Portable Rotatable Mast
I have been trying to make my antenna system as portable-friendly as possible for operating on my family's island. That has been much easier said than done, however. I run fairly large beams on 50, 144, and 432mHz - all rotatable . 5 elements on 50, 13 elements on 144, and 21 elements on 432. These antennas require a significant amount of strength to hold up. We get very strong winds on the island in storms so nothing too light duty would work. However, at the same time everything needs to fit in the back of my pickup truck for the 2 day trip there and back (that's one way, its roughly 20 hours - of driving).
To start with the theory behind my current mast system Ill say this: I am pretty much a one-man-band when it comes to setting up this stuff. That's 3 relatively large beam antennas and a rotator for positioning that need to be raised in a portable setup by one person. Sounds like a lot of work. To add to it I am setting up on a slab of granite that slopes fairly sharply down then drops off in to the lake. It doesn't sound like a very good place to set this up. Its not. Click on the picture to and enlarge it.See where the guy lines go? Right down to the edge of the water. A couple years ago I went for a swim setting up.
The first problem in my mast set up was to figure out how to make my upper mast rotatable and mast mountable. For one antenna this is a piece of cake - you stick about 2' of mast in the top of the rotator, bolt a mast adapter to the bottom of the rotator, and stick it up as high as you want to go on a main support mast. Done. Maybe some guy lines for added support, if needed.
This is all fine and dandy for 1 antenna. However, when you stack antennas you have to consider the distance between those antennas. Every antenna interacts with every other antenna in close proximity. To minimize loss of performance you need to separate these antennas. In portable setups, the general rule of thumb is to separate the antennas by as much as you possibly can. Getting to the edge of interaction is probably impractical. Since there is only one direction to go, up, that means the antennas need to be separated by stacking one on top of the other going up the rotating upper mast.
Back to the 1 antenna on the rotator. The distance between the antenna and rotator is not that much of an issue, however as the frequency goes up the image of the rotator increases thus the distance (in wavelengths) between the antenna and rotator increases. Theoretically, you could mount a 50mHz beam right to the top of the rotator (no spacing) and it would be fine (there would be interaction, but not too much). The image the roator shows to the antenna is small. There would need to be much more space (in wavelengths) at 432mHz between the rotator and antenna because the image of the rotator is MUCH larger to the smaller frequency, it is almost 1/2wl wide. However, this spacing would only be roughly 1/2wl - or a tad over 1'. At this distance the force on the upper mast on the rotator when the wind blows would be very minimal. The rotator would have no trouble holding all of this.
To bring this all back to my setup: I have 3 antennas. The upper mast I have is roughly 18' (it is a section telescoping mast and fits in my 6' truck bed). To get the most performence out of my antennas I need to space them out as much as I can on the mast. The further from the rotator I go the more the wind loading of the antenna stresses the rotator. 18' of upper mast is WAYYYYY too much for the mast clamp to hold. It would snap off like a 100lb weight would do to a broom stick if it was only held at the base. The solution to this wind loading problem is the force applied to the mast clamp on the rotator must be RE-DIRECTED. The amount of force can not be changed, it is what it is. However, the direction that the force acts on the rotator CAN be changed. Currently, the force on the clamp is a vertical snapping force. Hold a pencil straight up by the eraser and push the lead tip over - see how hard it is to hold back the pencil from leaning over. You cant do it. Its a lever - and a lever with a TREMENDOUS amount of mechanical advantage. Instead of a snapping force, why don't I change the force to a horizontal shove so the mast clamp is holding the mast in place rather than the mast trying to break off the clamp? Can I do that? With therotator by itself, no I can not. I need something else. I need to support the upper mast at some point above the rotator. The higher this point is the more mechanical advantage the BOTTOM of the mast has to HOLD BACK the wind loading of the antennas above.
So now my problem has become how to support the upper mast at a point above the rotator to change the force applied to the mast clamp from a vertical snapping force to a horizontal shove. What this calls for is to mount therotator inside of something rather than on top of the main support mast. In permanent radio antenna installations the rotators are mounted inside of a tower. The upper mast goes from the mast clamp on top of the rotator, through a "thrust bearing" at the top of the tower, and up to the antennas. My rotator is mounted to a mast, not inside of a tower. What I need is a bracket to mount the rotator in that also supports an upper mast at a point higher than the rotator.
