Unless your tower needs are very simple, it’s likely that you will want more than one “thing” up in the air – be they mesh nodes; cameras; dishes, lights, antenna wires or standing antennas. To have the room to do all this, you may want to add a crossbar on the tower, at the place where its support is strong and yet the position is high enough above the ground for your purposes. I’ve used 1″ aluminum tubing but other materials also work. Make sure that whatever you put up there is sturdy and not coming down, or bouncing off on the freeway as the case may be. Stainless steel screws and hardware are best.
I’ve found that the length of the bar you can put in is limited mostly by whether its support mechanism is vertical or horizontal while the trailer is driving down the road. Horizontal bars must be limited to the width covered by the trailer, typically; while vertical bars can be longer as long as there is road clearance and height clearance. On my smaller trailer, the bar had to be short and horizontal because of the need of the rotational hinge on one site to rotate. I added a PL259 barrel adapter and a secondary, smaller antenna.
One of the towers proposed use cases requires two separate two meter antennas. This small antenna fits the bill for the “easy” close repeaters needed; while a higher gain primary antenna will be used for the more remote use cases. To keep things simple, I decided it was easier to wire two antennas than keep a duplexer aligned as it bounces down the road.
On the other tower trailer the mount permits a vertical alignment, which opens up more possibilities. More is not necessarily always better; as one still has to contend with the wind load and road-worthiness of the items attached to the bar. On my trailers for each I have actually bolted on the high-visibility flag so that one never can forget to (re)install them before rolling out. They naturally fall to the most extreme protruding part of the trailer, where other drivers can see them. Also, when the tower is set up they are visible on the mast and clearly mark the trailers location at a site.
With the second trailer we have much more room on the bar. One thing that will take up more space on this bar is a wire descent loop. This trailer does not have an effective way to keep the wire away from the heavy antenna mast sections as they collapse today (as evident by the original coax being utterly destroyed when I bought the trailer). Each section of the metal mast is a natural born wire-cutter for coax. I’m hoping that routing things through a u-bolt a foot away from the boom (with some Velcro for support) will keep the cables from being destroyed.
On the trailers that do have decent loops for routing the wire, there can still be the problem of having more wires to deal with than these loops can carry. So this might be something I have to revisit on both trailers – we’ll see how it goes.