Antenna tuning ?

My answer will not help you but it is my experience the on-board swr meters always seem to read a bit higher than actual. I cannot think of a single exception. Of course the key words are "in my experience". This is strictly anecdotal and your mileage may vary.

 

The only radio I have found that the swr reading is comparable to external meter is the new President Mckinley I just recently aquired, in most cases I wouldnt trust radios meter, that being said, yes if reading is higher on 40 then whip needs to be lowered, start trimming with 1/4 inch at a time. The advice given that your current readings are fine could be debated because generally school of thought has been 2.0 or less is good, but if radio is operated with high swr ( 3.0 or greater) over time could lead to the finals failing, I personally am not happy unless its below 1.5. I was in a 2018 T680 for a year and now an in a 2019 T680 and the best antenna that I have found is the 4 ft tunable Firestick II , the swr is 1.1, and the 5ft may work better but havnt tried them. The firesticks are top loaded so radiating coil is up and away from mirrors and cab and one of the reasons your swr readings are a little higher is due to reflect that the coil is experiencing.

 

Another issue to consider when addressing the swr is power loss. The wattage output will be less as the swr reading is higher. Firestick has a chart you can find on their website that shows power values with corresponding swr readings, as an example a swr of 2.0 equals 11.1% power loss so the radio is transmitting 88.9 % of rated power, so if you take a radio like yours that has rated pep of 70 watts at that swr it is only transmitting 62.23 watts . The Firestick II is the antenna that is tunable, it has a large screw at the top that allows you to raise it or lower it to adjust swr, there are also numerous other brands of fiberglass antennas that are adjustable and work as well, its just from my experience that unless you get a minimum of a 20 inch shaft to raise up the K40 , at best a 1.8 to 2.0 swr may be the lowest you will get.

 

The K40 and Wilson 2000 and other coil base loaded antennas are good they just dont work well on a truck that has a low mounting position like the T680, they are better suited for mirror mounts. I wouldnt recommend running linear amp with stock coax, that radio with low swr should do just fine on the airwaves, I switch radios out from time to time and use a Bearcat 980, Cobra 29 lx, and President Mckinley ( most recent addition) and they all have 4 watt carrier and pep to 12 to 16 watts ( give or take) and I usually average 5 to 10 miles Tx and Rx, so you may decide that amp may be worth more trouble than good.

 

Go to page 2 and look at thread by JRich, my setup in T680 and T880, and he has videos on how he routed his own coax.

 

Nothin factory is any good..
Op....Dont focus on the # of watts..Depending on who and how it was tuned it could still be crap....
Any trk with a low mount point need a top load ant(coiled wire up top)like firestock 2 and the skip shooters...Their tips are tuneable so no cutting req cause of the adjustable screw..
K0bg.com
Go study there a lil and watch some of the youtube vids on antenna set up..Alot of ham guys have bids that will point you in the right direction..All the same principles just doin it a lil different..

Also dont get hung up on swr,yes its good to get it low but there other measurements that need to be correct 1st..Find a shop with an analyzer...
Resonance(x=0) resistance(r=50) then swr will likely be near a 1.0-1.5...
Pick up an ohm meter and learn how to use it for coax and ant testing along with mount positions..