From: Doug Bade Date: Thu Oct 14, 2004 10:47 am Subject: Re: [AR902Mhz] 900 gear receive specs? The filters used in the commercial band in most all front mount radio's are centered around 935-940. Call it 938 for simplicity... The filters are usually +_12 +_14 mhz wide at best.. That would give a bandwidth of spec rx at 938 +_12 or 926-950 mhz... More or less.. Such is the case with all of them. Filters start rolling off at 927 in post and are maybe 3db down at 926, and fall pretty fast after that.... When you replace them with 915 centered filters you can get 902-927 spec, but that is the only way.. So... 927.. OK 926 still not bad 921 not so good, like 10-15 db down or more, 915 more like 40db down... 902 just as bad.. Choice is remove and bypass the filters if you need mid band and low end, or replace with 915 centered filters which can be found. I have never changed them in a GTX but as I recall they are surface mount and the Digikey versions should work. Basically the GTX mobile receiver is 1-2 dB poorer at 927.5 than it is at 939 MHz.. trivial. At 01:21 AM 10/14/2004, you wrote: >Well we got a pair of GTX mobiles working now. (Thanks for everyones help). What I now wonder (since I don't have access to a service monitor) what are some typical receive specs on this radio in various parts of the ham band. > Well, it's been a busy week. All sorts of testing and hacking going on. Myself and a group of experimenters have been busy playing up on the 900 MHz ham band. Our prerogative was to determine if it was worth putting up a 900 MHz repeater in town. Our initial test involved GTX, and MTS2000 portables. Both had been aligned to TX approx 2.9W in the 927-928 MHz area. Sensitivity tests yielded average results of 0.19-0.22uV for 12dB SINAD. Myself and another member of the group made arrangements to "borrow" the roof of the tallest building in town, which is about 430 feet up. The other participant went out for a drive in his vehicle, with the MTS2000 using a duckie. How did we fair? Not bad actually. We found that 900 MHz worked FULL QUIETING right up until the line-of-sight factor hampered us. We managed to talk from Winnipeg to 5km north of an indian reservation called "Scanterburry". This is about 65km as the crow flys. (Maybe 40mi or something?) The mobile station was able to use the low-power setting on the MTS2000 for 1W and talk back to us. Very impressive. I've been using a Motorola R-2001A service monitor to set everything up, and measure specs. Here's what I've found so far: - GTX Portables - Very reliable for the most part. Sensitivity is typically better than 0.25uV for 12dB SINAD. Most times around 0.20uV. I've had one so far out of 10, that is completely deaf. I suspect a hardware issue with this one, as they are all used/ ebay radios. TX power is easily set up for 2.5-3.0W output for 927-928 MHz, and they also yield 2.0-3.0W in the 902-903 MHz repeater input area. (We use 927-928 MHz for talk-around, as the radios are COMPLETELY deaf in 902+ MHz Sensitivity picks up again (sharply) at 926 MHz, but it's not acceptable until about 927.4MHz) We were aiming for deviation of about 4.0 - 4.5 KHz through the 902-903 MHz spectrum, and 927-928 MHz. They aren't stable in this regard, and the best I've been able to do is 4.2-4.3 KHz deviation for 927-928, and 2.3-3.4 KHz in the 927-928. That's still not too bad. You can set them up for 4.5 KHz deviation in 902-903, but then it's either over deviated in the 927-928 MHz area (10KHz!!) or under deviated. (less than 1KHz) - GTX Mobiles - They are about the same as the portable. I've been able to get 11.2W - 13.0 W TXPO for 902-903 and 927-928. Deviation is slightly more stable (avg of 4KHz) and sensitivity is about 0.25uV for 12dB SINAD. Again, they are DEAF on 902-903 MHz, but it doesn't matter as this is repeater input spectrum. - MTS2000 - This portable has proven to be slightly more tricky to set up. They have issues (so far) with squelch hanging open, or being too high, regardless of what I do in the soft-pot menus. We use tone-squelch, so it's not that big of a problem. Deviation is low with the built-in mic, but is fine with a speaker-mic. It's almost as if the gain for the built-in mic is disabled. Yes, I've checked/played with the gain settings in the service menu. No go. TXPO is a healthy 2.9W, and sensitivity was 0.26uV for 12dB SINAD. "Stable" deviation (across the band) is about 3.4 KHz. Good enough - we can always turn it up later, and sacrifice talk-around once the repeater is up. - MCS2000 - Now this radio IS THE BOMB. I've been working on a 25W version over the last 24 hours. It is by far, the best sounding beast I've played with. The audio quality on this thing is killer. It is a model I. I had to do a few mods to it to get it to operate how I wanted, but they weren't scary. First of all, I didn't have an accessory plug, so I had to get the service/install manual, and look at the pinout. I had to jumper pins 1/2 to get the internal speaker working, 4/9 to disable the "tx on powerup" issue (emergency button) and I installed a jumper in the control-head/ faceplate to always enable the mic hang-up feature. This enables the tone/dpl decoder at all times. Ready for this? TXPO on 927-928 MHz was 31.8W. TXPO on 902 MHz was slightly less, around 24W. The only issue was with the squelch, which seems to have a lot of hysterisis, regardless of how low I go (improvements noted) with the squelch level. If I set squelch to open for 12dB SINAD, it won't close until the signal is completely gone. If I turn it up a value or two, it gets really tight, and won't open until I inject a signal of about 0.4uV. I set it for the lower value, and we use tone-squelch, so no big deal. The radio is DEAF at 902-903. Sensitivity was 0.19uV for 12dB SINAD at 927.5 MHz, so it's a decent beast. Deviation is about 4.0 KHz, and stable. It sounds AWESOME with a desk-mic. - LCS2000 - This radio sucks. TXPO on the inputs (902-903) is a max of 5.6W. Sensitivity at 927- 928 is about 0.33uV for 12dB SINAD. Deviation is not stable (I can get it to 4.1 KHz on the talk around, but it goes from 1.8 KHz to 11 KHz in one digital pot setting increment) Further exploration is needed before we use this radio. Hardware mods too, most likely. I was kind of surprised, as I thought it was basically a GTX. Maybe it's a bad radio?? I only have one to work on, so I can't play with any more. - LTS2000 - Another disappointment. I thought it was going to be a great HT, but the sensitivity is about 0.36uV for 12dB, and TXPO while good on talk-around (2.4W) is only 0.8W on the 902-903 MHz repeater input spectrum. Deviation was low too, in the order of 1.8 KHz. More experimentation is needed with this HT before we make use of it. Anyway, the MCS2000 is a killer base-station/mobile, and the GTX portables seem to be really decent for the most part. If any of you guys are getting in to 900 MHz, those are the killer radios. The GTX mobile also performs quite well, but has lower TXPO. I haven't tested any spectras yet, but I hear they can get quite sensitive after some basic mods. Next week is a bigger test. We're hauling up the MCS2000, and connecting it to a 21dB omni at 450+ feet (tower on the roof) and doing another range test. This should be fun, and I'll keep you guys updated. 900 MHz is working out better for us than 440 MHz, in terms of range. Play people, play! Shaun