I picked up a GR1225 model H5157B. I wanted to use it with 4 of my UHF business frequencies ranging from 463.0000 to 469.000. What I noticed when I programmed my radio was in the 2 higher UHF frequencies it had no power out put on my meter. The lower 2 came in at 40 watts or so. I know they recomend a 5Mhz split. I programed with the receive freq on the high end 468.xxxx and the transmit on the low end 463.xxxx with the recommended 5Mhz seperation. I figured the low end should be my transmit since it was one of the two on the low end that showed transmit power. I can get maybe 10 miles out then it is unusable. I have a base antenna mounted high up on the GR1225 and the mobile is a 5-10 watt handheld to a vehicle magmount. Is there something in the software that can tune it better or do I need a tech?
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GR1225 Setup
- Thread starter RDTS
- Start date
A GR1225 repeater has a duplexer built in and will only work well on the tuned frequencies. Did you retune the duplexer ? Normalyl you have one channel in a GR1225. Alot of times we programmed channels with same frequency with different tone info only.
A GR1225 seller on ebay commented that the duplexer will only pass frequencies that are within .25 MHz of each other if duplexer is tuned for that frequency; example: 463.675 thru 463.925. Is that true? Using a power/swr meter is there a process to tell where my transmit and receive frequencies that the duplexer will allow to pass thru?
The flat pack duplexers are notch only and at best will cover about +/- 100KHz with some degradation at the edges. The way these work is the receive side will notch out the transmit frequency to not only protect the receiver from damage but to also take the transmit energy down to a level where it won’t desense the receiver. The transmit side does the same in removing broad band transmit noise that might otherwise fall on the receive frequency and desense the receiver. The total isolation provided buy both transmit and receive notches is about 75dB.
If you get outside the the tuned frequency range far enough the full transmit power will be present at the receiver and you will damage the receiver. If the duplexer isolation specs are 75dB then it should meet specs on the center frequency it’s tuned for. At the useable edges maybe +/- 100KHz expect about 70dB or less isolation. Get 1MHz away and it may only be 20dB. further away and expect receiver damage.
It would be difficult and time consuming to use a wattmeter to try and find the tuned frequency but with a programmable or VFO radio you could get close. A wattmeter is only going to show maybe 25dB dynamic range at best and you want 80dB or more to see the tips of the notches. You’re only going to get that with a scalar/vector analyzer, spectrum analyzer with tracking generator or some other method with a spectrum analyzer and a signal generator or noise source.
Appreiciate the info. I have a Nano VNA-H4 Vector Network Analyzer from Amazon. I don't know if that's precise enough. Looking for someone in the North Phoenix, AZ area that can do it but I've learned DIY works also. Thanks again!!
