50 MHz DATV - plans and ideas...

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g8gtz
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50 MHz DATV - plans and ideas...

Post by g8gtz » Mon Oct 06, 2014 11:03 am

Hi

A new topic to discuss designs, plans on air times and dates etc for 6mt DATV. A few thoughts from me to get the ball rolling:

For DVB-S MPEG2 the Tx equipment is easy as the old broadcast equipment using separate encoders & modulators generally have an IF output which will cover down to 50 MHz. The DATV Express board will also go to 50 MHz.

Rx is more challenging – most of us a building up converters.

I use a 1450 MHz Ultram VCO (these are sold in the BATC shop as the 1249 MHz digilite LO but if you don’t connect any control, they run naturally at 1450 MHz). I mix this in a standard minicircuits high level mixer with the incoming RF to give an output at 1400 MHz and tune to it using a standard STB or PC card.

On the rx input I have a G4DDK SPF pre-amp in to a 50 MHz bandpass filter from here http://yu1lm.qrpradio.com/6m%20BP%20FILTER-YU1LM.pdf – some very nice 4 MHz wide easy to build filters. I then use a standard interdigital filter at 1400 MHz to get rid of the image and LO – if you use a lower LO you could use a 23cms interdigital design. Seems to work well down to around -97 dBm lock...

Whether we will ever work VK I don't know - but maybe we should apply for a high power station NoV just outside our band and have this controlled by a web interface. When any DX station hears the narrow band European beacons on 50 MHz, they could go to the password controlled web interface and turn the TV Tx on....

So what are you doing - and should we really be looking at using MPEG-4 and going to really Narrow Band?

73

Noel - G8GTZ

g8gtz
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Re: 50 MHz DATV - plans and ideas...

Post by g8gtz » Mon Oct 06, 2014 1:54 pm

A copy of the leaflet that Dave, G8ADM, was handing out at CAT14 is here:

http://www.batc.org.uk/club_stuff/6mArticle.pdf

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G8ADM
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Re: 50 MHz DATV - plans and ideas...

Post by G8ADM » Mon Oct 06, 2014 4:29 pm

I have done tests with three stations so far.
G3PYB/P Hoggs Back. He received me only.
G8GTZ Basingstoke. No signals.
G3SDQ Leyton, N London. No Signals. S9 +60dB on FM Audio.

I am currently using an Alteia Receiver, I will try some alternatives over the coming weeks.

I have also heard that G4KLB in Portsmouth and M0DTS Co. Durham, G4CPE Luton are all setting up for 6m DATV.

Please let me know if you wish to try any tests on six metres.

Dave.

g4wvu
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Re: 50 MHz DATV - plans and ideas...

Post by g4wvu » Tue Oct 07, 2014 9:38 am

I'm very interested in this - even thinking about adding an input to gb3TN - I have one worry though.....
Are other users of 6m going to get upset with wideband DVB signals putting "noise" over the upper part of the band ? I see looking at the band plan , in particular , there are 6m voice repeaters spread around 50.6 - 51.4mhz , these are obviously right in the 1.5Mhz DVB-s bandwidth. I wonder if 1MS/s might help slightly without having to use specialized receivers etc. (just playing devils advocate)

Mark

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Re: 50 MHz DATV - plans and ideas...

Post by G4EWJ » Tue Oct 07, 2014 1:08 pm

Coming from the narrowband angle, the 50MHz bandplan shows no assignments for 50.9MHz to 51.2MHz, between the repeater outputs and inputs. I'm not familiar with 50MHz at all, so I don't know if that part is actually used for anything specific. This 300kHz segment would allow a 222k symbols/s transmission to fit in without disturbing anyone.

All being well, DigiThin should be able to produce a 222kS, H.264, DVB-S, QPSK signal, but it may need mixing to that frequency as the modulator chip isn't specced down that far.

There don't appear to be any domestic receivers which will handle symbols rates that low, so it would have to be a PC plus satellite rx card until we design our own receiver.

