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Coax Cable and Connectors - For general discussion of all things pertaining to coax cable, wave guide or any other medium for passing RF energy. This includes connectors, weather proofing and grounding.

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Old 06-11-2018, 3:23 AM
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Default Removing mag mount coax and attaching my own coax?

Has anyone tried to unattach the coax that came with thier mag mount and re-soldering new coax?

I was looking at the spectrum force wideband magmount from scannermaster.com. The coax that comes with the antenna is to short though. 12 ft only.

Know of any wideband magmount antennas with longer coax?
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Old 06-11-2018, 3:34 AM
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You could just do an extension
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Old 06-11-2018, 5:09 AM
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Yeah I could do that too. Sounds like it might be easier. Never done either before.
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Old 06-11-2018, 6:17 AM
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It’s simple. Some RG-58 cable with some BNC connectors and a BNC barrel adapter will do it.
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Old 06-11-2018, 10:05 AM
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Okay , I"ll work on it. Already have a barrel adapter just need more coax.
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Old 06-11-2018, 10:31 AM
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The cable length was probably kept short because even at 12 feet long, RG58 already presents a 1.7db loss at 800MHz which is about 30% less signal. If you made the extension 10 feet long, that would drop signal by another 1.4 db and result in a total 50% less signal without connector losses. You will want to keep any extension as short as possible. The quality of your connections will also have an effect. If the connectors are properly installed on the cable and are of a decent quality, loss due to connectors should be pretty minimal but could be as much as 0.3 db for the BNC on each end of the extension cable plus the barrel connector for 800MHz.

You can use this online calculator to get cable loss information.
Welcome to Times Microwave | Coaxial Cable - Attenuation & Power Handling Calculator
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Old 06-11-2018, 11:03 AM
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Yeah I was thinking it might take some practice to make good connections. It's not RG-58 though which I do actually happen to have on hand, it's RG-174 which I don't have at the moment. Hopefully that has lower loss, I"ll try to make it as short as possible, I'm guessing without having measured yet around 16 to 18 ft. Yep, that's what I read as long as the connectors are good quality the loss is minimal.
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Old 06-11-2018, 11:59 AM
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RG58C and RG174 have the same loss at 400MHz. RG58 and RG58A have less loss.
At 800Mhz the RG58C/U have 18dB loss and RG174 have 27dB at a 100 feet lenght.

The cheap TV/sat RG6 cable have 6dB loss at 800MHz and 100 feet.

/Ubbe
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Old 06-11-2018, 2:41 PM
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Quote:
Originally Posted by Driverj30t9 View Post
Yeah I was thinking it might take some practice to make good connections. It's not RG-58 though which I do actually happen to have on hand, it's RG-174 which I don't have at the moment. Hopefully that has lower loss, I"ll try to make it as short as possible, I'm guessing without having measured yet around 16 to 18 ft. Yep, that's what I read as long as the connectors are good quality the loss is minimal.
RG174 exhibits much more loss than RG58. At 800Mhz, 12 feet of RG174 exhibits 3db loss. RG174 exhibits 2.1db loss at 400 MHz vs. 1.2db for RG58 at 400MHz. RG58 is a much better option. You can also use RG59. RG59 loss at 800MHz for 12 feet is 1.3db instead of 1.7db for RG58. RG59 is 72 ohms, but in a receive setup with a broadband antenna, you will not have 50 ohms across the entire range anyway. Actually only at one or maybe a few frequencies for a multiband antenna will there be 50 ohms impedance at the antenna. The impedance of RG59 is not going to matter at all for receive. There are flexible versions of RG59 instead of the common stiff kind often found. I use old RG6 leftover from a satellite installation for receive. It is also 72 ohms and it would be 1db loss at the same 12 feet, but RG6 is quite stiff and not a good option for mobile antennas.
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Old 06-11-2018, 3:17 PM
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Quote:
Originally Posted by Ubbe View Post
RG58C and RG174 have the same loss at 400MHz. RG58 and RG58A have less loss.
At 800Mhz the RG58C/U have 18dB loss and RG174 have 27dB at a 100 feet lenght.

