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Know Your FM Translator Basics

Here’s some help in staying atop this important industry segment

AM revitalization continues to move along, and thus 2018 will see many stations adding FM translators. Let’s review the latest FCC window, recent regulatory changes and some basic rules that remain unchanged, then consider a couple of real-world examples.

The FCC’s predicted 60 dBu contour for K264CQ, transmitting from a tower in the array of its associated AM, KWKW, with 60 W vertical polarity.

In December, the Federal Communications Commission released the public notice detailing the instructions for the final window for AM stations to get FM translators. This window will be open Jan. 25–31 and is primarily for Class A and B AM stations that were not permitted to file in summer 2017’s window, when Class C and D AM stations were allowed file for new FM translators.

However, any AM licensee that did not file in last summer’s window and that did not acquire a translator during the period when AM licensees could acquire existing FM translators (and move them up to 250 miles) to rebroadcast their AM station, can participate.

An online tutorial is available to provide an overview of the application filing requirements for applicants seeking a new cross-service FM translator, available at the Auction 100 website:

The tutorial intends to help potential applicants further their understanding of the application process, allowing them to navigate the presentation outline, to review written notes, to listen to audio recordings of the notes, and to search for topics. This tool will include links to auction-specific commission releases, email links for contacting commission licensing and auctions staff, and screenshots of the online application system.

[Read more in the Radio World eBook “AM Translators: What’s Next”]

Back in 2015, when the translator windows were first mentioned, the FCC solicited comments on whether it should relax the siting restrictions to permit FM translators to operate from transmitter sites further away from the AM station’s transmitter site. Eventually, the commission adopted rules to permit FM translators serving as a fill-in service for AM stations to be built such that their 60 dBu contour is contained within the greater of the 2 mV/m contour of the AM station, or a 25-mile radius centered on the AM station’s transmitter site.

One of two Kathrein CL-FMs, making up the K264CQ antenna, is hoisted to the top of one of KWKW’s towers.


Let’s review 10 basic translator rules, since our system design will have to fit within their guidelines.

• The maximum effective radiated power for any translator station is 250 watts.
• If the translator is a fill-in translator, the ERP may be further limited by the need to maintain the translator’s service contour within the primary station’s service contour. Composite antennas and antenna arrays may be used.
• A commercial fill-in translator may receive a primary station’s signal via any terrestrial transmission method, including (but not limited to) microwave, phone, internet, and dedicated fiber optic cable.
• An FM translator station rebroadcasting the signal of an AM or FM primary station shall not be permitted to radiate during extended periods when signals of the primary station are not being retransmitted.FM translators rebroadcasting Class D AM stations may continue to operate during nighttime hours only if the AM station has operated within the last 24 hours. (74.1263(b))
• If the translator site cannot be reached at all hours and all seasons, means shall be provided so that the transmitting apparatus can be turned on and off at will from a point that is readily accessible during all hours and all seasons.
• FM translator and booster stations may use FM broadcast transmitting apparatus verified or approved under the provisions of part 73. (74.1250(a)).
• Radio frequency harmonics and spurious emissions must conform with the specifications of § 74.1236. (74.1250(c)(1))
• The apparatus shall contain automatic circuits to maintain the power output in conformance with § 74.1235(e). (74.1250(c)(3))
• Apparatus rated for transmitter power output of more than 1 Watt shall be equipped with automatic circuits to place it in a non-radiating condition when no input signal is being received in conformance with § 74.1263(b) and to transmit the call sign in conformance with § 74.1283(c)(2). (74.1250(c)(4))
• For exciters, automatic means shall be provided for limiting the level of the audio frequency voltage applied to the modulator to ensure that a frequency swing in excess of 75 kHz will not occur under any condition of the modulation. (74.1250(c)(5))

Because K264CQ is on an energized tower, it needs isolation so that the AM and FM systems are kept separate. A Kintronic Labs iso-coupler, shown here, fulfills this function.


Some FM translators associated with AMs will colocate at the AM transmitter site, some will not. Each option presents a separate set of problems.

In the case of colocation:

• Is the tower physically strong enough to hold the FM antenna and transmission line specified?
• Will the inclusion of an FM transmission line running up the tower (by way of the isolator, as mentioned below) change the base impedance to the extent that it cannot be compensated?

Answering the first question will require consultation with a mechanical engineer — preferably the same firm and/or engineer that designed the tower in the first place. Don’t just assume the tower can hold up an FM antenna (no matter how small).

The second question should be put to your consultant — probably the same one you have used during the allocation and application process.

Use of a hot AM tower for the FM translator antenna is a common technique, and it’s typically done through an iso-coupler. The iso-coupler effectively isolates the AM and FM systems from one another. Sources for iso-couplers include Kintronic Labs, LBA Group and ERI.

Another approach to this issue is operating with the tower grounded and skirting the tower, converting it in to folded-unipole system. Nott Ltd. and LBA Group provide technical resources for that approach.

If your translator site is not co-located with the associated AM station, then you trade one set of issues for another.

In this second case, you are essentially building a low-power FM site, with all the attendant needs of STL and remote controls:

Program delivery. Just like any other remotely located transmitter site, you will need to deliver programming, and as we saw earlier from the rules, this can be done via microwave, wirelines from your local exchange carrier, or even via the public internet.
Remote control. Unless the site is accessible on a 24/7/365 basis, plan to install some sort of remote control so that the translator can be turned off and on remotely.

Now that the antenna for K284CM has been made omnidirectional, it covers a substantial portion of the Sacramento metro, inside of its FCC-predicted 60 dBu contour.


Let’s look at a couple real-world examples of translator builds.

Lotus Communications is one of the largest privately owned radio station groups in the United States. Founded in 1962, it 34 stations in Arizona, California and Nevada, along with two low-power television stations in Arizona and Texas. Jason Houts is the company’s director of engineering.

K264CQ is associated with the company’s first station, KWKW, and transmits from one of the towers in KWKW’s directional array, located about 6 miles west of downtown Los Angeles.

I asked Houts if the coverage of the translator (60 W, vertically polarized, 110 M AGL) is living up to its expectations.

“It actually meets the predictions we had for it, and we even have it advertised on bus benches and whatnot, showing that actual frequency for KWKW.” (The translator is on 100.7, and KWKW is on 1330.)

Lotus has been using Nautel VS300 transmitters for its recent translator projects. K264CQ uses a directional antenna made up of a pair of Kathrein CL-FMs.

K284CM is another translator recently put on-air by Lotus, near Sacramento. This translator is associated with KVMX. Unlike K264CQ, which is located at its parent station, K284CM is located away from the KVMX transmitter site — about 7 miles to the northeast.

“That one is the top performer. Really has some coverage,” said Houts. “It’s running 250 Watts. It went on the air directionalized, and then we had done some things with another translator and basically opened that up — we were able to remove the parasitics off the antennas and turned it into an omni. Doing that actually made another big difference, and that has really helped that AM up there. It’s been really useful.

“It’s a great signal for a translator. It actually deserves an auxiliary transmitter, which we are installing so we can have some kind of redundancy.”

Over the last several years, the AM revitalization effort, as manifested by previous translator windows, has provided a chance for many stations to attract more listeners by getting on the air in the FM band. At least one more opportunity exists — if you are an eligible. For AM owners, the time is right to gain an extra foothold.

Doug Irwin, CPBE AMD DRB, is vice president of engineering at iHeartMedia in Los Angeles.