Hello ECR Family, and welcome to The Antenna Farm. This is your friendly Antenna Farmer Charles, KC6UFM.

In this article, we’re going take a look at the general concepts of mobile antenna systems. See Figure 1. Many things we’ve seen prior to this are aimed at stationary antennas, but the same ideas still apply. There are, however, a number of special concepts that are important in a mobile installation and yet are meaningless in a stationary setting. First off, let’s define what mobile means in relation to ham radio.



The simple definition is that anything that can be moved and/or is not permanently installed is a mobile system. Specifically, we are usually talking about one of four possible situations:

    1) A system installed in a car/truck.

    2) A system installed in a boat.

    3) A system installed in an aircraft.

    4) A system that is portable and can be easily moved.

Each of these situations has special needs that are specific to them. Some share a few things, but most are stand-alone situations. The majority of hams will be most concerned with #1 and #4. A few will be interested in #2. A few more will need to know about #3. Most of #4 we will discuss in it’s own article on portable operations that will cover info needed for POTA/SOTA (and similar operations) as well as HT use.

The prime rule of mobile antenna systems is that they are all compromise systems. As you move lower in frequency, the compromises become more pronounced, but even in the SHF bands, there are still compromises that must be made. In other words, you will never get the same efficiency from a mobile that you will from a stationary antenna, and so you will never get the same performance.

There are four main areas of compromise…

    1) Physical Space Limits. A mobile antennas, particularly at VHF bands and below, must be smaller just to fit into the space allowed. Think about it…your car/truck/boat/plane is not as big as your house or the property it sits on. There are ways to make these smaller antennas work, but there will always be trade offs made.

    2) Elevation Limits. Again, your car/truck/boat isn’t as tall as your house or even a modest mast pole. Planes may have an advantage here because of their typical cruising altitude. You need to resist the urge to get your mobile antenna as high as possible, though. Many overpasses around the US are at just under 14 feet. Most fast food drive through windows are at a lot less, often in the 9 foot range. And that’s not even looking at trees and wires that overhang many streets and roads. Personally, I try to stay under about 8 feet or so.

    3) Grounding Issues. Your car, boat, and plane are isolated from the real ground. This means you must simulate a good ground for your mobile system using the body of the vehicle. This, in and of itself, isn’t a problem, but the implementation can be a challenge. See Article 8 on grounds for more information, but the closer you can get the simulated ground to look like a real ground, the better off you will be.

    4) Losses. All mobile systems, due to the compromises that must be made, have huge losses when compared to stationary systems. The only way to mitigate this is to make sure that any loss you can control is addressed.

Some works will combine the above into fewer categories, but I find keeping them as independent ideas makes it easier for me to think about. Even so, they are all somewhat related and, often, will play off of each other. For example, problems in your mobile ground system will increase losses.

Before we go any farther, I do want to mention something that the FCC Rules found in Part 97 make clear…when operating either maritime (on a boat) or aeronautical (on a plane) mobile, you MUST have the permission of the commanding officer of the boat or plane. If you’re just out on the lake in your john boat, you are the commanding officer (both practically and legally) and can, of course, give yourself permission. If you are on a Princess cruise ship to the Bahamas, you need permission from the master of the ship. Similarly, if you are a pilot flying around doing whatever it is that pilots do, you can give your permission. If you are on Southwest flying to Orlando, you need permission of the captain (usually called the Pilot In Command or PIC). While there have been few cases of this kind come before the FCC, in every single one, the ham who was operating without permission of the commanding officer lost their license for life. Many also faced financial penalties.

Now that we have that out of the way…

Our focus here on the Farm will be on the actual antennas and related systems like grounds and feed lines. That said, we do need to look at the radio itself for a few minutes. Today, many mobile stations use a handy talkie (HT) for the radio. The vast majority of these radios put out less than 5 watts and are typically VHF (mostly 2m), UHF (mostly 70cm), or both for dual band radios. There are variants that have 1.25m, 23cm, and even 6m available. While I have no solid research data, I would guess that perhaps 70% of the HTs in use today are one of the many Chinese rigs. Some of the more popular brands are: Baofeng, Wouxun, AnyTone, TYT, Radioddity (strictly speaking, this is not a brand but a reseller of brands that have been private-labeled for them), and a few others. There are those out there who will criticize the Chinese rigs, but they are actually a very good value in many ways and a perfect first radio for new hams…just don’t try to compare a $30 Baofeng to a $400 Icom. They are apples and oranges. The problems with using an HT in a mobile setting apply to ALL of the HTs out there, not just the Chinese radios. It comes down to a few important facts…

    1) Poor Filtering. Compared to actual mobile and base rigs, HTs have horrible filters on both transmit and receive. While not a direct, true loss, the noise from the poor filtering decreases your station performance on both transmit and receive. Remember, we need to minimize all losses in mobile systems.

