Those aren't substantive answers. I actually want to see a detailed explanation on what I'm allegedly missing by not buying a Larsen. If it's longevity and durability, great. I just would like to know.
Excellent questions...
If you take a permanent NMO mount antenna and mount it dead center on the roof of a vehicle, and compare it directly with a properly tuned magnetic mount antenna in the same location, it's about 0.1dB of loss on the mag mount. That's not something that any hobbyist is going to notice and nothing that can be properly measured without the right professional grade/calibrated test equipment on an antenna test range.
Seat of the pants will tell you there is no difference.
And you'll get no argument from me on that.
There are some other things to consider, however….
The capacitive coupling of the magnetic mount to the vehicle body isn't perfect. That's where the above 0.1dB penalty comes from. That can also vary depending on the size of the mag mount base, right down to thickness of the paint.
With a permanently mounted antenna the coax won't/shouldn't radiate energy.
With a mag mount and no direct connection to the ground plane, the coax can. Radiating coax means power isn't getting out the antenna. It can also create issues with vehicle electronics by letting RF into places it doesn't belong. It can also change the radiation pattern of the antenna.
The other challenge is that more and more vehicles are using aluminum or other non-ferrous materials in their construction. Aluminum has been used for a long time on fire trucks and ambulance bodies for weight savings and corrosion resistance. Mag mounts are not going to stick to that. My work and personal Ford trucks are aluminum, so mag mounts are not an option. Some cars are using composite body panels, so again, not an option. The permanent mounts are one of the few options that work well, and there are a lot more mounting options available where a mag mount won't work.
The other concern with mag mounts is getting the coax inside the vehicle. This may not be an issue for some, but for others it is. Having coaxial cable flapping around on the paint will lead to scratches/damage. Running coax through door/window seals can result in water intrusion into the vehicle. And then there's the issue with the coax cable itself getting damaged.
Coaxial cable has a characteristic impedance. This impedance is determined by the exact measurements of the cable, mainly spacing of the center conductor from the outer conductor. Pinching the cable in a door/window can change that and create a small 'bump' in the impedance of the cable.
Other big issue is that vibration, compression and movement of the coax can result in damage to the outer jacket. Unless you are using a direct bury rated cable, water is going to get inside and corrode the copper and eventually break down the cable and dielectric. That can lead to unwanted noise, a direct short, open, etc.
But some are OK with those trade offs. I get it and usually try to not hold it against anyone. We all do what we can to survive, and drilling 3/4" holes in the roof of a vehicle isn't for everyone. On the other hand, some of us enjoy the hell out of it and will do it every chance we can get.
The big benefit to the mag mounts is ease of installation, and ease of swapping one antenna between vehicles. That's pretty high value for some. Being able to drop an antenna on the roof and be on the air in a few minutes is a great option in some cases. For others it's quick satisfaction with little investment.
But magnetic mounts are not superior to a permanent mount antenna from a technical standpoint.
The permanent mount will work better, but may not be noticed by the end user. Having properly routed cable results in a cleaner and quieter installation. Not dealing with doors, windows, rear hatches, sliding rear windows, etc is also a benefit. There's no risk of paint damage, either.
The permanent mount will work better. Having a better ground plane connection makes a difference. It can also help with better radiation patterns, if installed correctly.
On the professional side, things need to look good. Permanent mount meets that requirement. Also, you don't want to be dealing with coax running through a window/door on a fire truck, ambulance or police car. That's a whole mess on it's own. The reliability needed in those applications requires taking the extra steps to do a full install. Knowing the antenna is going to work correctly every time and to the best of it's ability is a life safety concern. I can't guarantee that with a mag mount, and I wouldn't put one of our officers through that sort of risk.
As for the Larsen antennas….
I know brands like K40 have a good reputation. There's a ton of them out there and most have good reviews. Nothing at all wrong with those if they fit your needs.
Like you, I have antennas that are decades old and still working like new. I have a Larsen NMO-27 that is over 30 years old now. It's been on top of numerous trucks, hit tree branches, Texas bugs, low parking garages, etc. and it still works just fine.
Probably just like your K40 has. Both work as good as new, I'm guessing.
For me, it's the known name brand. It's having installed hundreds of them over my career. It's knowing they'll do the job and not fail. I know how they work, I understand the parts, and I understand how they go together. I can carry spare parts in my work truck and fix any issues I run across (rare). I've had to replace a lot of inferior brands/designs over the years where someone saw a chance to save $1 each on 20 antennas and took that easy route.
It's knowing that the antenna will be durable and work when it's needed. It's knowing that I don't have to worry about it failing at the wrong time.
But, yeah, I get it. Sounds like you are happy with your choices. If those brands and magnetic mounts work well for you, that's all that matters. Nothing wrong with it and I'm not going to try to convince you to drill a hole in your car for some sort of bragging rights. If what you have works, that's all that matters.
