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sarts9v1 / 9v1rs

Files relating to the Singapore 2m amateur repeater.
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Potential problem with higher output power desensitizing the input side #3

Open danweecc opened 3 years ago

danweecc commented 3 years ago

Arnold (9V1CD) had reported of a problem with difficulty or problems triggering the repeater when output power was set to 25W. It is uncertain what might be causing this problem but it is possible that:-

  1. The duplexer is unable to isolate the TX signal sufficiently from the RX
  2. The duplexer tuning could be improved to mitigate this problem
  3. Some other unidentified problem with the antenna system

Currently, the solution is to reduce the output power. It seems that we could profile the duplexer performance to see if it is within specification.

rolandturner commented 3 years ago

I asked Arnold about it and there's no report, just a hypothetical problem, but see also https://github.com/rolandturner/9v1rs/issues/5

danweecc commented 3 years ago

Last night, I observed a QSO between 9V1AI and 9V1AN. Initially, the power of the repeater was set to LOW (or LOW2) by Jaya when 9V1AN called. In the course of that QSO, the power was changed to high (25W) and then back to low, intentionally, in order to test the behaviour of the repeater on 9V1AN's signal.

  1. When repeater was at low power, 9V1AN was triggering it and the output from the repeater was only very mildly noisy. It was otherwise a 5-readability.

  2. 9V1AI then raised the power to 25W to see what the impact was. 9V1AN retransmitted, and this time, the output from the repeater was very noisy to the point that it was no longer readable though the repeater was still triggering.

  3. The power was then lowered again, and the situation reverted to where it was before with a fairly clear output from the repeater of 9V1AN's audio.

I do not recall if 9V1AN got a better reception of the repeater while at high power, probably because the signal became unreadable at that point.

danweecc commented 3 years ago

On my part - I am approximately 1.64km from the repeater, line of sight. When the repeater is at high power, I still get reports of a noisy signal even when I am outputting 10W. This includes from those who are living even closer to the repeater.

danweecc commented 3 years ago

(Repeater at 25W) Had a QSO with 9V1FH who is 7km west of the repeater, operating at 5W. I am copying the repeater signal at S9+, however, most of his audio is very noisy. Likewise, even with 10W at 1.6km from the repeater, the he reports a noisy (though strong) signal. I have recorded a sample of that audio.

https://user-images.githubusercontent.com/13930571/103355563-825e8500-4ae9-11eb-9f9c-722c67a6fdd5.mp4

danweecc commented 3 years ago

(Repeater at 25W) Had a QSO with 9V1AN in Serangoon Central (11km) NW of the repeater, operating at 5W. The incoming signal is holding the repeater well but audio is very noisy with occasionally popping. I am given a 59 report. I have a sample of this audio as well.

https://user-images.githubusercontent.com/13930571/103355643-b8036e00-4ae9-11eb-9459-92933c864e0a.mp4

danweecc commented 3 years ago

(Repeater at 25W) Had a QSO with 9V1LD in Yew Tee, 11km NNW of the repeater, operating at 2W, 4W, and 6W. At high power (6W) he was holding the repeater but with noise and popping sounds in the audio. He reports hearing a good signal but with "hiss in the background". At low power (2W), the repeater will trigger and drop repeatedly, resulting in a string of beeps without audio. Attached is what it sounded like with 2W.

https://user-images.githubusercontent.com/13930571/103359270-95c21e00-4af2-11eb-82ed-6f67c795f655.mp4

Here is where he mentions the hiss in the background from my signal.

https://user-images.githubusercontent.com/13930571/103359518-08cb9480-4af3-11eb-80e7-192f3da01b9d.mp4

danweecc commented 3 years ago

(Repeater at 25W) Had a QSO with 9V1PL who triggered the repeater with 1W into a 7-element Yagi pointed at the repeater (25m elevation). He was unable to trigger the repeater with a rubber-duck antenna from his QTH, 12.9km to the east of the repeater. He reports popping noises from many other users but my signal is doing well. His audio still has noise and corroborated by 9V1LD in the recordings below.

https://user-images.githubusercontent.com/13930571/103364408-ab3a4680-4af8-11eb-8634-5e9a0e425346.mp4

9V1LD's comment on the noise.

https://user-images.githubusercontent.com/13930571/103364424-b8efcc00-4af8-11eb-91bd-52002be48ca5.mp4

danweecc commented 3 years ago

(Repeater at 25W) Had a QSO with 9V1KT using 7W from about 9km east of the repeater. His signal was breaking in and out, often the beginning of his transmission will hold for a while before dropping out. This would happen repeatedly. The audio is also noisy like most of the others. The signal is strong enough to trigger the repeater but would immediately be dropped, thus repeating the cycle (negative feedback). 9V1PL is able to copy the transmission attempt from 9V1KT on the input frequency but the repeater would exhibit the above problem.

https://user-images.githubusercontent.com/13930571/103366163-d2931280-4afc-11eb-9c4d-c3e7511f7239.mp4

In this next clip, he could not even get any audio through with 7W. 9V1PL reported that he could hear the transmission on the input frequency.

https://user-images.githubusercontent.com/13930571/103366178-e179c500-4afc-11eb-9c11-b2c7cb262215.mp4

rolandturner commented 3 years ago

Thanks Daniel, that's pretty concrete. Give or take multipath considerations, that level of noise for a 10W source at <2km seems unreasonable, particularly if cutting TX power resolves it. This points to a significant TX-related desense problem which shouldn't be happening; the cavities have a notch deep enough that the TX at rated power should be below the noise threshold at the RX port.

