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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 03-16-2018, 2:08 AM
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Default Coax question

Hello, everybody, I have a silly question. I have a mag mount antenna on my roof because that's all the landlord will allow. It only has 17ft of rg58 coax, and I want to move it to a higher point. I was wondering if I would be seeing any extreme losses if I were to put an adapter in the line and attach about 50 ft of rg6 coax which has fewer losses in the frequencies I want to listen to than the rg58. Would I minimize losses by using the rg6 for the remainder of the length, or would I be better off getting more rg58? I read that using adapters is not really a problem because losses from using them are minuscule usually less than 1db, so I am more worried about the coax. I know this isn't ideal, buts its basically my only option since I can't mount anything onto the building.
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Old 03-16-2018, 2:31 AM
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Never mix coax with different impedance in the same run.

RG58 is 50 ohm, and RG6 is 75 ohm.

Mixing impedance gives you extra losses at each end of the 75 ohm coax segment. There are other 50 ohm options that would be preferable to RG6.
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Old 03-16-2018, 2:47 AM
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Originally Posted by jonwienke View Post
Never mix coax with different impedance in the same run.

RG58 is 50 ohm, and RG6 is 75 ohm.

Mixing impedance gives you extra losses at each end of the 75 ohm coax segment. There are other 50 ohm options that would be preferable to RG6.
Ok that makes sense, so do you think I would be alright going with 50 ohm RG59 then?
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Old 03-16-2018, 3:03 AM
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I wouldn't use either, I would use LMR400 or something like that. It's more expensive but has much lower loss than 58 or 59. Even LMR240 would be better.
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Old 03-16-2018, 3:51 AM
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If you have a compromised antenna with no alternative, then using it's existing higher loss cable and extending means the end result will be worse - which not simply modify the mag mount to let you use lower loss cable from end to end, that will show an improvement. RG59, bu the way IS 75Ohm cable, but it's great for video, and pretty rubbish for RF at UHF. Good quality adaptors are pretty good, loss wise. Cheap adaptors are usually much, much worse. However - the real issue with using adaptors outside is the need for waterproofing that is effective, because it takes a tiny bit of moisture to start the corrosion. This is why most folk avoid external joins. They work at first, but degrade rapidly and you don't notice what's going on.
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Old 03-16-2018, 7:37 AM
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RG59, bu the way IS 75Ohm cable, but it's great for video, and pretty rubbish for RF at UHF.
Sometimes RG59 are a video coax not intended for RF and sometimes it is equal to RG6, a very low loss coax for RF up to 2GHz. You have to read the specs on it.

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Originally Posted by paulears View Post
Good quality adaptors are pretty good, loss wise. Cheap adaptors are usually much, much worse. However - the real issue with using adaptors outside is the need for waterproofing that is effective, because it takes a tiny bit of moisture to start the corrosion. This is why most folk avoid external joins. They work at first, but degrade rapidly and you don't notice what's going on.
Both expensive and cheap adaptors usually have less than 0,1dB loss but the cheaper ones are not so mechanical stable and could be out of spec and often cause intermittent connections when you touch them. Agree about adaptors being used outside. You have to waterproof them properly and radio amateurs often say that adaptors attenuate the signal even when used inside the house and should be avoided. They probably have experience from bad waterprotection of their connectors outside. No problem using many adaptors and connectors that are not exposed to bad weather, if the adaptors work properly as they should.

/Ubbe
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Old 03-16-2018, 7:48 AM
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Would I minimize losses by using the rg6 for the remainder of the length, or would I be better off getting more rg58?
It will be much better to use RG6 than the lossy RG58. As mentioned connecting two different type of coax with different impedance will affect the perfomance a little but the difference to use LMR400 or equal are nowhere justifying the cost of that higher grade 50 ohm coax and especially the connectors needed.

/Ubbe
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Old 03-16-2018, 9:03 AM
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And to change the conversation completely... Have you considered a different antenna? Something that is much less visible than your mag mount... and will work much better than a mag mount on the rooftop without a proper groundplane. Here's an antenna that doesn't need a groundplane.... and only you will know it's an antenna - Ventenna
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Old 03-16-2018, 11:47 AM
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Quote:
Originally Posted by BillH1 View Post
Hello, everybody, I have a silly question. I have a mag mount antenna on my roof because that's all the landlord will allow. It only has 17ft of rg58 coax, and I want to move it to a higher point. I was wondering if I would be seeing any extreme losses if I were to put an adapter in the line and attach about 50 ft of rg6 coax which has fewer losses in the frequencies I want to listen to than the rg58. Would I minimize losses by using the rg6 for the remainder of the length, or would I be better off getting more rg58? I read that using adapters is not really a problem because losses from using them are minuscule usually less than 1db, so I am more worried about the coax. I know this isn't ideal, buts its basically my only option since I can't mount anything onto the building.
I don't see any mention of what the antenna will be used for. Is it receive only? Transmit too? Also, the frequencies of use will determine just how much loss will be realized by any particular coax of a given length. As a general rule, the higher the frequency, the higher the loss and therefore, the better the coax you'll want to use.
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Old 03-16-2018, 12:45 PM
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Some folks at this site, continually refer to RG-59/Rg-6/RG-11 coaxes as "low loss" cables. At 800 mhz, these cables are rated in the 7-10 db loss range, per 100' length, when new and prime. The main reason they get away with using them with ourdoor antennas, is that the antenna height gain equals or exceeds the high loss, or the signals are quite strong to begin with. LMR400 type cable has about 4 db loss, per 100' @ 800 mhz. That 3-6 db lower loss can be a major improvement when trying to receive weaker signals. Forget the things like 50 versus 75 ohm cable, adapters, etc., as these are of minor consequence. RG-6 and such only have one thing going for them, they're cheap! If you can go that route and use them, do so.
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Old 03-16-2018, 3:43 PM
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What frequencies are the target? Without knowing that, it is impossible to really spec the right coax.
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Old 03-16-2018, 4:37 PM
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Quote:
Originally Posted by Rred View Post
What frequencies are the target? Without knowing that, it is impossible to really spec the right coax.
Loss in coax versions increases as frequency increases, but they generally run parallel courses from MF/HF thru UHF/SHF. The only thing that changes is that the coax selection versus cost versus loss may tilt the selection to a lower cost version as you go downward in frequency.
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Old 03-16-2018, 5:48 PM
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Just saying.........

