No need for the screen - four radials of #14 wire are quite adequate for an elevated mount, are lighter and
cheaper, and can be bent to fit in the truck as needed.
If you have a reasonably clear line of sight to the repeater it doesn't take much of an antenna: I'm using
an omni with 3dB gain over a dipole and hit repeaters 100 miles away. On 2 watts or less. Of course,
range is more limited when there are intervening hills.
My usual recommendations for those planning to provide portable 2m communications are:
(1) portable mast 12' to 20' feet tall that breaks down to a convenient length for you to carry. Some
folks use telescoping painter's poles or pool cleaning poles (from Lowe's?). I've got telescoping
squeegee pole that is nearly 12', and a bundle of aluminum tent poles that I found at Goodwill that
I can combine in various ways to get to 15'. (I've also got a bunch of the 4' military mast sections -
they are great for Field Day and portable antennas, but a bit heavier and bulkier than some of the
other options.)
(2) ground plane or equivalent 1/2 wave antenna (dipole, J-pole, etc.) Yes, you can get omni antennas
with more gain than this, but you still have to get them up in the air to take advantage of it. For example
here is a simple ground plane antenna you can build:
http://www.hamuniverse.com/2metergp.html(I use different dimensions and connect the radial wires to the coax connector differently, but the
principle is the same. With brazing rod (stiffer) for the vertical part and #14 copper house wire for the
radials, the whole thing takes virtually no space to store if I bend the radials down flat.) Some folks
prefer J-poles, either the roll-up type, copper pipe, or whatever. (You could use screw-on elements
to allow the J-pole to collapse into a 20" bag for transport, or arrange for the radiator and radials
of a ground plane antenna to screw on.)
(3) sufficient coax - you can use RG-58 up to 25' or so, but RG-8X is better, and RG-213 is the best
choice for longer distances. (I would normally carry about 25' of coax for my portable antenna, with
a barrel connector and 50' of RG-213 (or more) in case I have to place the antenna further from the
radio.)
(4) for difficult paths, some sort of directional gain antenna. A reasonable 3-element yagi will have about
6dB gain over a dipole, equivalent to 4 times the power. A well-designed 2-element quad is 1dB less.
A boom length less than 5' will give you a couple more dB. I use aluminum ground wire (from Radio Shack)
stuck through holes in a 3/4" PVC boom and the antennas are very light. For transport I pull out the
elements and slip them inside the boom. Depending on the length of your threaded cleaning rods you may
be able to put a threaded rod through the boom (with a couple nuts to hold it in place) and screw the
elements onto each end of it. I'm experimenting with some fibreglass rod with similar threaded ends and
wire elements running along it.
Many of my 2m yagis are made using measuring tape for the elements, but then I have a particular
application: I use them for radio direction-finding while running through the woods. The elements bend
when they hit a branch, then snap back into place. A 3-element version is less than a pound using
1/2" thinwall PVC and fittings. I have another 3-element 2m antenna using copper wire stuck through
a wood boom and it is 8 ounces - the elements fold down against the boom by simply bending the
wires, then straightening them when I use it.
But the limit on your support won't be weight as much as it will be wind loading: you can make a very
light 6-element using aluminum wire elements, but the force of the wind blowing against it will cause
more problems with the tripod than the weight itself.
As a practical note, I find some way for all of my antennas and masts to mate with 3/4 PVC slip connectors.
It might be a piece of PVC that slips over the top of the mast, or is taped to the bottom of the antenna,
but that gives me a common mounting interface so any antenna can be mounted on any mast if needed.
Also, for masts, I find that the easiest way to put them up is from the BOTTOM. That is, I put the
antenna on the top section, raise it up in the air, and slip the next mast section on underneath. With
a telescoping mast I push up the sections one at a time starting with the TOP, while holding the mast
and antenna vertical. This is much easier than trying to assemble the antenna on a complete mast
and tilting it up into place. (I was going to put the HF triband yagi up by myself this way for Field
Day, but someone insisted on helping. However the two of us didn't have any problems, or even
have to work hard to get the antenna up to 24'.)
So my suggestion would be to start with a ground plane and see how it works over the paths that
you want to cover. Remember that height makes the biggest difference for signal strength, so even
you might try some temporary masts (perhaps wood or PVC pipe) to see if that works any better
than what you have now. The next step might be to try a yagi: here is a simple construction method
that is easy and inexpensive to build and works well:
http://wa5vjb.com/yagi-pdf/cheapyagi.pdf(we can adjust the element lengths a bit if you are going to be operating near the top end of the band.)
Once you find what type of antenna meets your needs, we can look at how to build a more durable
version using the materials you have handy.
