SteveShannon

Everything you say is true, but I’ve never seen an NMO mount (that comes with a cable) that only has 3.8 dB loss at 467 MHz. Most come with much lossier cable.  For instance, the following Laird magnetic NMO mount comes with a cable that would have 12.5 dB loss if it were 100 feet.  It’s only a 12 foot cable so it loses about 1.5 dB for that distance.
https://www.arcantenna.com/products/laird-antenex-gb8t-magnetic-nmo-mount-vehicular-nmo-antenna-roof-mount-12-ft-rg-58a-u-cable-tnc-male-connector-included?variant=32297030811779&currency=USD&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gad=1&gclid=CjwKCAjwjOunBhB4EiwA94JWsNBrxPjgMgjWlB56LSL_hmzYUfMLDg2FMcehTYdnfrryxVeyNCFQOhoCk0EQAvD_BwE

As mentioned by @Borage257, especially due to the time of day and the time of year and what the forecast has been, you are near certainly experiencing tropo ducting.
 
That said, you touch on something that many people grossly overlook, including radio pros.  Although we discuss VHF and UHF radio being a "line of sight service"... that doesn't mean that atmospheric conditions impacted by solar cycle don't impact VHF and UHF... particularly in the case of GMRS.  In fact, during the day, they can have a profound affect in either increasing range or swinging the other way, causing radio blackouts.
 
UHF signals actually travel comfortably beyond the visual line of sight, by about 15%.  This is due to refraction and scatter affects in the atmosphere.  The more loose, electrically charged ion particles are in the air, the more scatter and refraction you get.  This translates to better signal quality at distance and often, a small bump in range.  The ionization in the F region increases roughly 2 × 106 e/cm3 during maximum sunspot activity.  The F1 layer is the principle reflecting layer during the day in the summer.  This is why some bands perform much better during the day.
 
While these principles have more and more of an impact as you go lower in frequency, the lower the takeoff angle and the more the solar weather is spicing things up, the more likely you are to have a significant amount of your UHF signal refracted and/or scattered back to the earth beyond the horizon.
 
As an example, during a contest back in 2009, I was able to use a stacked beam array and 1,500w to make a contact on 146.52 MHz from a mountain top in Virginia to central Texas.  On that same day, I was also able to make a contact on 446 MHz to Tennessee, near the Arkansas state line, using the same kind of setup as the VHF contact.

What is acceptable is up to the individual and if you are getting the job done.  Losses in a 17' run will be difficult to measure since it's so small and most equipment the casual operator owns and uses.
In order to have your signal impacted by 1 signal unit (out of a possible 9, typically) you would have to cut your power by 4.  Meaning, if you have a total of 10 watts out of your radio, but only 2.5 watts makes it to the antenna, you will see that the receiver's signal strength meter only falls 1 signal unit.  Same concept on your receive sensitivity. 
Well, lets say your cable has a 3.8 dB loss at 467 MHz.  You would need to run almost 200 feet of cable before you impact your signal 1 signal unit.  at 17 feet, you only lose about 0.65 dB.  At 10 watts out of the radio, you will still have 9.2 or 9.3 watts going to the antenna.  While the result is measurable, its meaningless from a user perspective.

DPL is a digital code rather than the analog tone used in CTCSS. On many radios it’s called DTCSS or DCS. Almost every radio supports it but some companies have included some non-standard codes. 
The Motorola version of CTCSS is simply called PL, not DPL. 

DPL is a digital code rather than the analog tone used in CTCSS. On many radios it’s called DTCSS or DCS. Almost every radio supports it but some companies have included some non-standard codes. 
The Motorola version of CTCSS is simply called PL, not DPL. 

Looks good but you have the wrong channel!  Everyone knows it should be CH19..
But, also, i think the "average" GMRS/FRS adventurer probably does not know what a "PL" is, and probably does not know how to set/change it.  Of course, the radio-dorks do, but nobody wants to talk with them. 

Driving to Moab UT tomorrow, for a week.  I thought I would try this sign.
They were made by friends, for free.  They could not find magnetic sign material on short notice (which I would rather have).
I guess I will see if I get any takers. 

