meter band exacerbates the losses at higher frequencies of operation. If your favorite band is 17 meters or 15 meters, this is not the coil for you. Further, it is not only large it is heavy. It requires heavy-duty mounts and springs, and does not lend itself to body mounting, although it can be.

If Q is the only criteria and the required inductance is lower than 25 uh, a coil with a diameter to length ratio of two is the ideal. Coils with this form factor will have a static Q in the range of 400 to 600, but 800 is possible with proper construction. If we need more inductance than this, the coil diameter gets really large as does the distributed capacitance, and the self-resonant point goes down. In short, these large coils are best used on the lower bands.

For inductance in the 25 to 75 uh range, the ideal diameter to length ratio is closer to one. Properly designed, coils with this form factor will have a static Q in the 300 to 400 range, but 500 is possible with good construction techniques.

For large inductance over 75 uh, a form factor of one would still be ideal, but again physical coil size starts to be a problem, so typically you see the form factor of these coils closer to one half (.5, or twice as long as they are in diameter). Average static Q is in the 200 to 300 range, and top out at about 400.

Form factors lower than .5 become very lossy in terms of Q. If your goal is efficiency, cigar-sized coils are out. And the smaller pencil thin ones are no more than wire wound resistors.

Please keep in mind, these are static Q factors, and once mounted as part of the antenna, the Q factor goes down. The addition of a weather shield, metal end caps, mounting ferrules, and the installation of the mast and whip will lower the Q. These reductions may seem insignificant, but they are not as we shall see later on.

It is a given that short tapping (shorting out unneeded turns) lowers Q, but by how much is the meat of the question, and the answer isn't easy. Let me add short tapping is a way of life if you use the same coil for more than one band. If you think you can leave one end of the coil open (not connected to either mast or whip) in an effort to maximize Q, think again! Doing so will cause the unconnected end to arc and possibly destroying the coil in the process. It's called Flywheel Effect in case you're interested.

The factors described in the paragraph before last all effect the change in Q due to short tapping. As a rule, however, short tapping has little affect on coil Q until approximately one half of the coil is shorted out. With three quarters of the coil shorted out, the Q will drop by 25%, and continue to decrease until the coil is shorted out completely. I should point out the lower the form factor the more short tapping lowers Q, but the differences between 2 and .5 are small.

Screwdriver designs claim to circumvent this reduction by storing the unused part of the coil inside and at the same potential as the mast portion of the antenna. While it appears on the surface to work well, the added size of the mast greatly increases the stray capacitance with respect to the coil, to ground, and to the body of the vehicle negating most of the advantages. This is particularly true of the behemoth 6 diameter ones. Screwdriver designs also present wind loading and weight problems, and are difficult to body mount.

One of the more popular lightweight antennas uses a series of small coils distributed along its length. The form factor is maybe .02. This helical winding scheme has a Q in the order of 20 to 40. Short tapping such an antenna lowers its already miniscule Q factor and you end up with little more than a dummy load on a stick. With an efficiency of less than 2% average, one wonders why so many people think it is the cat's meow. -- It just maybe the ease with which they're mounted and matched.

Of late the SGC series of autotuners and Icom's AH-4 entry have in some eyes revolutionized HF mobile operation. They're many times faster tuning than any screwdriver antenna especially when changing bands. They don't require any meters or other external devices to set resonance. They seem ideal, but they do have drawbacks. For example, to keep the stray capacitance losses low, the tuner must be mounted directly at the base of the antenna (this is after all a form of base loading) and this is not always mechanically possible. The radiation resistance (Rr) of a base loaded antenna is approximately one half that of a center loaded antenna. Some will argue the higher the Rr, the better the field strength all else being equal. Without getting into a really long discussion about this, the differences between this antenna solution and most of the others from band to band are typically less than 3 db (an S unit is 6 db), sometimes favoring one, sometimes the other.

As a general rule, the larger the antenna the better it performs. It would be nice if we could drive around with a full quarter wave 75-meter antenna; alas we cannot. Full quarter waves are not very practical even on 20 meters. On 15, 12 and 10, they're almost commonplace nowadays. But just how much better do they perform? Good question, but you have to lay a little groundwork first.

