The ‘Aerial’ Antenna Section
The balloon kite arrangement I use involves the Helikite Sky Hook or the Lightweight Helikite . The Sky Hook is larger than the ‘bird scaring’ range of lightweight Helikites, and will fly in wet weather. The two types of Helikite are highlighted.
Wind Max Altitude_Helium Capacity Lift No wind Lift in Wind Max
Vigilante Helikite 0.15 m3 0.03 Kg 0.15 Kg 25 mph 1,000 ft
Lightweight Helikite 0.15 m3 0.06 Kg 0.18 Kg 25 mph 1,300 ft
Skyhook Helikite 1.0 m3 0.4 Kg 1.5 Kg 28 mph 2,000 ft
Skyhook Helikite 1.3 m3 0.7 Kg 2.5 Kg 31 mph 2,500 ft
The Helikite Sky Hook will just about lift the quarter wave (40 metres) of number 14 flexiweave wire without wind, but any breeze at all takes it right up with the additional generated lift. The designed angle of flight of the Helikites is 45 degrees or a bit more, making it not quite a true vertical but certainly a lot more vertical than an inverted ‘L’.
To deploy the balloon/kite antenna, the 12 metre SCAM12 mast is nested at 2 metres, a 4 metre fibreglass stub is inserted into the top at the 40mm spigot. A 150 kg. breaking strain fishing swivel is attached to the top of the stub to keep twisting to a minimum. All attachments are made with 200 kg. breaking strain Kevlar line. On each side of the swivel is about 10 cm. of Kevlar line. The antenna wire to the Sky Hook is run from a small porcelain insulator at the swivel to the Helikite, with the last two metres being Kevlar attached to a small high strength porcelain insulator. Another swivel at the Helikite attachment point takes care of the last of the twisting. The final bit of additional Kevlar keeps any arc away from the attachment point and saves chasing the Helikite across the country as it flies to 1 to 2 thousand feet and drifts rapidly away. (My insurance has paid for two escapees that took off)
The Sky Hook is then released, the 40 metres of flexiweave antenna wire played out, and then the pump up mast is put into the full up position, SLOWLY, to avoid strain on the kite antenna wire. The soil of MY QTH (have a look at this link) is very sandy, but I am not far from the North Sea. The antennas for Top Band work well. Here is a plot of the Helikite antenna, shown with a ground loss of about 10 ohms which is a bit high. I have about 60 radials from 45 to 120 feet in length. I think this plot is conservative.
First, Balloons – How Big?????
Now we come to the part about launching a balloon 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 to use a bleeding resistor (or inductor) to the antenna’s ground, high enough wattage )non-inductive) to discharge the static build up at an adequate rate while preserving the electrical characteristics of the antenna. (r > 100,000 ohms, an inductor value depends on frequency). Another way is to do it by making a spark gap. Use a very large gauge of copper wire, say #8. Solder the wire to your ground connection, and to the antenna wire, 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.
Unless you exceed a quarter wave by a significant amount on Top Band, 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 metres if I go ‘VHF’. Sometimes an ‘L’ network is used for high impedances encountered at 3/8 wave.
Back to the physics lesson, what about the physical (structural) ‘reality’ and security of such a setup. First, there are a few physical laws that control the balloon’s behaviour and here is the lesson.
Sizing for still-air conditions
Material specification. (also good for a small to medium kite)
Thin copper wire: AWG 28 : 2081f/lb (.0005 lb/f); 65.31ohm/1000′ (source ARRL Handbook.)
Support line: 40 pound test BRAIDED NOT MONOFILAMENT at .0001 lb/f (conservative estimate).
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 Archimedes’s 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) + the weight of the helium (s.g. = .1308 that of air) it carries + the weight of the balloon envelope.
For example, at 500′ ASL you want to launch a 200′ high (= 700′ ASL) balloon able to procure sufficient lift for a .8045 lb antenna and line payload.
