Carl, I like it. You could call it "Splash Radio." And the irradiated wildlife could provide extra income by charging visitors admission for a look at the glow-in-the-dark animals, birds and reptiles. And who knows, maybe the egrets and turtles could learn to line dance to the stations music. OH, and the gators could do a guest shot on the Low Power Hour. I'm just sayin'.
By the time everything that's been said about grounding of antenna radials has been sifted into my thoughts I find myself confused.
It seems on the one hand that emersion in a moist soil is more effective at extending transmission distance than dry soil, but Marshall has found that 4 radials above ground is more effective than many more buried radials...This implies that something about being underground is a negative... adding resistance? Is it the soil itself? Soil absorbs frequencies... what are the signal loss expectations induced by soil...no doubt that varies widely based on particular soil composition. Salt water is excellent... should we be adding salt to the ground (goodbye grass). But I remember someone said that a flat grassy ground makes a good ground plane. Does the grass reduce the signal absorbtion as perhaps also does ice and snow, which definitely improves signal outreach, maybe for the exact reason that the layer of ice/snow prevents blocks ground absorbtion and sends the signal skimming on top.
The problem of oxidation is not confusing, and I would suspect is to be expected in any buried installation, requiring always welding connections for best result.
In all the considerations I am left with the feeling that (generally) the dirt itself is not helping as much as it is hindering the signal. If so, perhaps there is an acceptable "ground cover," which would do what ice does... if grass does it than maybe thicker grasses would do best... what about black top or concrete (probably not).
What do AM stations do with side-of-mountain towers? Or, maybe AMs don't use the sides of mountains.
This has been actual thinking aloud in the struggle to understand what's going on with AM signal grounding.
Sure, the capacitive effect of above-ground radials will be better ... buried ones are at the mercy of whatever is in the dirt. But there is unlikely to be any two disparate locations the same, so like nearly everything else with Part 15 and ham radio, much of it is experimental.
The FCC wants AM radio to have buried radials, i.e., they aren't visible, don't form a true dipole effect, etc., etc. In a Part 15 setup, it would make an obvious attempt to radiate more energy, and likely to be questioned. As with other things, it's up to the agent, but I've heard a number of times that above ground radials for Part 15 AM would be considered part of the antenna length, as would a capacitive hat. If true, if an agent would look at a Part 15 setup that way, then you would end up losing more than you gain.
My personal take on this issue, having read reams of data and in many cases attempted to run numbers, is to put a a ring or disk about 18"-24" above ground to improve the near-field, clamp all your radials to that, then run them out below ground just enough to allow you to mow over them safely (1"-2")
"What do AM stations do with side-of-mountain towers? Or, maybe AMs don't use the sides of mountains."
Commercial ones? I'd say that's true ... they don't. I've never seen or heard of one, but you could still do it. The ground system will be weird to deal with is all.
You could excavate the portion where you will mount the antenna ... make a 20' sq or circular flat level space. If you are limited to less, then make it as big as you can get away with. At least that will allow you to retain as much near-field as possible.
But in your case perhaps the best thing would be an isotron, which doesn't need ground radials.
There are hundreds of AM broadcast stations with antenna systems constructed on hillsides. The feed point impedance is the only consideration when these antennas are designed and constructed. The feed point impedance determines the base current allowed for the antenna. With these parameters the power input to the antenna system can be determined using Ohm's Law.
Unless the antenna system is a directional array, the shape of the signal coverage area is a minor concern (see below). With directional arrays (2 antenna elements or more) signal phase and amplitude along with element spacing become the design parameters for producing the proper shape for the signal coverage area. I know of, and have worked on, directional arrays on hillsides and they work as designed. The only measured result that matters with these systems is the far-field signal strength at predetermined distances and points. If the signal from the system hits the right numbers, you're good.
The FCC, with licensed stations, only really care about the far field signal strength (1 kilometer and more). Non-directional stations are to have a circular signal contour that is within about 10% of the theoretical value. This would mean building an AM antenna system on a very steep grade with a high crest could disqualify the design because of contour non-linearity. The FCC could still be petitioned for a waiver of this rule; and it has been granted in the past.
And by the way, the FCC does allow elevated ground radial systems for AM broadcast. Elevated ground radial systems are REQUIRED to perform engineering signal measurements to determine if the system meets the FCC minimum signal strength requirements of the construction permit before the commission grants a license. On stations that are replacing the in-ground radial system with elevated radials, the commission still requires signal measurements to continue licensed operation. Elevated radial systems are considered by the commission as a "major change". The process for these measurements are laid out in Part 73 of the rules.
For near-field system enhancement, many engineers put an 8-12 foot copper ground screen at the base of the tower/antenna to increase the near-field capacitance of the system. This lowers the system feed point impedance and increases efficiency and signal launch. The ground radials are usually bonded to the outer edge of the screen. This practice helps to ameliorate the effects of poor ground conductivity of the soil around the antenna system. The ground screen is usually covered with fine sand or pea gravel to protect it.
I certainly defer to Marshall's experience with commercial stations, but the problem is dealing with such low power and short antennas, per Part 15 antenna rules.
