Ama­teur tele­vi­sion (ATV) is the trans­mis­sion of Broad­cast qual­i­ty video and audio over the wide range of fre­quen­cies of (radio waves) allo­cat­ed for Radio ama­teur (Ham) use. ATV is used for non-com­mer­cial exper­i­men­ta­tion, plea­sure and pub­lic ser­vice events. Ham TV sta­tions were on the air in many cities before com­mer­cial tele­vi­sion sta­tions came on the air. Var­i­ous trans­mis­sion stan­dards are used, these include the broad­cast trans­mis­sion stan­dards of NTSC in North Amer­i­ca and Japan, and PAL or SECAM else­where, util­is­ing the full refresh rates of those stan­dards. ATV includes the study of build­ing of such trans­mit­ters and receivers, and the study of radio prop­a­ga­tion of sig­nals trav­el­ling between trans­mit­ting and receiv­ing sta­tions.

ATV is an exten­sion of ama­teur radio. It is also called HAM TV or Fast Scan TV (FSTV) (as opposed to slow-scan tele­vi­sion (SSTV), which can be trans­mit­ted on short­wave ham bands due to its nar­row­band struc­ture, but is not decod­able by a com­mer­cial­ly avail­able tele­vi­sion receiv­er).

In North Amer­i­ca, ama­teur radio bands that are suit­able for a tele­vi­sion sig­nal (wide enough to fit such a sig­nal) are high­er in fre­quen­cy than VHF broad­cast TV. The low­est fre­quen­cy ham band suit­able for tele­vi­sion trans­mis­sion is 70 cen­time­ters, which is between broad­cast chan­nels 13 and 14. While out­side of broad­cast tele­vi­sion chan­nels, this fre­quen­cy falls into CATV fre­quen­cies, on chan­nels 57 to 61 (420−450 MHz). As such, ATV trans­mis­sions can be viewed by set­ting a tele­vi­sion to cable input and attach­ing a ter­res­tri­al anten­na. For more sen­si­tive recep­tion, some users may use a pur­pose­ly-built ATV down-con­vert­er, which is a kind of set-top-box. Oth­er bands are also used for ATV, most of them in the UHF region on fre­quen­cies high­er than UHF broad­cast TV. 33 cen­time­ters and 23 cen­time­ters are two oth­er com­mon­ly used bands for ATV, but recep­tion of these high­er bands requires the use of a down-con­vert­er.

Most ATV sig­nals are trans­mit­ted in either Ampli­tude mod­u­la­tion (AM) or ves­ti­gial side­band (VSB) NTSC (North Amer­i­can ana­log TV broad­cast mod­u­la­tion stan­dard). AM and VSB sig­nals are inher­ent­ly com­pat­i­ble with each oth­er, and most tele­vi­sions can receive either. AM sig­nals are wider than VSB sig­nals, but VSB trans­mit­ters are more dif­fi­cult and expen­sive to con­struct. For prac­ti­cal rea­sons, most indi­vid­ual ATV users trans­mit in AM, and VSB is trans­mit­ted by repeater sta­tions. On the 33 cm and high­er bands, Fre­quen­cy mod­u­la­tion (FM) ATV may be used, and on the SHF and EHF ham bands, FM is more com­mon­ly used than VSB or AM. FM ATV is incom­pat­i­ble with AM/VSB ATV, and a sep­a­rate demod­u­la­tor is nec­es­sary to receive sig­nals.

70 cm band

The 70 cen­time­ters ham band is the most used band for ATV. Sig­nals trans­mit­ted on this band usu­al­ly prop­a­gate longer dis­tances than on high fre­quen­cy bands, for a giv­en trans­mit­ter pow­er and anten­na gain.