https://www.youtube.com/watch?v=5AgFum5K7bw
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Old 03-17-2018, 4:51 AM
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If you are tight on money and would like a quick solution, buy that RG6 for $10. LMR400 are $100 for a 50 feet run.
For the money saved you could get a good antenna, but the one you have are probably ok, and a preamplifier with a variable attenuator and say goodbay to any kind of coax loss or impedance missmatch loss.

/Ubbe
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Old 03-17-2018, 8:15 AM
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Quote:
Originally Posted by BillH1 View Post
...I have a mag mount antenna on my roof because that's all the landlord will allow. It only has 17ft of rg58 coax, and I want to move it to a higher point. I was wondering if I would be seeing any extreme losses if I were to put an adapter in the line and attach about 50 ft of rg6 coax which has fewer losses in the frequencies I want to listen to than the rg58...
According to this coax attenuation chart, Coax Attenuation Chart, RG6 coax has a 6.0 dB loss per 100 feet at 900 MHz and LMR400 coax has a 3.9 dB loss per 100 feet at 900 MHz. Your total coax length will be 67 feet (17 feet plus 50 feet mentioned in your original post). Based on that coax length RG6 would have a total loss of 4.02 dB at 900 MHz (0.67 x 6.0) and LMR400 would have a total loss of 2.61 dB at 900 MHz (0.67 x 3.9). The difference between the two is 1.41 dB at 900 MHz.

Personally I don't feel the increased cost of the LMR400 and its associated connectors vs. the RG6 coax would be worth it for a 1.41 dB gain at 900 MHz with your relatively short 67-foot coax length. Also keep in mind that 900 MHz is typically the upper limit of conventional scanner monitoring. Coax attenuation (see the above chart) is less at lower frequencies.

Regarding your current magnetic mount antenna, if it is an NMO mount magnetic antenna you might want to consider an NMO ground-plane kit to mount it at your home. The Laird MBCN with an N connector would be a good choice: https://www.theantennafarm.com/catal...gies-mbcn-7531

If you decide to go with the NMO ground-plane kit with the N connector and use RG6 coax, you can use an F-to-N adapter at the antenna end of your RG6 coax to connect to the ground-plane kit: https://www.theantennafarm.com/catalog/enp-26-8020-5908
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Last edited by trp2525; 03-17-2018 at 8:19 AM.. Reason: Correction
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Old 03-17-2018, 4:37 PM
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Quote:
Originally Posted by BillH1 View Post
Hello, everybody, I have a silly question. I have a mag mount antenna on my roof because that's all the landlord will allow. It only has 17ft of rg58 coax, snip .
And is your mag mount antenna mounted to a flat metal surface at least 36 inches in diameter?

You may have forgotten the other half of the antenna system, the groundplane.

And what 'mag mount antenna' are you using?

And of course, what frequencies are you wanting to receive?
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Old 03-17-2018, 6:25 PM
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Thanks for the replies everyone. I was able to get the landlord to agree to let me install a dedicated outdoor antenna.
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Old 03-17-2018, 9:34 PM
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RG6 will work just fine.
.......
I use nothing but RG6 on ALL receivers.
.......
50 ohm coax is for TRANSMITTING that is the ONLY reason it was invented.
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Old 03-17-2018, 10:17 PM
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Actually 50-ohm coax was invented because many antenna designs have an impedance of approximately 50 ohms. And 75-ohm coax was invented because numerous other antenna designs (particularly dipoles) have an impedance of approximately 75 ohms when a balun transformer is used.

Matching the feedline impedance to the antenna is good practice whether transmitting or receiving. Failing to do so isn't going to liberate magic smoke when receiving as can happen when transmitting, but it's still beneficial when receiving.

There's nothing magical about RG6. It's decent-performing coax for the price, but not the best choice when connecting a 50-ohm antenna to a 50-ohm receiver.
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Old 03-18-2018, 3:53 AM
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Most antenna designs are only at their nominal impedance at one particular frequency. Deviating from that frequency will change the impedance dramaticly. A discone and a logperiodic antenna design are one of the few that will have a more or less constant impedance.

75 ohm coax are actually better to use than 50 ohm. The voltage will be higher in the coax and have higher level above the noise floor. Balanced 300 ohm ladderline are even better, you can't make coax that high impedance, but ladderline are extreamly sensitive to nearby metal objects and need to use nonmetallic standoffs, making it difficult to use in normal homes.

You can google the scientific facts about 50 ohm versus 75 ohm coax used in a receive configuration.

/Ubbe
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