LMR 400 OR RG6 coax

w6act

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Hello everyone,

I have both LMR 400 and triple shielded RG6 coax for my Comet discone base antenna. I have a Uniden SDS100 scanner and would like to use the triple shielded RG6 because lots of folks say it's better then the LMR400. The RG6 is 75ohm and is better for receive only devices. I would like some opinions on this by some of you folks on this matter.

Thanks Alex
 

jonwienke

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RG6 is not magic.

75 ohms is not magically better for reception. All else equal, the coax impedance should match the device. In some cases the lower loss of RG6 offsets the impedance mismatch loss.

LMR400 is more expensive than RG6, and isn't as flexible, but will perform better.
 

pro106import

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I use rg6 quad shield. Much easier to work with and even on 800mhz. I see such a small loss it is negligible at 50 foot lengths. It also depends on your particular situation as far as total length. I have 10 yagi antennas up, most are 450 or 800 mhz. Yagis, and all have RG6 Quad shield cable. It is so easy to crimp the connectors also with this cable. Make sure you get the correct compression connectors and compression tools for your type coax. The impedance mismatch only matters if transmitting, for all practical purposes as far as I know
 

wa8pyr

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I use rg6 quad shield. Much easier to work with and even on 800mhz. I see such a small loss it is negligible at 50 foot lengths. It also depends on your particular situation as far as total length. I have 10 yagi antennas up, most are 450 or 800 mhz. Yagis, and all have RG6 Quad shield cable. It is so easy to crimp the connectors also with this cable. Make sure you get the correct compression connectors and compression tools for your type coax. The impedance mismatch only matters if transmitting, for all practical purposes as far as I know
Impedance mismatch does matter if it's a pretty bad mismatch, but the difference between 75-ohm coax and a 50-ohm receiver is so slight you won't even be able to notice the difference (seriously... the human ear isn't that good).

I have several quad-shield RG6 runs to the attic where my antenna farm is located; the RG6 has never failed me for receive-only purposes, and because of it's added shielding, has worked better than standard 50-ohm coax in that particular application.

Worked well for me at the old house as well, where I had a 30 foot mast behind the house with a couple of Channel Master Monitenna clones on it; both were fed with quad-shield RG6 and it worked great, even with the 75 foot run.

For receive only applications and modest cable runs (around 100 feet or less), multi-shield RG6 will be much cheaper and easier to work with.
 

vagrant

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@w6act As you have both, I recommend you experiment. Find a handful or more of weak signal frequencies on different bands using one coax and then try the other. Let us know if the results were the same or different. You can take your time finding the frequencies over a week or so. When you finally test, log the results of one and then switch to the other and log. Signal strength can change during the course of the day, so don’t wait long between.
 

pro106import

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@w6act As you have both, I recommend you experiment. Find a handful or more of weak signal frequencies on different bands using one coax and then try the other. Let us know if the results were the same or different. You can take your time finding the frequencies over a week or so. When you finally test, log the results of one and then switch to the other and log. Signal strength can change during the course of the day, so don’t wait long between.
I experimented a different way. I did a line sweep test on it first, and then a real static check. I had one 800 mhz yagi aimed at a system 60 miles from me in nyc. Using a short jumper I made consisting of 3 feet of the coax, I made note of the receivability of the control channel. Then I swapped the jumper with the 50 foot length I had made up to use for the antenna. I found positively no loss in signal. Tried this on a few different signals just to be sure. At 450 mhz. I didn't even have to check it.
 

prcguy

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There is always a right tool for the job and RG-6 is not necessarily the right tool for connecting scanner antennas to receivers. Looking at probably the worst common frequency range for scanner folks at 850MHz, 100ft of low loss foam dielectric RG-6 has about 5.8dB loss. Single shield, dual, tri, quad doesn't matter, it all has the same loss and the extra shielding does nothing for loss.

Now look at 100ft of Times or any of its clones, 100ft at 850MHz is about 3.8dB or 2dB less than really good RG-6 for the same run. Then you have the 50/75 ohm impedance mismatch problem. For an antenna like a Discone or a resonant antenna operated within its frequency range where its impedance is close to 50 ohms, using 75 ohm cable will incur an additional loss. It might be only be .2dB or so at some frequencies but at others it could be much more. So that 2dB extra loss for using RG-6 is now more than 2dB and could be 3dB or ?? at some frequencies.

If your cheap you can skimp and use RG-6 to connect your scanner antenna to your receiver but please don't claim its the right thing to do, because its not. Its a compromise to save $$ and some of us are not willing to compromise our reception. LMR-400 is also not required for all situations but its a known standard for a certain level of performance. You can go better or worse depending on many factors and your budget.



I forgot to specify my coax I use, it is PPC Perfect Flex 2016 quad shield . About one tenth the cost of LMR400.
 

gmclam

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First, all RG-6 is not created equal. The most important factor is signal loss at the highest frequency you want to receive.

