I would like to experiment with a 3m radius ground screen, laid on the ground at the base of the transmitter. I am wondering if anyone here has any ideas on an inexpensive way to create this easily and quickly; possibly with the ability to fold or roll it up when not in use (or when necessary to transport the system). Ideally the ground screen would be highly conductive, although it certainly does not have to be made of copper. I would prefer something that approximates a sheet as closely as possible rather than a rat’s nest of wires. I have used things like gutter cover and chicken wire for ground screens in the past, but they don’t really come in the right size and shape. Something like this would not only be useful for Part 15, but could also be used for ham radio (augmented, of course, with radial wires).
Tin Foil?
I hate the fact I live in an apartment. If I had my own house, I would have an extensive grounding system and a nice large ground plane. Right now the wire antenna is just hanging vertically on the balcony. If I covered the balcony in sheet metal would that help boost my signal?
The plus is I am on the top floor, 5 stories up.
That is a tough situation
If I were you, I would ground the transmitter to the best available ground I could find. You’ll have to be the judge of that. A counterpoise would work better, but probably would not be permissible under the FCC rules.
I am going to take another look at ground screens in the Spring. I wondered about using tin foil for a temporary test setup. It will be necessary to get a good connection between strips.
RE: That is a tough situation
“A counterpoise would work better, but probably would not be permissible under the FCC rules.”
Why? A counterpoise isn’t even attached to anything. I have some other questions about this sort of thing, but I think it might deserve its own thread. Look for it: “Marine Grounding Exercise”
A Counterpoise = Raised Radials
A counterpoise does not need to be connected to earth ground. The counterpoise however creates a sort-of artificial RF ground potential for the transmitting system and raises the radiation resistance of the system sufficiently to raise the efficiency of the radiator. Run small wires along the base boards of your apartment and the hook them to the ground terminal on your transmitter. Test it out and see what happens. HF, VHF and UHF antennas have used raised radial systems for years. They work great.
If the antenna is hanging
If the antenna is hanging from the balcony ceiling, wouldn’t it have poor proximity to the wires proposed?
I’d think that if it’s an AM BCB system, it would benefit more from a loading coil. Then, of course, why not both?
Limited space and ground plane issues are another reason I want to continue experimenting with loop and fractal antennas in the AM BCB frequencies. I have some ideas, but, especially with fractals, I have no idea how that would work with regulations geared strictly to stick electrics.
If anyone else does, please feel free to contact me off the forum.
Proximity…
The wires proposed are the missing half of a dipole antenna. An AM BCB vertical tower is only half of the antenna system. The other half is the ground radial system. The radiator is hooked to the RF out and the other wires (in close proximity) are connected to the ground terminal. The radials are buried not to be in contact with the earth, but to act as the other half of the antenna. True the ground system does not radiate, but it does add capacitance to the antenna load. Radiator and counterpoise wires can run in close proximity and parallel to each other and still work- (see Bazooka antenna). One additional note: The counterpoise wires need only be 5 percent longer than the radiator antenna wire. Extra length adds nothing.
A loading coil is included to add inductive reactance (XL) and reducing ca[capacitive reactance to the antenna system to raise the radiation resistance and gain a little efficiency. The coil effectively lengthens the radiator. You can do a similar task by putting a variable capacitor across the input to the antenna system. Try it, the results may surprise you.
Again, other antenna designs have been tried for MW propagation over the past 90 years or so. The ability to produce ground wave sufficient for an AM broadcast receiver to capture and demodulate the signal still requires the use of the traditional BCB radiator. For broadcast antennas, towers are still the antenna of choice for the broadcast industry. However, if you ever get the time, research the KinStar antenna system for AM BCB. There is an actual license and operating system in Florence, Oregon.
KinStar station
“the first Kinstar antenna was placed on the air by KCST-AM in Florence, OR, operating on 1250kHz with 900W daytime power and 37W nighttime power”
Would it not be an absolute blast to be running a 900 watt AMer?
Too cool!
Tin Foil? ?
You can buy an “artificial RF ground” from MFJ enterprises in MS. It’s the MFJ-931.
http://www.mfjenterprises.com/Product.php?productid=MFJ-931
Or you can make one, which is more educational. An artificial ground simply consists of a series resonant LC (inductor-capacitor) network in series with a random wire as long as you can make it. Connect the capacitor (a 10-250pF variable) to the chassis of the transmitter, and connect its ungrounded terminal to an inductor of about 80uH (for 1.7MHz) in series with the wire. The wire should be at least 25 feet long. The trick is knowing when the whole network (L, C and wire) is resonant. You can tell this by putting a simple, cheap analog V-O-M probe near the coil and tuning the capacitor for maximum soup while you are transmitting. Do not connect the probe directly to the wire.
Happy grounding (artificially, that is)
According to the page, the
According to the page, the frequency range of the MFJ-931 is nowhere near the AM BCB. Not sure how that affects its performance.
Also, concerning the KinStar … well … heh-heh … try getting that past the Part 15 FCC Inspectors 😉
There are manufacturers that make copper foil and tin-plated copper foil products. Sheets and tapes.
inductors and copper tape
The low end frequency of the MFJ is 1.8MHz, not really that far from the AM BC band. Unclear how they determine this, but they are not going to advertise something that works at 500kHz even if it did. Since it is simply a series L-C network I expect you could decrease the resonant frequency by adding inductance. Inductors are pretty easy to wind.
In any case, using wire (or wires) has to be much cheaper and easier than putting down a bunch of metal sheet or copper tape. It’s cheaper too.
Thanks for the ideas!
