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FM Antennas: The Silent Component
Hypothetical but typical scenarios
So now that we've discussed the generic questions regarding the purchase of a new antenna, let's look at a couple of hypothetical installations. First, let's say you are participating in the construction of a brand new FM station. The allocation aspects are outside the scope of this article; so let's just say that you've found a potential site (a mountain top outside of town) with two potential tower locations. At one, your ERP will be 55kW; at the other (which has a slightly lower height above average terrain, or HAAT) your ERP will be 57kW. At the higher tower location, 70' are potentially available for your antenna; at the lower location, 63' are potentially available.
I'll present multiple antenna products in this article, and I'll start with ERI. When visiting the website you'll notice an antenna size calculator as well as a chart of antenna gain figures listed by the number of elements and antenna spacing (full or half-wave) for the various antenna types. Starting with the 55kW number, you decide (for starters) to look at a six-bay, full-wave spaced antenna with circular polarization. From the website, you get the antenna power gain figure (3.3028). Divide the ERP by the power gain: 55/3.3028 = 16.7kW (rounding up). The SHP-6 will give a large margin of safety on the input power (16.7kW compared to the 39kW input power rating). 50Ω 3" Heliax will also work in this application; in our hypothetical situation we find we need 225' of the line to reach the transmitter. According to ERI, the efficiency of this line length (at 93.3MHz, as an example) is 93.2 percent; so, excluding any loss attributed to connectors, our TPO is: 16.7/.932 = 17.92kW (again rounding up). Clearly a new 20kW transmitter will be adequate in this case. Also you can see on the website that this antenna requires an aperture of 65', and fortunately that's available on the higher antenna location.
However, it turns out that the lessor for the lower location will charge less, so it makes sense to see if that location will work. As I mentioned earlier, only 63' are available, so the full-wave spaced antenna won't fit. Referring back to the online calculator, we see that the SHP-10AC-HW will give us close to the same power gain. Doing our TPO calculation again: 57/3.126 = 18.3kW (rounding up) is needed at the antenna input. We'll use the same transmission line type and length, so our necessary TPO is: 18.3/.932 = 19.65kW (again rounding up). The aperture necessary for this antenna is 60', so it will fit.
At this point you'll need to decide if the extra height is worth the extra monthly expense. You'll have to consider the cost of the proposed 1/2-wave-spaced antenna for the lower location as well -- it'll likely be more money (since it's a lot more material and there's more labor involved in making it). After deciding on the preferred location, have your FCC consultant calculate the RF levels directly below the antenna to make sure they do not exceed the occupational limits (referring to OET bulletin 65) and have your structural engineering consultant determine whether or not the tower can safely hold the antenna.
Jampro offers the JHPC line of FM antennas, and that is obviously worth looking at in this same application. Its website gives antenna gain figures, so you can do the same set of calculations as shown above. Likewise, Shively Labs offers the 6810 high-power series, which can be seen in detail on its website. Shively offers all the details about this antenna series -- the gain figures, along with size and weight, to help you do all the same calculations that were described earlier. Dielectric also offers its DCR-C line.
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