Batboard

Hello all, Might start a lengthy discussion about this topic, but here goes.

Have a two story wood frame building (fire house). Mounting 6 fiberglass whips on metal galv frame. 1/2 hardline about 90 feet down to the 1st floor radio equipment room (not a direct path). Installed a Qwikport roof entry port on roof with ground bar installed inside enclosure (entry port within 6 feet of antenna frame). 1st floor radio room has a separate ground bar with proper poly phaser lightning arrestors installed. Electric service ground and cold water ground are in building basement.

Questions:

1) Where would one ground the antenna frame to?
2) Hard Line Ground kits are connected to ground bar in the roof entry port, where does this ground bar get grounded to?
3) Need to use stranded gorund cable, what cable size from roof ground bar to X location is proper?
4) Should the roof ground bar be bonded to the radio room ground bar with the lightning arrestors installed on it?
5) Where should the entire "ground system" be grounded to.

I have seen many a time the antennas, ground bars w/arrestors, equipment racks, equipment, etc get grounded right to the incoming electric service ground point?

Thoughts, comments, questions, conerns, nasty thoughts??

Thanks, Rob

Rob,

I believe that your best course of action would be to consult the NEC rules on grounding. There may also be some NFPA data available on the same subject. My reason for suggesting you go this route is that if six people answer here and give you advice the chances are you're going to get six different methods, some or which may be correct but there's no guarantee of that.

Regards,

Also refer to the R56 standards book. NEC supercedes R56. But, to answer your question so you have a direction to go - you need a ground halo around the compound. The size and depth of the rods and halo will be determined by the soil resistivity and comp test that you have performed. You'll drop #2 from each ground bar to the halo, or to a building common ground bar that is connected to the halo. All connections to the ground bars can be compression type. All attachments to the halo will be exothermic.

By any chance does the building have a lightning rod system on the roof?

If there is a lightning rod system on the roof, you really need to tie the antennas to this grounded system. If there is no
lightning rod system on the roof, then you need to follow the comments that Bill has made and put your own ground
system in.

Point to remember, that just one ground rod in the ground is not enough.

Point to remember, no sharp bends in the ground wire going from roof, ground bars and radio equipment.

All outside ground connections should be exothermic type welded connections. This way you will not have to worry about
them over time. This includes the ground wire to the ground rods. If you ask around, you should be able to find an
electrical contractor that may have the proper molds for the exothermic connections.

Recommended wire used for grounding should be # 2 solid plated. You should be able to obtain this from your local
electrical supply house. Home Depot and Lowe's will not generally have this wire.

Connection to the ground bars with the ground wire itself should be made with a 2 hole compression wire lug. Put
a small amount of an anti oxident compound like "Pentrox" between the lug and the ground bar. The compression
fitting is put on with a special crimping tool. The electrical contractor should have the tool.

Ground systems are not cheap. Takes time to install the grounding system. You need to dig up the yard and make
sure it is below the frost line or at least 30 inches below the surface. A bit of time spent with the National Electrical
Code (NEC) will provide the details. You will also fine similar info in the R56 standard from Motorola in their grounding
section.

Don't forget that once you install your grounding system, it also needs to tie to the electrical ground for the power
and telephone cable connections. This is also covered in both the NEC and the R56 standard.

Jim

Thanks all. The electrical contractor for the FD will be handling the grounding to my ground bars (top side and in the radio room). I just wanted to see some different opinions as to what you folks do elsewhere. I sorta kinda knew the answers, but was looking for some other expert thoughts. My thoughts, after expressing my comments to the FD is that I am out of breathe, and if it blows up I didnt do the grounding, and stated in my contract, if you catch my drift.

Any ideas on where a copy of Motorola R56 can be had, is it in PDF version somewhere? Thanks, Rob


P.S. The location that the FD is at has very sandy soil, so I would assume that ground rods would nned to go deep to get a good contact?

Sandy soils require ground enhancement material. There are several permanent and semi-permanent products out there.

The cheapest is rock salt. Very effective at lowering soil resistivity. Also effective at corroding the copper rods and conductors. Equally effective at washing out over time requiring replenishment. There are permanent "salts" that can be applied that are EPA approved that do not degrade the copper, and do not wash out as easily. Consult your local electrical supplier.

Bentonite, aka kitty litter, is the next best cheap solution. Augur an oversized hole, drive in the rod, pour in the bentonite adding a little water as you go, backfill the top 10 inches of the hole with augured material. However, bentonite shrinks when it dries, and expands when it's wet. Depending on the depth of the hole and overall soil moisture, the material can pull away leaving you with seasonal high ground resistivity.

There are conductive cements for special applications that neither corrode the conductors nor shrink as the soil dries. They maintain high surface area and low resistivity even under low soil moisture conditions. Again, consult your local electrical supplier.

All of these are reasons why modern site preparation use a combination of multiple vertical rods tied to a buried halo. Even excellent black soils will dry out seasonally leaving you without a ground. The three rules to an effective ground system are surface area, surface area, and surface area. Buried deeply enough over a large enough area, even sandy soils will provide adequate earth ground.

eta: You can order the R56 standards book or disc from M - 68P81089E50 is the latest.

FMROB wrote: Any ideas on where a copy of Motorola R56 can be had, is it in PDF version somewhere?

I did a Google on 68P81089E50 and found a PDF copy in the first 5 hits.

I did the same thing and got the same results.. Thanks