Paul: Yes, you can install the box at ground level. You can ground it there, even put down some ground radials if you can, 4 to 8, stuck in the ground by splitting it with a shovel and pushing the wire in. As long as practical too. I had mine in an inverted L about 25 ft vertical, the rest strung out back to the house. The feed point is down in the back yard, there is a ground rod there next to a shed where I connected the transformer ground. My coax comes through a conduit about 65 ft to the house where I have a metal box with surge protection inside for that antenna and also the scanner antenna on the roof. The ground from the box is tied directly to the service entrance ground about 3 ft away with #6 wire. The ground (I used 1" braid) from my 2nd floor shack goes to that point also.
I later changed the feed point by just pulling the whole 45 feet of it up into the air on the theory that the more wire in the air the better. In this config the transformer was not grounded except back via the coax shield, the only ground on the coax was 65ft away at the house. It seemed to receive better that way, and I didn't see much difference in noise levels even though I'm on a small city lot with sodium vapor streetlights only 20-25 ft away. I was worried about them.
Currently, I have a 40 meter dipole there, off center fed (sometimes called a Windom) with 44 ft toward the house and 22 ft hanging in the tree toward the alley. This was so i could get a transmitter on the air. As I look out my shack window, the center insulator is eye level from me so its not up very high due to the fact that I have exactly one tree to work with! I think the PAR worked much better receiving on lower freqs, especially AM broadcast, and I'm going to try to get it back up somewhere - maybe on the roof. The dipole works great, every morning I have a band full of broadcasters from China, India, far east, south asia etc on 40 meters just after sunrise.
Yes, sloping is perfectly fine if thats the best way available, and if you can ground the business end that would be great, just be sure to ground it somewhere as it enters the house, preferably with a lightning protector of some kind. Lacking that, I've used a double female SO-239 (Barrel adapter), with the connectors from outside and inside coax runs screwed into it, and grounded with a clamp from the hardware store like ones used for connecting a wire to a cold water pipe. Of course, weatherproofing it is required, that's why mine are in a box that keeps them dry. Like they say, experiment with the ground jumper on the box for lowest noise/best signal. When I first got the PAR, I had the transformer inside the grounding box and the wire stapled loosely to the wood fence and even that worked fairly well.
The Wellbrook article describes keeping the RF ground away from the electrical ground to reduce noise pickup that may come from the electrical service. If you don't have a lot of electrical noise (loud buzzing mostly) then that elaborate system is not necessary.
