50 MHz DATV

(2020-01-22) PE1ITR

I started experimenting with DATV on 50 MHz and 144 MHz. There seem to be nice opportunities and challenges on 50 MHz. On the one hand, nice radio propagation is possible at 50 MHz. On the other hand, it is a challenge to make the equipment suitable for this low VHF band. On this website I describe my setup and the adjustments that I had to make in my station.
Do not hesitate to contact me if you also have possibilities to be QRV at 6m with DATV. Even if you can only receive. I am very interested in skeds.

My station at this moment: Portsdown DATV transmitter with a limesdr mini. Followed by a bandpass filter. Then amplifiers up to 30w output. The antenna is a 5 element yagi tuned at the top of the 6m band. I use the DVB-S2 mode with symbol rates from 125k to 333k.
The receiver is a minitiouner with a up converter in front. The 51 MHz is converted up to 161 MHz.

Fig 1: block diagram of the setup.

The experiments take place at the top of the band where it is quiet. Mainly 51.250 and 51.750 MHz are used. Below the band plan with the RB-DATV channels as I use them.

Fig 2: the provisional bandplan as it is currently being used.

The center of the DATV activities is around 51.5 MHz. Because the stuff in my station is normally used for SSB / CW activities around 50.2 MHz, I had to make the antenna and filters suitable for the DATV activities. When it comes to weak signals, we have the best results with DVB-S2, 125k symbol rate and FEC 1/2. I have the impression that sometimes it goes better with a higher symbol rate of 250k. In any case, the image quality is also a lot better.

Antenna

I have a 5 elements 6m yagi that I use for dxpeditions. This antenna can be completely dismantled and stored in a small bag. When assembling, the elements are pushed together and secured with a hose clamp. I simulated the antenna in nec and this showed that the elements on each side must be about 4 cm shorter to make the antenna suitable for 51.5 MHz. So by sliding the elements in a little further, I made the antenna suitable. Below a picture of the antenna and the swr diagram after modification.

This antenna was originally designed as a low impedance antenna. It is 12.5 ohm. Lots of gain, clean pattern and narrow bandwidth. Not really suitable for broadband DATV applications at all, but I'll just do it with it. You can also see the narrowband in the SWR diagram.

Fig 2: 5 elements 1 wl 50 MHz yagi

Fig 3: Antenna swr diagram

Transmitter Bandpass filter

The bandpass filter used in the transmitter chain is an old filter that I made back in 1985 for my analog AM 70cm TV transmitter. I then started from 50.2 MHz as the middle frequency. With a crystal of 96 MHz I made 384 MHz by multiplying by 6. Then this was mixed with the MF 50.2 + 384 = 434.2 MHz. The 50 MHz was the residual sideband filter of this TV transmitter. Then only the 5 MHz above the video carrier was allowed to pass.
The same filter now starts a new life in the DATV transmitter.

Fig 4: TX bandpassfilter

Fig 5: TX bandpassfilter

Upconverter

The upconverter is still experimental and still consists of separate boxes. When the configuration becomes clear it will become single a circuit. From the antenna the signal goes to a pre-amp. Then to a band filter, again a 20 dB amplifier, another band filter then in an SBL-1 mixer. I still use an external frequency generator as a 110 MHz oscillator.

Fig 6: UP converter showing: Pre-amp with build in bandpassfilter, RF2320 amplifier, Bandfilter, Mixer, Minitiouner.

RX Bandpassfilter

I searched the internet for a simple band filter for the receiver with sufficient bandwidth that is flat within the 6m band. I found this design that I have build. The intention is to make more variants and choose one for the definitive circuit.

Fig 7: RX bandpassfilter

Fig 8: RX bandpassfilter

Results

Stations wkd (two-way QSO): PE1GLX, PA3GUO, PE1MIX.

Who wants to be next? Skeds are welcome.

Received	Received by
PE1GLX Reception with RTL stick	My transmission received by PE1GLX
PA3GUO Reception	My transmission received by PA3GUO

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