Since I started dabbling in CB again, I wanted a rugged, easy to store, easy to deploy base antenna that could reach out 10+ miles. Cheap and DIY would be a pluses too. A wire dipole could work, and there are several plans for these floating around on the net.
I built up this one: http://advancedsurvivalguide.com/2010/09/01/diy-build-a-portable-cb-base-antenna/
It worked, but I wasn’t entirely happy with it. The connection between the braided shield wire and the ground leg was flimsy, adjustment at the connected ends made SWR tuning a pain, and when not strung up it’s a tangle-matic. It does function though and the price is right, but there was a lot of room for improvement.
Improvements... Being able to detach the coax feed line at a standard PL-259/SO-239 plug-and-socket would make setting up and stowing the thing easier, and make the junction between it and the dipole legs stronger. Tuning the SWR by trimming the ends of the antenna would be easier than pulling it apart and re-crimping every step of the way. And having some kind of storage for this rat’s nest of wires would help.
So here are the upgrades: mount an SO-239 on a plastic jar lid, and build in strain relief for the dipole legs so that they don’t yank loose from the connector. Make the dipole legs a little longer than the standard quarter-wave 102” to give room to trim (105” is about right), and attach a paracord loop near each end to provide support independently of the ends that’ll need trimming. Rig it so that the dipole legs can be stuffed into the matching jar for storage. I used a 40oz plastic peanut butter jar, that’s what I had handy, but anything similar will work. The lid is 3 1/2” wide, much smaller than that would be hard to work with, but the 40oz jar itself is a little large.
SO-239 chassis mount UHF connectors are about $1–$2 online.
20’ of 12 gauge hook-up wire was about $5. Lighter 14 gauge will work as well.
50’ of coax with CB-standard PL-259 connectors on each end. $20-$30.
Four 6-32 x 3/8” machine screws with nuts.
Matching #6 washers & lock washers are optional, but will make things stronger.
One eye lug that’ll crimp onto the lower dipole wire.
2 18” lengths of paracord.
For tools, all that’s needed are the basics. Screwdriver, pliers, knife, the usual stuff. Plus a soldering iron & solder (just one connection to make, to the center pin on the connector) and a 5/8” paddle bit ($4), to put a neat hole for the connector into the jar lid. Will need a couple of smaller drill bits too, for the #6 screw holes in the lid, and another slightly larger than the antenna wires.
The only dimensions that are important are a 5/8” hole for the SO-239 connector and having the wire dipole legs originally cut to slightly longer than 102”. The rest can be made to “about right, yeah that works” tolerances.
The first two pictures show how the connector is installed in the lid, and how the dipole legs are threaded through their holes to provide strain relief when this thing is hung up. The only trick to drilling for the connector is to drill the main 5/8” hole first, put the connector in and drill one of the screw holes, put in that one screw to hold things steady, then drill the other 3 holes. This keeps the remaining screws pretty well aligned. A small drill press is best, but a hand drill will do almost as well.
The third picture shows how paracord is attached to the ends of the dipole legs with zip-ties. Three hold it all together, while a fourth zip-tie marks where 102” is exactly. Notice how the wire’s end can be trimmed to adjust the SWR without affecting how how the thing hangs up.
The last picture shows the completed antenna, tucked in for storage. Don’t try to transmit with the wires all coiled up in the jar, it could fry your radio.
There are many ways to hang a dipole. Sometimes they’re use horizontally at a half-wavelength above the ground for trying to shoot skip. Here’s some info on doing it that way: http://www.ventenna.com/files/Rad-Pattern.pdf
Then there are various “L” and “inverted V” configurations that give you some directionality, http://www.tscm.com/radiapat.pdf
gives an overview. There’s a Marine field manual at http://www.everyspec.com/USMC/MCRP_6-22D_14371/
that has tons more on this, almost too much.
To get started I just wanted a omnidirectional pattern, so I hanged the driven leg (the wire connected to the center coax conductor) from a tree limb, and stretched it up vertically. Some paracord and a tent peg on the ground end kept it straight and tight. Run the coax in as nearly perpendicular to the antenna as possible for at least 10’ or so to keep the signal from leaking back up the feed line. Another length of paracord and another tree limb kept the coax properly placed. I’d post pics, but the thing is almost invisible up in my oak tree.
It’s not really weathertight, but a plastic cone hat over the connectors could help a lot. Will add one later.
About tuning, you’ll need an SWR meter. Some radios have them built-in, which is what I used. Hung up as described, I chopped the ends 1/2” at a time until the SWR reading got down to 1.5 and 1.7 on Ch. 1 and 40 respectively. Good enough. Cut the same amount off each leg at each step as you tune. Mine came out to just over 102”. If you’re operating in other frequency bands, you’re probably already a ham operator and know how to calculate the right length.
The antenna works pretty well for local comms. Just after dark last Sunday evening, a friend and I with identical setups (this antenna with the bottom of the ground leg 4’ off the ground, box-stock Cobra 148 GTL’s, oak trees for support) were easily talking to each other at 10 miles using SSB mode. Rolled up and stashed inside, it stays safe from high winds and is simple to put up after a hurricane passes. Local communications problem solved, on to the next project.