Question about coax in a vehicle

[dragon_slayer79]

Hello all,

I've been thinking and wondering for the last few days about coax use in a vehicle. What I'm wondering is this... Most mobile antennas mounts both scanner and HAM almost always uses RG58 cable, but you always read about how lossey RG58 is and how it's not shielded all that well. I know a typical cable run in a vehicle isn't more than 20 feet or so, but there is so much "noise" in a vehicle that it seems counter productive to use (for lack of a better term) cheap cable in a harsh environment.

I know it's small and easy to hide but wouldn't you be better off using something like RG6 QS, LMR400 or something that has lots of shielding to help fight against all the "noise" that exist in a vehicle? Granted it's a bigger cable etc etc, but if a person would be willing to put forth the effort would it be worth it in the end?

I know some will just go mad with such a question, but at the same time it's a genuine question I have, be it stupid or not.

If there are mounts out there that use better coax than the RG58 that you know of please share for I've never came across such a mount. I'm referring to the NMO style mounts just to be clear.

 

[burner50]

Using 75ohm cable (RG6) on a 50ohm application doesnt yield the best results in receiving, and will damage your radio if you transmit.


If you're in a vehicle, you're using short runs which helps avoid signal loss, and as far as shielding goes, it doesnt matter what "size" the cable is, it probably comes in various shielding options. On the shielding option, what is the point of "Shielding" a coax that is connected to a vertical antenna 4 feet away that is designed to receive radio signals?

Also it depends on what bands we're talking about here. Signals have different losses at different frequencies... The higher the frequency, the higher the signal loss.


But really, since we're talking about a vehicle, the runs are going to be short enough that any gains by using a huge cable are negligible... Just stick with whats commonly available and save yourself the extra cost and headache.*

 

[n5ims]

There are some mounts that use LMR-200 (or quite similar) coax, but they are quite difficult to find and cost a bit more. There are also various different qualities of RG-58 out there and it's hard to figure out what's what.

For example, Larsen makes several different NMO mounts with one option being the type of coax that's attached. First would be their NMOKHFCX which uses the normal "RG-58 A/U cable" that's the lowest cost and easiest to find. Next up would be their NMOKHFUD which uses "RG-58/U Dual Shield, low loss cable". Lastly would be their NMOKHF200NCI which uses "LMR200, low loss cable" (it also has a crimp N connector attached while the models given have no connector attached - you can order with many types of connectors for all but the LMR200 though).

If you notice on the model numbers, they start out the same, indicating that they all use the same NMO High Frequency Thin mount ("NMOKHF"). Next indicates what coax is supplied. "CX" is the low end, "UD" the double shield, (there's also a teflon dual sheld with the code "DS"), and "200" for the LMR200. The last part indicates what connector, if any, is supplied or attached.

The mounts are designed to be used by commercial 2-way radios and as such must use 50 ohm cable so RG-6 wouldn't be an option for them since it's 75 ohm cable. LMR-400 is quite large and stiff so it doesn't make a very good mobile coax. The cables are normally 12 - 17 feet long, so there's not enough loss to be noticed in actual operation. Use of even the lowest grade is good for VHF and UHF signals, but for 800 the middle grade is often used. The LMR-200 is normally reserved for the very high frequencies (not VHF, but in the gigahertz range), due to the cost.

You also asked about the noise in a mobile environment. If the DC is properly supplied, direct from the battery or other isolated power distribution unit, much of the electrical noise is taken care of. The antenna should also be well grounded to the vehicle to prevent noise from getting in that way. Grounding of the radio is also generally done, but this should be done according to the radio manufacturer's instructions. A failure here may cause not only additional noise, but other failures, including blown fuses or even fires.

 

[firemedic2150]

I like the coax that Stico uses on their CCAS-SB concealed antenna's. I use a lot of these antennas on vehicles and the coax that comes with them is very stiff and has a tough outer jacket and has a full braid as well as a foil shield under the braid. I am not sure who makes it, or a type designation. It is also a bit of a pain to put a conector on the first couple of times you use it but after you get the hang of working with it it is great.

