Basics

I have expounded about playing the Gain Game on this web site numerous times, as have others. Apparently that isn't enough as some amateurs just don't get the concept. This is an effort to further explain the concept in as simple of terms as possible.

As a prerequisite, it is necessary to understand several basic antenna terms, and what they relate to, but there is much more to it than what is presented here. This is why I recommend purchasing an ARRL Handbook, as no other publication has more amateur defined information within its covers. In other words, it's beyond the scope of this article to delve into dB any deeper than what follows. For the record, we're dealing with FM operation, which is typically mobile to repeater to mobile. Although there are a lot of direct mobile to mobile communications, in the big scheme of things, it's not as prevalent.

Antenna gain is measured in Decibels (the real term is Bel), which is named after Alexander Graham Bell, and expressed as dB. It is defined as a logarithmic unit of relative power measurement, and is expressed as a ratio. Like any other measurement, it must be referenced to some standard. There are two such standards commonly used when dealing with antennas. They are: dBi and dBd. The term dBi refers to dB gain over an isotropic source. An isotropic source is defined as an infinitely small, single point source with radiation being equal in every direction. The other term, dBd, is referenced to a dipole antenna in free space. In practical terms, there isn't such a thing as a free space dipole, but it is a good reference as we all know what a dipole is, or at least should.

When dealing with antenna gain, part of the confusion comes from the indiscriminate use of the term dB, without the requisite suffixes, i or d. Since there is a difference of approximately 2.5 dB, published figures without the suffix are misleading. Further, published gain figures may be referenced to some standard other than an isotropic or dipole source. In the case of one import antenna manufacturer, it is referenced to a typical handheld antenna. Read that as a Rubber Ducky, which sure puts new meaning into the term dBd! This is not just misleading, it borders on outright fraud. This propensity of the antenna industry to mislead potential purchasers is the main reason the ARRL does not publish antenna gain figures in its publications, unless such claims are properly documented.

The two drawings below are a loose representation of the antenna pattern of a quarter wave antenna, and a typical collinear antenna (a collinear in this context consists of several stacked elements with phasing coils in between). We're really not interested in how much gain the collinear has over the quarter wave, instead we're interested in the take off angle of the radiation. Properly designed, the collinear will have a lower take off angle. In very simple terms this is how an antenna "gets" gain; it compresses the radiation pattern, and typically (not always) lowers the angle as well.

Before we go any further, I have to repeat, this is an over simplification of what actually happens. The point I'm trying to make is this; the angle of radiation from a properly designed collinear antenna will be lower than a quarter wave antenna. But....

This oversimplification does not take into account several very important factors. First, most collinear antennas are far from optimum in their design, especially imported ones. And it doesn't take into account any inherent losses the aforementioned phasing coils may have. It also doesn't take into account its sturdiness or suitability (excessive length, mounting difficulties, etc.).

HAAT

The term HAAT stands for Height Above Average Terrain. Repeater licensees must include HAAT figures when applying for frequency allocations. This is done in an effort to minimize interaction between FM repeaters operating on the same frequency pair, but located a fair distance away from one another. In most cases, repeater licensees try to position their antennas at the maximum HAAT local terrain and interference overlaps will allow. This affords the maximum operating radius while minimizing dead spots in the coverage area.

Mobile stations have HAAT too. Unfortunately, it isn't always optimum for maximum coverage. We also have to contend with our surroundings which include tall building, trees, other vehicles, dead spots, etc. This fact leads some uninformed amateurs to play the Gain Game.

In simple terms, the maximum distance a mobile can effectively communicate via a repeater (or mobile to mobile) has a lot more to do with location circumstances than antenna gain, output power, or HAAT of either one, albeit they have some impact. It's sort of like the old adage about the three most important attributes of commercial real estate; location, location, and location. We'll discount these factors for a moment and keep with our HAAT scenario.

Urban Areas

I've done my best to place the drawings on the page to give the reader a good representation of real world circumstances between the repeater and a mobile HAAT. Depending on the width of your browser page, they may not be exactly as I intended them to be. With that in mind, we'll assume the top left drawing is a quarter wave antenna mounted on top of a vehicle of some sort. The drawing at the upper right is our repeater site.

Extending the line which indicates the radiation takeoff angle of a quarter wave antenna, it appears to be optimum, and it is in the urban areas most of us drive in.

In comparison, the collinear antenna's take off angle is lower. If we extend its radiation takeoff angle, we'll see it's well below optimum with respect to the HAAT of the repeater's antenna. In other words, in an urban area the quarter wave antenna will have a better signal at the repeater site than the collinear antenna will.

Outlying Areas

I suspect one could present a good argument for more gain in a suburban area, and they might be correct. Again it depends on the HAAT of the mobile and the repeater, and giving a pat answer is a case of presumption. I've been operating FM mobile for well over 30 years, and I've never found a situation where additional gain would have appreciably added to the length of the communications path. This includes rural areas.

FM Radio is a wonderful mode of operation, and without doubt there are more FM amateur radios nowadays than there ever has been of all the other mode types put together. Nonetheless, FM has its limitations, and long distance mobile to mobile is one of them. You can increase the distance by increasing antenna gain and adding power amplifiers. However, within reason, these additions don't magically double or triple the length of a reliable communications path. In most cases, a 10% increase would be optimistic. If long distance mobile to mobile is required on a regular basis, one would be much better off switching to the SSB mode, and using horizontally polarize antennas like a halo or squalo.

Other considerations

Another rather important aspect deals more with the physical nature of the two types of antennas we're dealing with, and that is length. Remembering the prerequisite for the maximum HAAT, mounting a collinear antenna atop a vehicle to maximize its HAAT is nearly impossible due to its length. You can of course, but you'll be hitting just about every low hanging tree branch. Considering just how poorly made most collinear antennas are, the life span isn't going to be very long. Once you break it, the SWR goes too high to be used.

On the other hand, smack a quarter wave antenna and about all you'll do is bend it, and you might not even know unless you see it bent. Straighten it up, and you're back where you started. With the collinear, you'll be back too, but this time to the dealers buying another.

Currently, the most popular amateur FM mobile antenna is not a quarter wave, but Larsen's NMO2/70. It's gain? Who cares! It does its job very well, it is sturdy, and best of all it is short enough to be roof mounted with little concern of height above ground. If your garage door is too low or your vehicle has a sun roof, then mount it on the trunk lid. Performance wise, you'll never know the difference.

And speaking of mounts; Trunk lip mounts, bumper mounts, license plate mounts and similar schemes further negate what little advantage a collinear antenna might have. If you're a frequent visitor to the eham.net mobile forum, you know all of the facts concerning proper mounting. In short, if you want to have the longest communications path you can muster, then drill the proper hole.

A few last comments. The last time I looked, the NMO2/70 was selling for ¡Ö$60. Most of the imported collinear antennas are about $90 and up. I have an NMO2/70 mounted in the center of the trunk lid. This is the third vehicle I have had that antenna mounted on. It still looks new. One of my amateur friends in Denver, has an imported collinear mounted on the rear bumper of his 2004 Honda Civic. Last week he replaced the collinear for the fourth time, but this time with a Larsen. His comments to me via e-mail was a major driving force behind this article.

The other was the post asking for information on the highest gain 2 meter FM mobile antenna. Hopefully, I've addressed that person's post.

Alan Applegate, K0BG

www.k0bg.com