The 70 cm band chan­nels (with car­ri­er fre­quen­cy for video and audio) are:

57: 420–426 MHz (421.25 video, 425.75 audio)

58: 425–431 MHz (426.25 video, 430.75 audio)

58: 426–432 MHz (427.25 video, 431.75 audio)

59: 432.75−438.75 MHz (434.00 video, 438.50 audio)

59: 432–438 MHz (433.25 video, 437.75 audio)

60: 438–444 MHz (439.25 video, 443.75 audio)

61: 444–450 MHz (445.25 video, 449.75 audio)

33 cm band

The 33 cen­time­ters ham band is next high­est fre­quen­cy band avail­able for ATV in North Amer­i­ca. This ham band is unique to ITU Region 2, and it is rarely avail­able for ama­teur use in ITU Regions 1 or 3. This band is also shared with many users, includ­ing ISM devices and Part 15 users, so inter­fer­ence issues are more like­ly than on oth­er bands. This is also the low­est fre­quen­cy band on which FM ATV occurs. This band does fall with­in the CATV chan­nel scheme; how­ev­er, most cur­rent con­sumer equip­ment does not tune to chan­nels above 125.

The 33 cm AM/VSB chan­nels (with car­ri­er fre­quen­cy for video and audio) are:

143: 906–912 MHz (907.25 video, 911.75 audio)

N/A: 909–915 MHz (910.25 video, 914.75 audio)

N/A: 910–916 MHz (911.25 video, 915.75 audio)

144: 912–918 MHz (913.25 video, 917.75 audio)

145: 918–924 MHz (919.25 video, 923.75 audio)

N/A: 922–928 MHz (923.25 video, 927.75 audio)

23 cm band

The 23 cen­time­ters ham band is the third high­est fre­quen­cy band avail­able for ATV. Due to the wider nature of this band over the low­er ones, and the expense of VSB-fil­ters, VSB is rarely used. Most ATV is either in AM or FM. Old TVRO receivers may be used to watch FM ATV when an anten­na is con­nect­ed to the LNB input. The IF of these receivers over­laps this ham band.

The 23 cm AM/VSB chan­nels (with car­ri­er fre­quen­cy for video and audio) are:

1240–1246 MHz (1241.25 video, 1245.75 audio)

1252–1258 MHz (1253.25 video, 1257.75 audio)

1264–1270 MHz (1265.25 video, 1269.75 audio)

1276–1282 MHz (1277.25 video, 1281.75 audio)

1288–1294 MHz (1289.25 video, 1293.75 audio)

Typ­i­cal­ly fre­quen­cy mod­u­lat­ed TV is used on fre­quen­cies above 1240 MHz (1.24 GHz), where there is enough band­width for such wide­band trans­mis­sions. This is often used as a repeater’s input fre­quen­cy, with out­put being stan­dard VSB on the four chan­nels list­ed above.

below 1.24 GHz: Ves­ti­gial Side­band, above 1.24 GHz: FM, PSK etc…

The qual­i­ty of trans­mis­sion is expressed as a “p-lev­el”; “p” stand­ing for “pic­ture”. P-lev­els range from zero to five, increas­ing as the pic­ture becomes more view­able. P-0 sig­ni­fies a state in which sync bars are vis­i­ble, but the pic­ture is too snowy to be seen; this occurs at a min­i­mum sig­nal strength of 3db. Each lev­el rep­re­sents an increase of 6db over the pre­vi­ous; P-5 is 30db above P-0 and rep­re­sents a per­fect­ly clear pic­ture.

As trans­mis­sion fre­quen­cy increas­es, atmos­pher­ic path loss­es become greater, par­tic­u­lar­ly at fre­quen­cies above 10 GHz. Addi­tion­al­ly, long-dis­tance prop­a­ga­tion by F-lay­er ionos­pher­ic skip over the hori­zon does not typ­i­cal­ly occur at high­er fre­quen­cies, and ter­rain and man-made struc­tures can affect prop­a­ga­tion of sig­nals, block­ing or redi­rect­ing sig­nals. Fac­tors such as E-lay­er skip prop­a­ga­tion, tro­pos­pher­ic enhance­ment, and knife-edge dif­frac­tion can extend the use­ful range of sig­nals.

 

 


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