The cheap TV/sat RG6 cable have 6dB loss at 800MHz and 100 feet.

/Ubbe
Interesting. I had never observed that RG-58C/U and RG-174 had similar loss at 400 MHz. 14.0 db at 100 ft for RG-58C/U vs. 14.7db at 100 ft for RG-174. Pretty much the same. But at 800MHz which I imagine he will want to receive, even RG-58C/U would be far better than RG-174.

http://rfelektronik.se/manuals/Datas...tion_Chart.pdf
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Old 06-11-2018, 4:00 PM
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Quote:
Originally Posted by Driverj30t9 View Post
Has anyone tried to unattach the coax that came with thier mag mount and re-soldering new coax?

I was looking at the spectrum force wideband magmount from scannermaster.com. The coax that comes with the antenna is to short though. 12 ft only.

Know of any wideband magmount antennas with longer coax?
Quote:
Originally Posted by Driverj30t9 View Post
Has anyone tried to unattach the coax that came with thier mag mount and re-soldering new coax?

I was looking at the spectrum force wideband magmount from scannermaster.com. The coax that comes with the antenna is to short though. 12 ft only.

Know of any wideband magmount antennas with longer coax?
I did not look for the exact antenna you were referring to earlier. I just now looked it up. As suggested, I would not try to take that base apart. I have owned a few of these types of antennas for easy portable use with HAM radios and they are pretty much glued shut.

Anyway, I have a similar antenna I bough off eBay for receive on my car. I cut the RG-174 about 2 feet from the antenna and inserted a low noise 10db amplifier with the rest of the RG-174 leading into the car to hookup to my BCD436HP. It works quite well. I had to put SMA connectors on the cut ends to connect to the amplifier I also got off eBay. I was skeptical at first about the amp, but it turned out to be pretty good and it compensates well for the loss in the remaining RG-174 leading into the car. And that amp is cheap!

This is the amplifier. Sold by a few others on eBay. https://www.ebay.com/itm/DC-12V-10KH...item2a6afa34a1

You don't want a high gain amplifier, and it should only be used to overcome cable loss. Any amplifier is going to amplify anything at the antenna and too much gain will overload the receiver and make reception much worse. It will also generate some noise itself, so you have to try and find something with a low noise figure and you only want just enough gain to compensate for cable loss.

I hooked the power for it up to the hot side of the trunk light. When I measured it only draws about 20mA current so it will never drain the car battery with it being on all the time. The car voltage is a bit too high for it. It got very warm so I put a dropping resistor inline on the positive side. I used a 100 ohm 1/4 watt resistor to drop the voltage down about 2 volts and now it runs cool. Been that way for about a year now and works very well and never an issue with the car battery. The rest of the car probably draws way more than 20mA when it isn't running with all the other electronics onboard.
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Old 06-11-2018, 4:35 PM
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What is RG-58C and RG-58C/U? I've heard of RG-58A and RG-58A/U but not RG-58C or C/U.

Okay, RG-174 kinda lossy. Maybe I should look for an antenna that uses different coax.

Any idea what the lowest loss coax I could get would be? Just curious.
RG-213 Mil spec?

So impedance is a non issue with receivers? Wouldn't a mismatch cause standing waves that would impede incoming signals?

That'd be nice having free pickings at the sattelite installation.

Is there any bad effects from using different grade feedlines connected together?

Indeed, I listen to 800 MHz regularly.
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Old 06-12-2018, 9:01 AM
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Looked like to me you'd have to take a saw or somthing idk to get the mag mount base open but thought I'd ask. I"ll just go with an extension.

I also bought some LNAs , two of them off of amazon.com, guess I only need to use one if it's hooked up at the antenna.

