A Home Brew 224 MHz Repeater Project. May 2009 Part 3 The exciter (transmitter) First Post May 2, 2009, Second Post May 6, 2009 - This Post May 8, 2009
Moving right along... This text is part 3 of a description of a recently completed 224 MHz Repeater Project. One could easily apply the same techniques toward a repeater project in different frequency ranges. Pictures of the completed repeater project reside in the group photos section. http://groups.yahoo.com/group/Repeater-Builder/photos/album/1157128983/pic/list I selected a Hamtronics T301-6 exciter (transmitter) for the project for some of the following reasons. The exciter is frequency synthesized, relatively low in cost (about $229 each at the time of this post), respectable in performance and fairly straight forward to interface. Hamtronics normally requests the frequency of operation at the time of purchase so the receiver arrives "pre- tuned" and ready to interface after mounting. Unlike the receiver portion of this project I did not have an available original Hamtronics Equipment box. I did have a basic LMB type Aluminum box with a matching cover, its dimension about 7x5 inches. I drilled the proper locations for 3/8 inch threaded standoffs at the PC-Board mounting holes. There are no external controls so one only need be concerned with the RF Pipe (coax) and wire connections normally made through the rear panel. Be sure to pay attention to the box height requirement as the coil forms can "stick up" quite a bit. I've used 2.5 inch high boxes but the luxury of a 3 inch height box assures every coil form should easily clear the box cover lid. I again chose the feed-through capacitor method for routing the power, logic and audio type connections through the box wall (rear panel). The picture(s) should give a fair example of the same 3X3 hole drill pattern I used for both the receiver and transmitter FT Caps. Also note the solder ground lugs used mounted with some of the capacitors on both sides of the box wall panel. Let's talk about the coax connector being an RCA jack on the exciter PC Board. Why bother with a box chassis mount connector added to the path when in many cases it's more loss than it's worth. So... I soldered an RCA connector on the end of a section of quality small size (brown tan) Teflon coax and routed it through a hole made just large enough (to pass the RCA plug) with a Unibit Step Drill (bit). A low cost clone set (three in one package) of Unibit type step drill bits are almost a must have expense of less than $15 at Harbor Freight. The other end of the Mini Teflon Coax routes directly out of the box to the RF Amplifier mounted right next door. My only grief was trying to find the original one piece RCA plug. Everyone wants to sell the two-piece plug more suited toward the audio crowd. I used one from the salvage bin while the replacement parts RCA Plug stock back-order gets sorted out. CTCSS encoder audio is routed from the TS-32 in the receiver box to the proper feed through capacitor, the wire on the box inside makes the connection to the proper point on the exciter. All the connection points are clearly described in the Hamtronics Manual. The output of the TS-32 has a lot of available voltage so a knock down series resistor (like say 15K) might put the level adjustment pot up off the almost off - zero position. I didn't bother but I will say setting the CTCSS injection level is something to do with great care. Hamtronics like a number of other Manufactures of current and previous transmitter modules provide a choice of two methods to "key the exciter" RF on and off. A classic method is to operate the low level oscillator section and switch supply voltage to the trailing higher level RF stages. This allows the oscillator or synthesizer to run constantly, which in many examples place the transmitter on the air fairly fast. The disadvantage of leaving the oscillator on all the time is a constant "local" low level signal that is sometimes a lot stronger than you would expect. One of my 224 MHz Spectrum transmitter oscillators can be heard almost a half mile from the repeater site (which is not really very professional or something to be proud of). In the case of this model exciter, one can key the entire module each transmission knowing the synthesizer requires about 350 mS (Milli seconds) to come on the air. There is a provision on the exciter module to delay the RF until the synthesizer is stable and proving a valid lock signal so you're not sweeping a signal across the band as you power (key) the exciter each transmission. So I tied both the synthesizer and RF Chain supply leads together and switched both to the supply voltage with an active low logic made using a small 12vdc reed relay sold by Radio Shack. You can see the relay glued to the chassis right behind the exciter (transmitter) box. It's fairly clean, cheap and simple, which is half way to a free lunch deal. I'll trade the fast exciter key up time for no low level exciter signal in non repeater on-air times. The 12 vdc coil reed relay was out of a flea market junk box purchase but I just purchased another this week so it is a current Radio Shack Product. Shielded audio wire from the chassis rear supplies repeater transmit voice audio from the external repeater controller. Supply voltage to the reed relay is sourced from the Todd Engineering fuse block that gets described in another post. The keyed exciter outputs about 1.7 watts to the Service Monitor input lead and the actual on-air signal is clean and clear. I "shot the exciter box down" to the chassis with the same type of hybrid quasi sheet metal screws I'd been using everywhere else. I have used a number of other box mounting techniques, but in this time pressing case I was looking for a strong, reliable, easy and fast method to anchor the box in place. So you can probably see the screws on the box floor each side of the exciter PC Board. It doesn't get much simpler than what you see in the pictures. Since the exciter arrived complete and pre-tuned... all I did was mount it in a covered box, make the proper connections and interface key logic (the reed relay set up to "ground start" or key the exciter) before securing the assembly down on the main chassis (board). Part 4 will be about the RF Amplifier Cheers, skipp skipp025 at yahoo.com
