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Having an issue receiving UHF 470-512mhz in house

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LZJSR

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Looking for a little guidance..

I am using an approximately 75 foot run of RG-58 from my rooftop (25-30 foot AGL) base station antenna with a dual band vhf-uhf commercial NMO mount antenna into my home office, to provide signal to my BCT15 Uniden Scanner. I get pretty good reception on low band (45 Mhz), VHF (150 Mhz) from more than 30 miles away, but UHF signals from a mountain top repeater about 10 miles away or less is nearly completely deaf. If I turn my squelch to zero, I can make out some static and intermittent poor quality voice on what should be a very strong signal. The setup is in my home office, but I just don't think my computer is causing the issue. We moved last year from a few blocks over, and I did not have this issue. I have tried several different antenna scenarios on the roof, from a UHF only antenna whip style with 5db gain, to a half wave no ground plane, to a surveillance style antenna but no luck. I have also tried changing the scanner but have the same issue, so I have to assume it is an antenna issue.

Any guidance would be helpful
 
popnokick

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ALSO - What you wrote makes it sound as though you are using antennas intended for use on a vehicle on the roof of a building ("dual band vhf-uhf commercial NMO mount" "UHF only antenna whip style with 5db gain, to a half wave no ground plane, to a surveillance style antenna").... which is not very efficient. You can do much better with an antenna designed for use on a building. When you change the coax consider using a proper antenna as well.
 
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LZJSR

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Thanks for the replies. I will try switching out the coax. In regards to the antennas, I was basically stating that I had tried different options, and didn't have success. I have used commercial antennas designed for rooftop, and other permutations, but not switching out coax. I will try that and see what happens....
 
mmckenna

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Sometimes it helps to look at things from a different angle…
Coax Calculator

This online calculator will tell you what sort of losses a particular type of coaxial cable will have.

Just for giggles, I punched in 75 feet of Radio Shack RG-58 at 490MHz. Let's just say you were transmitting (I know you aren't, but as an example) at 10 watts from your radio. About 1.7 watts would make it to the antenna. The other 8.3 watts would be lost to coaxial cable losses.
The losses work in reverse, so what ever is hitting your antenna on the UHF band is getting knocked down by about 83% before it even reaches your radio.

If you can upgrade and/or shorten your cable, you'll see an improvement.

Using the same length, but substituting LMR-400, you'll see a bit over 2 dB of loss. That would be like transmitting with 10 watts, and a little over 6 watts making it to the antenna.

There is better coaxial cable than LMR-400, but that's a reasonable cable for what you are doing, affordable and easy enough to work with.
 
jonwienke

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Worse.

In addition to somewhat higher loss in the coax itself, you have to deal with double impedance mismatch losses at each end, when the 50Ω antenna connects to the 75Ω coax, and again when the 75Ω coax connects to the 50Ω scanner input.
 
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mrweather

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Actually according to the coax calculator posted above, RG-6 would be slightly better than RG-58 in terms straight attenuation. It's still crappy though.

Impedance mismatch doesn't add very much.
 
jonwienke

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mrweather said:
Actually according to the coax calculator posted above, RG-6 would be slightly better than RG-58 in terms straight attenuation.
Click to expand...

That wasn't the question, which was how RG-6 would compare to LMR-400. LMR-400 is a clear winner on all frequencies, compared to RG-58.
 
popnokick

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spongella said:
What about 75 ohm RG/6? Any better or worse than LMR-400?
Click to expand...
RG/6 is going to be only about 1.1 dB more loss than LMR-400 for a 75 foot run at 500 mHz. So it's probably worth comparing the price (and simplicity of installation) of RG-6 vs LMR-400. And if it matters, you can buy RG-6 in your local home / hardware store.... where you're not likely to find LMR-400 or the connectors for it.
 
spongella

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Thanks for the replies on RG/6. I like to use it as it's much easier to work with for hobby installations. Use it for FTA satelite reception too. Most of my coax runs are less than 40' though.Diameter is smaller, easier to bend, smaller holes to drill into the house, available at big-box stores, easy to cut and crimp connectors onto, etc., hi hi.
 
jonwienke

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The Chinese RG-6 you get at wally world probably won't match the specs of actual RG-6 though.
 
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lmrtek

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Hair splitting over 1 db is beyond ridiculous but rg58 of that length
is quite useless at uhf frequencies.
........
RG6 would be a huge improvement
 
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