Here is the link for the amp I bought if you care to see it----

https://www.amazon.com/HiLetgo-0-1-2...QDL&ref=plSrch

It is high gain though, up to 32db. Maybe I can get it to work by using minimal power. Didn't know lower gain was better when I bought them. I plan on running two scanners from a splitter so perhaps that would help cut down the db increase from the amp as well. Splitters have a fair amount of db loss. Plus I'd be feeding two scanners so the signals to each would already be cut in half.

How do you protect the amp from the weather? What kind of case do you keep it in? So it's best to put the amp around a foot or two away from the antenna?

I guess it's a fair price for a PCB. Cool that it uses such little power.

Good thinking on installing the resistor.
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Old 06-12-2018, 10:10 AM
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Quote:
Originally Posted by Driverj30t9 View Post
What is RG-58C and RG-58C/U? I've heard of RG-58A and RG-58A/U but not RG-58C or C/U.

Okay, RG-174 kinda lossy. Maybe I should look for an antenna that uses different coax.

Any idea what the lowest loss coax I could get would be? Just curious.
RG-213 Mil spec?

So impedance is a non issue with receivers? Wouldn't a mismatch cause standing waves that would impede incoming signals?

That'd be nice having free pickings at the sattelite installation.

Is there any bad effects from using different grade feedlines connected together?

Indeed, I listen to 800 MHz regularly.
Quote:
Originally Posted by Driverj30t9 View Post
What is RG-58C and RG-58C/U? I've heard of RG-58A and RG-58A/U but not RG-58C or C/U.

Okay, RG-174 kinda lossy. Maybe I should look for an antenna that uses different coax.

Any idea what the lowest loss coax I could get would be? Just curious.
RG-213 Mil spec?

So impedance is a non issue with receivers? Wouldn't a mismatch cause standing waves that would impede incoming signals?

That'd be nice having free pickings at the sattelite installation.

Is there any bad effects from using different grade feedlines connected together?

Indeed, I listen to 800 MHz regularly.
Any coax with less loss is going to be thicker. RG-58 and RG-59 are about as thick as you want to get in a mobile installation especially with a mag mount.

RG-58C/U was used years ago for 10base2 Ethernet. It is much more flexible than the other RG-58. Due to some differences in overall design, it is a bit more lossy than the other versions of RG-58. Sometimes it is used for short runs in radio applications. I have used it due to its flexibility for the 2 meter band and HF in short lead-in runs.

We are really starting to get into the weeds here, but your questions are valid concerns.

A mismatch of impedance does have an effect on receive, but not exactly the same way as transmit and not nearly as detrimental. Any mismatch causes some loss of transfer of energy.The reality is, that antenna will not present a 50 ohm load across the entire range of frequencies that it is itended to be used for. If a had to guess, I would imagine that for the range of frequencies it is designed to be used for, if you were to check it at various frequencies in that entire range the impedance would swing from a few ohms to hundreds or more. It can only present a 50 ohm load at the frequencies in which it is resonate, which for a multiband will be maybe 2 or more depending on design. An antenna for a very wide band of frequencies is a compromise. If you want the best performance, you would have to pick a single frequency or a narrow band and choose an antenna tuned for that.

Regarding standing waves or VSWR values, it will surprise many to know that a 1:1 ratio which is not going to happen in any antenna system, is not much better than 1.5:1. At 1.5:1 the loss is only 4% of the transmitted power to the antenna besides any loss due to attenuation in the cable. A VSWR of 2.0:1 causes a loss of 11%.

I am not sure what you may mean about different grade of feed lines. If you mean quality, probably not depending on how poorly the cable is made. If you mean mating different impedances of feed lines connected together, you can probably do just fine mating a length of 50 ohm to a length of 72 or 75 ohm for receive. Sometimes it is done on purpose to transform impedances.

RG-213 is an excellent cable, but quite thick and excessive for a short run in a mobile situation where it is not practical to use. I have a lot of it for HAM radio use.

RG58 is just fine for short runs in a mobile situation even at 800 Mhz. For 16ft of RG-58 at 800 MHz the loss is 2.2db. Not great, but not terrible either. The same length of RG-174 is 4db loss. If your main target is 700 or 800 MHz, you would be best off with an antenna for those bands that has a bit of gain which will compensate for the loss in the cable. In which case you would probably need to look at an antenna designed for a mobile transceiver, rather than one designed for a scanner to get something good.

