I'm surprised that shifting 1MHz makes a difference Noel. I suppose it depends on the strength.
Brian
70cm DATV RX Experiences
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This forum is run by the BATC (British Amateur Television Club), it is service made freely available to all interested parties, please do not abuse this privilege.
Thank you
Re: 70cm DATV RX Experiences
Brian,
Dave G8ADM can get in on 437MHz with 100 milliwatts over a 50Km path when the "interference" is not on but requires 8 watts when it is (almost 20dB difference). It would be interesting to measure the strength of the interferer to achieve this level of degradation.
I'm no expert but as the signal is actually within the 2MHz signal bandwidth I presume it depends on the resilience of QPSK to other carriers being present in the constellation.
Moving 1 MHz takes the interference out of the signal bandwidth so unless the interferer is very strong and pulls the rx AFC (which is what I suffer from at my site) the rxr doesn't see it and can decode the QPSK correctly.
Noel
Dave G8ADM can get in on 437MHz with 100 milliwatts over a 50Km path when the "interference" is not on but requires 8 watts when it is (almost 20dB difference). It would be interesting to measure the strength of the interferer to achieve this level of degradation.
I'm no expert but as the signal is actually within the 2MHz signal bandwidth I presume it depends on the resilience of QPSK to other carriers being present in the constellation.
Moving 1 MHz takes the interference out of the signal bandwidth so unless the interferer is very strong and pulls the rx AFC (which is what I suffer from at my site) the rxr doesn't see it and can decode the QPSK correctly.
Noel
Re: 437 vs 438 MHz
Tony,
"What is the situation regarding switching to 438Mhz TX?"
I think there are 2 issues to be aware of - firstly to make sure you are legal and clearly you can not transmit outside the band edges.
The other issue is the guidelines laid down in the RSGB bandplan. The band plan is not the law but just as we like it when people respect 144.750 as the ATV talk back channel, we do need to be careful not to interfere with other modes. Bearing in mind that your signal if centered on 438 MHz will stretch from 437 MHz to 439 MHz and at a minimum this will take you across the 7.6 MHz split repeater inputs.... probably best to at least check that one out on http://www.ukrepeater.net/repeaterlist1.htm before transmitting!
73
Noel
"What is the situation regarding switching to 438Mhz TX?"
I think there are 2 issues to be aware of - firstly to make sure you are legal and clearly you can not transmit outside the band edges.
The other issue is the guidelines laid down in the RSGB bandplan. The band plan is not the law but just as we like it when people respect 144.750 as the ATV talk back channel, we do need to be careful not to interfere with other modes. Bearing in mind that your signal if centered on 438 MHz will stretch from 437 MHz to 439 MHz and at a minimum this will take you across the 7.6 MHz split repeater inputs.... probably best to at least check that one out on http://www.ukrepeater.net/repeaterlist1.htm before transmitting!
73
Noel
Re: 70cm DATV RX Experiences
Something vaguely relevant but a few years ago I was playing around with Stanag 4285
(military HF PSK modem) for a project I was working on at the time for a customer.
When there was an interfering tone in the passband the modem would not decode. However if a
notch filter was used to remove the tone the modem would start decoding again correctly.
4285 operates in a 3 KHz BW so the notch filter removed quite a large part of the wanted signal.
A narrowband interferer at 70 cms would only remove a fraction of a DVB-S signal so
a similar approach could be used to null out know interferers in the receivers passband.
So all may not be lost!
- Charles
(military HF PSK modem) for a project I was working on at the time for a customer.
When there was an interfering tone in the passband the modem would not decode. However if a
notch filter was used to remove the tone the modem would start decoding again correctly.
4285 operates in a 3 KHz BW so the notch filter removed quite a large part of the wanted signal.
A narrowband interferer at 70 cms would only remove a fraction of a DVB-S signal so
a similar approach could be used to null out know interferers in the receivers passband.
So all may not be lost!
- Charles
NEW NEW dvb-t Receiver
New product of HiDes, a DVB-T HD receiver for all bandwidths 2 ~ 8 Mhz, MPEG2 and H264, Frequenzrange 170 ~ 950 MHz, with remote control.
Test results on: http://www.oe7forum.at/viewtopic.php?f= ... =120#p1309
Test results on: http://www.oe7forum.at/viewtopic.php?f= ... =120#p1309
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