By Tom O'Hara W6ORG - Email:
Retired owner of P.C. Electronics, The Leader in Amateur Television Equipment for 50 years - history
Last update January 1, 2015
This app note describes a basic inband ATV repeater on the 70cm band .
The primary advantage of an inband 70cm ATV repeater is that users can use a single band transceiver and one antenna. With the 70cm band, only one common antenna is necessary and many can just try ATV by using a cable ready TV set before investing in a transceiver or transmitter. 70cm goes farther than higher bands given the same power and antenna gains.
Shielding, bypassing and connections must be very tight so that transmitter RF sidebands dont feed around the filters and get into the receiver. Note that an additional VSB filter in the transmit line is necessary if using a single antenna - the filter makes up for the 50-60 dB isolation if 2 separated antennas were used. All TX sideband intermod energy, noise or interfering signals need to be less than 1 microvolt (-107 dBm) at the receiver input.
Frequency selection and antenna polarity is critical and varies from area to area. This should be determined first. Shown is 434.0 in and 421.25 out with vertical polarization. Horizontal would be suggested if 439.25 input so as to be cross polarized to FM voice repeaters below 444 MHz. But often the polarity is not chosen for technical reasons, but by what ever ATVers already have up. See the 70cm ATV DX chart for possible coverage.
Component Sources and Prices as of 2010
|Vertical Antenna 406-512 MHz||ANT450D6-9||Telewave||Telewave||$1145|
|Horizontal Antenna 420 - 444 MHz||Quad Wheels||Olde Antenna Lab||P. C. Electronics||$395|
|ATV Downconv. 70cm||TVC-4S||P. C. Electronics||P. C. Electronics||$149|
|Ch 3 receiver||Rch3||P. C. Electronics||P. C. Electronics||$89|
|ATV Transmitter 70cm||TX70-5s||P. C. Electronics||P. C. Electronics||$499|
|ATV Transmitter 70cm||VM-70X + parts||Videolynx||PC Electronics + others||$360|
|35 W Amp||7025PACK||Downeast Microwave||DEMI||$160|
|50 W Amp||D26N||Mirage||AES, eBay||$279.99|
|80 W Rpt Amp - mod to D100||D1010ATVNR||Mirage||AES, HRO||$629.99|
|ATV Repeater Control board||ATVC-4+||Intuitive Circuits||P. C. Electronics||$349|
|70cm VSB filter 421.25 MHz||DCI-421.25-6C||DCI||DCI||$399|
|VSB Duplexer 434.0/421.25||DCI 70cm Duplexer||DCI||DCI||$999|
|Die Cast Aluminum Box||1590D-BK||Hammond||Mouser Electronics||$24|
|70cm Low Pass Filter||NLP-550+||Mini-Circuit Lab||Mini-Circuit Lab||$35.95|
|Power Supply 13.8 Vdc @ 2.5A||RS 22-504||Radio Shack||Radio Shack||$79.99|
|Power Supply 13.8 Vdc @ 20A||RS-20A||Astron||AES||$99.99|
|50 Ohm Coax Cable, 250 ft roll||9913 or LMR400||Belden or Times||AES or RF Parts||$129.95|
|Video Identifier board||OSD-ID+w/c||Intuitive Circuits||P. C. Electronics||$129|
Total repeater system cost is about $3000 to $4000 depending on antenna polarity and going with two separated antennas or single antenna with duplexer. Sketch your block diagram similar to ours above, then make your parts list. Prices are as of Dec. 2010, so check current prices and sources.
Choose the antenna polarity already in use in your area if you want to attract other ATVers to use your machine so they dont have to change polarity or get a new antenna system. Technically it is best to select the polarity to be opposite of your adjacent mode - 439.25 horizontal due to FM voice repeaters being vertical above; 434.0 vertical due to SSB being horizontal on 432. If your application is with mobiles in motion then vertical. If from an emergency site where you could set up a small beam like a OAL 5L-70cm, it could be mounted for either polarity.
The receiver and control boxes can be mounted in a Bud CB-1372 5.25" high 19" rack panel chassis available from Mouser Electronics.
If you have 50 ft vertical separation on the repeater tower, you can consider a less cost approach with two antennas and eliminate the duplexer. The transmitter VSB filter changes from an 8 pole to a 10 pole at $449. You may have to find a magic space separation antenna position on the tower to get more than 50 dB attenuation especilly if horizontally polarized.
The Mirage Amp needs the DC power cable removed and replaced with a feedthrough cap to prevent RF being radiated on the cable. Likewise, if you use a Mirage D100-ATVNR or Downeast 7025PA amplifier.
If you think you will never add a link, crossband alternate input, tower cam, etc., you can substitute the VOR-3 Video Operated Relay board in place of the ATVR-4+ ATV Repeater Controller board and save $270. Also see the crossband Portable Public Service ATV Repeater for info on building the repeater control box in a Hammond 1590D die cast aluminum chassis.
The low pass filter in the receiver antenna line is necessary because cavity or transmission line type bandpass filters repeat at odd harmonics with very little attenuation. If it were not there, the 3rd harmonic of the transmitter could pass through the VSB filters and duplexer to overload the receiver.
See the Crossband ATV Repeater Example.
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