Paul Knupke Jr. N4PK wrote:
- A group of us in the Saint Petersburg, FL area are putting a new 1.2G repeater up for all of us T81A owners. I've not had much success finding duplexers for this band and am looking for some direction to pass on to the group.
Sinclair, TX-RX, and Wacom all make 1.2 GHz duplexers. Angle Linear was mentioned as well. As an alternative, if you don't mind a little work, you can play around with UHF cavities. They will resonate as 3/4 wavelength on 1.2 GHz. You may have to cut down the loops a bit. Plain bandpass cavities are easiest to work with, but many pass/reject designs will cooperate on 1.2 as well.
I'm not sure if they have any left, but Sinclair was blowing out their
1.2 GHz "Sinclabs" compact duplexers last year, so you might check with
I've used the Diamond F1230 (?) 1.2G monobander. It worked pretty well. Comet makes several 1.2G monobanders, but I haven't tried them. Austin Antenna also makes 1.2G antennas. I don't know of any of the commercial manufacturers that make them unfortunately.
If you don't mind, I'd like to make a suggestion. Consider using transmit and receive converters with good commercial radios as IF's/exciters. The transverters that Downeast Microwave sells are very affordable and work well. The cost of a transverter plus your favorite rig (MICOR, MASTR II, whatever) will be less than the 541's. You can get the transverters pre-assembled or as kits.
If you want to home brew, a varactor tripler is relatively easy to build for the transmit. However, it's not for the faint of heart, and you'll need good test equipment. It's very easy to throw spurs from a poorly designed or improperly tuned varactor tripler.
A simple receive downconverter with decent dynamic range and noise figure can be made using an MMIC mixer and a fixed 800 MHz oscillator as a low-side LO. An 800 MHz LO is easy to get; grab an 800 MHz radio (practically worthless on the used market these days thanks to Nextel) and use its LO. Or rob the LO off an 800 MHz MICOR receiver. Doesn't matter what the exact frequency is; just order UHF crystals to match your makeshift IF frequency. Just make sure your IF doesn't fall on anything in use nearby. This method (the bare-bones MMIC mixer) isn't as good as a real receive converter with a tuned front end and additional IF filtering, but something to experiment with. Considering a commercial IF radio has a nice tight front end, you might be able to get away with it as long as you have enough Tx carrier suppression in the duplexer itself.
The commercial rig route also has another benefit. If you use something like a UHF MICOR as an IF, you get the benefit of AFC which at 1.2 GHz is about as important as a noise blanker on 6m. Not only because of drift on the part of the repeater's LO(s), but the user radios as well. 2PPM at 1.2G is 2.4 kHz, 5PPM is 6 kHz. A lot of the ham 1.2G radios have AFC built in, so as long as your repeater's transmit is 2PPM you should be OK.
Downeast also sells PA kits. But remember that most of your users on 1.2 FM are going to be running 10 watt radios max., so there's no need for a big PA on the repeater. Make up as much as you can in antenna gain and height. As a friend of mine says, on 1.2 GHz, "If you can see 'em you can work 'em."
In most cases, provided you have enough duplexer isolation, a good preamp is easy to get away with on 1.2G due to the lack of pollution from other services and a nice naturally low noise floor.
Just a few more things for you to ponder...
Building 1.2G repeaters in the Netherlands....
Reacting to the recent interest in 1.2G repeaters, and realizing that my 1.2G repeater (PI6EHN) was put into test operation only several weeks ago, I thought a couple of comments might be useful. Hopefully this inspires other people to start activity; the band desperately more people using it.
The band plan here is different than the American one. We have 1240-1300 MHz. The top and bottom MHz are used for digital links (full duplex, 59 MHz shift). The approach for repeaters is very different: inputs in the middle of the band (1270.200-1270.700), with 28 MHz shift either up (1298.200-1298.700) or down (1242.200-1242.700). Assuming that input signals are weaker than the repeater itself, this allows for more repeaters to be used; also this allows easy compatibility with the full duplex digital links if the node and the repeater are at the same site (if a node has it's outputs high, then the repeater will also be high and vice versa).
Also, this keeps the user's transmitters above 1260 MHz; Japanese 'J-mark' equipment is TX blocked below 1260 MHz (which cannot be overridden with a diode) but this is no problem for 1270 MHz inputs. At any rate, this means that the shift here is 28 MHz; more on that below.
There's quite a bit of development for packet in Germany. This includes so-called 'link-trx' transceivers. The 'Link-TRX IIIb' is very suitable to build a repeater; it consists of a receiver module (1.2G in, audio out), a transmitter module (audio in, few milliwatts out), and a power amplifier module (choice of two, 1.5W out, using the well known Mitsubishi M67715 module, or 15 W out, using the Mitsubishi M57762 module). (These power modules are very useful BTW, even if you use a different path, you may want to consider using them as power amplifiers; rumor has it however that Mitsubishi is taking some out of production). Both receiver and transmitter use a simple phase-locked loop with a prescaler (divide by 128), and a crystal for reference. As a result, the spectrum is very clean and has a low noise floor. I'm using the 15W module; I always try to run amplifiers below their ratings (they last longer with high duty cycles) and with the cable losses, the filter losses, the 15W module is a good start.
