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Amateur Television

Portable ATV Pictures 

Equipment

                                                

The Amateur Bands used for Fast-Scan TV range from 70cm up to the high GHz Bands. The 70cm band has been used for black-and-white TV for many years, but more recently the wider bandwidth available in the 23, 13 and 3cm bands have made them suitable for full colour picture and sound transmission. The PAL colour system is used and is compatible with commercial TV systems.
 
Reception of 23 and 13cm
Until recently the favoured route for reception, was with the use of a surplus satellite receiver, available at very low cost from various sources, rallys, local TV shop, local council tip etc:. This route is still not to be frowned upon, and can provide an excellent introduction to this mode provided a few minor modifications and adjustments are undertaken.
As most aerials present a short circuit as far as D.C. is concerned the LNB supply voltage that is present on the RF input should be disabled or made switchable.
As our commercial friends for whose these receivers were designed for,  use greater video deviation than we do the first requirement is to increase the video gain of the receiver, this can be often as simple as turning up the appropriate preset (usually marked VIDEO GAIN), if not enough adjustment is available then a change in values of a few components in this area usually does the trick.
In the UK we use 6mhz intercarrier sound so the receiver should be set accordingly, this may be variable on older equipment or software controlled on later examples.
Since these receivers were intended to be nothing more than a tuneable IF it is no surprise that they are quite deaf and have a noise figure of around 6db. The LNB providing some 40db or so of stage gain so for 23cm reception a preamp is advisable for all but the strongest of signals.

The route most operators now take, is to use the various Comtech evaluation receivers that are available. The leading supplier in the UK seems to be Giles G1MFG, full info and prices, mods can be found on his website at www.g1mfg.com. These modules seem to be very sensitive and the fact that they are processor controlled removes the guess work as to where you are receiving. Versions are available for the 23 and 13cm band, dip switch control as standard, but LCD and scanning upgrades are available as seperate boards. There is much information available on the web for modifications and improvement.

Transmission
Transmission equipment is also available from G1MFG for 23 and 13cm,  again based on the Comtech modules, the output from these devices is however only in the mw range so further amplification is required for serious work, however using these barefoot to a high gain aerial can bring interesting results.

Aerials
Various types are available but must have a bandwidth of around 70mhz for single aerial working. By far the most common aerial in use is the QUADLOOP, usually based on a design by G3JVL, providing a useful amount of gain and wide bandwidth. Various YAGI types are available, usually cut for the bottom end or top end, a design available from one of the repeater groups that is very popular is a wideband YAGI in 2 versions 10db and 14db starting at less than £20. Some stations also use Flatplate or bowtie types, these are very easy to make, expect around 10db gain and quite a wide beamwidth.

At these frequencies use should be made of the best coax you can obtain, westflex 103 is a good starting point.

Listening on the ATV callback frequency of 144.750mhz can give an indication as to the amount of activity in your area.

G8NWC Amateur Television