MORE TO COME SOON, INCLUDING DIGITAL TV , SOFTWARE CONTROL & BRAIN SURGERY
One of the first things to do if you are interested in getting involved in Amateur TV is to establish whether there are any ATV signals being sent in your area. Monitoring on 2 metres should give you an idea of any local activity.The usual ATV talkback channel on 2 meters band is 144.75 Mhz. VHF polarity is usually vertical though some stations do use horizontal. This may vary from area to area and on different repeaters.70cms is used for some talkback,especially if a suitable repeater is available.You may only hear one side of the conversation of course until you can receive on 23cms. Look at the LINKS page for suppliers of equipment described here.
If
you have a repeater near you then an old Amstrad
analogue satellite receiver can be used but normally requires a preamp
and they do suffer from being wideband fm rather than the standard ATV
fm signal giving a rather weak video. A decent aerial and possibly a pre-amp will also be required.
A far more elegant solution is to use a Comtec receiver . Available from
2 sources in the uk and on Ebay (see links page), they give 625 line colour
picture from a 23cms signal input. Given a decent yagi up on the chimney
etc will work well with local signals even without a preamp. If your local repeater is digital output buy a suitable FTA satellite box. See here for more details
For
2 metre talk-back vertical polarity is generally used and in many areas
a dipole aerial roof /chimney will work quite well with a 5/10watt transceiver.
Low cost 4 element yagis are available from Amateur Radio shops, but being
directional will generally need to be mounted on a rotator. Horizontally
polarised yagis are used for tv on 23cms and are reasonably easy to make
or can be bought from about £25. If you decide that gain is less
important than 360 degree radiation then a JVL/Alford slot aerial will
be useful, thus avoiding the need for a rotator. A 23cms slot and a 2 mtr dipole will save the price of a rotator
etc but at some loss of gain.
Many local spoilsports authorities will insist that aerials
should not not exceed the highest parts of the building.
If you need the extra height then you will need to submit drawings/plans
and get planning permission. Alternatively build a sixty-foot chimney at the rear of the house and fix the aerials to that!
A quick change-over from tx to rx is not normally important with tv as
overs tend to be long, coax changeover relays are available but expensive ans rather lossy
for these frequencies. A supply of ex-equipment relays are often found
at rallies.They will normally be either N or
SMA connectors.
An important piece of test equipment you will certainly need at some point is a 23cms swr/power meter. It allows you to tweak the equipment for maximum power out and equally important to ensure that as little as possible returns back from the aerial. Ironically this little piece of equipment actually costs more than the single PA. If you are setting up for ATV you will probably be able to borrow one from a local operator or take a look for a secondhand one. A homebrew pcb version will appear on this website later .
TV
camera.
An old camcorder- perhaps one with a broken tape mechanism- is ideal,
although in a temporary set-up a webcam through the computer using a graphics
card with 'tv out' feature will get you on air. You dont have to send BBC quality pictures, just getting pictures out for others to see will be fine to start with, and remember the guys on air already will know just what an achievement it is to get a tv signal up and running and may well have bits and pieces in their junk boxes that you can use.
Switching
Some form of switching between different video sources will eventually be needed, a simple video switchbox as used for video recorders etc is fine. A basic mixer with preview and level adjustment per channel will appear on this site later this year.A true mixer with total fade is not normally an option as this requires all equipment to be syncronised to a central timebase, not normally possible to achieve with domestic camcorders etc. Mixers such as the Panasonic MJAVE5 convert both inputs to digital and then back to analogue allowing mixing and crossfade effects to be produced and can be found on Ebay for a few hundred pounds.
Sound.
A gooseneck microphone through the sound card of the computer is a way
of producing sound, some camcorders have in-built microphones but can
sound rather 'distant' unless a wide angle lens allows you to be just
a few feet from the camera. If you are listening to the 2 metre talkback
through the speaker of the receiver it is best to keep the microphone
as far from the speaker of the receiver as possible to avoid feedback.This
is prefferred to headphones, apart from the inconvenience of headphones,
it can be useful for other viewers to be able to hear the other half of
the conversation where the 2metre coverage is poor.However on digital sighnals it can be a problem because of the half-second delay during encoding.
Monitor
tv
Any 625 line colour tv with either a SCART socket
or an RCA/phono video input.
Computer
Any computer
with a TV out card
Some useful links...
Making printed circuit boards
CQTV
Newcomers pages
Using
old satellite receivers for receiving atv
23cms KITCH yagi
Information
on connectors
and for aerial suppliers see the LINKS page