Brian

g7ocd
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Re: 50 MHz DATV - plans and ideas...

Post by g7ocd » Sat Oct 18, 2014 6:17 pm

g8gtz wrote: For DVB-S MPEG2 the Tx equipment is easy as the old broadcast equipment using separate encoders & modulators generally have an IF output which will cover down to 50 MHz.
I'm fully supportive of 'sweating' the 6m (and 10m where we are primary!) sub-bands but we do need be mindful of other users.

I have to disagree with the comment above, TX using old b/c equipment is NOT easy.
The spectrum plot in the flyer that I took away from the AGM only showed the occupancy at -30dB and the one in November Radcom shows the -60dB point in a rather unwanted point.

So, whilst the idea of filling up parts of the band which see less use is great, we need to be a bit smarter with the technology.
I'm not convinced that throwing broadcast standards and kit at narrowband operation will meet our needs.

g8gtz
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Re: 50 MHz DATV - plans and ideas...

Post by g8gtz » Wed Oct 22, 2014 6:12 pm

Hi Jason and Brian,

I think you are both right - we should regard the use of DVB-S and MPEG-2 as initial experiments only and quickly move towards the true narrow band approach that is being developed for the new 146 MHz band in order to limit potential interference to other users.

http://www.batc.org.uk/forum/viewtopic.php?f=76&t=3912

http://www.batc.org.uk/forum/viewtopic.php?f=76&t=3958

If there is an argument to continue with DVB-S for interoperability reasons during the Sporadic E season then we should apply for a high power station NoVs just outside our band...

I also think the spectral regrowth issue will become a major issue for us on all bands and we need to think about setting up a list of regional experts / mentors with suitable test equipment who are prepared to help people ensure their equipment is set up and operating within the recommended limits.

73

Noel

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G8ADM
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Re: 50 MHz DATV - plans and ideas...

Post by G8ADM » Wed Feb 11, 2015 7:18 pm

New Contact on 6m DATV
10 February 2015 at about 22.30 hours my 6m DATV pictures were receiver by Mike, G8LES North of Alton in Hampshire, 40 miles distance.
Mike was using a 6 element beam and I was using just a dipole at 60ft above ground. My location is 450ft ASL. Mike's is somewhat higher.
I was transmitting 200W from a 300W LDMOS PA to minimise any distortion.

I tried receiving Mike with no success, there are many interfering signals on or near the band and my dipole did not do a very good job of reducing the interference.
So the next step is for me to put up a 6m beam, this will have to wait until the weather warms up a bit, then try again.

Frequency: 51.200 MHz. Bandwidth ~1.4 MHz. Symbol Rate: 1.1330 MS/Sec FEC=3/4. Using surplus commercial equipment. Propagation: Normal.

See previous entry above for notes on tests with other 6m stations.

Dave. G8adm

F4WBG
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Re: 50 MHz DATV - plans and ideas...

Post by F4WBG » Wed Feb 17, 2016 9:07 am

Hi,
I am new on this forum. I am french radioamateur. Sorry for my bad english.

I am also interested in DATV on 50 Mhz with low SR. Arround 50.9 MHz +/- 200 Khz it seems to be free here in France.
Is it the same in UK ?
I will be ready end of february for a first test here in France in JN26AO with 120W DATV, SR250, and a Yagi antenna with 6 el. with 11.4 dbi gain

Best 73
Henry F4WBG

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Re: 50 MHz DATV - plans and ideas...

Post by M0DTS » Thu Feb 18, 2016 6:43 pm

Hi Henry,

I will have my 6m yagi back up in April before E's season starts.
I have 5el LFA yagi but no power amplifier yet, but Good Rx :-) If there is activity i will get an amplifier built.

I was monitoring the band last year during Es season and 50.9 is a good choice for centre, there is very little narrow band activity there.
See attached plot for an idea of band use seen from IO94 last year.

Rob
M0DTS
IO94IL
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6m full band usage 280515.png
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