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Friday, 5 July 2013

Home made repeaters

I have been experimenting with building a repeater at home with parts I already have. Not buying any duplexers or cavities or anything. I've tried 3 different types: A cross band, a simplex, and a erm same band repeater.
The repeater with both input and output in the same band seemed to work okay, until I got a friend who lives about 2 miles away to try and get in to it. When he keyed up his radio, it completely desensitized the receiver which lost the signal straight away. The effect of this, heard on the repeater output, was just a load of noise. I would hear noise, then the repeater would close the squelch, then it would stop transmitting, and the receiver would be able to hear the signal again, and it would start all over again.

This might have been to blame on my use of a scanner as the receiver, but as I said, it was a simple design using only parts I already had. All I used was a PMR446 radio as the transmitting side, and a Uniden USC230-E scanner as the receiver. When the scanner heard a signal, it would send the audio through a standard 3.5mm audio cable, to the Binatone Action 950, via a 3.5mm to 2.5mm converted. The Binatone would then key up using vox. I told you it was simple!
 Someone told me afterwards that the scanner probably has a very wide front end, meaning that it would be desensitized very easily. I should try it with another radio, such as one of my Chinese PMRs, but I need to buy another 2.5 to 3.5mm adapter to do that, since the audio out on the Chinese radios I have is 2.5mm. With a large split, and a better suited reciever, I think the same band repeater should work a lot better.

The other type of repeater I tried was the cross band type. I used 446 as the output, and one of the UK simple business channels and the input, on VHF. I know this is not strictly within the rules of the bands, but it is only for experimentation, not permanent use, and I set it up in a way that would cause minimal disruption.
This setup did not have any problems with desensitization, but it did have problems with signal propogation being different on VHF and UHF. I asked the same friend again to try and talk through this repeater, but this time, he could not get in at all, even though he could hear it perfectly fine. His VHF signal just did not reach the repeater.
I suspect this may have been down to the antenna he was using. It was the stock antenna that came with a dualbander. These stock antennas are usually good on UHF, but awful on VHF, since they are just too short. I think if he had used a longer antenna, like a dual band whip made for amateur use, he might have been able to access the repeater fine.
I also tried this repeater out from slightly closer, but still not a location where the signal would be very strong, and it worked fine from there.
So I think the fix for this repeater would just be to use extendable antennas on the radios, so that they could be extended to transmit effectively on VHF.

After these two problems, I decided I would try a simplex repeater, using the scanner again, and the Binatone radio on vox. This seemed to work pretty well for range, but it did have a bit of a problem of cutting off the first second or two of audio, so you would have to repeat messages a lot. It's also quite tedious having to wait for your message to be played back again after you send it.

I think the cross band repeater is the most effective. Maybe if I were to switch the output and input, and use a VHF radio to transmit, instead of the 446 radio, it might work better. Although I would then have to use a computer and a com port keyer to key up the radio, since all of the radios I have don't key up the vox when you use an audio cable plugged in to them.

2 comments:

  1. One thing you could try is a notch filter on the receiving radio, set for the exact frequency of the transmitting radio. A quarter wave stub of coax, open ended, calculated by the velocity factor for the correct length. The better quality the coax the sharper the notch. Not sure if it would work for your split or not. We use a 5mhz split here on UHF, and it may just do the trick for you if that's the case.

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    Replies
    1. The split on UHF was very large. 13MHz to be precise. It still desensed. I may have to try a notch filter as you say.

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