If an antenna RF ground is elevated above ground, it becomes a ground plane, a resonant element (not a dipole, but close), and I believe it would have to be counted as part of the antenna length, like a hat, and also would need to be tuned, or else become part of a combined system of tuning. But there's no harm in experimenting and learning.
I like the idea of a copper ground screen mesh, but as indicated, it should be bonded to the whole ground system. That will surely involve a lot of work, but 12' would make a good foray into the near field. I don't have to worry because my system is on a boat, but at my chosen frequency, the near field should be about 60'. So 32 ground radial wires extending out 48' beyond the screen (R=12') should make a pretty darn good system, even if the terrain is a hillside.
I'm still curious about an isotron antenna. If you fold the 10' pipe at the top so that the capacitive elements are in the right relationship with each other, it ought to work fairly well. But how would a safety ground be dealt with?
Awhile back, someone said a vertical antenna as short as we're talking about IS a capacitor.
I noticed that the range of my Talking House Transmitter doubles at night. I am only using the wire antenna.
Did any one else experience this?
Nope, I don't have one, I wish I did so I
could try it. If I had the money I would
buy one and take the chance.
The antenna is less than 3 meters long,
but because it has so many parts, it is
physically wide. Nobody has ever talked
about this being a regulatory problem,
and I don't think it is.
I do think that the Part 15 AM transmitter
output would have to be 50 ohms, and that
the transmitter would have to be mounted
right on the antenna. The antenna does not
need a ground. I have no opinion on the
safety issue. Well, except that I have taken
down antennas before thunderstorms have reached
my area.
Best Wishes,
Bruce, MICRO1690/1700
Yeah, I can't see my way to pay out the $$$ for one either, but there have been some interesting home-brew versions: http://www.pcs-electronics.com/phpBB2/viewtopic.php?t=1092
As I understand them, they must be sharply tuned, and DO need a counterpoise. Some claim the feed line is the radiator and the isotron doesn't do much.
Range goes up at night
I noticed that the range of my Talking House Transmitter doubles at night. I am only using the wire antenna. Did any one else experience this?
I can't? I doubt you are getting sky wave action. However night time FM goes up a little at night I recall. My effective Talking House range goes down at night simply because night time signals increase at night, aka the noise floor goes up. During the day as I get to the edge of my "range" all is well. At night the night time strong (weak) stations crowd my station (1380 Khz). I did Freq surveys day and night. There were some day Freqs that went to pot at night. I limit my self to sub 1610 Khz for my old tube radios. 1610 to 1710 tends to be quiet because there is on 54 station in the whole country on those Freqs. Where there might be 54 stations on one Freq below 1610. In the house at night, my station overrides night signals easily.
ELEVEATED GROUND
Practically it's not practical, you will trip over it. Some just lay it on the dirt and let it cover up naturally with grass. As far as the FCC I don't know what a ground is to them. I am sure the FCC could if they wanted argue with any ground system for part 15 above, on or below the dirt. Wire with insulation should work well for a long time. If the dirt has lots of lime and is real acidic it my be bad. NOTHING LAST FOR EVER. You may need to replace it every X yrs.
Weld, braze, solder is OK but mechanical connections do well, because of the contact pressure. Metals like stainless are good, but this is expensive stuff. Crimp on Lug connectors are usually coated copper and resist corrosion. But because it's different from stainless there can be galvanic corrosion action going on. So you have to get the right cadmium or passivated coatings. Sealing them is a good and needed step. However mechanical connections is time honored, but not with out problems. Even commercial operators have issues with this, the "rusty bolt" effect.
en.wikipedia.org/wiki/Rusty_bolt_effect
I agree consumer "silicone" sealant has acids in it. This stuff is fine on wood, tile or counter tops but not electrical connections. The electrical grease idea is good.
Weld, braze, solder is OK but mechanical connections do well, because of the contact pressure.
I'm accumulating #12 copper wire, and after done with constructing a ground radial I planned to take it into town to a friend of mine who has a welding shop to arch weld it at all the places it needs it.. I assumed this would be the strongest solid connection one could hope for.
But you say that a mechanics connection would be better?
I'm not comprehending why that would be so..?
Rich,
First, connections that are wires only wound together or bolted/screwed together (mechanical connection) will eventually corrode and break down, causing the resistance at the connections to go up considerably (sometimes several thousand Ohms due to the corrosion caused metal oxide). Regardless of the torque pressure of the connection, moisture will "wick" into the connection and cause failure. If you must have a mechanical connection (mentioned above), coating the connection with clear silicone grease helps to extend the service life of the connection. However, when the connection collects enough dirt and grime it will corrode also. Coax connectors are "mechanical" (not mechanics) connections as well and require the same kind of care.
Copper wire can be brazed or soldered, not welded. The copper wire to be brazed would have to be 6 gauge or larger to survive the high temp of acetylene gas. Smaller wires would need to be soldered with a silver-content solder (silver solder is usually high temp soldering requiring MAP gas). The arc welding process on copper would destroy the base metal without creating a quality bond. Welding is usually reserved for steel, iron, stainless steel and aluminum. The welding process for stainless steel and aluminum is expensive (TIG welding). Galvanized steel can be welded, but the zinc coating is destroyed in the process. There is a bonding process that you can acquire at electrical supply stores called, "Cad Weld". It is expensive and is used primarily for bonding heavy ground cables to large ground rods, usually used in "Halo" ground systems.