I’ve done a bit of searching of my own and have come to the conclusion that I will use inexpensive stranded wire rather than a solid plate. Here are some of the driving factors:
Installation: at ground level
Type of earth: sandy soil
Location: near the shore of a lake
Operation: seasonal (i.e. summer only)
Deployment: semi-portable. Transmitter and radials will be removed for the winter
Purpose: experimental. To evaluate how the number of radials affects range and be able to compare performance as radial wires are added or removed
After giving this some thought, my plan is to install up to 50 3m-long radial wires by laying them on the surface of the ground. If this is successful, they might eventually be replaced with a more permanent installation that is buried a few inches below the surface. I determined that the use of a flat plate or wire screen would be too much trouble for my application.
Because the ground is sandy, it has very low conductivity so burying the radials will not have much of an effect on performance. For the wire I plan to use either zip cord or flat multi-conductor antenna rotator control cable, splitting the individual wires apart to get more footage. I need approximately 500 feet of wire to run this experiment. I plan to anchor the far ends of the wires by tying them around nails pressed into the ground. The wires will be soldered to a common ring at the base of the transmitter and will be rolled up when not in use.
The FCC recommends 1/2 inch
The FCC recommends 1/2 inch hardware cloth that is readily available from hardware, home centers, farm supplies, etc.. for open area test sites. You can buy a spool for under 50 bucks, shape it the way you want it, bond it together with copper wire and solder it with a torch.
Cut the grass really short, and when it grows it will hide the mat.
Really Cheap and Disgusting 😉
I used a bunch of electric fence wire and made a ground area. Works OK and it buries easily.
FCC recommends 1/2 inch
Is there any particular pattern that works best? How many feet out?
Respectfully,
Mike
testing near lake
The test is interesting and the plan sounds great. Being near a lake gives you an opportunity to see what difference the water makes on radiation efficiency. Fresh water is not as good a ground as salt water but probably better than sand. Maybe an area near the lake where the ground is kind of mushy and has lots of plant growth would be more conductive due to ionization. If anything, the open area over the lake would give your EM waves a better chance of launching from a vertical antenna.
Update from the lake
It’s been awhile, so I thought I would give you an update. I finally constructed my lakeside test station. Originally I was planning to use a new transmitter but that did not work out, so I implemented Plan B and brought my Rangemaster from home. I had already taken it down from the garage earlier this spring, due to the “elevated ground controversy”. So it wasn’t currently in service.
A friend of mine ran some simulations suggesting that longer radial wires would be preferable to a plethora of short (e.g. 10′) radials. I purchased a 500′ spool of insulated stranded #14 THHN wire at Home Depot very reasonably. That was my raw material. That, and a four foot length of #12 solid wire.
The only site that I have available is 25′ from the shore of the lake and 20′ from the neighbor’s property line. I pounded in about a 4′ piece of water pipe, which I had threaded first to make it possible to install a cap. This made it easier to drive into the ground. Next to the pipe I put in a 6′ ground rod. At this location the water table is only about 3′ down.
I took the #12 solid wire, bent it into about a 1′ diameter circle, made a loop in each end, and soldered them together. This would serve as the ground ring. I decided to go with 8 radials initially. I made two of them 20′ long and the other six 25′ long, taking advantage of the area I had available. I cut the wires and soldered them to the ring. I added two additional wires to connect to the transmitter ground lug and the ground rod. Then I carried out the 8 coils of wire attached to the ring and slipped it over the mounting pipe. A moment later I had the Rangemaster bolted into place and hooked up to the ground ring.
For testing purposes, I just laid the radials directly on the ground. I pushed the ends into the sand to hold them in place (the ground here is very sandy). Then I applied power and tuned the transmitter. Tuning went smoothly. Flipping on the radio, I was rewarded with a fierce hum on my monitor radio. I thought it would go down when I connected the mixer, but it didn’t. More on that later.
I put on some program material and went for a test drive. Passing the first three houses, the signal was solid as a rock; then it dropped off precipitously. At 1/4 mile, it was barely audible. This was not what I was hoping for. I drove to the other side of the lake, hoping for a miracle, but no such luck. After I got back, I decided that since I still had about 300 feet of wire left, I would try another experiment. I cut another 125′ or so of wire and attached that to the 25′ radial that went straight toward the lake. You would be surprised how long that much wire looks when you have it unspooled! I found a large, waterlogged branch and tied the end to that. Then I carried it straight out into the lake and dropped it on the bottom. Now I had one quarter wave radial mostly underwater. Repeating the range test, I found to my dismay that things had not improved significantly. I could just barely hear a trace of my signal on the other side of the lake, but that is all.
Unfortunately, my time at the lake has come to an end for this visit, so I packed everything up and put it away, leaving the ground radials in place for now. Now, about that hum: it went away as soon as I drove a short distance from the house. I came to the conclusion that it is probably due to RF following the audio/power cable from the transmitter back to the mixer. The cable appears to be “hot” with RF– I can get a slight reading if I wrap it around my field strength meter. Likewise, the ends of the radials will produce a reading in the same manner.
One advantage of this location is that I have one frequency that is extremely quiet in the daytime. It is as close to perfect as you could hope for. On the other hand, the ground here is very low in conductivity. I do not think the presence of the lake improves things very much (if it were salt water, that would be another story). When I return next time, I plan to repeat some of the experiments, and compare this setup to the one I tried a couple of years ago, in which I had the transmitter bolted to the end of my metal dock. The dock is only 30′ long, but I have a feeling that worked better than my current setup. Some other ideas I have include adding the capacitive top hat that I described elsewhere, and perhaps trying just a few tuned radials. The ground conductivity is so low that in earlier experiments, I found that a 40m horizontal full wave loop just one foot off the ground worked quite well. So perhaps three or four tuned radials lying on the ground would be effective.
I took some digital photos of the test setup; as soon as I get them online, I’ll post links to them here.