Remembering the days in licensed radio it was always exciting to hear the station signal over the monitor system coming from the transmitter and tower located many miles away. That gave the real sense of controlling a powerful signal.
What I would like to do with part 15 is get the AM transmitter as far from the house as possible and have that sense of “distant listening” by hearing it over the radio.
Fortunately there is a spot out back which is 60 to 75 feet away where the whole ball of wax could be installed including 10-foot stick and buried radials.
The hesitation that is keeping me back is lack of a plan for getting the control cables out to the location. It would need to be safely laid underground, with power and audio.
Got to declare a starting moment and begin taking action.
A Neat Thought
Check you local tool rental stores for a “wire trencher” or “dog fence trencher”
These things are basically a gas powered machine that cuts a groove in the ground about 3″ deep where you can stuff your wires. The groove closes itself after the next rain.
A few years ago I laid a wire along the perimeter of my property for a radio fence for my dog. The total length was 1000 ft. With my son’s help we did it in one day. Cutting the slot wasn’t much work, but stuffing the wire wore us out!
For a hundred ft or so, you can probably rent the trencher for a half day or whatever minimum and easily do the whole job in one day without exhausting yourself.
Trenching
I knew a ham who used a chainsaw as a trencher. It didn’t cut wood too well after that.
Neil
Further Discussion
The “trencher” approach sounds like underground-grade wires would be needed, and I only know that underground Romex is available, but I will need to learn about underground balanced audio wire.
Perhaps I was thinking too fancy, since I envisioned a water-tight pipe-conduit containing nice dry wires.
Throw Back…
You could always do a pole line and run the cables overhead. 3 or 4 4×4 posts ought to do it.
I remember radio station WADC serving Akron, Ohio had their transmitter site about 5 miles north of here. They used OPEN WIRE COAX for lack of a better term to connect the 5 kw transmitter to the phased vertical antennes located a few hundred feet from the transmitter.
The open wire coax consisted of a center conductor with 4 ground conductors parallel to it forming an open cage. They used that for about 40 years.
Make your stick antenna look like the famous WLW Diamond antenna and run the overhead lines to it for a real old time look.
Interesting
That’s interesting, MRAM, and I’ve never before heard of the concept of an open-wire coax like you describe. Very fascinating.
Would it be any more prone to lightning hit than cables just below ground by a few inches?
So, the four wire “cage” ground is enough to keep the center signal wire from radiating? How often does the cage-wires need to be tied to ground rods to preserve their low impedance? At least at both ends, I’m guessing.
Are you suggesting having the transmitter indoors with the center conductor of the open coax carrying RF? Would the fcc count that as “too much length?”
Underground Wire
If you mount the transmitter at the remote antenna site you could use the wire available for outdoor low voltage lighting for the power and audio feed. Due to the length of the run it would be a good idea to operate the audio line differential balanced by means of small audio transformers at each end. I installed this wire 20 years ago below ground for lighting and it is still serviceable.
Another option is to bury a 1/2 CPVC pipe and thread the wire into this. It is a bit of work but it will protect any type of wire you use if you seal water out of the ends.
A third option is to place the wire on top of the lawn and let the grass knit it into place. Most jacketed multiconductor wire will survive outdoors for a few years.
Also consider how to keep lightning out of your building.
Neil
What Is Known
Out there at the tower will definitely be a grounding rod for the lightning protection it will provide, so if lightning hits the tower the transmitter will be the only possible casualty.
Of course we must ask, what of lightning takes a side-track and heads over the cables toward the building?
What if every leg of every line had a very delicate quick-blow fuse. Would lightning voltage be stopped? Or would it jump the gap?
I’ve also heard that if cables are put into a circle every so many feet a voltage burst will tend to be discouraged.
Indoors will be a total disconnect switch but what if no one catches it in time?
And absolutely the audio will be balanced all the way down the line. Got plenty of transformers for that.
Me and My Poker Face…
Carl, that’s the only place I’ve ever seen open wire coax like that. It probably went into service like that when the transmitter site was built all those years ago. And budgets being what they are, if it wasn’t broke they didn’t replace it.
