I bury cables every time it is practical. This includes both transmission lines and control lines. I have thousands of feet of buried cable. This keeps cables safe from farm machinery, yard work, antenna maintenance, most critters (including human!) and other mishaps. Of course the cables must be suitable for direct burial. I do not use conduit.
There are several places where I run cables overground. For the Beverage system in particular there are long runs of RG6 that lie directly on the ground. I can get away with that in the bush where no one goes but me. Tree roots and other obstructions can be major impediments to burial, but it is convenient to go overground for long runs where it is not unsightly and the risk of damage is low.
The most important overhead cable run is from the edge of the hay field adjacent to the house yard to the Trylon tower where most of the switching and cable interconnections is located. The overhead run was a temporary measure until I had time to bury them. Since then I have come to realize that overhead is better. Maintenance and changes are easier. The mat of tree roots a few inches below grade make burial a formidable challenge.
The original overhead run was not sufficiently robust to survive for many years. Construction was admittedly rushed to leave more time for major tower and antenna projects. This year that had to change. Sagging and anchor woes coupled with the addition of more cables spurred me to act. The first step was to replace the ground anchor with one that is better able to support the messenger cable and the weight of the many cables. Once the concrete cured I got to work.
It is helpful to describe the problem and the underlying theory before diving into its construction. Let's do that using the following diagram.
The messenger cable is anchored to the tower and the ground. The post outside the stone wall keeps it at a safe height to walk beneath. Tension is needed to resist sag due the weight of the messenger cable and the cables it supports. The greater the tension the less the sag. This is a critical design parameter since there is stress due to tension on the tower and on the ground anchor.
This brings us to the catenary. It is a common subject in the ham literature since our antennas bend and sag due to wind and gravity, affecting performance and survival. Catenaries are more discussed with respect to wire antennas where the effect is particularly noticable. The physics and mathematics of catenaries is deceptively complex, and that is annoying when we want a quick answer to what appears to be a simple problem.
My "simple" catenary question: how much tension is required to have no more than 2' of sag in a 60' horizontal messenger cable and its cable load? Finding the equations that provide a reasonably accurate answer wasn't quick. I found the answer first by measuring the tension and the sag. The required tension is more than I can tolerate for the constraints I am faced with.
Before diving into the mathematics I'll describe my practical approach. I ran the new ¼" EHS messenger cable above the existing overhead run, anchoring it to the tower with galvanized chain, to a replacement 10' steel post, and down to the recently installed ground anchor. I used several rubber tie-down straps to tie the cable bundle to the new messenger cable. A come-along at the tower anchor was used to increase the tension and measure the sag. As tension increased, the old messenger cable went slack and the cable load was taken up by the new messenger cable.
The old ⅛" aircraft cable messenger and aluminum post remain tied to the cable bundle. Had there been a mishap it would be there to retake the load. The old cable was temporarily transferred to the new ground anchor.
At right you can see a yellow rope that runs from the old post to the 150' tower 100' behind the photographer. I did that when the screw anchor failed. There is a steel peg under the post that provides good lateral support, but that is not reliable enough on its own.
That peg gave me the time I needed to install the rope support. Whenever possible never have a single point of failure in a critical system.
My Loos cable tension gauge was mounted on the new messenger cable to monitor the tension during construction. I did not do this for the original cable, relying on "feel" to keep the tension within reason.
To meet my sag target on the original system I had to install two intermediate 8' posts. They divide the span into 3 approximately equal sections which greatly reduced the sag for the amount of tension I was comfortable putting on the messenger cable, post and anchors. My hope was to eliminate the intermediate posts with the improved messenger cable. They can be a nuisance.
The top picture immediately above shows detail of the old and new posts during the project. The one on the bottom shows the Trylon anchors, old and new, with grips for the messenger cable and for attachment of the come-along. Look closely and you'll see that that plated chain for the old tower anchor is rusted.
Galvanized hardware is more durable than plated hardware. This is especially important on smaller hardware since there is less base metal. I recommend spraying metal (rust) or cold galvanizing paint on plated hardware every couple of years. Galvanized hardware will eventually wear and also need to be painted.
