Cable Pinching

[Eltee]

There is only a small gap between the top of my rear hatch and the roof of my SUV where I was hoping to snake my antenna cable. It squeezes RG-58a/u cable a little. I don't know if this cable is "forgiving" enough to survive the pinch or if I should grind a small notch to prevent the pinch.

Any input appreciated.

[djxs]

1 hour ago, Eltee said:

There is only a small gap between the top of my rear hatch and the roof of my SUV where I was hoping to snake my antenna cable. It squeezes RG-58a/u cable a little. I don't know if this cable is "forgiving" enough to survive the pinch or if I should grind a small notch to prevent the pinch.

Any input appreciated.

I think in order to help, we need to know what kind of mounting system you are planning to use? The MICROMOBILE® MXTA27 UNIVERSAL LIP MOUNT is what I am using on my hood and trunk of both my vehicles and allows enough clearance for when both the hood and trunk close. I hope this helps. 

[mbrun]

It could affect it depending upon the degree of deformation. The most absolute way to know is to check the SWR before and after. My mobile feed-line experiences very slight compression and the SWR remains in the excellent range.

You can probably do some tests by using your fingers to compress the cable while you are taking readings to see how much compression the cable takes between your readings go hay-wire.


Michael
WRHS965
KE8PLM

[WRNA236]

The impedance of coax relies upon the inner and outer diameters being constant.  It's best not to pinch any coax but especially if it's used to transmit or reception is critical it really needs to stay round.

More technical stuff.  In this the diagram the 'εr' stands for the dielectric constant, which varies depending on the material used.

Take a typical coax, RG-58/U.  This has an inner conductor O.D. of 0.036", which is the same as the dielectric I.D. (Di), and the dielectric O.D. (Do) is 0.116".  With a solid polyethylene dielectric constant of 2.25 this gives your characteristic impedance of roughly 50Ω (calculates to 46.8Ω).  With the shield and outer insulation thicknesses the overall O.D. will be 0.195". 

For the sake of argument say you pinch the coax so it's outside is 5/32" (0.15625").  The inner conductor and shield are unlikely to compress being copper and tin.  So it'll be a little bit of the outer jacket but mostly the dielectric that distorts.  So say the dielectric becomes 0.07725" in O.D. now.  The impedance at that pinch be calculated and will be about 30.5Ω. 

With that a lot of other stuff can be found.  Reflection coefficient can then be calculated, that will be 0.25.  With this you can estimate VSWR, that's going to be 1.67.  Other things can be estimated, mismatch loss for a reflection coefficient of 0.25 is 0.28 dB.  Return loss will be 12 dB, which means about 6.25% of the transmitter power is reflected back due to this example pinch.

If you need coax to go through narrow spots you can use a smaller in diameter coax, such as RG-316.  There's a downside in distributed loss, meaning to avoid a mismatch in impedance (it'll remain a constant 50Ω) the cable has higher loss (at UHF this is substantial).  So you'd want to especially keep it short.  There are some antenna mounts that mix coax types, a short length of RG-316 at the end to get behind a seal that then transitions to RG-58 or RG-8X for the length inside the vehicle. 

 

[Eltee]

On 7/18/2021 at 11:23 AM, djxs said:

I think in order to help, we need to know what kind of mounting system you are planning to use? The MICROMOBILE® MXTA27 UNIVERSAL LIP MOUNT is what I am using on my hood and trunk of both my vehicles and allows enough clearance for when both the hood and trunk close. I hope this helps. 

Using a Laird magnetic mount. 

[Eltee]

I want to route the cable through the weatherstrip. What is the recommended way to create a small hole to push the cable through. Can I drill it, use a hot phillips screwdriver, cut a small "X", etc.?

 

[BoxCar]

You may try slitting the weatherstripping and then sealing the slit with some Gorilla Glue or silicone glue.

[djxs]

Water mitigation is of utmost importance. Whatever you do, try and keep the weatherstrip in tact as much as possible. Send us some pictures.

[maddogrecurve]

On 7/19/2021 at 6:47 AM, WRNA236 said:

The impedance of coax relies upon the inner and outer diameters being constant.  It's best not to pinch any coax but especially if it's used to transmit or reception is critical it really needs to stay round.

More technical stuff.  In this the diagram the 'εr' stands for the dielectric constant, which varies depending on the material used.

Take a typical coax, RG-58/U.  This has an inner conductor O.D. of 0.036", which is the same as the dielectric I.D. (Di), and the dielectric O.D. (Do) is 0.116".  With a solid polyethylene dielectric constant of 2.25 this gives your characteristic impedance of roughly 50Ω (calculates to 46.8Ω).  With the shield and outer insulation thicknesses the overall O.D. will be 0.195". 

For the sake of argument say you pinch the coax so it's outside is 5/32" (0.15625").  The inner conductor and shield are unlikely to compress being copper and tin.  So it'll be a little bit of the outer jacket but mostly the dielectric that distorts.  So say the dielectric becomes 0.07725" in O.D. now.  The impedance at that pinch be calculated and will be about 30.5Ω. 

