Saturday, December 10, 2016

Military Masts, Part 3, Bases

Bases can be home made or modified military surplus. An AB-154/U swivel base from an AB-155/*/U mast set works fine and the mast can fit more snugly by
 covering the base with 4 or 5 evenly spaced #216 o-rings. Another base that fits the masts is the OE-254? mast swivel base. This base has 2 diameters. The top part fits into the mast, but I didn't like the small outer diameter of the bottom half. The first time I used the base I placed a spare guy plate over it for a larger area for the end of the mast to rest upon. After that I modified the base by turning the O.D. of the bottom part to match the I.D. of the mast. If you do not have a lathe and machining ability the washer works better than just the thin O.D. of the bottom half of the base. The concern is that with a small contact area the downward pressure could split a fiberglass mast.
Both of the above mentioned bases are available at surplus dealers at times and also on Ebay. The cost runs between $10.00 and $30.00 depending on the base and condition.
Some bases I made. AB-154/U with o-rings to use with the 1080 poles
Tent Stake with a ring to protect rope
I use a 1.5 inch diameter welded ring from McMaster-Carr slipped over a tent stake before driving the stake into the ground to protect the rope from chafing from the rough edges of the tent stake. I use a bowline or a ring hitch to secure the rope to the ring.
Here are 2 homebrew bases made to fit the 1080 series fiberglass or aluminum camouflage support poles when used for an antenna mast.
The bottom is an OE-254 antenna mast swivel spike base with the larger diameter bottom machined to fit the 1080 camouflage support system masts.
The OE-254 base top half will fit into the 1080 series poles, but the lower diameter is only a few thousandths of an inch larger. For a more secure installation I machined the bottom half to just make a nice slide fit into the pole allowing the pole and rest of the mast to rest on the larger base of the swivel.

Tripod Base:
This is an adapter I made for deploying the mast. It is made from pipe so it is heavy. I would have used aluminum if it were available. The ends were machined from round stock that was bored to clear the outside diameter of the mast for a sliding fit. The leg receiver pipes were cut from standard pipe, miter cut to 45 deg. Stability is great, but I should have used a shallower angle. 45 degrees puts too much stress on the legs when using 2 legs for more height. The entire assembly is painted with Krylon camo paint.
The angle of the legs would work better at 22.5 degrees to make 45 degree leg angles rather than the 45 degrees that make a leg angle of 90 degrees.
The base requires adapting a short piece of mast to create the downward force to stabilize the mast.This was done by taking a damaged section and cutting the top part, securing it with a shaft collar and painting the entire section with Krylon green camo paint. It is the closest to olive drab I could find.
Without this stub the mast will rest on the ground and it will not be stable in wind.

Thie is the base with one section and only one section used per leg. The mast is more stable using 2 sections of mast per leg. If the adapter pipe or a swivel-stake base is not used the mast can still be unstable even if guyed. We get quite a few unexpected thunder storms in Florida and the mast has been deployed with 2 sections per leg and the mast through the center resting on the ground without a swivel-stake base and it will rock quite a bit in the tripod adapter that it seem unstable enough to fall. It is much more stable with the center adapter section or a base stake. I do not know how stable this would be on a hard surface in the wind. On a hard surface without wind it seems stable, but I do not trust un-anchored or un-guyed masts even for only a simple vertical on top. I would not try any kind of wire antenna such as a dipole or random wire or even a G5RV even in an inverted-v configuration if the mast cannot be guyed.
There are advantages and disadvantages of using the mast with the tripod. With the mast to the ground the vertical force is to one point on the ground and the legs provide the horizontal support and distribute the horizontal forces of the wind and antenna on the leg or legs. Using the mast adapter section distributes the downward force to the three legs forming the tripod. The legs take all the forces. Guying in both situations will keep the mast stable. If the mast goes through the tripod to a ground stake base the pivotal force that could take place is canceled by the stake base. This is most likely to happen if only one mast section per leg is used. In other words if only one mast section is used in each leg the mast is not as stable in the wind and the if mast extends through to the ground the mast will kick out like a ladder improperly used and the mast will topple -- unless properly guyed. We've had this problem at our club.
There is also a tripod adapter for the AB-155/U base that will be posted with the AB-155/U page.

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