Sunday, June 9, 2019

Antennas and drones - some questions answered

DJI Phantom 3
My recent video of using a drone to hang an antenna line has generated a number of emails requesting more details on what is available, what is required, how much does it cost, etc. I'll answer those questions here to the best of my ability.

Why use a drone and not a ___________ ?

Drones go higher, drones are (with minimal practice) more accurate. I won't accidentally overshoot into someone else's yard, picnic, etc.

Which drones are suitable?

The only drones I know anything about are those I've owned: DJI Phantoms (Series 1, 2 and 3) and a DJI Mavic Air. I have not tried hanging a line with the Mavic Air - it is a very small drone and probably lacks the lifting capability. Any of the Phantoms have more than enough lift. I currently use a Phantom 3 (no longer available new).

How hard are they to fly?

You'll be amazed at how easy they are. The drones mentioned are GPS controlled. Start them up, let them get a fix, take off. Remove your hands from the controls and the drone stays put - in both position and altitude. Even a moderate breeze is compensated for. Of course "stays put" is a relative term - I don't think I've ever noticed my Phantom drifting more than 1-1/2 to 2 feet. Your mileage (driftage) may vary.

 
What is the total (minimum) cost for a complete antenna-raising set-up?

DJI Phantoms are on the 4th generation of drones and they are pricy. DJI doesn't sell their older generation drones new but they do have refurbished older models available. These still cost a lot and have feature unnecessary for antenna installation work - ie, you don't need a 4k or even 2.7k camera to put a line over a tree. The ideal drone would be a Phantom 1 and these can be had for $170 at Adorama Camera and probably other places as well. The Phantom 1's are made to carry a GoPro camera aloft. The camera is not included.

Bare minimum price then would be around $215 if you have a GoPro or decide not to use one - but please do use a camera (see below).


Do I need a camera at all for this usage?

You'd think not, but...

On the first few of my many uses of the Phantom 3 for hanging an antenna, I didn't pay attention to the video from the drone's camera at all. I just watched the drone go up and, when I thought it was higher than the tree I moved it forward to go over the tree and drop the line. Unfortunately, I was not higher than the tree and flew right into it.

The perspective can be very deceptive and I should have known this from previous experience.

On a later attempt, I flew up 300-foot tower (no, not to hang a wire!) and, when I thought I was above it I looked at the camera - I was still some distance below it. I flew to the same altitude as confirmed by the camera and then looked at the drone. At that point, it appeared much higher than the tower although I was now even with it.

At heights over 75 feet, the relative sizes of the drone and the top of the object you're flying near can trick you into thinking you're higher than you are. After several years of doing this, I still trust only the camera. So if you don't already own a GoPro, you might consider a Phantom 2 (or later) model which comes with a camera built in. The video from the camera is presented in real-time on your smart phone or tablet.


What else do I need?

  • A release mechanism - I'm very happy with this one
  • A 1-1/2 to 3 ounce weight, depending on several factors: height of object (taller = heavier weight), the amount, if any, of branches your weight will have to fall through
  • A spool of fishing line (I use 20lb test) at least three times as long as the object you intend to support your antenna
  • Nylon cord
  • Rope?

For portable operation I usually omit the nylon cord and just use the fishing line to haul up the antenna - if the antenna itself is lightweight. For heavier antennas I use the fishing line to pull up a strong nylon line, then attach the antenna to that. At home, I used the nylon cord to pull up a rope into two trees 120-foot trees about 100-feet apart. This supports my 30-40-80 meter dipole at 110 feet and it is a DX-workin' mofo on the low bands.

Regardless of the installation, I never wind the fishing line onto a reel - I just leave it on the spool, sitting on the ground in an area where it won't snag on something on the way up.


Antenna considerations

Obviously, we're talking wire antennas here. At home, the dipole is King. For park activations my preferred antenna is the PAR EFT-MTR. PAR makes other end-fed halfwave dipoles. I like them because they can be put up as horizontal dipoles, vertical dipoles or as an inverted V. Installed vertically or as an inverted V only requires one support so this is the configuration I normally use.

Additionally, a vertical dipole is a DX-worthy antenna. For Field Day, a dipole may be more suitable.

If I had it to do over again I wouldn't idiotically spend $100 on a piece of frickin' wire and a toroid - I would read and study Steve AA5TB's fine work and build my own for a fraction of that price. But I was in a hurry and, like stupidity, being in a hurry costs money...


Have you ever omitted the fishing line and just hauled up the nylon cord directly?

No, but it is on the To Do list. I think the Phantom has the necessary lift to haul up the line and a sufficiently heavy weight to fall once on the other side of the target tree.


When to not fly a drone:

I'm not a lawyer and don't play one on TV, so I'll leave the legal research and safety considerations up to you.

Apart from that category, wind is your enemy. Even a slight breeze makes me cancel the operation - not of flying, but of antenna hanging - and this has to do entirely with the fishing line. Lift the line up 75 feet or so in the slightest of breezes and it is no longer anywhere near vertical. It's not unusual to have well over 100 feet of line played out just to ascend a 75-foot tree.

I need dead calm or I'm back to a slingshot. Mornings are almost always calm.


Any other precautions?
How'd I do?


Yes, and it's a biggie:

Once up, the weight/line has to fall. You cannot change your mind and decide not to drop the weight. If you do, and try to land the drone with the weight/line still attached, you'll crash because as you descend, you'll be descending into the line you've already lifted and your propellers will eat it up like a Texan eats brisket and the line will win.

I hear your question: won't the line fall away and drop as the drone slowly descends? No - because of prop wash and the lack of mass of 20lb test line, the line will whip around if you try to descend. It will no longer be a vertical line and you will fly down into it. Ax me how I know...

Just so there's no doubt: if your release mechanism fails, you will crash your drone. Test your release mechanism on the ground right before flying.


Alright, I have all this stuff - now what?

I've learned that a weight falls through a tree with the least resistance when it is pulled from the spool as vertically as possible. I don't want the line to be pulled away laterally at all - this is what would happen if you positioned the spool further from the tree than necessary. The farther away the spool is from the tree, the more of an angle the deployed line will be. This makes for greater resistance in pulling the line up and, more importantly, in allowing the line to fall on the far side of the tree.

I usually place the spool about 10 feet further than the farthest branches reach.

Then I fly straight up until I'm 25 feet above the tree, then - with camera looking straight down, fly over the tree until I'm an equal distance on the other side from where I took off. Hit the release button on the release servo and behold the glory of technology. If I'm feeling philosophical, I'll ponder Sir Isaac Newton and the apple as I watch the weight fall, but that part's up to you.


What else can I do with a drone?

Apart from hanging an antenna, you may have to use your drone to get the antenna down.

I take my dipole down every summer prior to thunderstorm season. Normally, I loosen the rope at each tree, then pull on the ladderline that feeds the dipole at its center. Both ends come down - except this one time...the feedline broke and the dipole was left hanging up there - way up there. The only way to get the dipole down was to fly a line (then a rope) up and over it and pull it down with that.

You may also want to measure the height of a dipole. An easy way to do this is to fly a fishing line up and over, then retrieve the line, measure its length and divide by two.

And every now you may want to show off your bad-ass flying skillz:


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