With that a lot of other stuff can be found.  Reflection coefficient can then be calculated, that will be 0.25.  With this you can estimate VSWR, that's going to be 1.67.  Other things can be estimated, mismatch loss for a reflection coefficient of 0.25 is 0.28 dB.  Return loss will be 12 dB, which means about 6.25% of the transmitter power is reflected back due to this example pinch.

If you need coax to go through narrow spots you can use a smaller in diameter coax, such as RG-316.  There's a downside in distributed loss, meaning to avoid a mismatch in impedance (it'll remain a constant 50Ω) the cable has higher loss (at UHF this is substantial).  So you'd want to especially keep it short.  There are some antenna mounts that mix coax types, a short length of RG-316 at the end to get behind a seal that then transitions to RG-58 or RG-8X for the length inside the vehicle. 

 

Sadly, I just looked at my coax that is going under my hood!  it has two nasty creases in it from me closing my hood onto it!  I have a fender mount oand am running RG58 coax.  I did not know that the foam padding made a difference at all.

I may need to notch my hood a little or re rout it??  The damage is done though, or can I try to re-fluff the insulation?

 

Any help would be appreciated please!

 

Jeff WRMQ982

[djxs]

@maddogrecurve If you do cut out a notch, just be sure to seal it back up to prevent rust. 

[mbrun]

Sadly, I just looked at my coax that is going under my hood!  it has two nasty creases in it from me closing my hood onto it!  I have a fender mount oand am running RG58 coax.  I did not know that the foam padding made a difference at all.
I may need to notch my hood a little or re rout it??  The damage is done though, or can I try to re-fluff the insulation?
 
Any help would be appreciated please!
 
Jeff WRMQ982

While I agree with 236’s post on how characteristic impedance is calculated, what is most import to you is what effect that pinch is having on you in your situation. If you can find a way to eliminate it then I advise doing so. But you may not be in imminent danger. If you have an SWR meter or NanoVNA, measure the effective SWR. If the numbers you get are within acceptable range you can use what you have until you can find a better solution.

I will let those that do mobile installs offer further input how you might improve your installation.

Regards


Michael
WRHS965
KE8PLM

[WRNA236]

On 8/5/2021 at 12:22 AM, maddogrecurve said:

Sadly, I just looked at my coax that is going under my hood!  it has two nasty creases in it from me closing my hood onto it!  I have a fender mount oand am running RG58 coax.  I did not know that the foam padding made a difference at all.

I may need to notch my hood a little or re rout it??  The damage is done though, or can I try to re-fluff the insulation?

 

Any help would be appreciated please!

 

Jeff WRMQ982

Depends on how bad it's pinched.  Removing the pinch will help but it's not going to really fix it either.  I should also admit that it's not a situation where any damage is automatically bad necessarily.  Ideally you have perfect coax but that's also not realistic and a little damage isn't going to take your SWR from perfect to terrible. 

So if you can test you should but unless you've completely flattened the coax I'd probably not worry too much.  I would not notch my sheet metal personally, that's not necessary.  I'd just run it until the jacket abrades or the UV takes it toll and be more careful next time. 

BTW there exist mounts with a short length of tiny coax (RG-174 or RG-316) that transitions to RG-58 after about a foot.  This is the slick way to get through gaps.

[maddogrecurve]

On 8/8/2021 at 5:18 AM, mbrun said:

While I agree with 236’s post on how characteristic impedance is calculated, what is most import to you is what effect that pinch is having on you in your situation. If you can find a way to eliminate it then I advise doing so. But you may not be in imminent danger. If you have an SWR meter or NanoVNA, measure the effective SWR. If the numbers you get are within acceptable range you can use what you have until you can find a better solution.

I will let those that do mobile installs offer further input how you might improve your installation.

Regards


Michael
WRHS965
KE8PLM

My SWR has not changed, it is very good, around 1.3 all through GMRS/FRS.  So I am not worried about it, and since I am only running HT's, I am not worried about loosing power, I have none anyhow LOL>

[maddogrecurve]

On 8/9/2021 at 5:10 PM, WRNA236 said:

Depends on how bad it's pinched.  Removing the pinch will help but it's not going to really fix it either.  I should also admit that it's not a situation where any damage is automatically bad necessarily.  Ideally you have perfect coax but that's also not realistic and a little damage isn't going to take your SWR from perfect to terrible. 

So if you can test you should but unless you've completely flattened the coax I'd probably not worry too much.  I would not notch my sheet metal personally, that's not necessary.  I'd just run it until the jacket abrades or the UV takes it toll and be more careful next time. 

BTW there exist mounts with a short length of tiny coax (RG-174 or RG-316) that transitions to RG-58 after about a foot.  This is the slick way to get through gaps.

Very cool!  I thought about those kind of mounts, but where I would mount them, I have room for large coax.  It's my fenders that I have no space, and looking at my Ford Focus, it is even tighter.  I put a fender mount on it but am trying to figure out how to get the coax ran through.  I may have to stick with maggie mounts.