I'm as guilty as the next guy in quoting efficiency percentages and they don't mean much if we don't know what we're comparing our antenna against. Certainly HF mobile antennas better than a dummy load, but not as good as a beam at 60 feet. So let's set 100% as a lossless quarter wave. This places the average, good-quality, 20 meter HF mobile antenna at about 50% at best, and on the lower HF bands maybe as little as 1%. Remember, we always have some ground loss, some stray capacitance loss, and some coil loss. So it behooves us to minimize the losses (except Rr). Incidentally, a full quarter wave HF mobile antenna only achieves about 80% efficiency primarily due to ground losses.

While I'm on the subject of radiation resistance, I want to cover a few important points. The position of the coil with respect to mast and whip length ratio is a hotly debated subject. Placing the coil higher in the antenna does increase the Rr, but only in that part of the antenna below the coil, not the whole antenna, as some folks erroneously believe. Raising the coil higher also increases coil loss as more inductance is needed; nearly twice as much for center loading versus base loading. The notion that increasing the Rr automatically increases field strength ignores the other losses induced in the process. Further, the higher the coil is, the more likely it could be damaged hitting a tree limb or other obstruction. I'd rather straighten the whip than replace the coil.

I have written several previous articles extolling the virtues of properly mounting antennas to minimize ground losses and stray capacitance losses. And I still stand behind those recommendations. But I'm never going to convince the casual mobile operator to drill a hole in his precious lease vehicle's left quarter panel. So each and every mobileer is going to have to decide which is his/her perfect HF mobile antenna and how they're going to mount it, regardless of efficiency.

Just for the fun of it, let's select a few basic types and look at the pluses and minuses. (Excuse me if a little prejudice sneaks in at this point.) The types are: Single frequency, lumped constant (i.e.: Hustler®), Single frequency distributed constant (i.e.: Hamstick® and Outbacker®), the various screwdriver antennas (too numerous to mention them all, and clearly some better than others), and the autotuner/whip regardless of which brand.

In the first example, Hustler® rules the roost. They're inexpensive (some would say cheap), they fold over easily, they're light weight, and they're reasonably efficient considering their length and coil Q. You're always better off with the lower power coils as the high power ones have large metal end caps which reduces coil Q drastically to speak nothing of wind resistance. Because they are lightweight, they may be mounted using a variety of methods. Since the coil is mounted high, stray capacitance is lower than it is with most screwdriver types when bumper or frame mounted. They match well too; because the coil losses are high enough additional matching usually isn't required. With the addition of spider brackets, several resonators can be mounting on one mast making band changes easy. They are so popular they're almost ubiquitous.

Hamstick® and Outbacker® antennas are also popular because of their lightweight, short length, and in the case of the Hamstick®, low cost (the Outbacker® is as expensive as a good screwdriver antenna). They are also the lossiest of the bunch. This is perhaps an advantage because no matter where or how you mount them, the VSWR is always low.

At a recent Hamfest, I saw one of those behemoth 6 diameter screwdriver antennas for sale used with a $400 price tag. The seller told me he paid over $800 for it new. Just the mast/coil combination weighted close to 15 pounds. I ask him how he had it mounted, and his reply was profane. Other screwdriver antennas are of course lighter, but they're still much heavier than most others. This, and they require a power lead to the base as well as the coax feed. This all but negates their mounting via a heavy-duty ballmount. So in these cases, we have not only the weight to consider, we have to be extra observant of stray capacitance to the body of any vehicle we mount them on. As I hinted previously, large bottom masts add greatly to both ground losses and stray capacitance, and any mounting scheme needs careful consideration for these loss-inducing factors. To a lesser degree, the position of the coil (the ratio between mast and whip length) in the antenna is also a consideration. In other words, some are near the bottom, some are near the top, and both have their advantages and disadvantages.

I personally use an Icom AH-4 coupled to a 13 foot, 20-meter resonant antenna to combine the best of both worlds. It perhaps is a little better on 40 than an autotuner and unloaded whip combination. It is equal to any screwdriver I've compared it to, and without a doubt better than any spirally wound one. Others who have tried this combination also agree. One amateur I have known for many years uses an AH-4 and a stainless steel whip with a large tophat in the center. He reports better overall signals than when using his monoband units. I not too surprised, because the lack of a loading coil has allowed him to mount the whip portion on top of his vehicle -- the optimal location for minimizing ground and stray capacitance losses. Don't try this with a screwdriver antenna! The most important consideration is keeping the lead between the tuner and antenna as short as possible.

