Cindy Hutter Cavell is director of the Cavell-Mertz Spectrum Division at Capitol Airspace Group. She’s had a 48-year career in network, television and cable broadcasting and is a recipient of the NAB Television Engineering Achievement Award. She discussed STL links this year at the SBE Ennes Workshop at the NAB Show, and we caught up with her subsequently for a current ebook.
Credit: LinkedIn
Radio World: What’s the most notable trend in STL, Cindy?
Cindy Hutter Cavell: It’s been trending for a while, but the refinement of IP delivery and the development of more radio transmitters with IP signal inputs has allowed for a much larger range of options for signal delivery to the transmitter.
RW: How can a broadcaster choose the best option from among the choices available, including licensed vs. unlicensed, the different bands and many options available?
Hutter Cavell: The higher the frequency band, the wider the channel widths available, which translates into more throughput.
However, the higher the frequency band, the more fragile the link becomes. Higher-frequency paths will stay reliable only over a shorter STL distance and are more susceptible to rain fade.
For example, an 11 GHz link is far more “line of sight” and impacted by obstacles than a 950 MHz link. Often, a 950 MHz link will “bend around” some tree growth along the path, where the 11 GHz link would be stopped by the tree growth.
But an 11 GHz link has far more data capacity (150 Mbps) than a 950 MHz link (about 1 Mbps), which is much more suitable for a regional station hub where there are several owned stations in a market transmitting from a central tower, or a station that has several HD channels.
Meanwhile if it’s raining hard, an 11 GHz link becomes much less stable than a 950 MHz link because the wavelength of an 11 GHz transmission is approaching the size of a raindrop, so it’s more impacted by heavy rain.
As far as analog vs. digital microwave, here are some considerations:
950 MHz links are the only microwave links still available for analog transmission. A “refresh” of the old analog units is commercially available for a very attractive price.
Remember that analog links are NOT bidirectional, so they can’t be used for return signaling from the transmitter site.
Also, new transmitters may not have analog inputs any longer, so the analog signal may have to be converted to AES or IP before the transmitter input.
Microwave STLs in all frequency bands are available in a digital version. They are bidirectional, so transmitter data, remote control GUIs, security camera signals and other transmitter site-related signals can be transmitted on the return (TSL) link.
Each direction must be licensed, so the link has two licenses rather than one.
And throughout this discussion, keep in mind that the possible combinations of transmitter power, antenna size, frequency band, modulation scheme and Forward Error Correction (FEC) to create the “perfect” IP microwave link are endless. If microwave path engineering is not your forte, find someone who is an expert to help you. They can help you figure out what the “sweet spot” is.
RW: For radio-based STLs, which frequency ranges are best now? What are the licensed and unlicensed options?
Cavell: There are always unlicensed options in 2.4 GHz, 5.8 GHz and 24 GHz. Several manufacturers carry and provision unlicensed microwave products that are very cost-effective and easy to set up.
However, while it’s cost-effective and simple, unlicensed microwave may not always be the right answer for an STL, as there is no recourse for interference to your signal.
If the tower is in a very densely populated area with several tenants on the tower, unlicensed microwave can be a big risk. Note that if there is a cell carrier on your tower, many use unlicensed microwave as a temporary backhaul — their own STL back to the central office — which could interfere with any unlicensed microwave the station may want to use.
Licensed microwave comes many different frequency bands. They are licensed either under Part 74 as a Broadcast Auxiliary Service (BAS), or under Part 101 as a commercial microwave service.
There is only one Part 74 frequency band that can be used for radio: 950 MHz, either analog or digital. The license renews along with the radio station main license, so it’s not one extra thing to worry about.
There are several frequency bands available for digital microwave, 6 GHz, 11 GHz, 18 GHz and 23 GHz, in addition to other higher-frequency bands not suitable for STLs. These licenses are good for 10 years, and do NOT renew with the main station license; they must be renewed separately.
RW: Some stations are now running much of the air chain in the cloud, a few even with no studios at all. What are the implications of this trend for STL infrastructure?
Cavell: “The Cloud” needs access to the internet to send content to the transmitter site. Some sort of wireless STL is advisable as a backup.
However, if there is no longer any studio at all, a handy backup is to have a server at the transmitter site that can play content, commercials and station IDs in case the link to the cloud is lost.
RW: Many radio transmitter sites still do not have cell or hardwire connectivity. What options do these stations have?
Cavell: WISPs (Wireless Internet Service Providers) are often willing to provide free or nearly free access for your station in exchange for being on your tower.
And satellite internet access is an option but this is not always reliable due to weather. Heavy rain will interrupt Very Small Aperture Satellite (VSAT) reception.
This story is an excerpt from the free ebook “Getting Data Up the Hill.”
