Diamond X510 Antenna Manual
Times Microwave LMR400 is pretty good stuff and fairly easy to find. When measuring, allow extra for bends because this stuff is somewhat inflexible. There's a more flexible version, but at a cost. $1.35 per foot vs.88 per foot. RFConnection can cut you a custom length and install the proper connectors. As for the antenna, read the eHam reviews carefully, as most are a bit subjective.
There are usually a few that run true and a few bogus ones. Both Diamond and Comet have good products and the differences are small. On the large dual band verticals, I believe the Comet to have a slight edge in performance due to the design of the connectors that couple the 3 pieces together. When assembling this antenna, there's one part that needs attention and the use of small tools. If you're careful and pay attention, it's not a problem. It's a good idea to have someone check it if you've never done this kind of stuff before. Antenna assembly is something you want done right the first time.
The Diamond antenna has good quality parts. In a windy environment, the long Diamonds, like the X510, have a tendency to wave back and forth. Diamond X-510MA product reviews by real people. Another operator recommended this antenna to me as I was looking to. X510 to x510 at strength 9 on D-star.
I've had my share of projects out in the driveway.beams, verticals, wire, whatever. On the large dual band verticals, I believe the Comet to have a slight edge in performance due to the design of the connectors that couple the 3 pieces together. Well, the difference is that the inner brass elements use two screws (GP9) and one screw (X510.) I don't think anyone could claim any 'performance' gains there.
When I have to choose between two products, one thing I look at is the instructions. The Comet GP9 instructions are poorly written and rather sloppy. Granted, it's not a complicated design but these need a serious re-write. The Diamond instructions are 1000x better. I did read quite a few eHam reviews and was leaning toward the GP9 at first but then I read that the GP9 uses ABS plastic joints. One reviewer had issues with these over time. Now, he probably didn't seal them or waterproof them in any way.
The X510HDN uses metal joints that screw in. Custom wrenches are included to tighten these joints. (They do require some uber strength to completely tighten.) Either way, these joints should be waterproofed. Bought a Diamond X6000 144/430/1296Mhz Tribander around 2 years ago. It is about 30ft above the ground mounted to a steel mast pipe.
We regularly get 40knot winds here, being only a few miles from the bay. Sometimes they can gust up to 60knots. The Diamond seemed to be on a nice lean so I took it down (tilting mast) and investigated why. There is a bolt which secures the base of the antenna into the mounting tube. The bolt had worked loose and fallen out, leaving the antenna to twist around in the mounting tube which had worked the connecting coax loose (Andrew 3/8' Hardline). I tried to replace the bolt, but the metal used at the antenna base (which the ground radials also screw into) is very soft and easily stripped when trying to apply sufficient torque to hold the antenna tight in the mounting tube.
Diamond X510 Antenna Price
In the end I gave up and used my Dremel with a cutting disc to put some 2.5' slits at the top of the mounting tube. I large stainless steel radiator hose clamp has been used to constrict the tube around the base of the X6000. The whole assembly was then covered in self amalgamating tape to keep out the moisture. Very disappointed in how Diamond approaches the mounting method, which really is not good enough given the amount of flexing and twisting that a vertical can be subjected to. If this antenna was mounted at 100ft on a tower, it would have been dangerous and rendered useless in no time, with only the coax connector holding the antenna in place.
Dealers
No doubt there are many, many Diamond antennas out there that are just fine and it is a proven design. I was very surprised how the bolt had worked loose so quickly. As far as RF performance goes, the X6000 is a real winner.