Then there is the home brew crew. Once upon a time all amateurs were home brewers because you couldn't buy any gear commercially. Nowadays, it's almost become a lost art. -- That notwithstanding, a good quality, efficient, HF mobile antenna can be made from readily available parts. I know at least two amateurs who have made their bottom masts from rigid 1/2 copper water pipe. A 4-foot chunk of pipe, a couple of end caps and a 3/8 in bolt or two, an hour or so of labor, and you're done. The coil can be hand wound, or perhaps a commercial B&W unit, or maybe something from your junk box. You can use PVC as a coil form, but an air wound unit is better. The center insulator can be PVC pipe, phenolic, or even varnished wood. You can use a variety of material for the whip including welding rod, an old stainless steel CB antenna, hobby shop purchased bronze rod, or just about any metallic springy material.

This is a good time to bring up the 3/8? X 24-bolt size. I don't know who decided to make this bolt size ubiquitous, but we're more or less stuck with it. The problem is most 3/8 bolts and studs do not have enough strength to hold a big, heavy antenna. Even the stainless steel ones are suspect. When I set out to make my own, I discovered most hardware store bolts are not threaded their whole length. This is particularly true of the extra strength Grade 5 and higher bolts. I found what I wanted at my local Caterpillar tractor dealer (Fastenal also carries them); a Grade 9 bolt, 3/8? X 24 x 4. At $2 each they're not cheap, but I was able to make three extra long studs by cutting the bolt and dressing the threads with my Dremel Tool. I bet these babies won't snap off!

So every amateur has options depending upon his/her tastes, cost factors, efficiency, and mounting possibilities. You just have to decide what factor is most important to you. One thing every reader needs to keep in mind is just how serious you are about mobile operation. Here's something you can try at home.

Take your dummy load (every shack needs one of these and if you don't have one, shame on you!) and put it out in the back yard. Attach the coax to it using a T connector. At the tee, attach an 8 or 10-foot wire and string it more or less vertically. Now turn on your rig (low power now, no cheating with the amp on), and see how many contacts you can make on your favorite band(s). You just might be surprised at just how many you can make. This then, is the essence of HF mobile antenna operation. Decide on how much efficiency you need, what type of mounting you're willing to use, how much you're willing to spend, how much convenience you want, and make your choice accordingly.

Now that you have made your selection, it's time to think about your mounting method and there are a few basic rules, which need to be followed. I know I have harped about stray capacitance and for good reason. It is the single biggest factor effecting HF mobile antenna efficiency. I covered this well in one of my previous articles, but part of that article needs to be repeated here.

The average short, 80 through 15 meter HF mobile antenna (8 to 11 feet in length) exhibits a capacitive reactance of between 20 and 45 pf, some amount of radiation resistance, and other losses. Some form of coil cancels the reactive component and resonates the antenna on the frequency of choice. Below is a schematic of the average mobile antenna.

The nomenclature is as follows: -Xa is the capacitive reactance of the antenna; +Xl the inductive reactance which cancels -Xa and brings the antenna into resonance; Rl which is the resistive losses in the coil; Rg which is the ground losses; Rr the radiation loss; and Cs which is the capacitive shunt losses. It is this latter loss we're most concerned with here because it bypasses the radiation loss component of the antenna as can be easily seen by looking at the schematic.

An 8-foot mobile antenna at 7.2 MHz has a capacitance of 28 pf. If by poor mounting location and position we were to introduce 10 pf of stray (shunt) capacitance, our antenna efficiency would be cut by 35%. This is not rocket science; it is Ohm's law! (Yes you can tune out the stray capacitive reactance by increasing the inductance of the coil, but this does not negate the loss and in fact adds to it.)

Thus it behooves us to keep the various antenna parts as far away from the body of the vehicle as possible, especially the coil. This is nearly impossible when mounting a large screwdriver antenna on a van or SUV. In these cases, it is always better (from an efficiency standpoint) to mount the antenna at the front of the vehicle.

