Eke More Power Out of Your HD-R Tx?

Is there enough utility power available for the new installation, do I need to upgrade the emergency power and transfer switch? Will the installation survive future technology advances and new requirements?
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The pending request to the FCC by the NAB, a consortium of broadcasters and manufacturers for a power increase for FM HD-R by up to 10 dB has ramifications for planned HD-R installations. It is prudent to think through all aspects of a construction project to avoid increased costs in the future. A well planned installation will reduce future costs.

Common questions to ask: What technology method will I employ? Is my current installation obsolete or can I reuse some or all of the new gear?

Is there enough utility power available for the new installation, do I need to upgrade the emergency power and transfer switch? Will the installation survive future technology advances and new requirements?

Existing installations

The last question is the most important for anyone planning an HD-R installation.

The proposal for an increase in effective radiated power for HD-R provides unique challenges for existing installations. We can plan around these issues in new installations.

An important point is that the proposed increase is for power levels up to 10 dB. If this is approved and enacted, stations will be able to choose any level from –10 to –20 dB below the analog to transmit FM HD Radio.

If doing a new installation, choose the technology you will install carefully. The three current methodologies may be narrowed to just two with this proposal.

High-level and split-level systems are no longer practical due to the losses involved when operating at –10 dB digital vs. the current –20 dB. That leaves low-level and separate amplification as possible methods to employ.

However, facilities that have previously installed 10 dB high-level systems may only need to install a new antenna and line — either a dual-fed main replacement antenna or a separate antenna.

Many stations chose to do high level combining to avoid a tower or antenna change. That was driven by the fact that many towers and community antennas are leased from other companies that want big surcharges to add HD Radio capability with more feedlines, hybrid combiner retrofits, etc.

The same high level HD-R transmitter may be reused and will be capable of the new maximum power level, assuming the gain and losses of a new separate antenna are the same as the main. You will eliminate the lossy reject load and remove the injector/combiner.

Don’t forget, you will reduce the analog TPO by 10 percent. Utility costs may be reduced as the total HD-R and analog power generated will be radiated. You will produce less analog output than current high level systems require because the injector and reject will not be used. Power will no longer be lost in a reject load.

Facilities that have previously installed 8 dB injector/combiners to take advantage of producing power in the more efficient Class C amplifier operation (analog) may still be compliant with the proposed increase by reusing the HD-R transmitter. Under certain conditions stations chose to use 8 dB splits instead of 10 dB. This allowed an 85:15 split in power in which 85 percent of the digital and 15 percent of the analog power were burned off in the load.

The advantage is power loss in the load is generated more efficiently in the Class C amplifier operation of the analog and thus a smaller digital transmitter could be used. This is not the same thing as split level which uses the digital transmitter to generate some of the analog.

As an example, let’s look at a station with a TPO of 30 kW. Using 10 dB injectors the digital transmitter will need to generate 3 kW digital and 33 kW analog, But with 8 dB high-level injection, the analog power required is 35.6 kW and digital power is only 1.89 kW. Using 8 dB injectors resulted in a 15 percent:85 percent power loss in load vs. 10 percent:90 percent of a 10 dB injector install, thus a smaller digital transmitter was required as well as less capital and ongoing utilities costs. At the time, this was a good solution for –20 dB operations.

If existing transmitters employed in 8 dB high level installations are reused, and antenna system gains/losses are matched, the current HD-R transmitter may only able to do an 8 dB increase in some cases. That is 12 dB below the analog, and within the proposed range of –10 to –20 dB below the analog carrier.

Some may argue that a 2 dB difference does not matter in these cases. Interleaving a digital antenna with analog main antenna is a good option if you plan to install a separate antenna. Interleaving will generate similar patterns and provide a duplicate antenna as backup while maintain similar elevation nulls close to the tower.