    2) No Fold Over. Remember back when we talked about SWR and the fact that many transmitters will reduce power—or Fold Over—when the SWR is too high? The vast majority of HTs have no fold over function. They will just keep pumping energy into a 10:1 system until the finals fail. On the other hand, almost all modern base rigs and most mobile rigs have fold over. This is needed because the feed line and antenna on mobile systems are subject to sudden, unexpected damage and other issues (like a lost ground) that can shoot the SWR through the roof.

    3) Low Power. That Baofeng you have that is rated at 8 watts? Guess what? They lied to you. Most reports from those who have connected their HT to a good, calibrated watt meter report that, on average, output from an HT is really about 60% of the “rated” RF output. So that 8 watt handheld is really putting out around 4.8 watts. Losses on a typical (as opposed to good) mobile installation can easily reach 12 dB or more. If we assume the antenna has 6 dBi of gain, your 4.8 watts just dropped to about 1 watt ERP. If you replace that HT with an actual mobile radio at 25 watts by simply changing the coax from the HT to a mobile, your ERP is now just north of 6 watts. If you invest in a 50 watt mobile, you will have over 12 watts ERP. What I’m saying here is to get a mobile radio and stop using an HT. You will be impressed. Mobile rigs in the 25+ watt range are easy to find in the $200-$300 price points.

The next general concept of mobile installations we need to look at is the power supply to the radio. Low voltage due to drop in the supply cables, noise from many sources, and inability to provide sufficient current are just three of the things that can cause poor performance.

There are two major rules here:

    1) Never EVER power your radio from the cigarette lighter or 12vdc auxiliary power jack. It doesn’t matter if it is a 5 watt HT or a top of the line 100 watt rig. Do NOT do this! There are several problems using the cigarette lighter outlet…first, it is typically supplied by very small gauge wire, usually 18 AWG. If we assume large gauge wire from the battery to the fuse block that has very small voltage drop (and there is no guarantee of this, particularly with European cars), running 18 AWG for, let’s say, 10 feet from the fuses to the outlet at just 3 amps gives a voltage drop of nearly 2 volts, so your power supply is only providing 11.6 volts instead of 13.6 volts. If you have a mobile rig drawing 10 amps (typical for a 25 watt radio), the wire gauge is only marginally able to carry the load, plus your voltage drop is just over 6 volts, nearly HALF of your nominal supply voltage. Yes, the cigarette lighter is a handy place to plug into. And also yes, it will never work right.

    2) Always run heavy gauge wire DIRECTLY from the battery (both positive and negative) to the radio. For a 25 watt radio, 10 AWG should do the trick. For a 50 watt mobile, use at least 8 AWG. These wire sizes will not only carry the electrical load but also eliminate excessive voltage drop. Be sure to properly fuse the positive lead as close to the battery as you can. The negative lead should NOT be fused. Not only will this practice make sure that your radio is getting the right voltage and current, but the battery itself will act as a filter and reduce noise. If you’re not comfortable running this wire, talk to a local car stereo installation place…they will often run the wires, install the needed fuses and blocks, and leave the wires dangling for you to finish for well under $100. And they do this sort of thing all day long for high-powered sound systems, so they’re good at it. Some shops will do the entire install for reasonable money if things are slow.

Lastly, before we get to the antenna itself, we need to consider grounding and bonding, and this can be problematic. Essentially, all metal parts of the vehicle should be at the same electrical potential. We’re talking the frame (if the vehicle has one), fenders, doors, hood, trunk lid, roof, and everything else that is metal. This may seem like a no-brainer, but the fact is that not all metal parts of your vehicle are always connected to each other electrically. Take fenders…the main body and the fenders are painted (with heavy duty automotive epoxy paint) before they are assembled. When assembled, the electrical contact of the fender to the main body will likely be someplace between very poor and not at all. If you should mount your antenna to the fender, it is now an isolated point and will not work very well, if at all. Things like hoods, fenders, trunk lids, doors, moon/sun roof units, and more can all be isolated pieces. At best, they will cause noise and static. At worst, they make it nearly impossible to get a mobile system to work well. All of these independent parts should be bonded together using flexible, low impedance straps. Yes, this is a lot of work, but almost always worth it. This problem is even worse on many newer vehicles (and boats/planes) where body panels are often made from various non-conductive composite materials. In other words, putting a mobile rig in a Corvette can be infuriating!