My solution does just this. If you look at the close-up picture (from 07 Sept.ARRL VHF contest) you can see the black bracket the rotator is bolted to. The mast is supported high enough to where the force on the rotator is redirected, yet isn't close enough to where the mast would just kink over. I did not do any calculations on this to show where and how much force is applied, I just guessed at it. For a portable system it works fine.
The next task in my mast evolution is figuring out how to raise this. I would estimate the weight of the rotator/antenna system to be 100-120lbs. Considering this has to sit at the top of a mast, that's a LOT of weight.
My first idea was to use a pulley system and tilt up the mast. Needless to say, this was a terrible idea. The lower mast bent too easily with the weight of the antennas and was very unstable once off the ground. It wanted to swing all over the place. At first the idea sounded like it might work, but in practice it did not.
The next idea was to set up the lower mast on the ground real short. Then mount the antenna system to it. Then raise the mast and antenna system and put mast sections underneath to build the height. Well, with over 100lbs of dead weight flying above your head this was NOT a smart idea, let alone the difficulty in holding on to a pipe less than 2"OD with that weight wanting to shove it in to the ground.
What I needed was a way to erect the main mast entirely before the antenna system. This required the antenna system to be on the ground while the mast went up first. To do this, I built a "tram". The rotator bracket mounts to a pipe that slides on the outside of the mast. With the help of the pulley blocks from the tilting method the antenna system is hoisted right up the mast.
This sounds simple. Well, its not THAT easy. As with the other ideas, there are problems. The biggest one is the wind loading and weight of the antenna system on the mast. The mast is not nearly strong enough by itself to hold the antennas in the wind. Maybe 1 or 2 sections could, but not up at 25'. Its a giant noodle - with over 100lbs of dead weight wanting to bend it back down to the ground. What it needs are guy lines.
So, I have a tram for the antenna system that needs to be guyed. The guy lines do one thing in two ways: They keep the mast standing by 1:transferring the force of the wind pushing on the antenna system to the guy points and 2: also sending that force down the mast in to the ground. Problem: the guy lines will prevent the tram from going up. So, the tram must be raised in increments. Each time the tram goes up I must loosen the guy lines and then raise some more. Its a cycle. Not fun, but it works. Now the antennas have a way of getting to the top of the mast and able to be rotated. Since the mast cant move at the top from the guy lines, I'm off to the races!
Not so fast. The antennas must be able to rotate, and the position must be calibrated so I know what compass heading my signals are going. The rotator solves the issue of rotating the antennas, but how do I hold that position? With guy lines attached to the tram the force of the wind could spin the whole thing around and tighten up the guy lines even more, like the reel on a winch, causing failure in one of the lines. This is a volitile system. A failure in ONE of the lines is a collapse of the mast - its that simple. That's a big safety hazard, not just for people but for the cabin. Imagine 100-some lbs worth of metal falling from 20 some feet on to the roof... or the raised deck...
What I need is to prevent the antenna system from winding up the guy lines - causing the loss of my heading first, then the failure of the whole system. To do this, the guy points on the tram must be spaced away from the tram and held rigidly in place. Having the guy points away from the tram increases the mechanical advantage the guy lines have on centering the mast. On commercial guyed towers they have a star mount. What this does is exactly what I described I needed - its a bracket that places the guy points out away from the tower. However, the star mounts on most commercial towers have twice as many guy lines attached - 6 rather than 3, 2 per face. Since my system is not a permanent installation I should be fine with 3.
My star mount is made from 5/8" square tubing with a flat 1/8"attachment point in each corner for my shackles (see side photo, the angle supports on the tram point down when erected). The star mount is small enough to fit in the truck bed under my fiberglass cap (the low one, not the tall one with the windows) yet as big as I could make it to maximize the mechanical advantage for the guy lines to hold the mast in the right position.
