cable type for 100' run

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trauts14

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to run a 100' length of cable from a scanner to outside +8db gain yagi, what is a good low loss cable that is physically similar to RG58 in size? LMR-400 seems to be great, but it is physically fatter and I am trying to avoid drilling a large hole in the floor to run the cable. I will be monitoring 850mhz. any suggestions are appreciated. my cable knowledge is minimal.
 

FKimble

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Are you going to be putting the ends on the cable yourself? If not then the cable doesn't matter, the ends will be the same size. Consider using good coax from antenna to just below the hole in the floor. Then switching to RG58 size coax with a UHF connector on one end and a BNC (or whatever is needed at scanner) to go thru the hole to the scanner. Usually better to have thinner coax at scanner to prevent putting too much stress on the cable connector in the scanner. or just use RG6 since your not going to be transmitting. More options and opinions will be posted soon!

Frank
 

trauts14

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i will be putting the connectors on myself. the handheld scanner uses a BNC and i am not sure what most outdoor yagis use as connector. thanks for the assistance.
 

buddrousa

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COAX is like water pipe the bigger the more that will go through it. For 100 foot 850 meg I would use LMR 600 which is bigger than LMR 400 but has less loss. At 850 LMR 400 loss is 4.2DB and LMR 600 loss is 2.5DB here is the chart http://www.w4rp.com/ref/coax.html
Buying a good antenna and putting cheap coax on it is like buying a sports car and putting wood wheels on it.
 

Ubbe

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The RG6 (CATV) cable have the same thicknes as RG58 but are much cheaper and attenuate only 6dB at 900MHz. I beleive 6dB are one S unit on the signal strenght meter. If you really want to have that half S unit increase compared to a much expensive cable, then go for a good low noise antenna amplifier, that also are needed if you want to split the signal to several scanners.

/Ubbe
 

buddrousa

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In case you did not know every 3db cuts the signal in half so 6db is 1/4 the signal at the antenna
 

n5ims

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In case you did not know every 3db cuts the signal in half so 6db is 1/4 the signal at the antenna
While this is true, it's not anywhere near the whole story. The amount of signal your radio gets, while is important, doesn't matter much once you get past a certain level and once you reach another level, you have too much signal and will start having issues. Anywhere in that large area between those two levels you're good. Now I can't just tell you exactly what those two levels are, they depend on your exact situation (radio, type of system, amount of RF "noise" in your area, etc.).

It's kinda like saying that you really need a 100 gallon gas tank in your car so you can drive to your destination. So long as you have enough to have it flow properly from your tank to your engine, any extra in the tank (beyond a safe margin you need to make it to the next filling station) is just extra weight you're carrying. Sure you need a tank large enough so you're not stopping at every station you pass, but beyond that it doesn't matter that much if it's a 10 gallon or a 100 gallon tank.

If the system you're trying to pick up are very strong at your location, the coax loss really doesn't matter. You can afford to get 1/64 of the signal at your antenna and still pick it up just fine. Now if you live in a rural area and are trying to pick up a system that's a very long distance away, you may need every microvolt of signal you can get. Chances are you wouldn't notice the difference between that cheap RG-6 and that expensive LMR-600 for normal scanning. Well, you'll probably notice a few differences, such as cost, ease of installation, etc.
 

trauts14

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Thank you. What if my +8db yagi is deployed and (for example) I use a RG line that will lose 5db when it travels from antenna to scanner. Does that mean my once +8db antenna is now a +3db antenna. Is there a direct relation between a high gain antenna and signal loss?
 

lmrtek

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ALWAYS use 75ohm coax for scanner antennas

Much lower loss than 50 ohm coax at a much lower price

buy some RG11

loss is about 3.5db at 850

RG 6 will have around 6 db at 850 but you can easily use a 75 ohm mast mounted preamp that will make it loss less

the secret to scanner listening is to make use of TV antennas and preamps since they are designed for the ideal 75 ohm impedance to start with

50 ohm coax was created as a COMPROMISE

its only useful application is for transmitting at medium to high power levels

75 oms is the lowest loss coax ideal for receive only and low to medium power transmit
 

n5ims

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Now if you really need the lower loss cable (and cost is less of a priority) you can use some 3/8" commercial grade cable that will do the job with only around 3dB of loss, LDF2-50. It will be stiffer than RG-6 and the connectors will be harder to find and more expensive, but will give you both low loss and small size in a 50 ohm coax. https://www.tessco.com/products/displayProductInfo.do?sku=458139
 

jcop225

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low loss cable that is physically similar to RG58 in size? .
LMR-195 is a drop in replacement for RG-58:

https://www.timesmicrowave.com/documents/resources/LMR-195.pdf

Take a look and see if that is is high enough performance for your application.

There is also LMR-240 and LMR-300 if you need you need slightly less loss.

(Hint: the number in LMR-### type cables is the nominal outer diameter of the cable in thousandths of inches. eg. LMR-400 has an OD of 0.40" and LMR-195 has an OD of 0.195")
 

Dog

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If I may ask is this for a scanner or SDR dongles? I ask because if it is for dongles there are alternatives to long coax run that may be cheaper and lower loss.
 

teufler

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Pretty much so, but the gain and loss, I have found it more noticeable for transmitting than receiving. Oh you do notice some drop off but comparing your setup to a rubber duck or telescopic antenna that came with the scanner, you will notice a big improvement. Now the loss in the coax will depend on the frequencies you monitor. The higher up, more loss but still an improvement on the scanner's stock antenna.
 

Ubbe

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One note about amplifiers, and I'm sure it already have been covered elsewhere, are that the amplifiers internal noise figure are the important factor, not so much the gain. If a television antenna amplfier doesn't state any noise figure it probably is in the 5-6dB figure. The noise figure pretty much equal to cable loss. Even if you have a good 2dB loss coax it will equal a 6dB loss with a bad tv amplifier.

There are good amplifiers with 2dB noise and even better ones with less than 1dB noise, those are the ones to look for. Even if you use the cheap RG6 coax at 900MHz it will still equal the finest coax with 1dB loss, with gain left to use passive splitters to several scanners.

And always important when dealing with amplifiers (or in a high RF level area), get a $10 variable 0-20dB attenuater and insert that beween scanner and coax. Set it halfway at about 10dB and tune in a very weak station in your most important frequency band. Adjust the attenuater to best reception. If that turns out to be with 0dB attenuation then connect coax directly to scanner, else leave the attenuator in place. If you have several scanners, repeat the procedure and if the attenuation needed is about the same then set the attenuator between splitter and amplifier and adjust it again.

/Ubbe
 
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