antenna / coax cable question

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Darkstar350

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I recently got a outdoor antenna to improve reception
it looks like one of those old tv antennas that would go on top of your house but its made for scanners supposedly is good for 25 - 1300mhz...

It came with a 50 foot RG 6 coax cable which is 75 ohms

i have a uniden BC355n and in the owners manual it says when connecting a outdoor antenna use a 50-70 ohm coax cable i also have a old radio shack scanner which says to use a RG 8 or RG 58 50 ohm coax cable which seems to be hard to come by seems like most of the coax cables sold are RG 6

being that those 2 scanners in question are outdated is the 50 ohm just a recommendation in regards to a older setup? or will using the RG6 which is 75 ohms and not a 50 - 70 ohm cause problems?

i have been reading up on this matter and what im getting is basically the RG8 will give better reception
I briefly tested the antenna out on the BC355n and seemed to work well with no problems - audio was much clearer on all signals and frequencys that would never come in with the stock telescoping antenna came in which is what was expected

what im thinking is what they mean by the 50 - 70 ohm thing is NOT LESS THAN 50 - 70 ohm i could be wrong...

also im sorry i posted this in general i didn't realize there was a place particular for antennas/coax cables and the like... i kinda figured there was just couldn't find it
 
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mmckenna

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50 ohm is "ideal", but your 75 ohm RG-6 will work just fine and will not damage your radio.

Stepping up to lower loss cable will mean more signal will make it to your receiver, and less will be lost to cable losses. Of course, cable length plays into that too. The longer the cable, the more loss.

RG-6 will work just fine. If you are hearing what you want to hear, then it's good enough. Stepping up to a higher grade cable will help with weak signals.
 

rbm

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For receive purposes either 50 or 75 ohm cable will work just fine.

Over the wide range that a scanner covers, the impedance will vary quite a bit no matter what.

I use RG6 and it serves my purpose very well.

BUT ..
You must use good quality cable and properly installed, good connectors.

Rich
 

Darkstar350

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thanks guys

yeah i only had it hooked up for just maybe a few hours and it seemed fine - i actually was very pleased with how much of an improvement the antenna made i guess its like the saying goes if it aint broke don't fix it...
but that's why i was a bit confused
im reading the manuals saying to use these RG8 50 ohm cables(which i had to look far and wide to find) then again im thinking "well this RG6 cable came with the antenna that's supposedly made for scanners and it seems to fit so i suppose i better go with what came with it"

but i did get the 50 ohm RG 8 upon research about the better reception however the ends are about 3 times the size of the RG6 they don't even fit on the transformer of the antenna without some kind of adapter which i also got
haven't hooked anything up yet tho ...

I live in a busy area with lots of electrical infrastructure and whatnot so you can imagine what im going up against in regards to radio signals coming in

That's what turned me on to this antenna - which obviously is meant to go on top of a roof however i don't think im going to do that

when i tried it out i had it bolted on to a pole on a gazebo i have in my yard
i really didnt want the antenna as a permanent deal although i see now that's kind of what its meant for so i may consider but i think ill just bolt it on a pipe or something in the middle of my yard that way i can take it down if i want
 

n5ims

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Just use the RG-6 that came with the antenna. That transformer is important and must be kept. It changes the antenna's 300 ohm balanced feed point to the 75 ohm unbalanced coax. This is how the common TV antennas are designed as well. To use 50 ohm coax, you'll need an adapter (most likely a male F to female SO-239 - the mate to the male PL-259 that's probably what's on your RG-8 coax) the loss from the adapter will just about eat up anything gained by changing coax for your 50 foot run.

Your other choice is to hunt real hard to find a 300 to 50 ohm transformer (also called balun) with the correct connection to mate with your RG-8 coax. These will be difficult to find and due to their rarity, rather expensive. For this trouble and expense, you'll most likely only notice that a very very scratchy signal is at best only very scratchy.

Although it appears that you have found the correct RG-8 coax, be aware that there's also a totally different type of coax that has a very similar sounding name, RG-8X (aka 8-Mini) that is thinner (about the size of your RG-6) and has lots more loss than the thicker RG-8 (about half an inch thick). Don't confuse the two or you may end up with much less signal than you expected.

For a receive application (like a scanner), don't worry about the 50 ohm vs. 75 ohm difference. It won't hurt your scanner. If you're going to transmit, it may be important, but not if you're only going to receive.
 

Ed_Seedhouse

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Impedance matching is important if you are going to be transmitting, but largely irrelevant for receiving. Modern receivers are incredibly sensitive and will pull in a signal quite nicely even with a very large impedance mismatch. The difference between 50 ohms and 75 ohms is not big enough to matter on receive, over the frequency range you are using the antenna for the impedance will vary much more than that.

Even on transmit a difference of 50 ohms to 75 ohms only represents a 1.5:1 mismatch and generally makes almost no difference at all. Lots of hams I know use 75 ohm coax even though their transmitters are designed for a 50 ohm output. At HF a match of 1.5:1 is considered a good match. Most output transmitters will fold back their power for impedance mismatches higher than that, but won't bother for 1.5:1.

In short, don't worry about it for receiving.
 

scotttish

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Radio signal weaker after new antenna

Today I installed a 14 foot high, UHF (460-70mhz) home rooftop antenna, using Beldon 9913F7 High Flex, low loss RG-8 coax (about 65 feet coax length from radio). My radio is 45 watts. After soldering both the antenna coax connectors, and the connector going to the radio, I tested it and my signal strength while fully connected with tightened coax connector to the radio, I get little or no reception and no signal strength bars. When I unscrew the antenna connecter from the radio, the signal gets stronger as I back the connector pin out of the radio. Either way, I am still not getting the stations I got with my indoor antenna. Antenna info: Comet CA-712EFC 460-470MHz UHF GMRS High Gain Base Antenna
How can I solve this problem??
 
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KK4JUG

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Today I installed a 14 foot high, UHF (450-465 mhz home rooftop antenna, using Beldon 9913F7 High Flex, low loss RG-8 coax (about 65 feet coax length from radio). My radio is 45 watts.
What kind of scanner are you using that's 45 watts??????
 
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buddrousa

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9913 is flexible but is is also breakable or shortable. There is a turning radius (how tight of a bend or turn the coax can make.
#1. undo the coax from the antenna and the radio and measure with a ohm meter it should be open.
#2. short one end and measure the other end with a ohm meter it should read a short.
If you do not get the expected results remove the ends retest #1 and #2 if you still do not get the expected results. See if you can find a TDR and test the line for a open or short. The TDR will tell you the length to the open or short. If I was guessing you have a shorted coax or connector.
 

ofd8001

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Did you use a meter to check the center conductor against the outer portion for a short? Sometimes just a single strand of the outer jacketing can short against the conductor and give issues.

Also do a continuity check on the center conductor.

(Both ends have to be disconnected or you read resistance of antenna or radio.)
 

buddrousa

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NP I just added using a TDR if all else fails. That is how test coax cat 5 cat 6 ect today as I do other things.
 

majoco

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Now I know what a TDR is having used one many years ago when installing instrument landing systems. I doubt if Joe in the street has a clue what it is or how to use it.
 

buddrousa

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TDR's can be bought in the US for under $200 now. My TDR and my FLUKE VOM cost about the same now.
 
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