This past summer (07) I was able to get this system up entirely by myself. It was a challenge. However, it went up in a controlled environment and worked flawlessly throughout the week I operated. The bigger challenge was getting this down. I ended up using the help of one other person because I forgot to undo a redundant guy line I attached and it left me holding on to the system on a ladder and I couldn't let go hehehe. So, there are still bugs that need to be worked out but a working system is in place.
To start with the theory behind my current mast system Ill say this: I am pretty much a one-man-band when it comes to setting up this stuff. That's 3 relatively large beam antennas and a rotator for positioning that need to be raised in a portable setup by one person. Sounds like a lot of work. To add to it I am setting up on a slab of granite that slopes fairly sharply down then drops off in to the lake. It doesn't sound like a very good place to set this up. Its not. Click on the picture to and enlarge it.See where the guy lines go? Right down to the edge of the water. A couple years ago I went for a swim setting up.
The first problem in my mast set up was to figure out how to make my upper mast rotatable and mast mountable. For one antenna this is a piece of cake - you stick about 2' of mast in the top of the rotator, bolt a mast adapter to the bottom of the rotator, and stick it up as high as you want to go on a main support mast. Done. Maybe some guy lines for added support, if needed.
This is all fine and dandy for 1 antenna. However, when you stack antennas you have to consider the distance between those antennas. Every antenna interacts with every other antenna in close proximity. To minimize loss of performance you need to separate these antennas. In portable setups, the general rule of thumb is to separate the antennas by as much as you possibly can. Getting to the edge of interaction is probably impractical. Since there is only one direction to go, up, that means the antennas need to be separated by stacking one on top of the other going up the rotating upper mast.
Back to the 1 antenna on the rotator. The distance between the antenna and rotator is not that much of an issue, however as the frequency goes up the image of the rotator increases thus the distance (in wavelengths) between the antenna and rotator increases. Theoretically, you could mount a 50mHz beam right to the top of the rotator (no spacing) and it would be fine (there would be interaction, but not too much). The image the roator shows to the antenna is small. There would need to be much more space (in wavelengths) at 432mHz between the rotator and antenna because the image of the rotator is MUCH larger to the smaller frequency, it is almost 1/2wl wide. However, this spacing would only be roughly 1/2wl - or a tad over 1'. At this distance the force on the upper mast on the rotator when the wind blows would be very minimal. The rotator would have no trouble holding all of this.
To bring this all back to my setup: I have 3 antennas. The upper mast I have is roughly 18' (it is a section telescoping mast and fits in my 6' truck bed). To get the most performence out of my antennas I need to space them out as much as I can on the mast. The further from the rotator I go the more the wind loading of the antenna stresses the rotator. 18' of upper mast is WAYYYYY too much for the mast clamp to hold. It would snap off like a 100lb weight would do to a broom stick if it was only held at the base. The solution to this wind loading problem is the force applied to the mast clamp on the rotator must be RE-DIRECTED. The amount of force can not be changed, it is what it is. However, the direction that the force acts on the rotator CAN be changed. Currently, the force on the clamp is a vertical snapping force. Hold a pencil straight up by the eraser and push the lead tip over - see how hard it is to hold back the pencil from leaning over. You cant do it. Its a lever - and a lever with a TREMENDOUS amount of mechanical advantage. Instead of a snapping force, why don't I change the force to a horizontal shove so the mast clamp is holding the mast in place rather than the mast trying to break off the clamp? Can I do that? With therotator by itself, no I can not. I need something else. I need to support the upper mast at some point above the rotator. The higher this point is the more mechanical advantage the BOTTOM of the mast has to HOLD BACK the wind loading of the antennas above.
So now my problem has become how to support the upper mast at a point above the rotator to change the force applied to the mast clamp from a vertical snapping force to a horizontal shove. What this calls for is to mount therotator inside of something rather than on top of the main support mast. In permanent radio antenna installations the rotators are mounted inside of a tower. The upper mast goes from the mast clamp on top of the rotator, through a "thrust bearing" at the top of the tower, and up to the antennas. My rotator is mounted to a mast, not inside of a tower. What I need is a bracket to mount the rotator in that also supports an upper mast at a point higher than the rotator.
