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Antennas

Antennas: Jumping antennas can be a lot of fun. They are usually away from urban areas making them easy to access without getting seen. They often have large grass landing areas, making them easier to jump. The potential for object strike is generally smaller compared to buildings and cliffs. And finally, some antennas go as high as 2000 feet, offering a reasonably safe introduction to terminal BASE jumps.



Antennas are generally considered more forgiving objects than buildings and cliffs, but more complicated than bridges. Obviously there are exceptions to this general rule, and even antennas come with a large amount of theory you should consider before your feet leave the edge.
In fact, antennas with guy wires can also be considered more dangerous than cliffs in a way. The area you can safely fly in might be reduced from 180 degrees on most cliffs to 120 degrees between two sets of wires. Fortunately you can use the wind to your advantage as described below.

Where To Find Them

Obviously most antennas are found because a BASE jumper happened to drive by one and notice it. Word spreads, and before you know it the first jumps are made. However, the real gems can often be quite remote and it takes some work to find the good ones.

Most countries publish freely accessible databases of antennas. Some go as far as offering altitudes above ground level and precise GPS coordinates. If you do a search online you might be able to find some databases. Alternatively, ask your fellow BASE jumpers because they might already have a copy of the database.

Radiation

Antennas generally come in four forms.

  1. AM
  2. FM
  3. Support for radar dishes
  4. Inactive
For the technically inclined, studies on the biological effects of antenna radiation have been reported by Motorola (for the Occupational Safety and Health Administration) and by the FCC. Both of these links are to PDF documents.

Todo, talk more about radiation.

Climbing

Different antennas offer different ways to get to the exit point. Below are some things to consider.

Take the Elevator

Often only high antennas offer an elevator. The elevator is often the best way to get up there, not just because it takes less effort, but also because it minimizes your exposure to other people. It takes less time, and people will sooner wonder why a person is climbing the ladder wearing a backpack and a helmet than they will about an elevator going up.

Avoid breaking other people’s property when you try to gain access to the elevator. Lock picking is often preferred because it leaves no trace. Note that some countries and states consider lock picking a criminal offense and you should consider if this is worth the risk.

An elevator can not always be sent down without a person riding it. In this case, make sure you bring ground crew along for the ride so he can ride the elevator down. The last thing you want is the tower crew finding an elevator at the top the next day.

Climb on the Inside

Most lower antennas don’t have an elevator but offer a ladder. Usually the ladder is caged, providing some safety in case you should fall. Unfortunately such caging can be quite snug and offer little room for both you and your gear bag. In this case, it is recommended to gear up on the ground and then climb. Some people prefer to put the rig on their chest during the climb. This not only allows them to keep an eye on their shrivel flap or closing pins but also makes for more comfortable climbing as you don’t have to press yourself to the ladder to make room for the rig.

Climb on the outside

For very low antennas you might find yourself climbing on the outside of the antenna, without the protection of a cage. Tower workers would carry carabiners and rope for safety but you might not have time for this. In this case, be confident in your climbing abilities and strongly consider gearing up on the ground and having your pilotchute ready to pitch at any time.

Winds

Unlike on most objects, on an antenna the wind can actually be your friend, but only if you understand the theory behind it and know when to apply it. Time and time again people are found jumping antennas in winds that worked against them, even though these jumpers had the option of turning it into a beneficial situation. Make sure you understand the idea below and apply it in the field!

The reason winds can help you on antennas unlike cliffs and buildings is that they let the wind blow through them. The idea is that you jump on the side that gives you a tailwind, such that in the case of an offheading you will be blown away from the antenna.

This bears repeating: use the wind to your advantage and jump antennas with a tailwind!

Obviously landing area considerations should be taken into account when deciding what side to jump from. If your landing area is tiny and far away, you might only be able to jump the antenna from a certain angle. Consider jumping this antenna only when the wind blows at the right angle, or possibly in zero wind conditions. Don’t be tempted to jump in a strong headwind, as it will easily blow you into the antenna even if you open on heading.

If the antenna has guy wires, the theory gets more involved. Consider the image below.

This depicts an antenna with guy wires in three directions. This is a common architecture, but antennas with other configurations exists too. The theory should easily translate.

Consider making a jump into the yellow area. In this case, the green arrow depicts the ideal wind. It blows up the wire that is directly behind you when you jump. The red arrows depicts the absolute worst wind you can jump in. It will blow you directly into the antenna or wires, potentially ruining your day.

With winds down the wire (e.g. the red arrow), the antenna can be jumped in another quadrant as safely (from the standpoint of object strike) as it could be jumped in no winds. Consider the diagrams below.

Note that, although the ground speed of the canopy is higher with winds down the wire, the worst-case closing speed in the quadrant shown is no faster than if there were no winds. The important thing to consider is the rate at which the wind will blow a canopy toward a wire (or tower) strike. Any wind that increases the rate at which you close on the tower or wire is dangerous. Any wind which blows you away from the tower is good. Both zero wind and wind down the wire have no effect on the rate at which you close toward object strike.

These ideas translate to other wind directions. For example, if the wind is blowing from the bottom left corner of your screen towards the top right, you should optimally jump in the top right segment, with the wind in your back.

Don’t fall to the temptation to jump an antenna on a certain side because it’s always being jumped on that side. Traditions don’t save your life in BASE, applied theory does. There have been many occasions where jumpers got stuck on the wires and were found jumping in the wrong segment relative to the wind. In such cases, this may not have been the deciding factor (many things come into play in an object strike) but it certainly contributed.

As mentioned before, landing areas need to be considered, and the side with the ideal wind might not be a side you can land in. Think about jumping another day in this case. Fortunately guy wired antennas are often built in large fields and offer landing areas on all angles.

Stuck on a Wire

If you do strike the wire and find yourself lucky enough not to fall to your death, but so unlucky to hang up and be stuck there, a few options come to mind.

  1. Call rescue services
  2. Initiate a self-rescue

Todo, talk more about self rescue from guy wires.




09 November 2005

13:37 by anonymous.

Often, antennas with uncaged inside or outside climbs will have a safety cable running up the front of the ladder to which maintenance personnel can attach a cable ascender. Usually, the distance between the cable and the ladder will be maintained by rubber stand-offs which grip the cable at regular intervals. The cable can be pulled out of the stand-off so that the ascender can pass by, after which the cable is reinserted.

Note that cable ascenders are typically constructed somewhat differently than rope ascenders.

25 November 2005

12:09 by BASE 460.

In the case of winds going down the wire as discussed above and as shown in the left diagram above, there are two other issues worth pointing out:
1) in extremely strong winds, the jumper will have to compensate his or her body in freefall to adjust for the relative (cross) wind. In the diagram on the left above, the jumper’s body (as defined along the axis from head to toe) will naturally tend to turn either down the left wire, or 90 degrees perpendicular to the windline. 2) Once open, it can be safer to not turn away from the wire and into the open section as one normally would. In the diagram above, the jumper would naturally turn right and try to fly out into the open section. There are situations where it is simply better to turn left toward the tower, direct the canopy into the headwind, and continue the turn, allowing the jumper to avoid hitting the wire with minimal risk of striking the tower.

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