| Home |
| Search |
| Today's Posts |
|
#8
November 22nd 04, 02:36 AM
|
|||
|
|||
Gary Schafer wrote in message . ..
On 20 Nov 2004 17:59:12 -0800, (Brian Kelly) wrote: .. . . I'm one of those who pulls the coax off the tower at around eight feet and hangs it on a carrier wire from the tower to the outside wall near the shack. Theref are many installations like yours in existance. It was the "common way" to do it some years ago. Not the best though. In the past I've had end insulators at both ends of the carrier wire. Your point about grounding the coax at the base of the tower is well taken but is obviously not possible in these situations. It occurs to me that the same effect can be accomplished by connecting a #6 or #8 solid wire between the the coax shields where they bend away from the tower and the base of the tower. Yes? No that won't do much good. If you ground the coax shield to the tower where it bends away from the tower you will have a much better (lower inductance) to ground with the tower than what the wire would provide. The wire would do almost no good at all when compared to the much larger tower in parallel. Got it. Taking it a bit further it also occurs to me that the carrier wire could be connected to the base of the tower at the point where the tower connects to the ground rods there, then up the tower and connected to both the coax shields at the eight foot level and the tower again. Same as above. Grounding the carrier to the tower will do much more than a wire to the ground rods at the tower. The carrier wire should not be insulated from the tower. It and the coax should both be grounded to the tower at the exit point. Otherwise you can have flashover's to the carrier. OK, cancel useless wire from base of tower. Then horizontally to the house wall with the coax, then down to the ground rods just outside the shack to which the equipment is also grounded. I'd also connect the coax shields to the carrier wire again at the point where they turn away from the wire and go through the wall. One hefty continuous, unbroken length of copper wire. There would still be voltage differentials involved because there is no escape from the inductances BUT . . . is my thinking in the right direction here? Connecting the carrier wire to the coax again at the house is a good idea for the same reason you should connect it at the tower. to prevent flashovers to the cables. The same situation exist on the tower itself with lines running down. That is why they should be grounded to the tower at several points. Especially on a tall tower. OK again. The tower has inductance just like any piece of wire has. Although the tower inductance is less than just a length of wire it still has inductance. When lightning strikes the top, the tower and lines all share the current to ground. The farther up from ground you are the higher the voltage will be with respect to ground. I got that from your prior post. It can be significant. Especially on a smaller tower. It took a few seconds to get your point but yes, it's a matter of how far up the tower the coax departs the tower as a percentage of the tower height. Since I'm planning a short (35-40 foot tubular crankup) tower I'll have both a "high inductance tower" and a high pulloff level in terms of percentage. Not good no matter how one looks at it. Leaving the tower only a few feet above ground with your coax line is putting that line at some point above ground that can have high voltage. The best way is to run the lines all the way to the bottom of the tower, ground them there, and then run underground to the house to your ground rods. Don't forget to also run a ground lead from your house ground to your tower ground system too. That's a given. Bury it along with the cables. That will give you more contact with the earth as well as tying the grounds together. The wire will be there but I doubt that I'll be able to bury it. The whole (small) property is part of a forest of huge old hardwoods several of which are crowded close to the house particularly along the wall thru which I need to feed the coax. You'd have to see it to believe it and it's only six miles from City Hall Philadelphia. Digging a trench is not possible thru the tangle of roots on any approach from the tower to the wall. I'm not looking forward to driving ground rods thru this maze of underground lumber but I'll do it even if it takes some serious power drilling to accomplish. What I could do is run all the cables to the bottom of the tower with shield bonds at the top of the moving section, another one at the top of the fixed section, another bond halfway down fixed section and the last one at the bottom of the tower. Which will also be surrounded by trees. There's a hole below the canopy big enough to accomodate a Hexbeam or some similar very compact HF antenna if I spot the tower correctly. Some contractor is going to have a really bad time digging the hole for tower base. From the base of the tower I'll run the cables and the carrier wire horizontally on the surface for a few feet then back up to the eight foot level to a tree trunk. Six feet would also work and the rest of the run would be per previous. The good news is that the soil is eternally damp highly conductive dark loam . . Gary K4FMX Thanks Gary. |
| Thread Tools | Search this Thread |
| Display Modes | |
|
|
Similar Threads
|
||||
| Thread | Forum | |||
| Inverted ground plane antenna: compared with normal GP and low dipole. | Antenna | |||
| Grounding Question | Antenna | |||
| QST Article: An Easy to Build, Dual-Band Collinear Antenna | Antenna | |||
Threaded Mode
