7
$\begingroup$

I'd like to install an outdoor dual band VHF/UHF base station antenna, a simple pole antenna like the Comet GP-1.

I live in an area where lightning is rare, but not unheard of. Should I drive ground rods into the ground for antenna/RF grounding? The antenna itself has several small ground radials, so I'm assuming that a good ground will aid performance, but don't know if using the existing building ground is "good enough".

I live in a multi-unit building which may complicate this since my understanding is that any antenna ground system should be bonded to the building ground to prevent differing ground potentials (and the bonding should be via a dedicated ground wire directly to the building ground, not through a ground conductor in the building wiring), however my building ground rod is in a separate structure. The service entrance is in a closet in the back of a garage structure, then it goes under a sidewalk and my front yard and then into a service panel in my kitchen in the middle of the house (either under or through the cement slab). There's no apparent grounding electrode for the living unit, and even if there was, there's no easy way to run a ground wire from the antenna to it since I'd either have to go around a neighbor's unit or over the roof to reach it. It is possible that the service panel is grounded to the slab rebar or to a ground electrode that's not visible.

My housing complex looks something like the diagram below (though there are more housing units than what I've drawn), where I'm in unit 2 and would mount the antenna outside a room on the back side of the unit, while the service entrance comes in from the garage on the other side, The "V" shows the location of the service panel inside the unit:

enter image description here

Since I'll be using it with a 12VDC mobile transceiver, I could conceivably keep the tranceiver isolated from ground, leaving the antenna & transceiver ground completely separate from the building ground, but that seems quite unsafe if there is a difference in ground potential between the back yard and front yard.

What is the best practice for grounding this antenna?

$\endgroup$
  • 1
    $\begingroup$ I suggest reading some of the material at W8JI. Follow the links: there's a lot of good information there. $\endgroup$ – Phil Frost - W8II Feb 26 '14 at 17:10
  • 1
    $\begingroup$ I've added an answer at How can I protect equipment against a lightning strike?, despite that it already had an accepted answer. It's just too important of a topic, with too much bad information out there, to let it sit. $\endgroup$ – Phil Frost - W8II Feb 27 '14 at 2:03
6
$\begingroup$

Should I drive ground rods into the ground for antenna/RF grounding?

Probably not. What you are likely to accomplish is this: (diagram from W8JI)

lightning hits power lines

If the lightning hits the antenna, have the same problem in the other direction. The important thing to realize is that the current from a lightning strike is so huge that it will go through all the grounds, and if one of those paths is through your equipment, expect something to be broken.

You might even be making things worse by adding a ground (B) to your station. Now your radio is part of the safety ground for your house's electrical system, even if you disconnect the feedline (A). Don't do that!

Grounding the antenna won't improve its performance. The important thing is to eliminate common-mode currents in your antenna system, perhaps with a balun. Once you've done that, there won't be any need to ground the antenna for performance.

A proper lightning protection scheme requires that you take all cables entering your station, including the feedline, AC mains, ethernet, telephone, and anything else, and passing them through a common ground point which is bonded really well with the Earth (read: multiple ground rods, wide, short copper strap). With only one ground connection, there's no path through your equipment, only around it.

This is costly and time consuming, and given your specific situation (relatively small VHF antenna on what I assume isn't an especially large tower, in a region with rare lightning) it may be difficult to justify the expense and effort.

If you don't add more grounds (such as B in the diagram above), then you can be reasonably protected by disconnecting the feedline. Your station at least won't be any more vulnerable than any other device you leaved plugged into mains, like your TV.

$\endgroup$
  • $\begingroup$ Thanks for the great answer -- you've validated what I thought, that I shouldn't create a separate antenna ground system separate from the building ground. ANd you're right that the VHF antenna is relatively small and low. Keeping the station disconnected from the antenna when not in use (and especially if lightning is in the forecast) is not a problem. $\endgroup$ – Johnny Feb 26 '14 at 21:45
-1
$\begingroup$

Given what you have presented, yes, put a ground rod down and bond the mast to it. It would also be a good idea to attach a ground wire from the radio (one of the mounting screws will do) to this same ground rod. Your power supply should have a 3 pin plug on it for connection to the building AC; one of these pins (usually the round one) should be the "safety" ground running back to the distribution panel and its earth ground. You should have no trouble here unless you mount your radio to the power supply (keep them isolated from each other).

There should be no potential difference between your new ground and the building service ground. If you are qualified to use a AC volt meter you can easily check this.

There are many articles on the net about grounding; what you are trying to create is a single point ground for your system. At VHF and UHF this is not as critical as at HF frequencies (read up on ground loops).

It is always a good idea, if bad weather is in the area, to disconnect the antenna feed line from the radio and isolate it or ground the center pin of the coax to your new station ground. Keep in mind that if your antenna is the tallest thing in the area it may / will become a lightning rod.

$\endgroup$
  • 1
    $\begingroup$ The isolation between the power supply and the radio is minimal, at best, compared to the power of a lightning strike. While there may be no potential difference between the grounds under normal circumstances, there will be when a few kiloamps of current from a strike is flowing through them. Then, you have problems, as one of the paths to ground is through the station equipment. $\endgroup$ – Phil Frost - W8II Feb 26 '14 at 17:12
  • 2
    $\begingroup$ The confusing thing about this answer is that you say "what you are trying to create is a single point ground for your system", but what you describe is the opposite: two grounds, one at the mast, and one in the mains wiring. After reading it multiple times, it sounds like a way to make things worse. Maybe I'm misunderstanding, but without a diagram to make it clear, I have to -1 for bad advice. $\endgroup$ – Phil Frost - W8II Feb 26 '14 at 17:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.