From 1988 ARRL Antenna Book p. 3-13
From 1988 ARRL Antenna Book p. 3-13
“In many cases where directivity is desirable or real estate dictates, longer, more closely spaces radials can be installed in one direction, and shorter, more widely spaced in another”
More From ARRL
TECHNICAL & TECHNIQUE
In September, a contester starts to get a little itchy about whether
his antenna farm is really up to snuff, particularly on the low bands.
Most of us are using verticals of one sort or another on those bands
and there are hardly ever "enough" radials. Therefore, one common
cure for the "dB Itch" is to go out and put some more radials on the
vertical. But why bust your budget over radial wire? Do you need #8
oxygen-free copper to get that last 0.01 dB? Not really, here's some
advice from two guys that should know--thanks Tom, W8JI and Rudy,
N6LF.
"(A) comprehensive RCA funded study's conclusion was that adding a
screen does nothing for efficiency if the installation has even a
modest radial ground system. You can find measured comparisons in the famous study "Ground Systems as a Factor in Efficiency", but the
conclusions were that a screen or radial system has to be at least
1/8th wl or so in radius to become reasonably efficient, and small
screens don't do much either by themselves or when used in conjunction with even a modest radial system.
The idea that a few wide strips used for radials are better than even
very fine wires covering the same footprint of area is not correct.
The distance out is important, and the distance between wires is
important. The individual conductor thickness or surface area is
almost entirely meaningless once the radial becomes so thick or wide
it is mechanically reliable. Other then life of the system, you'd
never know the difference between 50 #24 gauge copper wires or 50
six-inch-wide strips on 160 meters...as long as they were the same
length.
I wouldn't think twice about using #20 or smaller wire, if it was a
temporary system. I would never even consider using anything larger
than #10 wire for myself, let alone strap or flashing for radials,
unless it was somehow cheaper.
Broadcast stations almost always use ground screens near towers, but the primary reason is for physical installation reasons rather than
actual efficiency. It is easier to work around the antenna and to
ground all the equipment near the towers if a screen is used near the
towers.
It would take a huge screen (certainly over 1/8th wl in radius) to
equal or improve on a system with thirty 1/4 wl radials. Use a screen
if it is easier to install or cheaper, not to make things "better" or
to replace or augment a reasonable size radial system. 73, Tom W8JI"
"If you want to see where this all comes from, check out the article
in QST for July 2000, pages 38-44. You can take the equations in the
appendix, drop them into a spreadsheet and see for yourself how the
performance of a buried radial system behaves as you change things.
There is also a long list of references where all this comes from.
George Brown in his 1933 Ph.D. thesis pointed out that the size of the
wire had negligible effect! Radial wire size is a mechanical issue.
73, Rudy N6LF"
I would assume that the increased signal strength is in the direction of the longer radials.
That is correct. I think Keith H. may have also referenced that in one of his posts.
Probably answers every question I've ever seen on radials.
"Read it, learn it, live it"
Wow ... I agree with others, that's the best posted message I've seen on ground radial issues. Now all I have to do is get the antenna to resonate proprly.
In Dr. Tainotti's communiqué with Ermi, he state:
"Care must be taken if the area in which the antenna is installed can be flooded, to avoid water at the antenna base. In this case, the metallic ground plane can be made as kind of a mound, to raise the antenna feed point above the level of the soil. The mound must be as flat as possible, with an elevation at the antenna base of 1 to 2 meters above the soil level. The mound is made of ordinary earth. The metallic ground plane is placed along the surface of the mound. The antenna base can be installed over a small concrete pylon in the middle of the mound.
I'm struggling with the wording here. How can a 'mound' which is raised and rounded be 'flat' which is, well, flat? The two terms are contrary. Does someone have to draw me a picture to get it? Answer: Resounding YES! I cannot conceive of something being a mound while at the same time being flat. Does he mean something like an average golf green ... a kind of plateau?
"As you see, a lot of work must be done even for a very simple antenna like this one, but this is the only way to get an efficient radiating system in these poor conditions."
I sort of 'accidentally' seem to have gotten my antenna working decently with no connected RF ground, but it rained like crazy here for a few days prior to temporarily raising the antenna, so the ground had been wet, even though it dried out somewhat on Friday night, plus all day Saturday, when I temporarily raised the antenna, was dry. No 'earth', though, except for a linear crack under the tripod between the parking lot chip seal and along the old poured concrete sidewalk that abutts the block building. It has weeds growing out of it, tends to flood in a hard rain. My small dog has to jump over the flowing water when that happens.
Also, I completely coated the antenna coil with PVC glue. I don't think it will get wet through that plus the wire insulation, but I can certainly give it another coat. It's also 4" dia. ... pretty large for putting inside something else.
I think the large diameter of the thinner wall (schedule 30?) white PVC drain pipe coil substrate, plus the use of 18 AWG multi-strand insulated wire contributes. Also, I think trimming excess wire between the TX and the base coil output, and again between the base coil and the antenna, did a lot for it ... IOW, the alligator clip wires were probably messing with trim matching more than I thought.
If I were to put a metal box around the TX as suggested by Dr. Trainotti in that other thread, I believe the Shakespeare 'glass mast will be too flexible to handle any wind loads, and mounting will be tough. Should I maybe check to see if it will fit inside a PVC pipe to stiffen it?
I definitely think it will take more work ... especially if I want to understand what's happening with how it is now ... with no RF ground connection.
Comments?
Our senses tell us that the earth is flat. It's no wonder that ancient mariners thought that they would fall off the edge of the earth if they ventured too far out. That is because, although the earth is actually a sphere, it seems to be flat on the surface because it is so large.
For a mound to be reasonably flat, it has to be wide. It would take a lot of real estate and many truckfulls of dirt to follow Prof. Trainotti's instructions exactly. The Professor thinks in terms of antennas for licensed AM stations. His view is that good construction practices apply to the tiny 3 m antennas for Part 15 as well as for larger antennas for licensed stations. The point is that a mound is needed to keep the base of the antenna out of water, and the mound should be as wide as possible. If your lot is only about, say, 1/8 acre, you'll have to do the best you can with the land you have available.