Maybe something like this, but it will of course be more expensive. You will have to purchase a separate mount for it.
Laird B8065 Antenna, 806-866 MHz, 5db Gain | Scanner Master

Although that antenna is designed for 806-866 MHz, I imagine it will work just as well down in the 700MHz band for receive. A 1/4 wave antenna at 769MHz is 3.65 inches, and at 866 MHz it is 3.24 inches. Not a lot of difference. It would only matter for a transmitter.

Then there is this one for 136-174/380-520/760-870 , but that one is really expensive. Laird Multi Band Mobile Antenna | Scanner Master

You can see why I tried an antenna like you had picked and added an inline amplifier, It was much cheaper. I also did not want a large mag mount on the trunk of my Acura TL. The smaller magnet is not discoloring the paint. And the trunk groove is oddly shaped and will not accept a trunk groove mount. I am still trying to find an alternative for a couple of HAM radio antennas.

Laird are very high quality, but Maxrad are decent. This one will need an NMO mag mount. https://www.ebay.com/itm/PCTEL-Maxra...UAAOSwXJFaqphb

BTW. DO you have, or know someone with an old mag mount cellular antenna? Those were deigned for 800Mhz. And most of them were 3db gain. You will have to change the connector on the cable or convert it from the Motorola connector to a BNC.
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Old 06-12-2018, 10:25 AM
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Quote:
Originally Posted by Driverj30t9 View Post
Looked like to me you'd have to take a saw or somthing idk to get the mag mount base open but thought I'd ask. I"ll just go with an extension.

I also bought some LNAs , two of them off of amazon.com, guess I only need to use one if it's hooked up at the antenna.

Here is the link for the amp I bought if you care to see it----

https://www.amazon.com/HiLetgo-0-1-2...QDL&ref=plSrch

It is high gain though, up to 32db. Maybe I can get it to work by using minimal power. Didn't know lower gain was better when I bought them. I plan on running two scanners from a splitter so perhaps that would help cut down the db increase from the amp as well. Splitters have a fair amount of db loss. Plus I'd be feeding two scanners so the signals to each would already be cut in half.

How do you protect the amp from the weather? What kind of case do you keep it in? So it's best to put the amp around a foot or two away from the antenna?

I guess it's a fair price for a PCB. Cool that it uses such little power.

Good thinking on installing the resistor.
A 32db amplifier is going to cause a mess with signal overload on a short cable run. You need to use a 10db version. Plus it will also amplify a lot of noise.

A much lower voltage to that amplifier will not directly relate to a lower gain. It will still have lots of gain at lower voltages up to the point of it cutting off, and likely have a lot more noise at some level of lower voltage just before cutoff. And the response curve will no longer be relatively flat.

If you have a long cable run, and the cable run is say 20db loss, you could probably use that amplifier.

My amplifier is in the trunk wrapped in heat shrink. The short piece if RG-174 I left on the antenna connects to it coming down through the trunk groove.

I would also recommend the 10db AMP with the splitter. A two way splitter will attenuate each leg by 3db. Get the amplifier as close to the antenna as possible. You want to compensate for loss of the cable before the cable, not after.
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Old 06-12-2018, 11:13 AM
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Quote:
Originally Posted by K4FLB View Post
A 32db amplifier is going to cause a mess with signal overload on a short cable run. You need to use a 10db version. Plus it will also amplify a lot of noise.

A much lower voltage to that amplifier will not directly relate to a lower gain. It will still have lots of gain at lower voltages up to the point of it cutting off, and likely have a lot more noise at some level of lower voltage just before cutoff. And the response curve will no longer be relatively flat.

If you have a long cable run, and the cable run is say 20db loss, you could probably use that amplifier.

My amplifier is in the trunk wrapped in heat shrink. The short piece if RG-174 I left on the antenna connects to it coming down through the trunk groove.

