WHAT IS AN ACCEPTABLE AMOUNT OF COAX CABLE LOSS

[nokones]

Of course, the best amount of coax cable loss is zero but what would be the acceptable amount of loss in a mobile environment (17 feet) cable run and in a base station cable run (37 feet), if there is a difference in each run, in dBs, operating on freqs in the UHF range 450-470 MHz? Any help would be appreciated.

[WRXB215]

@nokones I can't give you the answers you are looking for but one thing I want to point out that may help is that you can buy mobile mount/cables with the connector shipped loose. That way you can cut the cable after install to reduce the amount of excess cable and therefore reduce the loss. Here is a mount with cable I have my eye on.

https://www.arcantenna.com/products/nmo58aucp-pctel-nmo-perm-mnt17-ft-rg58aucrimp-pl259-loose

[marcspaz]

What is acceptable is up to the individual and if you are getting the job done.  Losses in a 17' run will be difficult to measure since it's so small and most equipment the casual operator owns and uses.

In order to have your signal impacted by 1 signal unit (out of a possible 9, typically) you would have to cut your power by 4.  Meaning, if you have a total of 10 watts out of your radio, but only 2.5 watts makes it to the antenna, you will see that the receiver's signal strength meter only falls 1 signal unit.  Same concept on your receive sensitivity. 

Well, lets say your cable has a 3.8 dB loss at 467 MHz.  You would need to run almost 200 feet of cable before you impact your signal 1 signal unit.  at 17 feet, you only lose about 0.65 dB.  At 10 watts out of the radio, you will still have 9.2 or 9.3 watts going to the antenna.  While the result is measurable, its meaningless from a user perspective.

[SteveShannon]

31 minutes ago, marcspaz said:

What is acceptable is up to the individual and if you are getting the job done.  Losses in a 17' run will be difficult to measure since it's so small and most equipment the casual operator owns and uses.

In order to have your signal impacted by 1 signal unit (out of a possible 9, typically) you would have to cut your power by 4.  Meaning, if you have a total of 10 watts out of your radio, but only 2.5 watts makes it to the antenna, you will see that the receiver's signal strength meter only falls 1 signal unit.  Same concept on your receive sensitivity. 

Well, lets say your cable has a 3.8 dB loss at 467 MHz.  You would need to run almost 200 feet of cable before you impact your signal 1 signal unit.  at 17 feet, you only lose about 0.65 dB.  At 10 watts out of the radio, you will still have 9.2 or 9.3 watts going to the antenna.  While the result is measurable, its meaningless from a user perspective.

Everything you say is true, but I’ve never seen an NMO mount (that comes with a cable) that only has 3.8 dB loss at 467 MHz. Most come with much lossier cable.  For instance, the following Laird magnetic NMO mount comes with a cable that would have 12.5 dB loss if it were 100 feet.  It’s only a 12 foot cable so it loses about 1.5 dB for that distance.

https://www.arcantenna.com/products/laird-antenex-gb8t-magnetic-nmo-mount-vehicular-nmo-antenna-roof-mount-12-ft-rg-58a-u-cable-tnc-male-connector-included?variant=32297030811779&currency=USD&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gad=1&gclid=CjwKCAjwjOunBhB4EiwA94JWsNBrxPjgMgjWlB56LSL_hmzYUfMLDg2FMcehTYdnfrryxVeyNCFQOhoCk0EQAvD_BwE

[Lscott]

15 hours ago, nokones said:

Of course, the best amount of coax cable loss is zero but what would be the acceptable amount of loss in a mobile environment (17 feet) cable run and in a base station cable run (37 feet), if there is a difference in each run, in dBs, operating on freqs in the UHF range 450-470 MHz? Any help would be appreciated.

You really need to analyze everything after the radio as a “system” looking at cable losses and antenna gains. I did a simplified one for the antenna install I want to do on my Mazda. What you loose in the cable could be made up to a degree with gain in the antenna. See attached files.

