Grounding a dipole

Question

How do you properly ground a dipole (RX only for VHF scanning) to protect it against ESD/lightning?

In my set up i'm putting a dipole on my roof. I've connected the two dipole elements together with a 100k resistor, and have a balun and amplifier just under the roof, then 20m of coax to my receiver. So to protect the balun and amplifier I need ESD to shunt to ground before it reaches the amp.

Should I ground the antenna via the mast and some heavy gauge wire leading to a grounding rod? And then connect the shield/gnd element of the dipole to the grounded mast? I could then also use a gas discharge tube on the line before the balun and amp.

But if I do this, won't I affect the dipole by essentially lengthening the ground side element via the mast and grounding rod?

Answer 1

The typical amateur radio / SWL budget will rarely allow for sufficient protection from a direct lightning strike. More often than not, the measures taken help minimize damage from a direct strike but more importantly they mitigate damage from near lightning strikes and static build-up.

If the goal is to bleed static and minimize damage then mast should be grounded (this is also an NEC requirement). The grounding should be carried out with a heavy gauge conductor (minimum of #6 by NEC) leading to a ground rod. The efficacy of the ground can be improved by using wide copper straps. The ground rods should be tied to the house system ground rod(s) with a minimum of #6 AWG wire.

The cellular industry has promoted bringing the coax down to the ground level and installing a lightning arrestor bonded directly to the ground system at this point. This is considered a best practice for amateur radio installations as well. If you cannot route the coax in this manner, installing a lightning arrestor in the coax at the mast and bonding it to the mast ground may provide some level of protection.

Be aware that your mast mounted pre-amp is considered lightning fodder. There is little that can be done to protect it from a near lightning strike when it is located so far from an effective ground. Some people advocate the use of an RFC (radio frequency choke) inductor across the input. This will offer some degree of protection but due to the rapid current rise (di/dt) during near lightning events, it is not likely to stop all damage.

Your resistor across the dipole legs will prove helpful to bleed of static that is generated by winds and nearby storms. You should periodically inspect this resistor as it too will be sacrificial in the event of a nearby strike.

Answer 2

Simply grounding things by connecting them to some ground rod arbitrarily stuck in the ground will not protect anything from lightning: in fact it may make lightning damage more likely. The protection comes not from grounding the antenna per se, it's having a single point of entry to the station. For detail on how to ground things and for lightning protection generally, see:

How can I protect equipment against a lightning strike?

If you aren't going to go through the trouble of having a single point of entry as detailed in that question, my recommendation would be to not ground the antenna at all, or ground it with a separate system which is disconnected when not operating. Disconnecting the antenna will provide a degree of protection, but a direct strike will have no trouble blowing past the few feet between your station and the disconnected feedline. You can solve that problem with insurance.

If you are going to properly ground the antenna to a single point ground, assuming you are using a coax feedline, use a balun and ground the coax shield. Otherwise you'll unbalance the antenna. If you are using a balanced feedline it's probably still best to run coax from the transceiver to the station entrance ground, then have a balun outside. It's difficult to make a low impedance connection to ground with a balanced feedline.

You also say you've connected the dipole halves with a 100kΩ resistor. This is already an effective bleeder for static buildup, without any need for a discharge tube. Though if you want to include a discharge tube at the station entrance also it can't hurt.

Answer 3

Lightning takes the path of least resistance I believe. If you have a larger conductor just above the highest point of your antenna going to ground that is a better conductor than your antenna I would think that would cover you.

Answer 4

You should ground the shield of the coax before or at the connection point to the balun, not between the balun and antenna. Remember that the balun conversion between the balanced dipole antenna and the unbalanced coax cable.

Also remember that grounding does not protect you or your equipment from a direct lighting hit.

Answer 5

Here is what is advertised (and recommended by Elmers in my ham club) to connect your coax to a local ground. It is called a Lightning Arrester.

It has an internal spark gap that will only be jumped by a high voltage coming in from your antenna.

There is no direct connection that will have any effect on the antenna, as long as this on at the near end of the coax, and not right at the antenna.

They are available from the usual sources.

I recommend grounding this the same place your radio chassis is grounded. That way this is nothing but a pass-through connector with an internal spark gap.