Best way to run a coax cable through a car window or sunroof

[WSHA485]

Hi, I am new to GMRS. I am trying to figure out the best way to run the coax cable by using a magnetic mount antenna in my car with my handheld or mobile radio. I have been leaving the sunroof open, but I get a lot of wind and road noise. If I close the sunroof or car window completely, will it damage the cable or affect the quality of the transmission?

Thanks.

[OffRoaderX]

Pinching the cable with the sunroof or window will eventually damage the cable.. The cable will work fine until then, but once the cable gets to the point where its broken or shorted inside it will either work very poorly or not at all - and at that point there is also the potential it could damage your radio.

On some cars the weatherstrip is so thick you can close the door on the cable without damaging it at all so you may want to check to see if your door is one of the lucky ones.

[WSHA485]

22 minutes ago, Socalgmrs said:

The is not good for a cable. Doing it correctly is the correct way to do it. This is not the correct way to do it.  

Thanks

 

[WSHA485]

6 minutes ago, OffRoaderX said:

Pinching the cable with the sunroof or window will eventually damage the cable.. The cable will work fine until then, but once the cable gets to the point where its broken or shorted inside it will either work very poorly or not at all - and at that point there is also the potential it could damage your radio.

On some cars the weatherstrip is so thick you can close the door on the cable without damaging it at all so you may want to check to see if your door is one of the lucky ones.

Thanks

 

[WSAM454]

Thick door rubber striping would be good.  You would not be able to completely close a window on the cable without damaging it.

Some owners use window deflectors like this:

https://huskyliners.com/c/exterior-accessories/vent-visors/?srsltid=AfmBOoqNIPV7B5LNpdnK1Nh6o4QrKHh7Dj8KkBcLiucq96wTuvt_dDHZ

Not recommending that particular one, but something like that would allow you to keep the window open a bit and pass the cable through it more safely.   Not sure if your vehicle would lend itself to this, or if you even care for the look, but just another suggestion.

[SteveShannon]

Often you can lift the edge of the weatherstripping and fit the cable in under it. Follow it to the bottom of the door and then sometimes you can squeeze it into the recess next to the end of the rocker panel to get into the vehicle. 

[WSHA485]

On 3/2/2025 at 4:41 PM, SteveShannon said:

Often you can lift the edge of the weatherstripping and fit the cable in under it. Follow it to the bottom of the door and then sometimes you can squeeze it into the recess next to the end of the rocker panel to get into the vehicle. 

Thanks for the photos!

[SteveShannon]

7 minutes ago, WSHA485 said:

Thanks for the photos!

You bet.  One other thing to be careful of is that the cable that’s used must have a solid dielectric rather than foam in order to withstand the typical stresses.  Commercial mounts with cable already fitted typically use RG-58, which does have a solid dielectric.  It’s lossy though at GMRS frequencies.  Some people might be tempted to use something less lossy, but most cables with less loss use foam dielectric. So it’s really important to understand what you’ve got.

To make things worse there are some brands of rig-58 that have foam dielectric.  

[HHD1]

On 3/2/2025 at 12:33 PM, Socalgmrs said:

The is not good for a cable. Doing it correctly is the correct way to do it. This is not the correct way to do it.  

What is the correct way?

[gortex2]

Best way is not to. Drill a hole. 

 

[Lscott]

On 3/2/2025 at 1:23 PM, WSHA485 said:

Hi, I am new to GMRS. I am trying to figure out the best way to run the coax cable by using a magnetic mount antenna in my car with my handheld or mobile radio. I have been leaving the sunroof open, but I get a lot of wind and road noise. If I close the sunroof or car window completely, will it damage the cable or affect the quality of the transmission?

Thanks.

The best way is anyway that works, doesn't damage the coax cable and doesn't let water leak into the vehicle.

You can look at other installations to get some ideas. The links below are for my CX-5.

https://forums.mygmrs.com/gallery/image/471-mobile-mount-with-antenna-rear-view/?context=new

https://forums.mygmrs.com/gallery/image/473-close-up-of-mobile-mount/?context=new

https://forums.mygmrs.com/gallery/image/474-mobile-mount-coax-routing/?context=new

https://forums.mygmrs.com/gallery/image/472-mobile-antenna-install-side-view/?context=new

https://forums.mygmrs.com/gallery/image/476-mobile-mount-drip-loop/?context=new

Some people will tell you that RG-58 has high losses at UHF, it does, but one has to look at the total system and make a judgement call to see what works the best. In my case I ended up using two types of coax, a very thin type to get inside and a lower loss type, better than RG-58, for the rest of the run. This might be over the top but I did the analysis for several installation cases.

https://forums.mygmrs.com/gallery/image/483-cx-5-antenna-system-analysisjpg/?context=new

 

[WRYZ926]

12 minutes ago, Lscott said:

Some people will tell you that RG-58 has high losses at UHF, it does, but one has to look at the total system and make a judgement call to see what works the best.

Lscott is correct. Yes RG58 has high losses on UHF. But most vehicle installations will only use an average of 12 feet of coax. You can easily make up for any loss with a good well tuned antenna with some gain.

I did the calculations for 12 feet of RG58 and antenna with 2 dBd of gain and an SWR of 1.8. You will be pretty much break even on power output. I used both 20 watt and 50 watt output for the calculations. 

12 feet of RG58 with a 2 dBd gain antenna with SWR of 1.8 output power is ERP (effective radiating power)

• 50 watt radio = 53.7 watts ERP  
• 20 watt radio = 21.5 watts ERP

As you can see, as long as your antenna has an SWR of 1.8 or less and at least 2 dBd of gain then you will be fine using 12 feet of RG58. I used 12 feet since most magnet mounts come with 12 feet of coax.

Most manufacturers list the gain in dBi and most loss calculators use dBd. Subtract 2.14 from the dBi value to get the dBd value.

https://kv5r.com/ham-radio/coax-loss-calculator/

[SteveShannon]

49 minutes ago, Lscott said:

The best way is anyway that works, doesn't damage the coax cable and doesn't let water leak into the vehicle.

You can look at other installations to get some ideas. The links below are for my CX-5.

https://forums.mygmrs.com/gallery/image/471-mobile-mount-with-antenna-rear-view/?context=new

https://forums.mygmrs.com/gallery/image/473-close-up-of-mobile-mount/?context=new

https://forums.mygmrs.com/gallery/image/474-mobile-mount-coax-routing/?context=new

https://forums.mygmrs.com/gallery/image/472-mobile-antenna-install-side-view/?context=new

https://forums.mygmrs.com/gallery/image/476-mobile-mount-drip-loop/?context=new

Some people will tell you that RG-58 has high losses at UHF, it does, but one has to look at the total system and make a judgement call to see what works the best. In my case I ended up using two types of coax, a very thin type to get inside and a lower loss type, better than RG-58, for the rest of the run. This might be over the top but I did the analysis for several installation cases.

https://forums.mygmrs.com/gallery/image/483-cx-5-antenna-system-analysisjpg/?context=new

 

That’s a nice series of pictures and a nice job of routing.

[Socalgmrs]

The is not good for a cable. Doing it correctly is the correct way to do it. This is not the correct way to do it.