I have seen (and you can still purchase) open wire ladder line. That is a much higher impedance balanced feedline. Some Hams still use it. It can either be bare conductors or insulated appearing similar to the old flat twin lead TV cable.
As for how effective a shield the four conductors provided, I would guess they didn’t care as long as it provided a stable impedance and the RF made it to the towers.
As for overhead cable for your installation, I assumed you would just be running power and audio to the outdoor transmitter.
As for lightning protection, I don’t think a fuse would help with a direct hit. For those with a weak heart I would suggest disconnecting and grounding the antenna when the lightning gets to close for comfort. Otherwise, using an in-line protector made for coax might help.
Lightning Gradients
When lightning strikes an object which is grounded a high current running mostly in all directions away from the object is established in the ground. This produces a voltage gradient which can exceed 1000 V/meter along the radial paths. It is possible that with a tower grounded and, for example say it is 30 meters from a dwelling, the tower will be at a potential of 30,000 volts referenced to the dwelling. So will any wires connected to the tower.
This is why it is necessary to provide a path to building ground for any wires entering the building. It keeps potential differences inside the building to safe levels in most cases.
Neil
Copy That
Neil I understand what you mean that the entrance ground at the building is important.
Is a copper ground rod out at the tower also vital?
Lightning Gradients
I can attest to this and have direct experience with the dog fence wire after a lightning strike.
Several years ago, lightning struck a tree about 30 ft from the house. There was a 2″ wide groove blown out of the bark all the way down the tree to ground. The buried dog fence wire passes within about 10 ft of the tree right about at the point where the wire loop passes into the house via about 20 ft of twisted wire. The entire loop is 18 gauge PVC insulated wire.
Inside the house, the dog fence transmitter is on the basement wall at the entry point of the twisted pair and is powered by a wall adapter.
The strike creamed the dog fence transmitter and exploded the wall adapter. The adapter was laying on the floor in pieces. About 30 ft of the buried wire near the tree was vaporized, and strangely a 6″ section about 100 ft away in the loop was vaporized. The tech from the dog fence company found the vaporized wire sections with a sensing meter he waved over the ground along the wire.
As I see it, a direct strike on an antenna, or anything nearby, will indeed create a voltage gradient in the ground that will do damage to the signal wires and connected electronics. Relatively small gauge wires will be “self fusing” when they vaporize. Any electronic equipment attached to the wires will also be “self fusing” when portions of the circuits vaporize.
The dog fence transmitter has a ground terminal, but there is no mention of how to connect it in the manual. In hindsight, if I had connected it to the house ground via a heavy gauge wire, I probably would have prevented the wall adapter explosion, but the transmitter would still have been cooked.
For part 15 outdoor installations, a heavy ground wire to a ground rod under the transmitter would divert much of the strike ground gradient voltage away from the signal lines to the house. How much it diverts would depend on where exactly, the lightning strikes. If it stakes the antenna directly, even a heavy gauge ground wire would vaporize and the signal wires would probably vaporize secondarily. Inside the house, the audio equipment would be damaged, but if the audio equipment is connected to the house ground with wire larger than the signal ground wire gauge, there wouldn’t seem to be a great fire risk in the house, because the signal wire would vaporize before the house ground wire would get too hot.
If lightning strikes a tree, for instance, halfway between the transmitter and the house, then the ground gradient voltage current in the signal wires will be divided between the transmitter ground rod and the source equipment in the house. Again, the fire hazard would seem to be reduced if the audio equipment is grounded to the house ground via a wire gauge larger than the signal wire gauge.
The bottom line is that the fire hazard inside the house would be reduced when the source equipment is grounded to house ground via a wire gauge larger than the signal wire gauge. AC outlets are grounded via either 14 gauge or 12 gauge. So the signal wires should be smaller than 14 gauge.
I don’t think anything will prevent transmitter or source audio equipment damage when a very local strike occurs, but fire the hazard inside the house can be minimized.