 

[fineshot1]

I have successfully used LMR 240 and LMR 400 in vehicles but its not easy.

I made up a specific length cable for each application and crimped N male
connectors on the ends and used a short pigtail of RG-58U with the appropriate
connectors. Running LMR 240 was much easier than the LMR 400 which is a
real PITA inside a vehicle. Both cables are rather inflexible and once routed and
positioned its best to secure them down if possible.

I have also made up my own NMO mount with both flavours of cable and this is
also a real PITA but much easier with the 240. When it comes to soldering with
these cables you gotta know what you are doing and be prepared to do it over
possibly several times until you get it right. The foam dialectric melts and distorts
if exposed to too much heat. This can compramise the impeadance of the cable.
If its just being used for receive its not a big problem but if you are also using a
transmitter it must gone done correctly.

 

[benbenrf]

Hello all,

I've been thinking and wondering for the last few days about coax use in a vehicle. What I'm wondering is this... Most mobile antennas mounts both scanner and HAM almost always uses RG58 cable, but you always read about how lossey RG58 is and how it's not shielded all that well. I know a typical cable run in a vehicle isn't more than 20 feet or so, but there is so much "noise" in a vehicle that it seems counter productive to use (for lack of a better term) cheap cable in a harsh environment.

I know it's small and easy to hide but wouldn't you be better off using something like RG6 QS, LMR400 or something that has lots of shielding to help fight against all the "noise" that exist in a vehicle? Granted it's a bigger cable etc etc, but if a person would be willing to put forth the effort would it be worth it in the end?

I know some will just go mad with such a question, but at the same time it's a genuine question I have, be it stupid or not.

If there are mounts out there that use better coax than the RG58 that you know of please share for I've never came across such a mount. I'm referring to the NMO style mounts just to be clear.

dragon_slayer79

You are 101% correct on all points raised:

- car interiors are noisy enviroments from an rf point of view.

- using 75ohm coax on 50oghm systems, when the extra cost for 50ohm is negligable, is pointless.

- screening/shielding is big time important.

Individualy these points may not be too imporntant in the overal context of things, but collectively its the small apparently insignificant considerations that add up to having a significant impact ultimately on system performance. So yes, it makes complete sense to employ a good quality 50ohm coax that has a nice dense shield (strip back the black plastic/rubber/pvc outer insulation a couple inches of whatever you intend to use, and check to see how thick/dense the shield is - the less seperating plastic/urethane/PVC you can see under the screen/sheild weave, the better the shielding is - and quite frankly, for the lengths you are talking about here, it will cost you little extra to invest in coax that offers a 100% screen/shield. I say go for it.

LMR200 willbe an excellent choice, but keep in mind that LMR200/400/600 etc etc .... have become generic terms, meaning that you get good qaulity LMR 200 and you get poor quality LMR200. It really comes down to who the actual manufacturer is - with many of the Far Eastern branded LMR200 type coaxs been presented, and sold with pretty crappy shielding. So don't just buy coax because its branded LMR200, or whatever - pull back a couple inches of the outer insulation and check to see how dense the screen/shielding actually is.

Second point: going to this effort of investing and installing a good quality coax will come to little if:

a) the receiver its self is not correctly tied to the chassis/earth, as well. Ensure it is.

b) the antenna (if its of a design/type that requires it to be grounded to chassis/body panel/earth) is equally well grounded. Ensure it is.

And lastly - in so far as is practcial and possible, route your coax away from any/all and as much as poss, from other wiring in the car. Coonect a scope to just about any car wiring and see how much "noise" is running around the loom - although, in reality I have to say that it seldom impacts on real world listening.

If a job is worth doing - its worth doing properly. Good on you.

 

[dragon_slayer79]

Thanks to all that have replied so far. Your thoughts and insights are informative and interesting to read. Thanks again for taking time to share your thoughts.