12 gauge wire should be soldered with paste resin flux, silver solder and a MAP gas hand torch. All of these should be available at your local chain hardware store.
As a final note, I have built and serviced ground systems in a marine (salt) environment for years. Salt air will make "Swiss cheese" out of solid cast aluminum enclosures within months of installation. I had one at a site overlooking the Pacific Ocean and within 8 months the expensive cast and machined box with real rubber gaskets was full of water. At your location, for reliability, you will need to put some thought into what you can do for a good ground radial system that is affordable. On several stations I have re-worked their ground radial systems in marine environments. After 15 years or so they required complete replacement. (1000 watts would reach out only 2 miles before replacing the ground system). Non-marine systems can last twice that long depending on soil acidity. It's all basic chemistry.
Well Marshall, I sure am glad I posted my intended plans. I had assumed that welding the copper would merge it together so that it would be in essence a solid piece of copper when it was done.
To be honest, I still don't get it; when copper is melted down it becomes a single chuck of copper once cooled.. So I really don't comprehend, but I trust your expertise, and will get the proper flux and solder for it as you advise. I saved your post to my desktop for reference when time comes.
I guess I misread jetpilots post, because it seemed to imply that mechanical connections worked better because of the pressure applied. That comment is what influenced me to post on the subject in the first place, which I'm glad I did, otherwise you wouldn't have made this post, and I would have continued with my plan.
As for the ground radials, I am planning on laying it flat on the roof - I realize there is some debate about doing that, but I have not seen anything really saying it's not permitted. Then running a couple grounds wires from the radials to ground rods located inbetween the buildings.
The transmitter being installed on top of the one story flat roof grounded to a short pipe coming out the roof of the building. I do not however plan to attach the ground lead to the radials. I really am attempting to keep it all legal.
I do have options of putting it on the roof of the building next door which has a metal roof, but what if they give up the lease and the next tenant objects? Then I'd have to move it. So, my first attempted install will be on the roof of the building I'm in, and see how that works.
Another thought I've played with is to somehow mount it on a very high chimney of the apartment building across the street and using a FM signal or microwave to feed the audio.. The guy that owns it is a established friend who used be my accountant years ago when I was in business, so I know it would be fine, but access to the transmitter would be a major ordeal, so I kind of scrapped that idea.
Installing on the ground simply is not an option, otherwise I would be giving it a try.
To be honest, the major reason I'm not in already in operation now is that I've been waiting on my tax returns for some working capital.. When I filed I put my damn street address on the forms instead of my PO box and they got returned to sender, and it is taking forever for it to go through the process and be returned back to me... I talked with the IRS again today, and all remedies are in place, but currently my check is still floating around somewhere in limbo during a very hectic time for them!
This has been going on since early March.
It all began about the same time I had a similar situation with some items bought on ebay.
It won't happen again.
Ah, but I'm drifting entirely of the subject.. It;s late and lack of sleep is catching up with me.
Thank you Marshall, as always, for your invaluable advice.
Goodnight.
As I recall from my experience in a motor manufacturing plant copper can be bonded by a process called heli-arc welding. This is similar to TIG but doesn't use tungsten. The heat involved in welding a metal can cause mechanical problems due to the effect of heat on what is called grain boundaries which changes the metallurgy. Motor lead wires were both spot welded (minimal heat damage) or brazed.
Is this necessary for part 15 ground radials? No. I have had success by approaching the problem of moisture by keeping the moisture away from the connection, be it soldered or mechanical. After making the connection I carefully wrap the connection completely in duct seal compound which is available at the big box home improvement stores for about $2 per pound. A pound is more than the average hobbyist will need. This compound is easily removed even years after application for changes or inspection.
Antenna and ground connections sealed in this manner at my home have remained tarnish and corrosion free for over 20 years. In fact, the coax insulation deteriorates before the connection. Eventually, any bonding and sealing method will fail but what I have mentioned works well for me.
Neil
What's wrong with using insulated wire for radials? Everywhere it comes up the consensus is that insulation has no effect at all on the grounding. However, many commercial insulations actually use copper strap in buried applications.
For our purposes, I think almost any gauge of insulated wire will be OK. It's the bonding joints where problems usually occur in any ground system. My theory is to use standard ground clamps to a coated copper ring or other device at the base, then coat these mechanical joints.
This way, it's very easy to replace things as necessary. Also, consider, in the case of galvanic corrosion, clamping a chunk of Zinc to the ground rod(s). The corrosion eats the zinc instead of the copper, and is very easy to inspect and replace.
Most structures already have such a ground system in place, but many people forget to inspect it. If the ground connection is lost, there is a potential for fire. We just had a house fire in Friday Harbor in which the owner, an employee of the town, lost her life. I bet that's one of the things under investigation.