I would also recommend the 10db AMP with the splitter. A two way splitter will attenuate each leg by 3db. Get the amplifier as close to the antenna as possible. You want to compensate for loss of the cable before the cable, not after.
Another note about your splitter. Place your amplifier before the input of the splitter. Do not use 2 amplifiers after the splitter to each scanner.

You don't want to overload your receiver with too much gain from noise before there is signal received at the antenna. You will wind up with a diminished signal-to-noise ratio.
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Old 06-12-2018, 7:05 PM
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It should be mandatory that an antenna amplifier comes with a variable 0-20dB attenuator, or a note that says you probably must use one to make the most out of the amplifier.
You absolutly have to balance the signal level to not overload the receiver, just a little overload will reduce its sensitivity.

That P1+10dBm amplifier has 30dB amplification and use two amplifier IC's in series, nothing you should ever do. Stay away from it. It doesn't even mention the internal noise level, which must be huge.

As soon as you alter the operational voltage of a circuit from what it was design to use, you also change where the circuit operates with it's best characteristics like how it withstands overload and adds its lowest level of own noise to the signal.

In mobile installations the most important thing is the coax shielding. The standard RG58 have only a single copper braid that will let interferencies pass thru from the electronics in a car. At least use a low loss RG58 version that have double shielding with both a foil and braid. You might get lucky if you route the standard RG58 away from other cables and electronic boxes.

/Ubbe
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Old 06-12-2018, 7:25 PM
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I took a chance on the 10db amplifier because it was cheap. It turned out to work very well and I bought a second to use portable when setting up an antenna on a balcony. I was skeptical but the results were surprising.
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Old 06-13-2018, 10:23 AM
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I have never encountered a situation where I needed double shielded coax in a mobile installation. I imagine double shield could solve some interference issues. One thing you do want to be sure of is to pick an RG-58 that is 95% shielded braid. There are some cheap ones out there that are not 95%. Belden 8219 is 95% braid as are other brands.
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Old 06-13-2018, 11:28 AM
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Good stuff to know. An antenna designed for specific bands or frequencies is something I'd like to try. I listen to 800 Mhz regularly but I also like to try to cover everything so all band is definitely the way to go right now. That's also why I want an all band antenna because it's cheap and I'm not sure if it will magnetize to my roof or not, not sure if I have a metal or fiberglass roof. I could try sticking a fridge magnent or neodyne on it to see but if it doesn't work I"ll probaly use it on another vehicle. It's for my semi truck and I haven't bothered climbing up there yet.

I think maybe I've heard of 10base2 ethernet. Interesting. Sound like the c/u could come in handy.

We are really starting to get into the weeds here, but your questions are valid concerns. Yes they are.

Okay I understand what your saying about impedence changing across frequencies and why an antenna would perform better if it is resonant for the intended frequency of use.

By grades I just meant coupling RG-174 with RG-58 or whatever else kind of coax they have available out there that's good for mobile use. Maybe that's about it, idk.

Glad you like the RG-213, think I"ll get some of that when I get my base station set up.

Yeah that interesting about the 1:1 VSWR. I heard 2: 1 or less is good to shoot for but at least I know now that I don't have to worry about that with the scanners.

Thanks for the antenna examples and brand info.

Welp, sounds like the 32db amp would just kill my signals so I"ll take your guys advice and go with the 10dbm, only a fool wouldn't.

Looks like I'm back im the market for some amps and double shielded foil coax.

Yeah I would prefer to buy an amp with an attenuator so I could tweak the amplification as I saw fit. I'm sure they must make some with attenuators. I suppose different equipment responds differently.

Your saying to stay away from the 10dbm amp also Ubbe? How does it put out 30db if it says 10dbm? P1? I don't understand db that much at all yet.

How well do mag mounts hold up to inclament weather? I need to figure out how you protect an amp outside the vehicle as well. There's no where close where I could put it inside the vehicle like you could with your trunk.
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