Grounding Tips for Wireless Outdoor Antennas

[Shorsch]

Hi guys,
There are 7-8 Outdoor Wireless AP Antennas to be installed around an Hotel, mounted on 2" Steel-Tube, with Surge-Arresters protecting the coaxial cable for the Antennas and another for the UTP CA5e Outdoor Cable. My Question is about the Grounding requirements. I'm recommending the customer to install one 8' Cooper Grounding-Rod for each Antenna and also to build/make a Grounding Ring, connecting all Rods's together to avoid Voltage-Diferential between de Grounds...does this make sense? (The customer is complaining about the extra cost involved).
Any comments will be very much appreciated.

 

[robertjo24]

I would recommend that each line be grounded at the point of entry to the building with all grounds bonded together. If they all enter at the same point, they can all be bonded to the same ground. If the antennas are all entering at different points around the building, your suggestion makes sense.
Keep in mind, these new grounds should be bonded to the building ground system.

 

[Higgler]

Why do these antennas need to be grounded? DC ground differentials won't affect high frequency wireless RF energy. Add DC blocks in the cables if the RF energy all come together at one point.

I would think that lightning protection for patrons is the only concern requiring good grounding.
kch (antenna engineer)

 

[SolderingGunslinger]

Adding to KCH's comment, I would add that the grounding should be with 4-6" wide copper strap as opposed to a jacketed cable.

Lightning transients are squarewaves. Squarewaves are made up of all of the odd harmonics of the fundimental frequency of the squarewave.

A typical lightning stroke is quite powerful. Significant RF voltages are present well into the GHz. Round cables look like inductors. The X sub L becomes a factor above around 400 MHZ. In broadcast facilities copper strap is used for the grounds in order to keep the reactances down and provide a low impedence path for the lightning currents. If you are worrying about current carrying capability, a 6" wide 1/16" thick piece of copper strip has a cross section of 447,464 circular mils. 0000 (4-0) cable has a cross section of 211,600 circular mils for reference.

Article 800 of the NEC should be followed in any commercial installation. This is for the safety of others and for your financial safety in the event someone is hurt or killed.

I remain,

The Old Soldering Gunslinger

 

[VEBill]

"Lightning transients are squarewaves."

Based on what I've read and seen, I don't think that the above statement is an accurate description. On the other hand, lightning does (at least at some scales) have sharp edges and lots of 'harmonics'. I just don't think that 'squarewave' is accurate.

It doesn't change the validity of the advice to use wide straps for grounding.

 

[SolderingGunslinger]

I must admit to being somewhat increadulous when I took the LNB apart to find the link from the DC grounded folded monopole probe blown in two. For grins I bridged the open link and tested the LNB. Tne noise figure was way out of spec. (Go figure!) But the unit did operate...after a fashion.

The DC grounded lambda/4 would indicate that it was probably a high AC potential at or near the resonant point of the probe caused the damage.

I realize that there could have been other things which would cause the link to blolw, however considering the cutoff frequency of the waveguide and the probe at DC ground leads me to my conclusion.

 

[VEBill]

The above posting should be in thread:

http://www.eng-tips.com/viewthread.cfm?qid=113825

 

[SolderingGunslinger]

Oh am I embarrassed.

Thanks!

--SGS

 

[dpc]

Check out

http://www.polyphaser.com

for a lot of info on antenna lightning protection.

 

[SolderingGunslinger]

Look at

http://www.harger.com

as well for lightning protection information and supplies.

--SGS

 

[rob46]

Look at Motorola r56.