Duplexer, Cable & Antenna-
The 'LinkTRX' designs include a duplex filter, built as two interdigital filters, which works very well and is easy to build. The large shift required helps quite a bit in filter requirements. Getting materials of the right size isn't easy though, I was lucky. The filter is short enough to fit in a 19" rack; the whole repeater is less than 5U high. I use H2000 coax cable; this cable has a loss of 15.7 dB for 100 meters, and allows a small bend radius. The antenna is a Diamond X5000, which is a triple bander (2, 70, 23), the other bands are used for other equipment on the site.
Getting the right stuff for CTCSS encode/decode wasn't easy. CML/MX-com chips are nearly impossible to get (in small quantities, for reasonable prices). Communication Specialists modules are also not easy to get here. Currently I'm running an experimental circuit where the CTCSS tone is generated as a bit stream from an EPROM; the same logic is also used to provide clocking for detection as well as bit streams for ID and such. These bit streams have low distortion and are reliable since the signals are generated digitally. I'm interested what other people have come up with; the repeater controller is still under development. Also, does anyone have experience with CVSD to digitally encode voice for repeater interlinks?
There is a missing trace in the LinkTRX receiver PCB. I forgot the details, but as a result part of the IF chip does not get bias; it's easy to find once you know what to look for. I don't know what the copyright situation is of the LinkTRX III design, but I only have paper copies of the design (in German, but a resistor remains a resistor). If there is sufficient interest I can try to ask the author, DF9IC.
Many thanks to Albert, PA3GCO, who explained much of this several years ago which finally resulted in getting PI6EHN built.
Geert Jan PE1HZG
1.2 GHz in Philadelphia Pennsylvania...
Over the summer of 2000 I was inspired to build my first repeater, a 1.2 GHz machine in center city Philadelphia when I was offered space at a site at 285 feet. I was able to find a reasonably priced Sinclair duplexer on Ebay and used 1.2 amateur mobiles. Due to frequency drift concerns in the older rigs, I had to break down and get a Kenwood TM-541 with AFC for my receiver to keep all my users on frequency (they would drift 6 kHz in each direction during a one minute QSO!). I do not have the expertise as yet to convert commercial equipment so this was basically an experiment.
I am very pleased with the results. I am currently using a Kenwood TM-541 receiver with Down East Microwave preamp. I am also using a band pass filter made by DCI in Canada set to pass 3 MHz on either side of my input.
Transmitter is an older Icom mobile with a Down East Microwave 65 watt PA. Antenna is a Diamond F-1230A monobander with 90 feet of 7/8 Andrews Hardline. I lose a little less than half my power getting to the antenna. 1200 is very lossy which is to be expected. I have found that I get 60 to 75% of the coverage of a MICOR 440 machine at the same site. I can work to 10 miles out with a handheld and around 20+ miles with a 10 watt mobile. 1200 is very unforgiving to any type of valley. It is true line of sight. Troposheric ducting occurs at 1200 through. Last October I was able to key the machine with 1 watt from the Outerbridge Crossing going into Staten Island, NY some 75 miles away.
The Icom T-81As work fairly well at 1200 however with the 9.6v battery you only get about 90 mW on low and about 400 mW on high. The Yaesu FT-911 is a better handheld because of the 100 mW on low and just over 1 watt on high with a 7 volt battery. Believe me, that extra 600 mW makes a difference when I switch from the T-81A to the 911. I have an Icom 901 with 1200 as my mobile with about 750 mW on low and about 8 watts on high at the antenna.
Overall I am very happy with the results and so are my users knowing they can at least warm up the final in their T-81's on the top band. Looks as though I'm the only coordinated machine in the Delaware Valley on 1200 MHz. We are considering a 10 meter FM remote base this summer to spark some interest. Anyone is welcome on my system if they are in or near Philadelphia (K3PHL/R 1294.950 MHz -20 MHz PL 77.0 Hz.). It feels like rare DX when a new user drops his call.
73 DE K3PHL
We did indeed put together a repeater in St. Pete which is on 1291.0 MHz, -20 MHz with 127.3 tone. The group decided to go the quick route and buy a pair of TM-541 mobiles. They work great. I bought one for my car this fall in fact so I could get into the repeater better driving around town.
Our repeater guru was able to tune a UHF duplexer at 3/4 wavelength successfully. We are on a 30' run of inch and 5/8th to a Comet GP-21 monobander at about 150' AGL. After losses, we are seeing 6 watts at the antenna. The repeater does hear better than its heard in spots. An amp may be a future add on.
HT coverage is good out about 10 miles as long as you find a hot spot. I'm 9 miles from the repeater and depending on where I stand I can get into it either full quieting or not at all. Pretty typical 23cm - line of site stuff.
Mobile coverage is decent out to 20-22 miles with 10 watts. There are maybe 6 or 7 of us with mobile rigs, the rest of the users are have the IC-T81A.
The owner of the other 1.2 machine in town has been keeping track of the people he has talked to on this repeater and last count he had talked to over 40 different people so in a year we went from 1 to 2 repeaters and about a dozen to over 50 hams with 1.2 gear. Our repeater gets drive time usage as well as is popular to get away from the masses. Almost everyone around the Tampa Bay area has UHF. Sure would make a good hideout if we are foxhunting a jammer.
I have also used the 1293.1 machine in Orlando (both from here in St. Pete and over in Orlando) and I was able to access at 1285.0 machine in Toledo last summer (no one came back to me.)
So anytime anyone might be in the Tampa Bay area vacationing or what not -- and if you have 23cm gear, give a shout on our repeater. Its an open repeater ... so far I've not found one objectionable person 23cm.
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