DATV on 70MHz.

The place to discuss reduced bandwidth fast scan amateur television on 70cms and below
g8gtz
Posts: 987
Joined: Wed Apr 07, 2010 6:26 pm

Re: DATV on 70MHz.

Post by g8gtz » Thu Jun 28, 2018 1:30 pm

Grrr - despite saying I wouldn't, I have!

It's amazing what the junk box throws up and I now have 20 watts out on 71MHz from a RA30H0608M module driven by the Portsdown / Express combo - I do wonder how far a barefoot Portsdown at 10mw would go.

On rx I have an upconverter (old 70MHz to 1100MHz unit designed for testing modulators), just needs some gain and a filter - found this which looks nice and easy to build http://pa3csg.hoeplakee.nl/WP/?page_id=62

I even found I had a birdthru line element covering this band!

Just got to sort an aerial - maybe a dipole is a good place to start??

EDIT - just knocked up one of these - http://www.70mhz.org/coaxdipole.htm - seems a reasonable match after some tweaking - not sure how it will work but cost me £5 for the tubing and an hours work so worth a try - but even this is very big!

73 Noel

BTW - for more ideas there's a 71 MHz feature in CQ-TV out this week

g8vpg
Posts: 254
Joined: Fri Oct 11, 2013 9:35 am

Re: DATV on 70MHz.

Post by g8vpg » Thu Jun 28, 2018 5:59 pm

Oh good, that means at least one station for me to work /portable! I have now built the Spectrum upconverter and it seems work quite well, with pictures from the Express being received across the bench. I have a borrowed Pye filter and lined it up on 71MHz today. It is a really lovely filter with 0.27dB insertion loss and 3.4MHz -1dB bandwidth. I have all the components needed to build the Mitsubishi based amplifier, so that is the next task. I have ordered an Innovantenna 3 ele OWL type yagi, which is lighter and more compact than their other models. After talking to Justin, he is going to cut me a special for 71MHz for the standard price, but it will take two weeks to arrive. With a bit of luck, I might be ready for the next activity day!
73 Shaun.

g0mjw
Posts: 603
Joined: Sat Sep 20, 2014 9:15 am

Re: DATV on 70MHz.

Post by g0mjw » Thu Jun 28, 2018 8:46 pm

M0SKM wrote:
Thu Jun 28, 2018 12:02 pm
Arthur G4CPE now has a 4m horizontal dipole on a rotator down the garden and my 3 element beam has just arrived. We were both wondering if anyone else apart from Mike G0MJW is ready to do 4m tests?

On Sunday morning I could be on one of the local hills for a test; either Dunstable Downs or Ivinghoe Beacon.

@Mike From this afternoon Arthur will be ready to test with you.

73 Steve
steve (at) m0skm.com
I can be around tomorrow afternoon hopefully. Was at work today.

Mike

g0mjw
Posts: 603
Joined: Sat Sep 20, 2014 9:15 am

Re: DATV on 70MHz.

Post by g0mjw » Thu Jun 28, 2018 8:48 pm

g8gtz wrote:
Thu Jun 28, 2018 1:30 pm
Grrr - despite saying I wouldn't, I have!

It's amazing what the junk box throws up and I now have 20 watts out on 71MHz from a RA30H0608M module driven by the Portsdown / Express combo - I do wonder how far a barefoot Portsdown at 10mw would go.
What level are the shoulders at 20W output? Seems a little high for a 30W module, I have a 5 element beam for 4m.

Mike

G4CPE
Posts: 39
Joined: Mon Oct 31, 2016 3:59 pm

Re: DATV on 70MHz.

Post by G4CPE » Fri Jul 20, 2018 3:53 pm

My second 2 way contact on 4mtrs. I have just worked Don G0WFT on 71MHz. Only 3.5km but Don was only running 10MW to a dipole.
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G1BVI
Posts: 107
Joined: Wed Aug 02, 2017 7:32 am

Re: DATV on 70MHz.

Post by G1BVI » Sun Jul 22, 2018 8:28 am

Hi Arthur
I wonder what you used to up convert to Mini Tiouner?
73

Nick

G4CPE
Posts: 39
Joined: Mon Oct 31, 2016 3:59 pm

Re: DATV on 70MHz.

Post by G4CPE » Sun Jul 22, 2018 12:00 pm

Hi Nick
The converter is an old CML TX up converter which has a L.O. on 930MHz and was used to convert 70MHz to !GHz.
I have also got a CML SAW filter amp on the I/P to keep out unwanted signals.
The set up appears to be good but my aerial is only a dipole which may grow a couple of extra elements when the XYL isnt looking.
73 Arthur

G1BVI
Posts: 107
Joined: Wed Aug 02, 2017 7:32 am

Re: DATV on 70MHz.