Zapped from a distance…
Lightning is a weird puppy…and quite unpredictable..just like how a real puppy is unpredictable when its time for it to go to the bathroom…..”ohhh that smell”….tinkle tinkle!!
Lightning can zap out stuff from miles away from induced EM fields from that massive spark. Even incredibly well grounded stuff can suffer from distant lightning strikes from very strong EM fields produced by that spark.
Its no different from how a spark gap transmitter works. When the spark takes place, very very large EM fields are produced and will be absorbed by anything conductive around..does not matter what it is…grounded or not.
There is no firm foundation formula or geek book calculation factor when dealing with lightning. It is a part of nature and nature tends to do her own thing on her own time and by her own rules. We merely try to apply some sort of starting point in an attempt to understand its physics and wonders.
Mother nature is very stubborn in giving up her secrets to her ways. Once you take that under advisement and place that at the base of every single number crunching calculation and plotting on graphs, it will become much easier to understand that just because us humans know and understand 4 percent of the universe…we are NOT the be all end all of everything…including what nature does on this planet.
Take every precaution and safety step to be safe. Go overkill if necessary. Its better to have too much protection than too little or none at all. Surprises come when surprises are least expected…always.
RFB
protection ideas
I tend to disagree with the lack of science relating to lightning protection,here is info you can use!
In the commercial 2 way biz,we use Polyphaser brand arrestors at the building entry. 99% of strikes will destroy the antenna but not the indoor equipment. The building has a halo ground ring around the outside with multiple ground rods and is tied to the tower base,a/c power and telco ground. There are polyphasers on incoming power and phone lines as well. The accepted idea is to tie it all together with all external connections protected so that when struck the whole building will raise in voltage above ground,eliminating any internal voltage differences and avoiding destruction. After 35 years and hundreds of tower sites,it DOES work! Be aware that one unprotected external connection will nullify all protection efforts and result in disaster.
Another device that helps protect equipment is the GEMOV.
Rated in breakdown volts and joules of clamping ability,they can be added to any circuit to reduce damaging spikes.They switch very quickly to catch the spike and usually sacrifice themselvs by going dead short. Common in surge outlet strips.
A neat trick for phone or audio lines is to make up a foot long pc board and lay out a couple of dozen low wattage (1/8w)low value film resistors,it will open up with minimal current flow.Add an air gap protector from each leg to ground. For phone lines,check the telco’s protector to see if it is the old style carbon buttons,if so have them update to the gas discharge type,usually at no charge!
Check all grounds to see if they are still connected,I found 3 power poles which had the grounds ripped out by cars and made them fix ’em. Ground to your water well,nothing better!
Want some interesting reading? try National Electrical Codes
grounding requirements,if you have a lightning claim and dont meet the NEC codes,the insurance may refuse to pay!
Very Good Tips
Very good tips WL. Always good to have baseline reference for such things as lightning and its many wonders.
I actually enjoyed being out at tower sites during lightning storms just to catch a bolt or two hitting the towers. Sort of like attending a freak-a-zoid pyrotechnics concert! And if anything got zapped..well I was already on site to take care of it! 😀
Even with the information available in literature and online, lighting still has so many variables we have yet to understand. That is what makes it so unpredictable. We can do things for those direct strikes pretty well. It is the incredibly powerful stray EM pulses that baffles everyone wondering why did my stuff get zapped..the lighting hit a pole or tree miles away.
Watch the display of a spec-an during a lightning storm at some distance…note the plentiful and rather large spiking taking place all across the spectrum. Every conductive object sitting in the air will be subject to those EM pulse spikes.
This is part of why power transformers are equipped with those electrostatic shields which not only cut off RF, but EM pulse spikes from lightning, although lines after the transformer will be victim to further EM pulse spikes. It is also why many devices include small MOV’s on the AC mains into the device. They are quite effective in shunting out the EM pulse spikes and serve as surge killers….sacrificial components found in power extension strips and surge protectors. Older stuff used to put .01mfd ceramic caps across the AC power leads which helped with the EM pulse spikes.
RFB