For myself, I've run LMR-400 equivalent (Air-802) from my antenna to the filters and multi-coupler. Then I run RG-6 from the multi-coupler to the receivers. The difference in loss for the short distances is small, and RG-6 is certainly more physically manageable.
 

w6act

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Thanks for all the replies, most of you are right I'm going to stay with the LMR400 only about a 40ft run. I have a comet discone antenna but I would like to replace with something better, any suggestions? Main scanning frequencies are 44mhz for CHP and 488MHZ and 772-856Mhz for police and fire. Would it be worth substituting the Comet discone for a better multiband antenna?
 

Ubbe

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The single best improvement you can do are to use a low noise amplifier.

You can use the RG6 coax and for the money saved from not going LMR400 you can buy an amplifier and a splitter.
Instead of speculations I can provide you with the facts. Below you will see the impedance that the SDS100 will have from 700 to 900MHz. It swings between 30 ohm and 120 ohm. A discone like the Sirio SD2000 have a SWR that swings between 1.0 and 3.5 in the same frequency band.
That 3.5 SWR equals to a 110 ohm mismatch to a 50 ohm coax.

When using an amplifier at the antenna it will work as an impedance buffer so that the coax will see a constant 50 ohm or 75 ohm impedance depending of if you use an amplifier designed for 50 or 75 ohm.

A CATV splitter at the scanner will also work as a buffer and makes the coax see an almost constant impedance of 75 ohm, instead of the scanners 30-120 ohm. There are also 50 ohm splitters to be had but are often more expensive. The middle of the scanners 30-120 ohm are actually 75 ohm.

The additional benefit are that you also do not have to worry about any coax loss and if the amplifier are low noise it will also improve your scanners reception as all scanners have at least a 3-4dB noise figure, usually more than that, and low noise amps have a 0,5 to 1dB noise figure. The total system noise figure will be the amplifiers plus something like 1/10th of a dB. Just from the noise figure alone you get 3-4 dB increase in signal sensitivity. That's like using twice as big antenna or stacking together two antennas. Then add the coax loss that you do not have any more and the improvement will be even bigger.

You will have to reduce the signal level to the scanner or it will overload. Either choose a splitter with a high attenuation like 10-15dB or use an additional attenuator, preferable a variable one that can be set more precisly.

Using any high quality/cost coax will always have a loss and the impedance mismatch between scanner-coax-antenna will still be there if you do not use a splitter and amplifier.




/Ubbe
 

w6act

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Thanks Abbe,

Thats some real great information. Can you suggest what brand and model #'s of the low noise amp and spliter?

Thanks Alex
 

Ubbe

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There are circuit boards and there are amplifiers in metal boxes and there are types that needs to be powered by a wire and others that can be feed thru the coax, bias-T.

The amplifier specs should preferable have a noise figure of 2dB NF or less and P1 of -10dBm and IP3 of +10dBm or higher at the input. The P1 and IP3 values are measured at the output of the amplifier and will be dependent of the gain. So take the spec sheet values and substract the gain value. If an amplifier has a 20dB gain it will be stated as having P1 of +10dBm and IP3 of +30dBm if you follow my recommended -10/+10 values.

If you have a transmit tower really close you might get values on your scanner that says -40dBm or even -30dBm. If the signal level goes up to -10dBm then that P1 value of max -10dBm will make that amplifier to start to overload and be problematic. Broadcast FM and television towers might come up to those values if they are close enough and you could need filters and/or a better spec amplifier.

CATV splitters are available with one port that has power pass where you power an amplifier that needs power thru the coax. Insert a choke coil in that port to power the amplifier. Many of them use 5volt so a cheap mobile charger will work where you cut the cable and use the red wire for +5v and the black for ground. If all ports in the splitter have power pass you will probably feed you scanner with 5 volt and often there are protection diods at the antenna port in the scanner that starts to shortcircuit at 0,7volt. You can use splitters with as many ports as you like as the attenuation will be compensated for by the amplifier. The choke can be anything from 3uH and higher and must be able to handle the current that the amplifer needs, if it can be bias-T powered. If you still want to receive the 25-50Mhz range you will have to avoid splitters that say they have a return channel at about 50Mhz.

I have some links below but you can get the components from anywhere and there are bias-T power devices in metal boxes you can buy but seems to always be over priced. You will also need connectors to match the devices and I prefere to use short lenght pigtails that have the correct type of connectors at each end and avoid adapters as much as possible as they sometimes are of poor quality.

Most amplifiers use SMA connectors as well as your scanner and CATV splitters and attenuator uses F connectors and your antenna have either a PL259 or N connector. Try and find pigtails that matches those connectors. It sometimes will be necessary to use a FM trapfilter that GPIOLabs also has in stock.

Mini-Circuits
GPIOLabs
Splitter with one power pass port
Choke coil
0-20dB Attenuator
Coax pigtails
F to sma pigtail
F to sma pigtail

/Ubbe
 

jonwienke

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Amplifying the signal and then attenuating it to prevent signal overload negates any advantage of amplifying, and leaves you with more noise than you started with.

Additionally, the impedance graph posted is misleading, because the input impedance by frequency changes depending on what specific frequency is tuned and which band filter is active. If you have the scanner connected to a VNA while it's running, the frequency/impedance plot fluctuates too much to be meaningful.
You will have to reduce the signal level to the scanner or it will overload. Either choose a splitter with a high attenuation like 10-15dB or use an additional attenuator, preferable a variable one that can be set more precisly.
 
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