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Thursday, 11 October 2018

Amateur television - a new adventure in radio for me

As you know I went to the National Hamfest this year for both days, giving me an opportunity to speak to more organisations than normal, one of which was the BATC, who were there with examples of their Portsdown transmitter used for DATV (which uses DVB-S and therefore can be received on a domestic satellite receiver but not Sky branded equipment as that is locked down in firmware and cannot be unlocked to tune to DATV) and a couple of other bits and pieces and an opportunity to join, after giving it some thought I plan to do so but lacking a suitable Raspberry Pi I cannot begin to construct the Portsdown myself so I looked at a cheaper and more challenging alternative that the BATC have on their website as a means to transmit ATV.

The alternative are the transmitter and receiver pairs used for FPV from drones, these operate in the 5.8GHz region, part of which falls within the 6cm band, since I gained my intermediate I also have privileges to operate on that band, something I did not have with my foundation, the agreed on frequency is 5665MHz, or 5.665GHz, which most of the units, though not all, can tune to, though I'd want to mount these in a diecast enclosure for two reasons, one so the enclosure acts as a heatsink for the device and two to keep RFI down if any, these units take a standard CVBS (composite) video feed though may need the appropriate sockets fitting to connect the camera to the TX and the monitor to the RX, the TX unit also needs a means to insert a callsign and send out a test card, the BATC suggest a memory card media player unit for this purpose, my idea is to add a video splitter to allow me to see what is being sent to the TX as well.

Camera wise I am sorted, I have a long redundant Samsung camcorder that, since I bought my JVC Everio camcorder, has been seldom used and been allowed to remain in a state of discharge, it had fitted a coin cell battery to back up the time and date, though not a feature I'd need and this battery has not leaked and the camera works fine and tested into a video source the picture is clean enough for transmission.

Antenna wise I need to look at, a single antenna requires a changeover relay, these can be had at rallies for a small amount, the antenna itself needs to be directional, horizontal polarisation, and equipped with an N type connector, 5.8GHz WiFi antennas would work here though these are expensive, a cheaper solution is to build an antenna, as I like building antennas I see no reason why not, a design I have seen involves copper wire and double-sided PCB and a chassis mount N type connector, as I lack any coax at this time I could simply build 2 antennas so I have one for TX and one for RX and the units connect directly, however these use RP-SMA and as such require an adaptor or an appointment with a soldering iron to swap the connector to standard SMA and again an adaptor, the former is quicker and easier.

Before I begin I plan to join the BATC as their annual fee is low, around £8 or so if my memory serves right and they send out a quarterly publication called CQ-TV, which sounds like a good read.

I watched a video from BATC's CAT17 conference in which the very person I spoke to at Newark discusses the 5.8 GHz FPV system being used for ATV, and a demo as well, including antenna information which include modifying a Sky Minidish (which are very common now and there are a few surplus ones doing the rounds) with a PCB log periodic antenna at the feed point, the BATC wiki details a homebrew antenna that I alluded to above.

Lastly at this frequency your normal coax is useless, so in the case of 2 antennas and no changeover relay the TX and RX are directly connected to them, 0dB loss, or if you're using a changeover relay, one antenna and possibly also a power amplifier (to get the TX up to 2 watts) then you'd need short coax links that are not lossy at these frequencies and fitted with SMA connectors for changeover relays and adaptors to convert the RP-SMA to standard, at my next rally attendance I of course will be keeping an eye out for microwave changeover relays as these frequencies are classed as microwave.

So in summary I have a camera, I have a mic to connect to the camera, I have a cable to connect the camera to a monitor/TV (which will go to a TX instead), all I need now is the parts to build the system, and because this station will be operated /p 99% of the time it will be created with this in mind, I also intend to build a self-contained receiver standalone to the main station to ensure the TX is actually working with help of an assistant.

I may, once I've got this up and running and had a couple of QSOs over it, demonstrate it to the local club, perhaps keeping the RX at the club and they talking back on 2-meters on the agreed talkback channel.

The best part about all of this is that everything can run off 12 volts so all I need is a battery, and while doing this I can still work on the Portsdown which will take a little while but I enjoy building things, part of the appeal of Amateur radio, the technical side of things too also is of interest hence this interest in Amateur television.

Time for further research and I hope to get something working real soon and I'll keep you updated on progress as it goes on.

73 de 2E0EIJ

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