(click thumbnail)In the hypothetical example in Fig. 1, a two-bay antenna will have one null at a downward angle of roughly 30 degrees from the flat horizon of the antenna, whereas a 12-bay will have 11 nulls spaced roughly every 5 degrees downward starting at about 5 degrees and ending at about 65 degrees. In this illustration, red indicates the 12-bay analog elevation plot. Blue is the 12-bay digital and nulls line up with analog. Black is a two-bay digital. Green reflects areas where the two-bay reflects different receive levels. Source: DielectricAntenna gains, losses

Each antenna should have the same gain (number of bays) to avoid self-interference close to the tower. If antennas with different gains and losses are employed you may need to address the increased power requirements differently.

Installing differing gains (number of bays) is not recommended. This is true if you are using interleaved antennas or a separate antenna at a different location on the tower.

If different antenna sizes are employed, the locations and number of elevation nulls will differ close to the tower. Two antennas with different power gains — and all antennas regardless of gain, mounted at significantly different heights — not interleaved will result in locations close to the tower site where the digital level transmitted is far greater than the analog and vice versa. This is due to the physics of a multiple element antenna.

In the hypothetical example in Fig. 1, a two-bay antenna will have one null at a downward angle of roughly 30 degrees from the flat horizon of the antenna, whereas a 12-bay will have 11 nulls spaced roughly every 5 degrees downward starting at about 5 degrees and ending at about 65 degrees.

In this example, there will be 10 locations near by the tower site, where the digital signal will be at maximum and the analog will be virtually zero. This assumes the analog is using the 12-bay and digital is using the two-bay. This is a greater concern to facilities with high population areas located near the tower, rather than those where the tower site is in a cow pasture miles out of town.

At least one company has been able to achieve greater than –40 dB of isolation between two interleaved antennas; see Fig 2. Isolation is important to minimize the potential of inter-modulation re-growth products as well as the need to protect one system from cross talking to the other.

A system with low isolation numbers, less than –35 dB, will result in analog power crosstalk into the digital operation and vice versa. The power will show up on directional couplers as returned power.

This will be seen by the opposing system as a “VSWR” indication. However it is not VSWR, it is only represented in the measurement device as independent power leaking back into the transmitter from the antenna and displayed as VSWR. Therefore it’s important to understand this crosstalk power is present on the output of your transmitter even if the transmitter is off and the opposing transmitter is on into the antenna.

Tower study vital

(click thumbnail)At least one company has been able to achieve greater than –40 dB of isolation between two interleaved antennas. Source: Dielectric
Low-level systems are reported by at least one manufacturer to require de-rating in excess of –50 percent to achieve an additional 10 dB of digital, and an additional –6 percent de-rating once the digital bandwidth MP11 mode is activated. Remember there are many modes of operation for HD-R. Each mode is represented by acronyms beginning with “MP.”

The current mode is MP1, Future MP3 and MP11 add additional OFDM carriers to the transmitted signal allowing greater amounts of data to be encoded. These additional carriers will result in greater peak-to- average ratios and thus may further de-rate transmitters by an additional –6 percent. This means the combining of two low-level transmitters is not likely an option, if you are running at near full rating of the transmitter now.

Don’t limit yourself to a particular antenna make; there are antennas available with less than half the wind loading of certain popular antennas. The obvious advantage of going with the lighter antenna is that you can put twice as many bays (interleave) with the same or less wind load of your existing antenna with few changes to the structural of your tower.

Don’t forget to have a tower study conducted that includes a separate additional transmission line if interleaving or installing a separate antenna.

Several manufacturers have recently announced new products that consolidate many of the different HD-R components into a single exciter, and both Harris and BE have announced shipping an embedded exporter standalone device. Both of these are seen as significant improvements resulting in lower capital cost and cost of ownership.

Data delivery is another area where costs may be controlled. Plan on using a wireless delivery system, either your existing 900 MHz path or a new unlicensed or licensed spread-spectrum path. These delivery options will tend to be more robust in emergencies than land lines and will also be less costly.

While these links are not affected by the power increase, they may be affected by any future change from MP1 operation to MP3 or MP11. Check with your transmitter manufacture for further information on bandwidth of different modes of HD-R operation, to ensure you design a link that will be sufficient for future growth.

Properly plan your installation and judge for yourself what options best suit your needs and if you have any doubts contact your corporate or consulting engineer.

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