Once you have selected the best mounting place, the mounting method is your next consideration. I'm personally an advocate of permanent mounting, and I don't mind drilling holes. I use an old GE Master ballmount, which is unfortunately no longer made, and I hoard the two I own. (If you know who originally made these mounts, I'd like to know.) These mounts are sturdy enough to hold a 15 pound antenna if mounted correctly. Although it would look a little funny mounted under one of those 6 behemoth screwdrivers, I think it would hold it. If you do decide on a ballmount, make absolutely sure there is room on the backside to attach the hardware. Pick a flat spot where there is a nearby crease in the sheet metal, and attach the mount near the crease. This will help keep flexing to a minimum. Of course, if your vehicle has a plastic or aluminum body you're almost out of luck using any sort of body or ballmount.

I also use a spring, but for just one reason; in case some nut decides to yank on the antenna just because it's there. The ones I use were once made by HyGain and unfortunately MFJ, the new owners of HyGain, have not seen fit to remanufacture them. Most CB type springs are not adequate for any antenna bigger than a standard CB whip. There are critics who say springs are a source of problems, and they can be if the internal braid opens or becomes intermittent. I've never had this happen to me, but I don't dispute that it does. However, mounting the antenna too close to the body causes the usual problems with springs. As the antenna sways, the stray capacitance changes, as does the resonant point. To use or not to use a spring is up to the user.

If you're not going to drill holes, then your choices become strained unless you've selected a lightweight antenna like one of those helically wound jobs. Then even a lightweight license plate mount or truck-lip mount will do. Adding 3 or 4 or more db of loss isn't going to make much difference to an already lossy antenna.

If you're mounting a heavier antenna like a screwdriver type, you'll need a more substantial mount. Several companies make trailer hitch mounts, but remember these add to the overall losses. An extension to your frame in the form of a thick chunk of aluminum may be the only method open to you, but these too add to the overall losses. This brings up an interesting anecdote.

Almost everyone I know who uses a screwdriver type antenna has them mounted on a homebrew mount. The lack of any common solution to mounting these antennas to any vehicle is a potential problem. Hi-Q Antennas does offer a bracket assembly for mounting theirs (as well as some neat-o-slick custom stuff), as do others, but you the user have to provide the attaching methodology. You should keep this in mind before selecting the brand and type of screwdriver antenna, as some are easier to mount than others, (remember the 6 behemoth?). Although it is important to keep the mast clear of body sheet metal, it's imperative to keep the coil away as far as possible. Some designs don't lend themselves to this, as their coils are to close to their bases. So pay attention to this potential problem before you buy.

Matching the low impedance of a decent HF mobile antenna used to be easy; we all had tunable tanks on our mobile rigs. Nowadays with the advent of solid-state rigs, we no longer have this ability. Some of the latest rigs have built-in tuners, which easily match the 2, or 3 to 1 VSWR present is good quality HF mobile antennas (albeit they cannot be used to match a 20 meter antenna to 40 meters). Some screwdriver designs have a built-in, shunt matching coil at the base intended to provide a 50-ohm match across their operating range. They're a compromise at best. You're much better off with a transmission line transformer (better know as an UNUN) almost no matter what kind of antenna you use. Properly made their insertion loss in under 2%, and typically one tap will give a low enough VSWR from 80 through 10 meters that your rig won't care. An UNUN is also a good way to DC ground your antenna. An RF choke is an alternative solution.

Using a VSWR bridge to match a mobile antenna is suspect for several reasons. Effects of shunt capacitance and secondary resonance can give false readings. If you're using a monoband antenna, and you don't own an antenna analyzer, then borrow one. It'll turn a very long job into a very short one. But if you're using a screwdriver type antenna, a VSWR bridge is about your only recourse while you're underway. Might I suggest you use one of the dual needle units, which allows you to see both forward and reverse reading simultaneously removing some of the ambiguity. Auto-controllers are nice too if you can afford one.

If you want to use a small, cockpit mounted antenna tuner to match your HF antenna to your solid-state rig, then do it. Even with a 5:1 VSWR, the additional losses through 20 feet of coax is minimal even at 30 MHz (<.3 db for RG8). But don't expect the same results tuning your 20-meter antenna to 40 meters through the coax! Even if you could, the losses would be nearly 100% and both the coax and tuner would have their voltage limits exceeded.

Every amateur should read Walter Maxwell's (W2DU) Another Look as Reflections (available on-line to ARRL members). I don't know who first coined the term “veswaritis” (pronounced vees-waa-er-itis), but Walt did his best to dispel the myth. Alas, far too many amateurs catch this bug every year.

