Maxtrac repeater combo

In the November 1990 issue of MRT, Carlton L. Tedrick, P.E., described how to link two Maxtrac mobile radios to serve as a link or repeater. In the May 1995 issue of MRT, William Plante, NCE, offered an improved version of the Tedrick design. In this issue, I offer you yet another version. This version is simpler to implement than the Plante design, yet sacrifices no features.

The old adage “Necessity is the mother of invention,” is quite true. In this particular instance, several needs were met by combining two Maxtrac mobile units to serve as a portable repeater or communications link. Much of the design work was already done for me by the previous designs. It was only necessary to simplify the second design (Plante’s design) to come up with the design that worked best for our situation in the South Carolina Forestry Commission.

We had procrastinated for some time about developing a portable repeater to help with communications during large forest fires where communications between handheld units was marginal. Other needs, such as a temporary spare repeater and a temporary communication link between state agencies on joint operations, spurred the development of this portable repeater.

Figure 1 below shows how the Maxtracs are modified and linked to form the repeater. The design by Plante in the May ’95 issue of MRT required a transistor, two resistors and a switch per Maxtrac. This design requires only a diode and one resistor per Maxtrac along with a couple of simple jumper connections. The additional circuitry for the modification is shown in red.

To understand how the setup works, refer to Figure 1. A key point in the circuitry is the collector of transistor Q552. This is on the logic board of the Maxtrac and near the front, allowing easy connection of diode D1 from the collector of Q552 to pin 8 or 9 on J8. When the radio is not receiving a signal, the collector of Q552 is at +9.6V. Let’s analyze the situation if radio A is receiving a proper signal. If a signal is received with the proper CTCSS (continuous tone coded squelch system) tone, Q552 is turned on placing the collector near ground potential. This turns on diode D1 which, through the interconnecting cable, places pin 11 of J8 on radio B (push-to-talk) at ground potential. This keys radio B. Audio from radio A (the receiver) passes from the volume control top through R1 (1.8K) and through the interconnecting cable to pin 12 (mic input) of J8 on radio B.

In this situation, radio A is the receiver, and radio B is the transmitter. The symmetrical arrangement allows for bi-directional operation. That is, radio B can become the receiver and radio A the transmitter. In certain situations this is desirable. But in some situations it is undesirable. Proper programming of the Maxtracs allows for great versatility in the repeater or link setup. It isn’t recommended, but the receiver can be operated using carrier squelch instead of CTCSS. The receiver can be programmed for carrier squelch, or the monitor button can be pressed to disable the CTCSS circuitry. Then the radio receiver will respond to any carrier on the channel. In this mode the collector of Q552 will go low on any on-channel signal-CTCSS or not.

The repeater or link can be operated several ways. The setup shown in Figure 2 on page 39 can be used to set up as a cross band link. Figure 3 at the right shows the setup as an in-band repeater. The duplexer that I selected is a mobile reject duplexer for the 150MHz band. It requires at least a 5MHz difference between receive and transmit frequencies. In our system, we receive at 151MHz and transmit at 159MHz, so this mobile duplexer works out fine for us.

A need arose where the South Carolina Forestry Commission and the South Carolina Department of Natural Resources needed to communicate on a joint operation. Since the DNR was operating at 151.xxxMHz and the SCFC was operating at 159.xxxMHz, the situation was ideal for the Maxtrac combo and duplexer. All that was required was to program the DNR frequency into one Maxtrac and the SCFC frequency into the other Maxtrac. Some tweaking of the duplexer provided good isolation between the two radios. With this arrangement connected to a short antenna, the two agencies could easily communicate-each using its own channel. The repeater setup was virtually transparent in operation. See Figure 4.

If you are using a duplexer for in-band operation, it is important that the duplexer is properly tuned to the operating frequencies for both radios. There will be a high side and a low side with a good frequency separation. In this case, the high side was at 159MHz and the low side at 151MHz. The key is to get maximum rejection at the undesired frequency. See Figure 5 for the following discussion.

The Maxtrac connected to the low-pass side of the duplexer (green) is operated at the lower frequency, in this case-151MHz. The Maxtrac connected to the high-pass side of the duplexer is operated at the higher frequency, in this case-159MHz. The low-pass side of the duplexer must be tuned to reject 159MHz. Thus, the Maxtrac connected to the low-pass side has a 159MHz frequency programmed into it to allow for tuning the notch. Simply set the Maxtrac to the 159MHz frequency, and set the signal generator to the same frequency. Adjust the tuning of the low-pass side of the duplexer for maximum rejection of the 159MHz signal as indicated on the sinad meter. As the notch deepens, you will have to increase the level of the signal generator until you have adjusted the notch to maximum rejection of the 159MHz signal.

To tune the other side of the duplexer, you will have to reverse the connections shown here and tune the high-pass side for maximum rejection of a 151MHz signal. You would have to have the other Maxtrac programmed for a 151MHz frequency just for duplexer tuning purposes. The deeper the duplexer notches are tuned, the better the performance you will get from the repeater or link.

The modifications to the Maxtracs were done at the pins of J8, keeping the connections close to the board on the pins so as not to interfere with the connection of the plug, P8. Users should take extreme care to avoid solder bridges. The diode connected from the collector of Q552 to pin 8 or 9 on J8 should be enclosed in spaghetti tubing to avoid a possible short circuit with other components on the logic board.

To make the unit more convenient to handle, we used a portable file box to house the Maxtrac and duplexer combo along with the associated wiring. As shown in the photo, we installed a voltmeter, two fuses (one for each Maxtrac), an on/off switch, cigarette lighter socket, banana jacks/binding posts and a ventilation fan on the side panel of the file box. Two 12-volt ventilation (muffin) fans are used in a push-pull type operation so that sufficient air is flowing through the box at all times. A UHF connector is installed on the side of the box to allow easy connection to the external antenna. The portable repeater can be placed in a vehicle, connected to the vehicular antenna, powered from the cigarette lighter jack and serve as a temporary repeater wherever needed.

Until next time-stay tuned!