Cable between station ground and ground rod.

[cwguy]

I am working on bonding my equipment and whether I use an RF ground plane or a bus bar, I haven't yet decided. But I do know that I need to connect this to a ground rod under my entry panel. This is a 25' run from my station to the entry panel. I'd like to get recommendations for a cable.

I've read that the goal is more “skin”. This 25' distance also passes behind a computer rack and other cables along that same run so I was hoping to find something insulated. Seems that running a bare wire like a tinned braid that would touch things might not be right.

Anyone have thoughts on this?

 

[Ham4CW]

Hi cwguy.
Personally I would use insulated wire rather than have a long run of exposed wire running around the shack. An alternative would be to use a metal strip attached to the shack wall (so that it is out of the way), and then use short bonding wires to that from each piece of equipment.

I'm puzzled about what you need it to do though. Is the idea simple electrical bonding between the items of equipment, or is the idea to use it as an RF ground for your antenna system?

Electrical bonding isn't a problem, but for RF grounding the length of the wire will have a huge impact on the impedance 'seen' by the radio equipment depending upon the frequency in use and the length of the wire run.

The rule of thumb for distances, or lengths of wire between an antenna feed-point and ground, or between a transmitter and RF ground, can be found by dividing the wavelength in metres by 10, and then use that number in feet to give the maximum length.

So for example, on the 160m band the longest length should be no more than 16 feet. On 40m that would be 4 feet, and by the time you get to the 10m band the length of wire is only 1 foot in length!

When those lengths are exceeded the phase shift becomes more of a problem, so the transceiver or antenna no longer 'sees' the ground, but instead appears as either an inductive or capacitive component in series with the ground wire, and in turn this affects the impedance.

So let's say for example you have a piece of wire that is about one quarter wave length long. If you connect your transceiver to it the wire will have a lowish (about 32 Ohms) impedance at the wires resonant frequency, or in other words you have created a quarter wave counterpoise. If you were then to ground the far end of the wire (to a ground rod or earth mat etc.), instead of a 32 Ohm impedance you would now see a very high impedance (3000 - 4000 Ohms or so). Clearly such a high impedance would be then useless as an RF ground (even though for electrical grounding purposes it would be fine).

The only way around this effect is to either choose a wire length of appropriate length for the wave length you intend to operate on, or else use a dedicated ground/earth tuning unit. Using one of those allows you to correct for inductive or capacitive reactance on your RF grounding wire, but it would still need to be retuned every time you changed bands, and on the lower bands would require retuning even for modest in-band frequency changes.

73, Mark...

 

[LW1DSE]

Be sure to NOT use a common rod for lightning rod(s) and for electrical/electronic apparatus. Use separate ground system, and as far as possible for both.

 

[cwguy]

The idea is electrical bonding of equipment like in chapter 3 of the ARRL book, Grounding and Bonding. Next I was going to work lightning protection which is chapter 4, then RF grounding, chapter 5 which is what I think you're referring to in the bottom half of your message.

When I read your rule of thumb, I'm at a loss but will just have to work each step as I read. If I divide the wavelength by 10, and use that number, for 40m @ four feet, that doesn't even get me past the desk my equipment is on.

You say I can use insulated wire, so that is what I will likely go with. Any thoughts as to the size of the wire?

 

[Ham4CW]

I'm surprised that the ARRL book does not mention any wire gauges? Some of the publications I've seen go as far as to use heavy braided copper earth straps between items of equipment, but I would think that unless you intend or think that large electrical current will flow between items in the shack then this is most certainly overkill.

For basic bonding I would have thought that 2.5mm wire should be more than enough to equalise the potentials between items of equipment. My main reason for suggesting insulated wire before is that there is just too much risk of accidentally shorting something out if you were to use bare wire.

Like Osvaldo said, keep the lightening grounding separate. I've seen a few ARRL (and other radio organisations), showing three independent systems of grounding/bonding/lightening protection. If you have or intend to erect a large antenna array then these things are essential for your safety of course. If you have a station like mine though, with low mounted antennas (and not much of them!), then such things are not really necessary.

That rule of thumb I mentioned came from a book "All About Vertical Antennas" by William Orr & Stuart Cowan. You are quite correct about the length for 40m as being four feet. Apart from which, using a ground rod as a counterpoise is a leftover from the early years of radio when folks were using wavelengths of thousands of metres. Once you get up to our relatively short wave lengths (by comparison to the frequencies used by experimenters) a single or even multiple ground rods will never be as efficient as elevated counterpoise wires, especially where the counterpoise is also resonant.

73, Mark...