Higher Power IBOC
Increasing IBOC power is no trivial matter; now is the time to plan.
If your station is interested in implementing higher power IBOC, then one of two cases must be true already: You currently transmit IBOC and need to consider the implications of raising the power to (at least) -14dBc; or, you are planning on transmitting IBOC for the first time. Let's assume the former, and take a look at how to increase your current IBOC power first. We'll look at new systems afterwards.
Perhaps the most common way to transmit IBOC has been by way of a -10dB coupler inserted in the output of the analog transmitter. At least four manufacturers (Dielectric, ERI, Shively and Jampro) make this type of device and assuming they all produce a new version (-4dB injection) then it should be a relatively easy matter to simply swap out the old with the new. The good news here is that the amount of waste heat formerly dumped into the waste load is going to be reduced.
One advantage to this type of coupler (at least at the -10dB injection level) is that there is an exceptional amount of isolation between the analog port and the digital port; a quick look at published specs of the four manufacturers shows 36dB minimum (with good VSWR). The rule of thumb has been -30dB of isolation between the analog and digital ports in an IBOC system (at minimum). In communications with these same manufacturers I was reminded (with respect to higher IBOC power levels) that isolation from the digital ports back in to the analog ports will now also be an important spec to consider, as the chances of generating intermod products in the output of the analog transmitter will now become a distinct possibility. Make sure you consider that specification when choosing a particular brand and/or model of coupler.
Another common method of getting IBOC on the air has been to use an auxiliary antenna. This was a very expedient method in many cases. However, because the new power level is four times that of the old, there isn't going to a simple way to achieve the increased ERP using the same old antenna. At minimum, you would need a new transmitter; and one that has four times the power capability as the old could be an expensive proposition. You would likely need to upgrade the ac power and air conditioning capability as well. A compromise might be changing the auxiliary antenna to one with more power gain (for example, doubling it) and then buying a new IBOC transmitter that has twice the power output capability as the old one. As in any power upgrade, be certain that the coax in place can handle the additional power; add 5.5dB to the nominal IBOC power to account for the peak-to-average ratio.
Probably the least common method of getting IBOC on the air has been the split-level combining method. This is a system that makes use of the analog transmitter; a combined-amplifier transmitter (meaning the digital and analog signals amplified together) and a high-level combiner that adds the two transmitter outputs together to generate the correct amount of analog and digital power in its output. This system works well especially when the analog transmitter has a lot of power capability, but not the 10 percent headroom necessary when using a -10dB IBOC injector. When increasing the IBOC power level, it will be necessary to derate the analog output level of the combined amplifier; but, depending upon how much headroom is available in the analog-only transmitter, you may be able to make up for that reduced analog relatively easy. The ratio of analog power contributed by the analog-only transmitter to that of the combined-amplifier will increase, so that the output of the combiner has the newly increased IBOC level, with the same analog level as before.
If you are considering implementing IBOC for the first time for your station, or if you are plan to quadruple the current IBOC power level, it is incumbent upon you to look in to either a combined-amplifier type of transmitter, or an antenna with an IBOC port. If your station uses high-level combining now (by way of the -10dB injector) you'll likely find the power upgrade relatively inexpensive and painless; however, if you are using space-combining I believe you will find that a new transmitter or antenna will be close in cost to the upgrades of the old system.
Let's consider a new antenna first. All four of the major antenna manufacturers offer at least one antenna model with both analog and IBOC input ports. In the case of Jampro, it's the JSHD; for ERI, it's the Lynx. For Dielectric, it's the HDR series of interleaved elements; Shively offers an interleaved antenna as well, such as its 6813.
One advantage to using the dual-input antenna type is that the characteristics with respect to the radiation pattern will match between analog and digital. The analog power doesn't change; simply feed the digital port with enough TPO to make the -14dBc ERP requirement. As an added bonus, the digital port can become a backup input for the analog transmission (albeit lower power). Ask any potential vendors about the digital-to-analog as well as the analog-to-digital port isolation.
As I wrote earlier, the rule of thumb has been a minimum of 30dB of isolation between ports. You could add isolators to the digital transmission line to improve the isolation from the analog port, but the other direction could be more of a problem. I've been assured by each of the four vendors mentioned that the digital input ports to their antennas will handle -10dBc, so obviously -14dBc will not be an issue.
-- continued on page 3
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