Lets suppose that a 40 inch (3.33′ ) diameter of .5 lb envelope weights 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
At 500′ ASL ; Weight of helium = 19.39 x .0754 x .1308 = .191 lb
Net buoyancy at 700′ ASL = 1.454 – .191 – .4 = .863 lb
In this case, two of these balloons would be ok for the proposed setup. One at the top of the support line, the other about ten 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 spherical object. (the balloons)
Fd = Cd p V2 A / 2
Fd : drag force ( lb ) Cd : drag coefficient (no dimension)
V2 : Velocity of the wind squared ( f2/sec2 )
A : Center area of the sphere ( f2 )
Cd = .4 for Re ( Reynold number) < 350,000 Cd = .2 for Re > 350,000
Re = D V / u
D diameter of the sphere (feet)
V Speed of the wind (feet/sec)
u (nu) Kinematic viscosity of air = ~.00016 (f2/sec)
Below is a table that shows the evolution of horizontal pressure (in pounds) induced by various wind velocities (mph) on various balloon diameters (feet)
Horizontal wind-induced pressures on a spherical balloon: Fd (pounds)
V (mph) 3′ 3.33′ 4′ 5′ 6′ Diam
10 .71 .88 .66 1.02 1.5
15 .83 1.03 1.5 10.7 15.4
20 1.5 1.83 2.63 4.1 5.9
30 3.33 4.1 5.9 9.2 13.3
40 5.9 7.3 10.5 16.5 23.7
Here, I will limit the analysis to a worst case scenario, where I will use only common sense to evaluate resulting strain on the antenna support wire. I did that to the table above to generate the table below. 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.
Estimated side wind-induced strain on the antenna wire (pounds)
V (mph) 3′ 3.33′ 4′ 5′ 6′ Diam
10 3.9 4.1 3.1 4.7 6.9 Safe
15 3.9 4.8 7.0 10.7 15.4
20 7.0 8.5 12.2 19.0 27.4
30 15.4 19.0 27.4 42.7 61.8 Risk
40 27.4 33.9 48.8 76.7 110.
One last table-the physical properties of air
Altitude (ASL) Air pressure Air density Air viscosity
feet lb/f2 lb/f3 f2/sec
0 2116.2 .0765 .000156
1000 2040.9 .0743 .000162
2000 1967.7 .0720 .000164
The balloon envelope material quality is also something you should consider before attempting any lift off. If not, you may experience a short lived project. We’ve all air blown party rubber balloons to their max, just to realize that the next day they had shrunk to half their size. The reason being that, due to pressure differential between inside and 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 atomic helium (He) can do that, you can imagine how helium would act considering the wide open barn doors these ‘holes’ are, relative to its 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 stock 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.
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.
Now…A word about safety
The DO NOT List
Do not fly your kite or balloon near power lines of any description. If the flying line becomes wet it will become a conductor of electricity with lethal consequences. A highly conductive aerial wire will only add to the considerable danger. Also, remember if the kite or balloon breaks its tether, the dragging wire can cause havoc with power lines some distance away.
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 so that is why I will use a balloon antenna lifter of some sort then. 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, bird or animal sanctuary. You could frighten or alarm 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.
Do not fly your kite or balloon near trees or buildings or your kite may become entangled. There is always air turbulence near them so they are best avoided.
The DO List
ALWAYS use a line in conjunction with the kite or balloon antenna element, 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 are not tangled together with the flying line or the antenna wire.
ALWAYS seek 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.
Near trees and buildings there is always air turbulence where the wind is tunneled or down draughts could upset the flying of your kite. If you fly upwind of the obstructions the kite is liable to become entangled in branches or obstructions on buildings. The wind close to the ground is usually not as smooth and laminar as it is higher up. Once the kite is airborne you will notice that it becomes steadier with height. You will also find that the wind is generally stronger the higher up the kite is flown. This can be felt with the increased pull on the flying line. Another thing to watch out for is the turbulence created by hills, as this turbulence can extend to a considerable height.
THE FLYING LINE
The choice of flying line is important. Braided fishing line is best and the required strength is at least 160 pounds.. Do not be tempted into using cheap polypropylene line, it will not stand up to the abuse kite flying will give it. Go to your nearest fishing tackle shop and consult with the shopkeeper. Use only the best available. 130 pounds will do, but it also reduces the safety margin.
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