First, Static Discharge Precautions
We now come to the part about actually launching a
balloon or kite supported tall vertical or protracted inverted L. First of all, you
should be concerned about potential shock hazards and high noise levels
resulting from wind-induced static.
The solution at low power levels is shown here. The discharge device is a bleeder resistor to the antenna's ground, high enough wattage (non-inductive) to discharge the static
buildup at an adequate rate while preserving the electrical characteristics of
the antenna. (r > 1,000,000 ohms) The circuit shown will handle about
100 watts. Ten watt resistors will handle about
200 watts of RF. An inductor can be substituted for
the resistor, the value depends on frequency. It will must act as an RF choke
at the frequency of operation but a DC path to ground for static electricity.
Another way is to do it by making a spark gap,
shown in the illustration to the right. Use a
very large gauge of copper wire, say #8. Solder the wire to your antenna's
ground connection inside the tuner, and to the coax centre conductor, then with a fine saw, cut the wire, leaving
just enough 'spark gap' to discharge the static build up, but not close enough
to arc under transmit conditions.
Normally unless you use a quarter wave or more on
Top Band, wind induced static discharge is not a real problem. A half wave can begin to be a
real problem. I have only tried an antenna higher than 60 metres once and there
was a significant amount of 'jolt' when I touched the antenna feed point before
attaching the antenna to the ATU. I normally use an SGC 230 auto tuner
which also gives me 80 metre capability if I want to go 'VHF'. However, for high
power, I use an 'L' network for the very highly reactive impedances encountered at
antennas exceeding a 1/4 and approaching a 3/8 wave.
The illustration using the 'L' matching network above is for antenna impedances
of < 50 +/- j. Reverse the position of the variable capacitor (move it to the
antenna side of the inductor) to match >1/4
wave antennas. Long or tall
antennas have a very reactive feed point once you exceed a quarter wave.
those in built up areas who will be using a shorter antenna, there is a third way to drain the
static AND match the antenna. I use this method with antennas in the neighbourhood of a
quarter wave or less. It is the auto transformer illustrated above. The coil is
in series with the vertical balloon or kite antenna, and you measure the
resonant point at your frequency of operation between the bottom of the coil and
ground. Then with the antenna analyser you find a point on the coil that
gives you a match of 1 to 1. The antenna is grounded, and all static is
passed to the earth system. Using this method I find that any length from
a quarter wave to just a few tens of feet can be resonated, then matched quite
well. I have used this method most effectively with balloon supported
If You Use Balloons...How Big???
A few lessons in physics. What are the 'physics' of a balloon, and also what about the security of such an airbourne antenna setup?
First, there are a few natural laws that control the balloon's behaviour. Here is the first lesson.
Sizing for still-air conditions
specification. (also good for a small to medium kite)
I use the wire and the line taped about every three feet. I leave about three feet of antenna wire dangling at the top to keep high voltage away from the support line. I learned this the hard way after bursting two balloons at key down (400 watts) and losing a balloon when the support line burned through. For higher power, larger wire is used, again, the source of the weight of the wire is the ARRL Handbook.
Balloons and Physics
As per the Archimedes or EUREKA principle, the upward force developed by a balloon is related to the weight of the air displaced by the balloon volume. Actually the balloon should be sized to carry this net buoyancy (.8045lb) plus the weight of the helium (specific gravity = .1308 that of air) that it must carry plus the weight of the balloon itself.
Lets suppose that a 40 inch (3.33' ) diameter balloon of .5 lb envelope weight is available.
By the way, I use Kent Balloons here in the UK for my balloons, but any large commercial balloon supplier will have them. Helium can be obtained from any commercial supplier of industrial gases, here I use BOC, which is listed in the Yellow Pages under industrial gases.
Density of air at 500' ASL = .0754 lb/pi3, at 700 ASL = .0750 lb/pi3
Volume = .5236 x d3 = 19.39 pi3
At 700' ASL ; Weight
of air = 19.39 x .0750 = 1.454 lb
Net buoyancy at 700' ASL = 1.454 - .191 - .4 = .863 lb
In this case, two of these balloons would work for the proposed setup. One at the top of the support line, the other about thirty feet down to dampen the sway of the antenna in the wind.
Effects of the wind
In fluid mechanics textbooks, you can find formulas to determine the drag force applied by moving air upon a stationary a spherical object. (the balloons).
Fd : drag force ( lb ) Cd
: drag coefficient (no dimension)
Cd = .4 for Re ( Reynold
number) < 350,000
Re = D V / u
D diameter of the sphere
Below is a table that shows the evolution of horizontal pressure (in pounds) induced by various wind velocities (mph) on various balloon diameters (feet)
Below is a worst case scenario. Let's keep in mind these values are somewhat on the safe side. Also, keep in mind that the balloon lifted antenna will have gone almost horizontal by the time the wind speed hits about 15 mph. This is a safety exercise after all.
Some Practical Considerations
Here is the second lesson in physics. The balloon envelope material quality is also something you should consider before attempting any launch of a balloon supported antenna. If not, you may find it is not in the air the next morning. Remember when you blew up party rubber balloons to their max, just to realise that the next day they had shrunk to half their size. The reason is due to the pressure differential between the inside and the outside of the stretched envelope. Air will just sift through it. Rubber is an elastomeric material built around long organic molecular chain attached one to the other through ramifications (something like a tree), and there are 'holes' between these chain elements.