When I increased tension to 700 lb tension (measured tension includes the weight of the cables) there was still too much sag. I couldn't safely walk beneath the cable. The cable and ground anchor can handle that tension but I am not so sure about the Trylon tower. Increasing the tension was out of the question since it rose alarmingly for little reduction in the sag. I lowered the tension and hit the internet to dig deeper into catenary mathematics.
Catenary theory is fascinating. For a mathematically minded person like myself it is easy to slip into the depths. Easy but unwise since I had an immediate problem to solve. Much of the material I encountered was far too general and required effort to simplify the equations to fit my project. More digging was required before I found what I needed.
There are many resources on the internet and I am sure there are better ones that what I found during the time I spent on researching the question. There are also spreadsheets to do the calculations Here is the link to the page that I found most helpful. It isn't perfect but it gets to the heart of the matter: the relationship between sag, tension and weight for a uniform horizontal catenary.
S = (WL²)/(8H)
S is the sag, W is the unit weight of the catenary (including the cable) and H is the tension. Any units can be used but they must be used consistently. I'll use English units in this discussion: feet and pounds. The only difficult variable to determine is W, which is the weight of the messenger cable per foot. From the known weights of the EHS, Heliax and heavy control lines that comprise a large majority of the weight, W is estimated to be 5 lb/ft. That comes to 300 lb for the 60' span.
For a tension of 700 lb the calculated sag is 3' at the midpoint of the 60' span. Since the ends are about 9.5' above ground and the bottom of the cable bundle hangs about 6" below the messenger cable, the ground clearance at the midpoint is 6'. This is surprisingly close to what I measured; it brushed the top of my head. Either I did the calculation correctly or I was lucky.
As I said, 700 lb of tension is too high, so the sag must increase. That is unacceptable. Even were higher tension permissible, the required tension rises rapidly. Notice that sag is inversely proportional to the tension. For example, to halve the sag to 1.5' (which would meet my objective) the tension is 1400 lb.
I concluded that the intermediate posts must remain. That makes 3 spans of 20' each. If I cut the tension in half to 350 lb the calculated sag for the shorter span is less than 1'. Since the posts are 8' high the bottom of the cable bundle at each of span midpoints should be almost 7'. And, indeed, that is what I measured. I think I'm getting the hang of catenaries!
In this picture the tension is less than 300 lb. Lots of rubber straps have been installed to hang the cable bundle; more were added after the picture was taken. I didn't use them for the old messenger cable, opting for plastic cable ties each time cables were added to the bundle. Although I no longer need the ties they might as well be left there until cables need to be removed. It's messy but not a problem that needs fixing.
Notice that straps are not placed close to the posts. When I tried, the cables were pressed hard against the post. It is possibly that could damage the more fragile cables over the coming years. By the time I was done there were about 15 rubber straps spaced an average of 4' apart. They're cheap and effective. I've heard that the rubber is prone to UV damage but the ones I've had outdoors for a few years do not show signs of deterioration. They'll be inspected at regular intervals.
There is one last item to discuss: lateral stability. The overhead run present a large surface area for the wind to press against. For a 60' run witha 4" cylindrical cross section, the total projected surface is 20 ft². That's a lot! For our wind zone it is recommended to build for a wind speed of 135 kph (85 mph). At that speed the wind load is 20 lb/ft², which adds up to a total force of 400 lb on the overhead run.
The actual force will be much lower. The overhead run is close to the ground and there are many obstacles that block wind that is orthogonal to the cables. The overhead run does lean to the leeward in high winds but not enough to be at risk.
Snow and ice increase the unit weight but, again, it hasn't proven to be a problem. What is a problem is snow on the ground which reduces walking clearance by a foot or more in late winter. I am not too concerned by that since few people other than myself are likely to be walking there when that happens.
There are a few more steps left to complete the project. Now that the strength and reliability of the new structure is confirmed, the old messenger cable, post and tower anchor will be removed. After that is done, the new post will be fitted to the ground peg the old post still occupies. The new post leans a bit since it had to be placed several inches from the old post. I should be done by Christmas.
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