Mobile Application Coax

[WRMG216]

I am doing battle trying to find a decent coax for my GMRS mobile setup. I used the RG-8X and it seems to be a big fail. I am losing power and the SWR is high. I am pretty sure it is the cable but how can I tell? When I put up my magnetic antenna on top of my 2020 Ram, The SWR is around 1.3 to 1.5 @ 20 watts. If I connect a 10 foot RG-8X extension then it goes south and the swr raises to 2.5 to 3.5. Yes the cable is ok. I tested it.  The magnet mount is a Midland with a Midland 6db antenna and it is using RG-58U cable. If I extend it with the RG-8X will that really cause the swr to raise that much? I am trying to hard mount an antenna using an NMO mount with good cable but want to make sure I have everything running before I invest a lot of time running the cable. Thanks for any help. 

[mbrun]

I am doing battle trying to find a decent coax for my GMRS mobile setup. I used the RG-8X and it seems to be a big fail. I am losing power and the SWR is high. I am pretty sure it is the cable but how can I tell? When I put up my magnetic antenna on top of my 2020 Ram, The SWR is around 1.3 to 1.5 @ 20 watts. If I connect a 10 foot RG-8X extension then it goes south and the swr raises to 2.5 to 3.5. Yes the cable is ok. I tested it.  The magnet mount is a Midland with a Midland 6db antenna and it is using RG-58U cable. If I extend it with the RG-8X will that really cause the swr to raise that much? I am trying to hard mount an antenna using an NMO mount with good cable but want to make sure I have everything running before I invest a lot of time running the cable. Thanks for any help. 

Extending the length of a coax should not typically raise the average SWR from a properly tuned antenna. If the primary and extension cable and connectors are good the only thing you should anticipate is average SWR going down bit (how much varies with length). However, any mismatch caused by impedance differences in the cable, connectors, adapters and antenna can and will manifest themselves in the form of elevated SWR readings.

For example, If I were to add a UHF barrel adapter in-between two otherwise good working short cables, I may see the SWR go up a GMRS frequencies as compared to use of only one cable. If I switch to a single cable of the same aggregate length the SWR will be lower. The frequency of test can result in different values based on cable length. Consider averaging your readings across the GMRS frequencies and see what you get.

If you owned a NanoVNA and new how to use it you could spot issues with the cables and adapters more readily.

One last thing, RG-8X cable has lower loss than RG-58. So if you separately compared the SWR of a given antenna with perfect 25’ samples of each of those cables I would expect the SWR on the RG-58 to be slightly lower because it has higher signal loss.


Michael
WRHS965
KE8PLM

[WRMG216]

Thank you Michael. Great info! I do have a barrel connector between the two when I tested the magnet antenna. Let me see if I have a different one somewhere and try that. 

[AdmiralCochrane]

Would a jumper with a female end on the coax be superior to using a barrel connector between 2 pieces of coax with male ends? 

[Lscott]

2 hours ago, AdmiralCochrane said:

Would a jumper with a female end on the coax be superior to using a barrel connector between 2 pieces of coax with male ends? 

That depends on the connector type. If you're using a constant impedance type like, "N" - BNC - SMA, it doesn't make much difference. There should be little loss in the connectors and the impedance bump should be nil so no real impact on the SWR. I wouldn't try this with PL-259/SO-239 connectors.

I use patch cables in my Jeep. I have several patch cables with BNC connectors spliced to a Mini RG8 type on the end of the cable to my luggage rack antenna mount. The Mini RG8, which look like miniature PL-259/SO-239 connectors, are really constant impedance type and low loss. 

[mbrun]

Would a jumper with a female end on the coax be superior to using a barrel connector between 2 pieces of coax with male ends? 

I agree with Lscott. It depends. The fewer the connections the better. Also, at UHF frequencies at which GMRS operates at, any adapter you put in line with a SO-239/PL-259 connector is a candidate to skew your numbers. That is absolutely my experience when measuring things with both a VNA and a standard SWR meter. While my radios may have the SO-239 on them, I try to stick with N-connectors for everything else. Adding barrels, right-angles, etc... does not skew the results to the same negative degree as the 239/259 products.


Michael
WRHS965
KE8PLM

[gortex2]

Jut order a NMO Mount with the cable. 90% of the NMO mounts come with 16' of cable reattached. That should be plenty for any size vehicle. 

[AdmiralCochrane]

15 hours ago, kb2ztx said:

Jut order a NMO Mount with the cable. 90% of the NMO mounts come with 16' of cable reattached. That should be plenty for any size vehicle. 

Was just asking in general use, not specific to mobile.  

 

Relating to my actual mobile application, 16' falls about 4' short on my company truck, a long wheelbase Ford cargo van; probably one of the few exceptions.  Instead of mounting the radio where I can actually see it, I have it Velcroed to the passenger arm rest.  

[gortex2]

NMO mount is for mobile use. They make a NMO-N Mount that allows you to use any length cable you would need. 

[OldRadioGuy]

Just remember that higher cable loss results in BETTER SWR.... (but not better radio performance).  I do not necessarily think this is what's going on in this particular situation though.... unless something is really wrong with the cable itself or a connector is defective. 

Minimum return loss is twice the cable loss.  So if your cable loss was 3dB your min return loss would be 6dB which means the worst SWR you could ever see is 3:1 even with no antenna at all.  (Of course, no mobile install would have such a high coax loss with only 20' of cable.)

You can see this if you just install a 3dB attenuator on the SWR meter with nothing at all attached after it.  

Ham guys are often puzzled when they upgrade (a 75' run or whatever) to low loss coax then end up with a worse SWR.  The antenna was always a poor match but it was masked by the lossy coax.  Of course their radio will actually radiate more power than before with the better coax even with the worse looking SWR.  More power reaches the antenna and therefore  more power is reflected back.... with the same antenna as before.

Length of coax does not affect SWR at all except for the difference in loss associated with it.  It can however affect forward radiated power if your radio output is not a true 50 ohms... and most are not.  They are designed to "work into 50 ohms."  This does not mean that they "look like 50 ohms"  when looking back into the output port.   There may be impedances other than 50 ohms which result in better (or worse) power output.  I've seen this on my KG805G with a ham radio 70cm antenna and a certain length of coax.  The radio will actually put out more than the typical 4-5W with some lengths of coax. 

Of course the "wrong" impedance can also cause instability or excess dissipation so you want to operate your radio into something reasonably close to 50 ohms.  Any SWR better than 3:1 should be safe though.  Many low power hand held or mobile radios would be "safe" into just about any impedance. 

Vince

[WRMG216]

On 6/4/2021 at 6:05 PM, kb2ztx said:

Jut order a NMO Mount with the cable. 90% of the NMO mounts come with 16' of cable reattached. That should be plenty for any size vehicle. 

Thats what I did but the cable was not long enough for the application. I am one of the 10%