Now, finally, we are to the antenna itself…

First off, let’s look at the various mounts that are available.

 

Everything else being equal in terms of performance, the best mount is a “Through-Hole” type that requires you to drill a hole in your vehicle. There are are essentially two types of through-hole mounts: the NMO (see Figure 2) and the UHF (see Figure 3). The NMO is a large mount using either a 3/4” or 3/8” hole. The UHF mount is, basically, an SO-239 connector that goes through a 5/8” hole. Contrary to popular belief, neither the NMO nor the UHF performs better than the other…they both work the same and are equal in performance. The NMO is, however, a little stronger mechanically due to its larger size. But you have to drill holes in your new car! Get over it. Such holes can be easily repaired by a body shop.



Next on the list of performance are the so-called “Clamp-On” mounts (see Figure 4) that physically clamp to some part of your vehicle like a mirror frame or bumper. These mounts work nearly as well as the through-hole types ASSUMING that the part they are clamped to is solidly bonded to the vehicle frame. The clamp-on mounts come with a UHF connector for the feed line and either a UHF, NMO, or 3/8”-16 threaded hole for the antenna to screw on to. The clamp-on mounts can be quite strong if the frame member they are clamped to is up to the task. Most over-the-road truck mobile systems use these for both ham and CB systems.



Here we have probably the most popular of the mobile antenna mounts, and that is the so-called “Mag-Mount” that is, basically, a big magnet (sometimes more than one) that sticks to your vehicle. See Figure 5. Again, mag-mounts are available with UHF, NMO, and 3/8”-16 connections for the antenna proper. Mag-mounts can be very strong and hold large antennas, but they rely on the vehicle having a steel body for the magnets to grab hold of and stick to firmly. Again, many newer vehicles use less steel and more composites, so a mag-mount may not even work for your vehicle. The advantages of a mag-mount are two: First, no holes need to be drilled, and Second, you can easily move the antenna between vehicles. With all this talk about grounding and the need for a good ground for proper antenna operation, how does the mag-mount work? If you look closely at the bottom of the magnet, you will see a thin sheet of insulation. Don’t try to replace this or put an “extra” layer of plastic wrap between the magnet and your car to protect the paint. This insulator, and the space it provides, are an actual part of how the antenna works…if you have two plates of metal (say a magnet and your car) separated by some insulator (say the automotive paint and that thin sheet of plastic), what do you have? That’s right! A capacitor! And under the effect of RF, what does a capacitor have? Yep…reactance. The mag-mount antenna is capacitively coupled to the car’s ground system. Change ANYTHING (like, say, the thickness/type of the insulator) and the reactance changes, so your mag-mount antenna will not work as it should. The problem is that this capacitive coupling is very inefficient to start with (typically showing about 6 dB of loss), and any changes tend to make it worse. Also, a mag-mount will leave a funny looking circle in your car’s paint after a while. All of these things add up to making the mag-mount a fairly poor option for an antenna mount.



But the mag mount is better than our last mount, the “Through-Glass” mount. See Figure 6. This mount works in a similar manner as the mag-mount in that it relies on capacitive coupling through a plate of glass. Usually, the inside and outside connectors are glued to the glass using, basically, Super Glue. The through-glass mounts have all of the problems of the mag mount, but there is one big issue that the through-glass mounts have all to their own…the thickness of the glass on cars—and therefore the capacitive reactance—varies. European cars tend to have the thinnest glass, American made cars have the thickest, and the far eastern cars are all over the map. This all means that the overall performance of through-glass mounts is, at best, unpredictable.

There are other variants as well, like systems that mount to trunk/hood lips, clip over the edge of your window, and many, many others. Some are OK. The rest are pretty bad.

So the bottom line is to use a through-hole mount if at all possible. It’s going to work best.

