I'd like to place a discone antenna in my attic. I have coax ran all through-out the house from an old cable service that I don't use. From the attic I have a 30 foot run of either rg6 or rg8 I think that goes into the basement into a big splitter. Im trying to decide if I should use that coax, but disconnect it from the splitter and send it directly down another piece of coax into my living room - total run is probably 60-80 feet - or do I run a new piece of shorter cable along the exterior of the house? obviously new is probably better in theory, but from a practical standpoint will I notice much difference? Mostly trying to improve my 400 range analog signal on an sds100
Use existing coax or run new stuff?
- Thread starter Tergar4
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- Aug 17, 2010
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tv cable is 75 ohm, the discone is 50 ohm. not only that but you would need adapters to make the f connectors mate up to the antenna and the scanner. all together that will be a lot of loss. so it may work, but if you really want to be sure run new cable, at 80 feet i honestly would run LMR400 with a smaller flex jumper like rg142 or similar at the scanner.
rf_patriot200
Active Member
I would use your length of Rg8 since it will likely have a lower loss figure at 400 mhz.
mmckenna
I ♥ Ø
Discone antennas are not a perfect 50Ω across their entire spectrum, so don't get hung up on the impedance difference. Your scanner isn't a perfect 50Ω load either, so there's lots of mismatches with this set up even without the cable.
75Ω cable would work fine for scanner use, if it's good stuff. Some older houses had cheap RG-59, avoid that stuff. If it's decent RG-6, you can use it.
The issue, though, is the length. As length increases, so do the losses, so 80 feet of cable isn't going to work as well as 30 feet of cable, if that's all you need.
New is always a good idea unless you can prove the existing cable is good. A shorter run is going to result in less loss.
Also,
Before you install an antenna in your attic, make sure your roofing material isn't metal, and that there isn't foil backed/vapor barrier insulation that will attenuate RF. Also pay attention to wiring, HVAC ducts, etc.
75Ω cable would work fine for scanner use, if it's good stuff. Some older houses had cheap RG-59, avoid that stuff. If it's decent RG-6, you can use it.
The issue, though, is the length. As length increases, so do the losses, so 80 feet of cable isn't going to work as well as 30 feet of cable, if that's all you need.
New is always a good idea unless you can prove the existing cable is good. A shorter run is going to result in less loss.
Also,
Before you install an antenna in your attic, make sure your roofing material isn't metal, and that there isn't foil backed/vapor barrier insulation that will attenuate RF. Also pay attention to wiring, HVAC ducts, etc.
Ubbe
Member
RG6 usually have a 4,5dB loss for a 100ft length at 400MHz and LMR400 2,5dB.
If you only have 50ft then it's a 1dB difference. It's not worth it to install shorter coax or switch to LMR400.
If you now have very weak signals that barely open squelch then you can install a low noise $30 amplifier and remove the coax from the equation and can probably use that splitter as well and still get a lower noise figure at the scanner compared to the most expensive coax you can buy without the preamp. But most gain would of course be had if that discone are placed on the roof without anything blocking the radio signals.
/Ubbe
If you only have 50ft then it's a 1dB difference. It's not worth it to install shorter coax or switch to LMR400.
If you now have very weak signals that barely open squelch then you can install a low noise $30 amplifier and remove the coax from the equation and can probably use that splitter as well and still get a lower noise figure at the scanner compared to the most expensive coax you can buy without the preamp. But most gain would of course be had if that discone are placed on the roof without anything blocking the radio signals.
/Ubbe
