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I have recently taken on programming radios for my workplace in Chicago, IL. We have an old base station (Kenwood TK863G-1) that I wanted to know if it would be possible to use as a repeater similar to how law enforcement does. What I mean by this is use the vehicle installed transceiver to relay or repeat the traffic to and from a uhf handheld. This would help get better coverage to our main building on campus when we are on the other end of campus or making a run to the gas station down the road.

I guess to break down this question I want to know two things... 1. How would I go about setting up a relay/repeater in a vehicle to relay traffic from nearby handhelds on our chosen frequencies (at least one but would be nice to relay/repeat our 3 to 5 channels)? 2. Can I use our existing Kenwood TK-863G-1 to accomplish the scenario above. If not, can you suggest some cheap alternatives?

Thanks everyone :)

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  • $\begingroup$ Welcome to hamSE! As a new user, be sure to take the Tour. Is this question about amateur radio or business radios? $\endgroup$
    – Mike Waters
    Commented Oct 4, 2018 at 6:40
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    $\begingroup$ @MikeWaters It is a little bit of both. I want to do this in practice on my business owned radios but really it is more for my own knowledge and interest. Thanks for pointing out the tour to me. $\endgroup$
    – Fireant456
    Commented Oct 4, 2018 at 7:00
  • $\begingroup$ I just Googled TK-863G-1 and it seems it is not an amateur radio. That's why I closed your other post as off-topic. Sorry. $\endgroup$
    – Mike Waters
    Commented Oct 4, 2018 at 7:32
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    $\begingroup$ @Mike Waters I am asking about principles not so much about the radio I have. I believe this very much is on topic for any space in the radio spectrum $\endgroup$
    – Fireant456
    Commented Oct 6, 2018 at 22:41
  • $\begingroup$ Okay. I just asked the other moderators, but none of them are in the mod chat room right now. The help doesn't seem to bar questions about business radio. $\endgroup$
    – Mike Waters
    Commented Oct 6, 2018 at 22:47

2 Answers 2

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The first requirement for any repeater is that it be able to simultaneously receive and transmit; this capability is known as full duplex operation. A duplexer - a pair of sharp filters - is required to prevent the repeater's transmitted signal from overwhelming the repeater's receiver. While an external duplexer could be added, I see no evidence in the manual for the Kenwood TK863G that it can operate in full duplex mode, so I don't believe it can serve your application.

Some mobile amateur radios, e.g., Yaesu FT-8900R, can operate cross band full duplex. That is, the mobile transceiver receives signals from radios on one band and retransmits those signals on another band, employing a built-in duplexer. Radios of this description were used by the hams on Dominica to temporarily replace repeaters destroyed by Hurricane Maria in 2017. Such installations can be "daisy chained" to expand the coverage of a single, central repeater.

In the urban environment you describe, I suspect that the locations of the "mobile" units would have to be carefully chosen to account for building reflections. Likewise, users would be required to change their "up/down" band pairs depending on their proximity to the mobile unit serving their location.

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First, let's start on the legal side of the equation. If you are in the US, you are required to have a license for your business radios as well as for your planned repeater. You do not get to pick the frequencies, they are assigned to you by an FCC approved frequency coordinator.

The license application is not particularly difficult or expensive but it is loaded with terminology that can be a barrier for the uninitiated. You would probably be better off to use a professional service the first time through the process to avoid having your application rejected by the FCC. You generally have one license that covers all of your radios, the repeater, and at least two assigned frequencies.

On the technical side, the transmit and receive frequencies will be 5 MHz apart. This is so that the repeater can simultaneously receive and transmit with minimal interference to itself. Even then, a very high isolation filter, called a duplexer, is required so that the receiver in the repeater is not "desensed" by its own transmitter. Desensing refers to the repeater receiver losing sensitivity for the desired signal due to a strong interfering signal - from the transmitter in this case. The coax cable used throughout must be at least double shielded to further prevent desense.

The repeater must sense a desired signal on its receiver and turn on the transmitter. It must then turn off the transmitter a short time after the receiver loses its signal. The repeater must transmit its FCC assigned license number, generally via morse code, generally during every 15 minutes of operation. All of these functions are typically performed by a piece of hardware called a repeater controller.

The transmitter used in a repeater must be FCC certified for the service. Generally the transmitter type is chosen to survive the tougher duty cycle of the repeater and to minimize any interference to the receiver due to phase noise and IMD.

I hope this introduction makes you aware that a successful repeater installation can be rather expensive and it requires a fair bit of technical expertise in addition to some expensive test equipment to get it to function properly and legally. In addition, the Chicago area is very "RF dense" which can raise many technical challenges. You may wish to get someone with the appropriate experience involved in your project.

As an alternative, see my suggestion for WiFi radios in my answer to your other question.

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