Separately, information I've received over the last 10 days also suggests that the cavities are significantly out of tune, quite apart from the implications of the tests above.

Jeff has indicated his willingness to retest, if necessary retune (presumably it is), and document. I'll work on logistics.

danweecc commented 3 years ago

Hi Roland,

That's great! BTW, saw you on TV the other day - CPO!

As for the repeater, we will need to come up with a plan to get the whole system looked at. I wonder if the cavity performance changes when it is tested in the lab, and when in the field. The other potential consideration is the ground for the system. Hopefully we can change up the RG-58 to something better - maybe a bit of RG213 or LMR400.

I'll be happy to help get the cavity to Jeff's place for him to look at too. Just let me know. I believe these are Telewave cavities and the one we have should be the TPRD-1484 which, if tuned correctly, should provide 80dB rejection at 500kHz offset. Hopefully we can return it to that level of tuning and I think there will be many happy hams.

Happy New Year!

rolandturner commented 3 years ago

Cable changes are desirable but problematic because of the limited gap between inside and outside. I'll see what I can do.

rolandturner commented 3 years ago

(And yes, the CNA documentary was interesting. The broad thesis has been obvious to me for some years, but a mainstream documentary communicating it is fascinating.)

fly-ing-otter commented 3 years ago

Good afternoon Roland

I have the same issue as above mentioned today during the SARTS Net Call. Was able to trigger the repeater but reports from others is that either the repeater will keep clipping or there is no audio at all.

I checked the antenna again and found that the antenna was resonant at 145.000 MHz as the centre frequency which is close enough to the repeater input frequency of 145.025 MHz. Therefore, there is nothing wrong with the antenna at this moment to my understanding.

I went to try and do simplex with 9V1KB and he was reading me 59 even though we are 12 km apart from our QTH. We even tried to go on Gunung Pulai (MARTS) repeater and there is no issue arises. Therefore, this means that my equipment is working properly.

Attached is the video recording (courtesy of 9V1KB) of the waterfall which shows that there is something wrong with the repeater.

https://user-images.githubusercontent.com/76770374/103478293-839cf400-4e00-11eb-976e-b202da168401.mp4

I thank you and the Repeater Community for the investigation and rectification works.

Cheers.

Best Regards Kevin Tan 9V1KT

rolandturner commented 3 years ago

Hi Kevin, I'm a bit puzzled about what happened during today's net. The repeater was behaving correctly when I set up at about 15:50, but when I tested again at 15:57 I was no longer able to key the repeater (5W HT into 3el yagi on Mt. Emily). I was still able to transmit via EchoLink and could hear myself clearly via RF, so the repeater was still on but, as we saw, noise was high throughout. As above, I'm reasonably convinced that the cavities are off-tune, or not working correctly for some other reason (RG-58 cable to the RX port for example :-( ). The next couple of steps are outlined above.

What is the recording from 9V1KB intended to demonstrate?

fly-ing-otter commented 3 years ago

Hi Roland. Same case as myself. Was surprised that 0.5 W couldn't trigger the repeater which normally I would be able to but not holding on the QSO. So I went on to 7 W to check in the Net and was informed by you that you could only hear beeping of the repeater. I did a total of about 6 attempts to check in but the issue was the same that there was no audio on the receiving end of the repeater.

I went to ask 9V1PL for his assistance to check if my equipment has any issue. We went on simplex and realised that both of our equipment was fine transmitting and receiving each other.

I tried again for a couple of time to check in and on the last attempt 9V1BC Whatsapp me that this time my audio made it through but was quite bad.

I also noted that 2m RF was not heard on Echolink and Echolink stations sounds very muffled on 2m RF.

At the end of the Net call, I arranged simplex with 9V1KB to just confirm that my station was actually ok transmitting long distance. He gave me a 59 for the signal report suggesting that there was no issue on my equipment as it was a 12 km distance with high rise building between us. Following which we went back to SARTS repeater and tried to establish a QSO. However, it was impossible.

The video taken by 9V1KB was intended to show that SARTS repeater automatically cut off my signal when I am transmitting and this could be suggesting that there is desensitizing of the repeater (which all of us by now should know about).

Thanks.

victor9v commented 3 years ago

Cable changes are desirable but problematic because of the limited gap between inside and outside. I'll see what I can do.

BELDEN RG-142 could help - smaller OD with same loss as RG213. de 9V1BH

rolandturner commented 3 years ago

Thanks Victor.

The hypothesised problem isn't loss, it's lack of screening, because transmission-power-linked-desense implies that transmitter power is leaking into the receiver. Belden doesn't rate 84142 for screening but of course super-low-loss cables are necessarily well screened. For comparison:

Cable Attenuation @100MHz (dB/100ft) EN 50117 Screening Efficiency/Attenuation ("SA") (dB)
Belden 84142 3.9 (not specified)
Messi UltraFlex 10 1.1 >105