Post by G1BVI » Sun Jul 22, 2018 7:13 pm

Hi Arthur
I noticed it said 10Gig on your RX Gui.

I am way behind most people on this project but enjoying putting enjoying putting everything together,

My problem is my eyes are not 20/20 any more hi hi,

Good luck with the upgrafes.

I keep looking at 4/6 antennas and imagining how big they are.... Not got one yet but whatever I get it will never be big enough :-)

Good luck on next contact

73

Nick

g0mjw
Posts: 603
Joined: Sat Sep 20, 2014 9:15 am

Re: DATV on 70MHz.

Post by g0mjw » Sun Sep 09, 2018 8:58 am

I came across this http://www.nooelec.com/store/sdr/sdr-ad ... it-up.html while looking for something else unrelated. There is a new version with a 125MHz LO and that is interesting because it starts to put 10m and 6m into the range of the SERIT tuner.

The reason it doesn't go higher is because it has a low pass filter in there. The design is open source and done in Kicad which is good because I can access the schematic and PCB design (noted - way too many stitching vias).
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It would not take a huge amount of effort to modify this design to cover 71MHz and perhaps also reject the lower HF region. It uses 0805 parts big enough to work with. They are cheap, $40, intended to allow TV dongles to work on HF (though they can now do this directly so what's the point?). I plan to order one to experiment. Note they also work for TX, so it's maybe a way to get a Pluto on 6m. They don't have a pre-amp built in nor the tight filtering you will need to keep out strong adjacent band signals.

Edit - a quick analysis with the excellent AADE Filter Designer shows only the low pass needs to be adjusted. It has an attenuation of ~20dB at 70MHz. The band pass covers ~140MHz-210MHz already which allows for up-conversion from ~15MHz to 85MHz.

AADEs transform function allows you to easily tweak without building and it came up with a set of updated values for a nice filter shape, but this means changing every part in the filter. Going to that trouble you might as well convert it into a band pass filter for the band you want. However, making C11 = 56p, C12 = 5.1p, C13 = 82p, C14 = 27p, C15 = 68p, C16 = 18p, C17 = 56p, L12 = 130n, L14 = 91n, L16 = 100n should result in a nicely matched filter. The loss will be 2dB at 72MHz and 30 dB at 88MHz.

A hack that is good enough is to change just the series resonators, only 6 parts and 2 can be recycled. C12 = 4.7p, C14 = 27p, C15 = 15p, L12 = 120n (formerly L16), L14 = 91n, L16 = 110n (formerly L14). Not quite as good but still has adequate rejection of the FM broadcast band, about 3dB loss at 72MHz and 30dB loss at 88MHz.

A lot of people have a good selection of capacitors but not inductors. If you don't have a 91n inductor to hand you could instead change all the capacitors, so C11 = 56p, C12 = 5.1p, C13 = 82p, C14 = 22p, C15 = 68p, C16 = 15p, C17 = 56p. This gives a response is similar to the above with about 3dB loss at 72MHz and 30dB loss at 88MHz.

Another hack is simply to bypass it and do your FM band filtering elsewhere, e.g. in the preamp.

g0mjw
Posts: 603
Joined: Sat Sep 20, 2014 9:15 am

Re: DATV on 70MHz.

Post by g0mjw » Sat Sep 15, 2018 8:34 am

The ham it up board arrived quickly, I was able to make the modifications and it worked.

The board as supplied has 10dB loss at HF rising to about 15 dB at its upper limit of 65MHz. Following the modification to the low pass filter the upper frequency limit becomes 71MHz as intended, with a loss of 16 dB. There is a steep rise in attenuation above 71MHz which is what we need to reject the strong signals in the 88-108MHz broadcast band. The board has a useful bypass ability so you can leave it in line to receive higher frequencies. So far, so good, a handy up/down converter. For DATV use it will need a pre-amp as there are no amplifiers on the board to overcome the mixer and filter losses.

I would not recommend using this board, largely because of the unexpected difficulty of working on it. Ideally it needs a hotplate and a high power iron. I have these things but many don't. The board uses many vias and there is no thermal relief for RF reasons. Pads connected to the ground plane have a very low thermal resistance to the large ground plane and as the board uses lead free solder with a high melting point, replacing parts is difficult.

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