Next, think about your feed line. I have never seen anything other than coax used in mobile systems, but open wire feeders would work well. Maybe. You need to keep open wire line away from other conductors (like the car body) to get the best performance and thinking about working with open wire line inside a vehicle makes my skin crawl. Use coax. But not just any coax. Most antenna mounts that come with coax are provided with RG-58 cable. Not a great choice. In fact, it’s pretty bad. If we assume a 20 foot run of RG-58 at 450 MHz (70cm) and an SWR of 1.5:1, we see there is a loss of 1.89 dB in the feed line. If we change to RG-8, the loss falls to 0.96 dB, but we have to deal with a much larger diameter and stiffer line. The same size stiffness issues apply to using LMR-400, but we have a loss of only 0.58 dB. If, however, we move to RG-8X, the loss is 1.75 dB and we still have a small, easy to work with cable, but little improvement in loss over the RG-58. But if—and here’s the money shot—we go to LMR-240, we have a nice, small cable and the loss is only 1.13 dB. Using LMR-240 instead of RG-58 just saved you 0.76 dB of loss, and that can make a noticeable change in you mobile station’s signal for not much money or extra work.

Now, let’s take a fast look at some popular antennas. For now, we’re going to look at antennas for VHF/UHF repeater work.

You will find all manner of antennas for mobile operation. Of particular popularity are: ¼ wave, 3/8 wave, ½ wave, and 5/8 wave verticals. All of these are just variations on the ½ wave dipole, and they all have certain advantages and disadvantages we’ll examine in our next visit. As repeater usage has expanded, the need for multi-band mobile antennas has grown. Beyond simple single band designs, there are dual band (usually 2m/70cm), tri-band (usually 2m,1.25m, and 70cm), quad-band (10m, 2m, 1.25m, and 70cm most often), and others. The design of a multi-band antenna requires some planning. You will often see on, for example, dual band (2m/70cm) designs with a relatively low gain on 2m and a higher gain on 70cm. This is accomplished through the use of “Traps” that force the use of the entire whip for 2m (usually 5/8 wave) and break the antenna up into multiple sections fed with the correct phasing to get more gain at 70cm. Strictly speaking, these “traps” are not really traps in the conventional meaning. In addition to isolating segments of the antenna, they also provide the proper phasing needed. We’re going to get more into this in a later article when we look at trapped antennas and phasing systems.

All of this said, if you are looking to save money by building your own mobile multi-band antenna, you might want to rethink things…you CAN save money building mono-band antennas, but not a multi-band unit. Once we cover the phasing and trapping systems, you’ll have all the knowledge needed to build such an antenna. Just know that it will likely cost more than buying a good antenna from someone like Comet or MFJ.

Now, since we have a mount, an antenna, the ground system, and the feed line, we need to decide where to put the antenna. In general terms, you want the antenna as close to the center of the vehicle (both front to back and side to side) as possible, and as high as possible (see above). If we’re talking about a car or truck, the center of the roof is a good choice. The idea is to get a ground plane as large as possible all the way around the antenna position distributed as even as you can. This is usually only possible with through-hole and mag mounts. Others usually place the antenna at some location away from the center of the vehicle.

When it comes to cable routing, that is something that literally varies from each installation to the next. If you can, place the feed line in such a way to protect it (kinks and pinches in coax will cause an impedance “hump” that decreases performance) and keep things neat. And by “neat,” I don’t mean draping it over the seat back. Same thing applies to power cables. And do NOT just coil up “extra” feed line or power cable…cut it off and make it just the right length. The thing here is, again, to minimize the controllable losses.

Then we have radio position. Make sure the rig is securely mounted (you don’t want a radio flying around inside your car in an accident!), does NOT interfere with the seat belts, airbags, and other safety systems, allows the safe and normal operation of the vehicle, and is conveniently located so you don’t bash your knee every time you get in and out of the car. Also think about visibility and accessibility so you can safely see and operate the radio. Most states and many local governments exclude ham radio from the various “distracted driver” or “hands free” rules, but being distracted because you can’t tell what frequency you are on is still not a good idea. Also think about mic location and how the cable will run when “hung up” and in use.

When I was in the Navy, we had a saying: “Plan the Work and Work the Plan.”

Finally, when dealing with boats and aircraft, there may be other actual regulations you need to follow that vary based on the size and use of the craft, federal (Coast Guard/FAA) defined installations, and other restrictions on how you install your gear. Always check to make sure you’re not running afoul of some obscure rule.

Next time, we’re going to take a closer look at the various types of common VHF/UHF mobile antennas.

Take Care & 73
de KC6UFM
Charles