Vertical Dipole Antenna
I was looking to build an easy to
breakdown 2 meter antenna for an event I was helping with. I have issues
with J-poles, and while easy to build, I think they interact with their
feedlines way to much. 1/4 wave ground planes work well, but will have
half the signal of a dipole. I was going to be in a canyon area, and I
actually didn't want a flattened pattern that most commercial antennas
give. The "dipole doughnut" was what I needed. The following is what I
came up with and has worked very nicely.
To start, a few warnings!!!
We're going to solder copper pipe with a torch. If you don't know how to do this, you may want to practice on scrap and check out some online videos of soldering copper pipe. We're also going to be using PVC pipe cement which is nasty on a couple of levels, beside being highly flammable (so make sure your torch is OUT before opening the cement). Be prepared to work with plenty of ventilation, disposable gloves and work clothes. The glue is dyed (blue or red) and it will stain you skin and clothes. This actually is fairly easy to assemble, but you'll spend more time getting the lengths trimmed if you're picky about your VSWR (and you should at least get it under 2:1 for your entire band).
My apologies to non-USA readers. I made this up from
supplies that were easy for me to find. The concept is simple though so
you should be able to adapt it to what you can find near
This is a pretty good club project, and some of the supplies
are much cheaper if you buy bigger quantities. The PVC/copper pipe and
fittings for instance.
saw for PVC pipe
#80 or #100 sandpaper
copper tubing cutter
propane torch for soldering (plus eye and hand protection)
pliers or crimper for terminals
antenna analyzer or good VSWR meter
drill and 3/8's drill bit
adjustable wrench that fits the copper threaded fittings
single edge razor blade or very sharp knife to cut outer jacket of coax
solder and flux
shrink tubing (1/2")
8" UV resistant tie wraps
RG58 coax and PL-259 (UHF) connector
More on this latter, but at 144Mhz you don't want much of this coax in your feed path. Definitely under 20 feet. I mostly use it on the arm due to its light weight. You want to switch to something with less loss for any significant run. You can cheat and buy a made up cable and just cut the connector off one end if you don't like terminating coax. Don't substitute anything with foil on the center insulator.
PVC: All PVC is schedule 40 1 inch.
tee's - 2
1/2" threaded slip fittings - 2
straight pipe - 4-5 feet
slip fit cap - 1 (maybe, read on)
Copper: all copper tubing is 1/2"
caps - 2
threaded fittings - 2ea 1/2"
32" straight pipe (we will cut this in half so if you end up with 2 16" pieces, that's OK)
10-24 x 1" brass machine screws - 4
10-24 brass hex nuts - 4
crimp on spade lug for #16-14 gauge wire - 2
Nice to have:
no prime paint for plastic - the glue
looks pretty ugly on the white pipe, and you may want to make it look
Getting Started! (See additional picture of the project
Start with two 16" pieces of the copper tubing. I know that's short for a 2 meter dipole, but read on. Buff one end of each piece and the inside of the threaded fitting. Your going to need to coat both surfaces with some kind of flux. Plumbers use acid flux which you need to make sure you wash off after you're done. I used rosin flux and electrical solder rather than acid flux and solid plumbing solder. It's not right for hot water, but it works fine for this. Do the flux and buff thing to both pipes and fittings. Ideally, put them so they're in the air and supported so that gravity keeps them together. Heat the fittings and a little of the pipe. The solder should just wick into the gap. Work around the pipe till you see a little line of solder around the entire edge. Let these cool for 10 minutes or so. DO NOT PUT CAPS ON YET.
Lets assume about an hour has gone by and the PVC glue is fairly set. Take the copper sections and thread them in. You may need an adjustable wrench to get them in, but don't twist too hard, their not meant to go in all the way. Note how deep each end went in and using a pencil, mark the copper and the PVC end so each will go back to the same hole. I actually used colored electrical tape. Now place the threaded end against the body and mark on the side of the tee, how far it went in. Taking the drill, place the edge of the bit (not the center) on the line so that it's closer to the center of the tee. The two holes should be aligned with each other and and approximately 90 degrees from the middle of the tee. Thread one of the hex nuts on each screw so that it's about 1/8th of an inch from the head. Thread or push the screw into the hole and then start the other hex nut on the inside. When you're done, the flat of the nut should face the opening, and the screw should be nearly in. (Editors note...the purpose here is that each end of the antenna copper tubing element (at the "T") touch tightly against the flat side of the nut inside the end of the "T". You should show continuity from the tips of each antenna element to it's respective end on the coax feed when your done)
Time to add the coax. Take the non-connector end (the end at the antenna) and strip off about 1 and 1/2 inches of the outer jacket. Pull the center conductor out through the braid so you now have two separate "wires". Slip about a one inch piece of the heat shrink past this area, you'll be bringing it back later. Strip about a 1/4" of the insulator off the center conductor. Crimp on the spade lugs. Put the center conductor lug on what will be the upper brass screw/terminal and the shield on the lower and tighten (If you have the Pentrox, a little between the brass and the lug would be a good idea). Now bring the heat shrink tubing tubing up so that the coax forms a "T". This is important, so check the photos at the bottom of the page if you need to. Shrink the tubing, and tie a wrap to hold in place. Get the electrical tape and a few ties handy. Wrap the coax around the pipe 5-6 turns, and while keeping the coils tight together us a tie wrap to hold them. Overwrap this with the electrical tape. It's very important that all the coils touch. This is what's called a solenoid balun. It is used to keep common mode currents off the coax. Now run the coax to the mount end with additional ties. Thread in the tubing and using a meter check to make sure you've got good conduct between the arms and the brass screws.
Okay, were nearly there. Assuming you have good contact between the antenna copper elements and the coax and no shorts, proceed to tune (you might want to lightly sand the bottom edge of the threaded copper fitting where it make contact with the brass and maybe even add a little Penetrox. Here we need either a good VSWR meter or an antenna analyzer. If you have an analyzer, sweep the 144-148 band. I'm guessing you'll find the antenna is too long, and you'll end up around 15" on each arm. I wouldn't remove more than 1/4" at a time. If you have an swr meter, check 144, 146 and 148Mhz on low power. You'll need to trim the same on both ends and the effective length is tip to tip. If you trim a little too much, the copper end caps can make up for that. Once you have a good match (and you should be under 1.3:1 at the middle freq), buff and flux the end caps and solder them on.
As you may have noticed, a little bit of the coax actually is part of the antenna which is why you can't use foil shield and why the arms are "short".
Mounting options: I opted to keep the tee and cap. I glued in a short piece of PVC tube and used threaded ends so I could add a removable mount tube. The mount tube has a notch cut in it and I use a hose clamp on the tripod section that fits into the notch so that it won't swing in the end. Another idea is to make an inverted "U" so you could hang it on a balcony railing.
Email Jim for questions here - mtnredhed AT gmail.com
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