FM Antennas: The Silent Component


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Acquiring a new antenna

The problem at hand is much more involved if you are buying a new antenna for a new FM installation. All seven of the previous questions need to be answered, and perhaps even more. There's also a philosophical question regarding the high TPO/low antenna gain versus the high antenna gain/low TPO question. Some examples will illustrate my point.

In San Francisco there were two FM stations that transmitted from nearly the same location on Sutro tower. One used a single-bay antenna with around 20kW of TPO; the other used a two-bay antenna with half of the TPO. On the Empire State Building, there are two master antennas; the one known as the mini-master has half the gain of the master FM antenna, and thus requires more TPO. (I'm sure there are many other examples like this scattered around the country.) I have never seen any studies, technical or based on ratings/revenue, that show that stations transmitting in the middle of town (such as those at Sutro Tower or the Empire State Building) benefit from the lower gain/higher TPO model. On the other hand, if your new FM station is to be located outside of town (as most are) and the majority of the audience is effectively at the horizon (plus or minus a few degrees) it doesn't seem logical to use the lower gain/higher TPO model, since a lot of power will be sent at low (and high) angles of elevation, likely in areas that have no potential listeners.

Another important consideration for a new antenna installation is its downward radiation characteristic -- meaning directly below, on the ground. If the available antenna location is somewhat low, you may opt for a 1/2-wave spaced antenna (as opposed to a full-wave spaced antenna). The former has less gain than does the latter, which would compel you to go with either more elements (to recover the power gain) or to use a higher TPO.

During my career, circular polarization has been the norm, and there is one compelling reason for it: Receive antennas come in all flavors and polarities. It would seem that most cars have vertical antennas but there are many that have horizontal antennas embedded in glass; and there are plenty of opportunities for user-installed antennas to be horizontal by chance (think of a folded-dipole sitting behind someone's desk). It's not surprising that circular polarization is used by just about every main antenna I've ever encountered. It's the best way to provide optimum reception in the vast majority of receive antennas in the field. There are some instances where vertical polarity seems like a reasonable compromise though.

The available space on a tower, and its height above ground, are of course related questions. These are more of a consideration when building a new FM site. Clearly the space available on a tower at a potential site will dictate many of the characteristics of the antenna that ends up being chosen.

Finally, it makes sense to engage the services of a structural engineer (who is a registered professional engineer) when determining if a tower will safely hold whatever antenna it is you end up choosing. Just about any site lessor is going to make you do this prior to the installation of said antenna; and if he doesn't, you should certainly be concerned about the mechanical viability of the tower in question.

- continued on page 3



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