 

Diamond C110 Extension Cable.pdf Diamond C101 Cable Assembly.pdf Diamond K550 Luggage Rack Mount.pdf Mazda 2023 CX-5 Antenna System Analysis Rev 5.pdf SG7500A.pdf CA-2x4MB Manual.pdf

https://forums.mygmrs.com/gallery/image/282-ca-2x4mb-scansjpg/?context=new

https://forums.mygmrs.com/gallery/image/283-sg7500a-swr-scans-2jpg/?context=new

 

[marcspaz]

@Sshannon good point.  Most of the cables in my mobile setups are rg316.  It's something like 18dB per 100. 

[SteveShannon]

8 minutes ago, marcspaz said:

@Sshannon good point.  Most of the cables in my mobile setups are rg316.  It's something like 18dB per 100. 

It’s great to have thin, flexible cable, but it’s sure lossy. We’re only saved by having very short runs. Now imagine the poor person who decides to reuse an existing 50 foot piece of coax for their cell phone booster!

[WRXB215]

@marcspaz, @Sshannon It sounds like my idea of cutting off the excess cable would only be for convenience and not for loss, correct?

[SteveShannon]

Just now, WRXB215 said:

@marcspaz, @Sshannon It sounds like my idea of cutting off the excess cable would only be for convenience and not for loss, correct?

It could go either way.  If it’s much too long I would cut it off.  If it’s just a few feet I would live with it, which is what I do with the few extra feet of cable on the floor behind the passenger seat now.

[marcspaz]

I agree with Steve.  Truth is, you need to measure to see if its worthwhile.

[SteveShannon]

Here’s a guide to losses expressed in dB:

Losses:

-1.5 dB = 25% loss of power

-3 dB = 50% loss of power

-6 dB = 75% loss of power

-9 dB = 87.5% loss of power

-10 dB = 90% loss of power

-20 dB = 99% loss of power

-30 dB = 99.9% loss of power.

 

[nokones]

Thanks guys.  I appreciate the opportunity you giving me in trying to learn this side of the subject.  In measuring the cable loss, is the measurement done open cable end or something terminating the cable end (Closed end)?

[nokones]

According to my manual, it appears that I should check the cable loss with no resistance/termination at the cable end.  I think I'm getting it.

[SteveShannon]

20 hours ago, nokones said:

Thanks guys.  I appreciate the opportunity you giving me in trying to learn this side of the subject.  In measuring the cable loss, is the measurement done open cable end or something terminating the cable end (Closed end)?

You can measure it in at least two different ways. The easiest is to place a wattmeter at each end of the cable and transmit. Then compare the readings on the two meters. (Or take two measurements with a single meter at each end of the cable). If using this method you must have the cable terminated when measuring before the cable, ideally with a 50 ohm dummy load. When measuring at the far end of the cable you would have a dummy load attached to the antenna output on the wattmeter. 
The other way is with a two port RF network/antenna analyzer. Connect the cable between the two ports and measure the insertion loss. 
In YouTube search for measuring coax loss. 
Here’s one video:

https://youtu.be/YnT1MA1wbSQ?si=ehBaLDnNKwg62xJ1

[Lscott]

2 hours ago, nokones said:

In measuring the cable loss, is the measurement done open cable end or something terminating the cable end (Closed end)?

You absolutely must have the cable terminated using a load that has the same impedance as the cable. A good 50 ohm dummy load is perfect. The next best thing is a tuned antenna, where the SWR is under 1.5:1. 

[WRYF638]

The amount of acceptable cable attenuation depends greatly on your antenna gain and its location. If you have an efficient antenna mounted high in the air, the antenna can make up for a good amount of cable attenuation. Conversely, if you have an inefficient antenna or the antenna is mounted poorly, you will need to almost eliminate cable attenuation in order to achieve any significant transmission distance.

 

For short cable runs, less than 10 meters, almost any conventional cable can be used without major impact on the signal strength. For long runs, as would be expected for a base station, low loss cable is far more important than the radio output power.