My solution does just this. If you look at the close-up picture (from 07 Sept.ARRL VHF contest) you can see the black bracket the rotator is bolted to. The mast is supported high enough to where the force on the rotator is redirected, yet isn't close enough to where the mast would just kink over. I did not do any calculations on this to show where and how much force is applied, I just guessed at it. For a portable system it works fine.The next task in my mast evolution is figuring out how to raise this. I would estimate the weight of the rotator/antenna system to be 100-120lbs. Considering this has to sit at the top of a mast, that's a LOT of weight.
My first idea was to use a pulley system and tilt up the mast. Needless to say, this was a terrible idea. The lower mast bent too easily with the weight of the antennas and was very unstable once off the ground. It wanted to swing all over the place. At first the idea sounded like it might work, but in practice it did not.
The next idea was to set up the lower mast on the ground real short. Then mount the antenna system to it. Then raise the mast and antenna system and put mast sections underneath to build the height. Well, with over 100lbs of dead weight flying above your head this was NOT a smart idea, let alone the difficulty in holding on to a pipe less than 2"OD with that weight wanting to shove it in to the ground.
What I needed was a way to erect the main mast entirely before the antenna system. This required the antenna system to be on the ground while the mast went up first. To do this, I built a "tram". The rotator bracket mounts to a pipe that slides on the outside of the mast. With the help of the pulley blocks from the tilting method the antenna system is hoisted right up the mast.
This sounds simple. Well, its not THAT easy. As with the other ideas, there are problems. The biggest one is the wind loading and weight of the antenna system on the mast. The mast is not nearly strong enough by itself to hold the antennas in the wind. Maybe 1 or 2 sections could, but not up at 25'. Its a giant noodle - with over 100lbs of dead weight wanting to bend it back down to the ground. What it needs are guy lines.
So, I have a tram for the antenna system that needs to be guyed. The guy lines do one thing in two ways: They keep the mast standing by 1:transferring the force of the wind pushing on the antenna system to the guy points and 2: also sending that force down the mast in to the ground. Problem: the guy lines will prevent the tram from going up. So, the tram must be raised in increments. Each time the tram goes up I must loosen the guy lines and then raise some more. Its a cycle. Not fun, but it works. Now the antennas have a way of getting to the top of the mast and able to be rotated. Since the mast cant move at the top from the guy lines, I'm off to the races!
Not so fast. The antennas must be able to rotate, and the position must be calibrated so I know what compass heading my signals are going. The rotator solves the issue of rotating the antennas, but how do I hold that position? With guy lines attached to the tram the force of the wind could spin the whole thing around and tighten up the guy lines even more, like the reel on a winch, causing failure in one of the lines. This is a volitile system. A failure in ONE of the lines is a collapse of the mast - its that simple. That's a big safety hazard, not just for people but for the cabin. Imagine 100-some lbs worth of metal falling from 20 some feet on to the roof... or the raised deck...
What I need is to prevent the antenna system from winding up the guy lines - causing the loss of my heading first, then the failure of the whole system. To do this, the guy points on the tram must be spaced away from the tram and held rigidly in place. Having the guy points away from the tram increases the mechanical advantage the guy lines have on centering the mast. On commercial guyed towers they have a star mount. What this does is exactly what I described I needed - its a bracket that places the guy points out away from the tower. However, the star mounts on most commercial towers have twice as many guy lines attached - 6 rather than 3, 2 per face. Since my system is not a permanent installation I should be fine with 3.
My star mount is made from 5/8" square tubing with a flat 1/8"attachment point in each corner for my shackles (see side photo, the angle supports on the tram point down when erected). The star mount is small enough to fit in the truck bed under my fiberglass cap (the low one, not the tall one with the windows) yet as big as I could make it to maximize the mechanical advantage for the guy lines to hold the mast in the right position.This past summer (07) I was able to get this system up entirely by myself. It was a challenge. However, it went up in a controlled environment and worked flawlessly throughout the week I operated. The bigger challenge was getting this down. I ended up using the help of one other person because I forgot to undo a redundant guy line I attached and it left me holding on to the system on a ladder and I couldn't let go hehehe. So, there are still bugs that need to be worked out but a working system is in place.
Labels: 2. Ham Radio, Portable Rotatable Mast
posted by Steve at
8:12 PM
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