Air made up of molecular oxygen, nitrogen, and carbon dioxide eventually finds its way through. So if molecular elements like O2 and N2, that are many times the size of helium (He) can do that, you can imagine how helium would act. This is because the holes are wide open barn doors, relative to helium's size. I don't think a normal small 'party' balloon would be able to retain its full size for more than a couple of hours. So not any off the shelf balloon material will suffice. That is why I use Mylar balloons instead of large rubber party balloons to support my antennas on calm-ish nights.
Sometimes I will use weather balloons that have enough helium to stay up even if a significant quantity gets out, but overall, Mylar is the only way to go. I have used 8 foot weather balloons, five foot rubber balloons, and Mylar balloons. The Mylar balloon is similar to the Helikite's and the Mylar envelope will stay inflated for days, whereas the rubber balloons are good for only about eight hours. The weather balloons stay up all night, but really lose size after about five hours. The five footers stay up about ten hours, but they are getting pretty sagged by then.
Note on a potential hazard. Some amateurs in the past have used hydrogen to lift balloon antennas. Although it is safe to use if handled properly, your insurance company may not like replacing your garage or back porch if something goes haywire and the hydrogen does what comes naturally to a highly combustible and dangerous gas! The very slight increase in lift is not worth the hassle, helium is the only safe way to go, and in some countries a HAZMAT license is needed to store bottled hydrogen at your QTH.
Now...A Few Words About Flight Safety!!!!
The DO NOT List
(This list is a compilation of my experience and good advice from the guys who have done this much more than I have.)
Do not fly your kite or balloon if there is the possibility of thunder or lightning. You don't want to become a lightning conductor. Benjamin Franklin was stupid enough for us back in 1752.
Read the above again!
Do not fly your kite or balloon if high winds are forecast. (Check the hour by hour forecast for your area at The Weather Channel.) You can put in your post code, zip code, or city and this site will give you a good indication if it is go or no go as you can see wind speed and direction forecasts hour by hour. Sometimes the winds will drop overnight. That is why I will use a balloon antenna lifter of some sort when the wind is not a dead cert to be steady all night. A dead calm will bring down the kite. In the dark, that is a pain!
Do not fly your kite or balloon near a road, car park, railway line or where there are any moving vehicles.
Do not fly your kite or balloon near farm animals, or a bird or animal sanctuary. You could frighten the hell out of them.
Do not fly your kite or balloon within 5 kilometres or 3 miles of an airfield but check the local regulations in your country.
Do not fly your kite or balloon higher than the legal limit in your country unless you have obtained permission from your local or national Air Regulation Authority. 60 metres is internationally recognised as the limit of vertical height above terrain. Any higher, you must tell the aviation authorities.
Do not fly your kite or balloon near trees or buildings or your kite will get tangled up. That is Murphy's Law and I can tell you from experience kites and balloons love to get tangled! Also, there is always air turbulence near them so they are best avoided.
The DO List
ALWAYS use a line with the kite or balloon antenna wire, NEVER just the antenna wire.
ALWAYS wear gloves to protect your hands. I use leather gardening gloves to prevent painful burns.
ALWAYS ensure that you or your clothing don't get tangled together with the flying line or the antenna wire.
ALWAYS ask permission of the landowner before flying, unless it is your property, then just soft soap the neighbours when your kites lands on their roof.
ALWAYS check the wind speed before flying, the higher the wind the more difficult the kite or balloon is to recover.
ALWAYS fly the kite on line the strength of which is determined by its size. The correct line for most lifter kites or balloons is at least 160 pounds.
ALWAYS launch and retrieve the kite or balloon from your hand....never run with it as you could fall and be injured, and look a prat like Charlie Brown!
ALWAYS carry sunglasses (Polarised ones are best) to keep an eye on your line/antenna in the daylight. At night, a torch or flashlight is handy also.
ALWAYS wind the flying line in a figure of eight onto the winding handle, otherwise you may never get the line off again!
ALWAYS replace the flying line if it shows any sign of becoming frayed or damaged.
ALWAYS untie any knots on the line which are not there by your intention. Incorrectly tied knots seriously weaken the flying line.
ALWAYS check the security of knots before flying. I have lost good kites when a knot went, and it was 200 feet in the air.
ALWAYS avoid anyone touching the antenna when you are transmitting. Believe it or not, kids do like kites.
ALWAYS prevent anyone else from interfering with the kite or your equipment while you are operating.
Another Few Bits of Information
It is important to understand the forces which are involved with kites and balloons. The larger the kite or the balloon, the more it will lift but also the forces of nature acting on it will also be that much more difficult to contend with. A larger kite will fly in mild winds but it certainly will become tougher to handle in strong winds. Should you launch a large kite in reasonable wind and some time later the wind strength increases you could be in some difficulties whenever you attempt to recover it. During the initial learning stages, just be sensible about the wind conditions you are attempting to fly in. The optimum size for a kite is its ability to lift things for any given wind and to be controllable by the operator. It is quite easy to launch a kite into the air but it is another matter entirely when you want to recover it. That is why my kites have been made to reasonable dimensions. They have to have enough lift and be within the ability of a single person to safely fly it in the widest range of wind conditions.
THE FLYING LINE
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