Since I'm talking about HF mobile antennas in this article, let's look at the input Z of the average antenna. The term Z is used to describe a complex impedance. -- In other words, the combination of the resistive and reactive components. On the lower bands the Z is typically 12 to 25 ohms, but some lossy antennas can have an input Z in excess of line 50 ohms. In general, however, if the unmatched Z exhibits a VSWR of about 2:1, which is about right, you need some form of matching, a.k.a. UNUN. If it is less than this (VSWR), you should start asking yourself “How lossy is my antenna?” Now I don't want to get into any long-length discussion about this subject; I'll just refer you back to Walt's dissertation. But I will say this: If the unmatched VSWR is less than 1.5:1, you need a better antenna.

One more little comment about VSWR: When your VSWR meter reads 2:1 and assuming the load is resistive, the resistance could be either 25 or 100 ohms. This is why I suggest using an antenna analyzer. It can show you the actual resistive value as well as the reactance.

Mounting location is important, and here in America it should always be on the left of the vehicle (or on top). There are several reasons for this. If it is mounted on the right side of the vehicle and you're parallel parked, there is always some frog who wants to meet Buster Brown and twang your magic twanger (if that doesn't date me, nothing will!). And have you even noticed; the tree and bridge clearances are always higher nearer the center of the road? And I prefer to mount mine where I can rotate the rearview mirror and reassure myself that my precious antenna is still there after hitting that poor defenseless pigeon. Front mounting is okay too, and contrary to popular opinion, it isn't any more electrically noisy than rear mounting. If you have a van or motorhome, this might be your best bet.

Here's a real hint I wish everyone would follow: Take as much time mounting your rig in your vehicle as you do your antenna. And please do yourself a favor. Don't use bungee cords or Velcro®. You might think they're safe at any speed, but they're not. The term "throw the rig in the car and go" isn't the way to go! Safety First! Always!

Headsets are great devices if they're legal where you drive. Coupled with the use of VOX, you can drive and talk at the same time. Just be mindful of the cord. If you have a choice, get one with just one earpiece. This way you can hear the siren of the cop whose pulling you over for wearing it where it isn't legal.

Don't you just love corona balls? They're the little thingies at the tip of your whips. If your antenna doesn't have one, then get one. -- The bigger the better. They help control QRN and they lesson the chance of injury in case the whip gets away from you while stowing your antenna in the trunk. Several companies market 1 corona balls which are adequate for power levels up to 500 watts. If you operate above 10,000 feet or run more than 500 watts, then a 2 one is more appropriate.

Tophats or more correctly capacity hats are typically disc-shaped devices mounted above the coil. They increase the capacitance above the coil at the expense of increased wind resistance (drag). This added capacitance has the effect of increasing the length of the whip thus less inductive reactance is needed (smaller coil) to resonant the antenna. The ideal placement is at the very top of the whip, which is impractical in a mobile venue. Mounting it too close to the coil increases losses, so a mid mounting position is the norm. From a practical standpoint they're not practical although a lot of folks use them. In reality they could be square and they don't need to be mounted perpendicular to the axis of the whip either. They could take the shape of a flag or pennant replete with your call sign in living color.

Quick disconnects can be a source of problems if they're not well made. Texas Bug Catcher and HiQ Antennas both market acceptable units. Expect to pay $35 to $50 for a good one. The small CB styles, which sell for $12 to $15, aren't worth the effort.

If you use stays or cords to help anchor your antenna, don't use nylon rope, Spectra®, or any other hygroscopic (moisture retaining) material. Delron® rod works well as does Plexiglas. Always endeavor to keep the stays or cords below the coil to minimize losses.

Let's readdress the subject of losses and antenna efficiency as I close this article. I have received a lot of responses to my various articles relating to how many DX countries a particular amateur has worked with his short and lossy trunk-lip mounted antenna. I refer you back to the paragraph about setting up your dummy load in the back yard and seeing how many contacts you can make. If the band conditions are good, the proverbial wet noodle will get the job done. But as I write this, 20 meters has been all but dead for the last few weeks. Signals on 17 meters are even more marginal, and 10 meters is a vast wasteland. As the sun spot cycle continues to wane, conditions will only get worse and every little bit of efficiency you can garner is worth the effort.

Alan Applegate, KØBG

Roswell, NM

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