DPL is a digital code rather than the analog tone used in CTCSS. On many radios it’s called DTCSS or DCS. Almost every radio supports it but some companies have included some non-standard codes. 
The Motorola version of CTCSS is simply called PL, not DPL. 

DPL is a digital code rather than the analog tone used in CTCSS. On many radios it’s called DTCSS or DCS. Almost every radio supports it but some companies have included some non-standard codes. 
The Motorola version of CTCSS is simply called PL, not DPL. 

This is the kind of info that’s useful.

Apparently emi gasket works. You’d have to take a wire wheel to the lip and inside of the seal are then apply the emi gasket.

I tested an ammo box using radios (2m HT with rubber duck antenna) and it still RXed.  I figured it was because of the rubber gasket.  Old gutted microwave worked though.

No

Good to know.  I’m surprised.  I wonder if the addition of metal tape around the lid would help.

I agree for smaller cans, but I don’t buy much (any) 30mm.  😄

I don’t know for sure, but I would be optimistic. But for God’s sake don’t pay $100 for one just because a preppier supply company sells it.  Find a local surplus store.  I think I paid $10 for a 30 mm ammo can a few years back.  If you absolutely cannot find something locally at least shop around:
https://www.cleanammocans.com/50-cal-ammo-cans.html

I spent the last 13 years of my career in the control center for an electric transmission system. I supervised the team that maintained the Energy Management System (SCADA) software and hardware that controlled the electric transmission system.  We spent nearly all our time making sure the Energy Management System worked well with hot swap redundancy at every level in the control center.  Every electric transmission system has such a control system which controls energy transmission substations across huge sections of our three national grids.  Loss of a single control center or our largest substations has significant effects on the grid. Fortunately, such losses happen very infrequently and our systems are built to be restored quickly.
But, those EMP weapons, tiny as you say, would be sufficient to completely destroy the controls for our largest electric substations and control centers. Or any other cyber or communications infrastructure.
I think an atmospheric nuke, causing an EMP as a side effect, is a highly unlikely occurrence.  There are only a few countries which have the capability and for the most part there are checks and balances.  
But, it’s just a matter of time before a suitcase sized EMP only device is deployed by some small political or religious extremist group.  It’s the ideal weapon for group that might wish to “bring our evil society to its knees” but that might also have a fundamental aversion to taking lives.

Got to get the 40 mike mike training rounds for the under-barrel. LoL 

I agree for smaller cans, but I don’t buy much (any) 30mm.  😄

What model Laird antenna did you use to get those high readings?

I agree with the comments above.  No adjustments in the radio.  The nl-770g antenna is a pre-tuned antenna, that does not require tuning.
 
I also agree that it's good to check your SWR anyway.  The reasons Kenny mentioned are all good, but the location of the antenna also impacts your SWR.  If your SWR is high, you may want to move the antenna to a different location.  Cutting a pre-tune antenna and changing its resonant frequency is typically not a good idea.

As Steve said, no but, it is always good to check the VSWRs on a new setup just to make sure you're not reflecting a lot of power back into the radio by having a bad connector of a defective coax cable. It does happen and a lot of reflected power will affect the power output and you will have a poor performing antenna system regardless if the antenna is properly tuned.

I have a few thoughts on that. While a lot of criticism was leveled at the government for waiting to shoot it down over the ocean there might have been a legitimate security reason for it. Getting photos is one thing, but I think the main purpose was to monitor our military communications systems and gage our response procedures and times to an unknown threat. If China invades Tiawan that would be very valuable Intel. Waiting until it was over the ocean to respond sort of wasted their efforts in that area.

I don’t know for sure, but I would be optimistic. But for God’s sake don’t pay $100 for one just because a preppier supply company sells it.  Find a local surplus store.  I think I paid $10 for a 30 mm ammo can a few years back.  If you absolutely cannot find something locally at least shop around:
https://www.cleanammocans.com/50-cal-ammo-cans.html

That would surprise me, but I have no actual knowledge.  My gut tells me it was more likely just for electronic surveillance, slowly passing over the US collecting signals while passing intelligence to Chinese satellites to be relayed back to China.