SWR change with same antenna?

[WRFV510]

I just installed my xpr5550e in my 21 Silverado crew cab 2500HD.  I had this radio and same antenna on my 17 Silverado crew cab 1500. A Laird BB4505CR. The only thing that has changed is the Coax. I was using RG8X. now im using LMR-240.  both are roof mount installs.  On the 17 I had swr of 1.2 on 467.725 and 1.5 on 462.5500. now i have 2.1 on 467.725 and 1.3 on 462.5550.  2.1 is not acceptable to me.  would the different length of coax make a difference? I had to use probably 5 more feet on the 2500.  The 17 had no sunroof so the antenna was in the middle. the 2500 has a sunroof and has a 16" ground plain. Being the swr on the higher channel is higher i guess i could shorten the antenna but i have another antenna. A Laird Technologies UTRA4301S3NB  that i wanted to use but the swr on that is just about the same. I solder my 1st nmo roof mount. is it possible i screwed it up? I checked it with My Fluke 88V and had no continuity between center and outer.  

[mbrun]

Yes, a change in cable and cable length can yield a different SWR. A higher loss and longer length cable will yield an artificially lower SWR when measured at the radio when contrasted against the same antenna but lower loss and shorter length coax. The greater the total loss in the coax, the lower the SWR.

 

The LMR-240 has lower loss than the 8X, so assuming the same length of cable you could see a small increase in SWR because you are able to see and measure the antenna’s reflected power better.

 

The integrity of the solder connection could be factor, as could the integrity of the antenna ground connection to the vehicle. Both are items to check.

 

If possible, use the shortest possible coaxial length with your antenna until you confirm that SWR is where you want it to be, just to eliminate the cable length variable from the equation. Once it is where you want it to be, then you can use longer cable, and if done well, the SWR will stay the same or drop a smidge.

 

Also, same antenna mounted in different vehicle(s) can yield different SWR values, as can the physical proximity of the antenna to other objects.

 

Just some thoughts for you.

 

I just installed my xpr5550e in my 21 Silverado crew cab 2500HD. I had this radio and same antenna on my 17 Silverado crew cab 1500. A Laird BB4505CR. The only thing that has changed is the Coax. I was using RG8X. now im using LMR-240. both are roof mount installs. On the 17 I had swr of 1.2 on 467.725 and 1.5 on 462.5500. now i have 2.1 on 467.725 and 1.3 on 462.5550. 2.1 is not acceptable to me. would the different length of coax make a difference? I had to use probably 5 more feet on the 2500. The 17 had no sunroof so the antenna was in the middle. the 2500 has a sunroof and has a 16" ground plain. Being the swr on the higher channel is higher i guess i could shorten the antenna but i have another antenna. A Laird Technologies UTRA4301S3NB that i wanted to use but the swr on that is just about the same. I solder my 1st nmo roof mount. is it possible i screwed it up? I checked it with My Fluke 88V and had no continuity between center and outer.

 

Michael

WRHS965

KE8PLM

[WRFV510]

well that puts my mind at ease some what. I have 2 BB4505CR antennas. the other one was too short and read 2.5 on low channel and 1.2 on high channel. so I know I just need to shorten the one antenna.  I also Have a BB4503. that is 1.5 across the board.  I cant make the cable shorter. The only place you can mount a radio my size in this truck is on top of the dash. So it had to go top to under the dash and under the center console to the back pillar then up the pillar to the center of the usable roof.  I wanted to post pics but i guess you're not allowed too. 

[OldRadioGuy]

As others have said SWR is the same as "return loss" which means how much power is reflected.

An SWR of 1:1 which is a very high (technically infinite) return loss or no reflected power.

Coax with more loss allows less power to be reflected and therefor a better SWR.

 

Adding a 3dB attenuator to your antenna cable will improve the SWR.

Of course, that would reduce your radiated power so no one would likely ever do that.

 

You can look up a table of SWR versus return loss and see the impact of cable loss on SWR.

 

 

Vince

[WRFV510]

I bought a Laird  RDE5798 450-470 MHz 3dB Phantom Elite, Black. 1.5swr on 462.5500 and 1.7 on 467.7250. tested it out and other then a little static in the worce areas for our repeater it works pretty good. i can talk to my dad the hole 40 miles home with it. The repeater is 5 miles away from my house.  I also tuned the Laird BB4505CR and got it to 1.5 on 462.5500.  1.2 on 465.0000 and 1.5 on 467.7250 so good as its going to get.