How to Make a KW4JM Portable Vertical
Before you start
There is no such thing as a "simple" DIY antenna project. Simple conceptually? Yes. Simple in terms of materials? Perhaps. Simple in terms of getting it "right?" NO! This is a simple antenna in terms of concept. It's a 20 meter 1/4 wave vertical with a center loading coil for lower bands. Think of it as a half wave dipole with one leg, the main element, vertical and the other leg, the radial(s), stretched out near but above the ground. If all is just right, at its resonant frequency, like a dipole, it will present a near 50 ohms to the coax that goes directly to your radio.
As with a half wave dipole, you can make it "look like" a resonant half wave at lower frequencies by adding a loading coil of a very specific inductance in the radiating element. The more turns or greater inductance you add, the lower the resulting resonant frequency. Rather than adding a loading coil to both the vertical element and the radial, this one uses one radial for each of the desired frequencies. So for a three band vertical for 20, 30 & 40 meters, there is one 16 ft radial for 20 meters a 23 ft radial for 30 meters and a 33 ft radial for 40 meters. All three radials are connected simultaneously. The loading coil which is about 1/5th of the way up from the bottom of the vertical element has a tap for each band as well. That's the simple concept!
By the way, this could be built with the coil at the bottom (bottom loaded) rather than part way up (center loaded) the vertical. Why center loaded which is somewhat more complicated to fabricate? I don't personally have a good answer to that. "Everyone" seems to say that raising the coil increases its radiation efficiency or something. I have no way to evaluate that. I've successfully used bottom loaded mobile HF antennas for years with good results. I went with the center loaded version just because I wanted to! The overall concept and construction are very similar for both. So make it the way YOU WANT to!
This antenna is NOT SIMPLE in terms of getting it right. I mean, selecting the overall length from the top to bottom, selecting the length of the upper element above the coil and the lower element beneath the coil. AND the biggie, finding the tap points on the coil that you'll be happy with. It is not as simple as counting turns. That will get you in the ball park but tweaking the tap points will require multiple cut-and-try efforts.
Materials for this antenna are very simple: stranded wire, a little hardware and a plastic coil form. You may have all you need lying around.
To tackle this or any antenna-making project, you'll need patience and a sense of enjoyment from futzing! And you'll need time. Projects such as these are fiddly and time consuming. You can't just throw it together and use it. You'll make dozens of trips between your antenna and rig and will likely "never" be fully satisfied that it can't be better. THAT is fundamental to making antennas. But when you finally say, "good enough" and make that first contact, you'll be thrilled! For me, both the "difficulty" of getting it right AND that first and successive QSOs are the reward - up to a point! I have a number of abandoned antenna projects as the futzing passed my own tolerance level! So settle in and enjoy the process!
What You'll Need
- Coil Form. A plastic pipe about 1-1/2 inch diameter and a foot long long. I used a piece cut from a Sink Tailpiece I got at the local hardware store for a couple bucks. It has a 1-1/2 inch outside diameter and is made from light weight plastic. The exact diameter is not critical. You might use a plastic core from kitchen wrap or something similar you already have lying around. The diameter will affect the number of turns of wire you'll need but that is all. I suggest you not cut the form to length until you have finished winding and testing the coil as the length will vary based on wire size, number of turns and the lowest frequency you plan to make it for. Give yourself some room to experiment.
- Wire for the coil, the top and bottom elements and the radials. I suggest stranded rather than solid as it is easier to work with. You'll need 50 to 75 feet. Wire gauge is not critical. Larger will require a longer coil form. I used a 100 Ft pack of 24 Gauge RCA Speaker Wire from our home improvement store. I split the two wires apart and used only one. I wouldn't go any smaller than 24 gauge as it will be difficult to add the taps. Larger will work, just making the antenna a bit heavier and the coil longer. However the taps will be easier to add to larger wire.
- Brass Screw Eyes. You want the smallest screw eyes you can find. I suggest brass but that's probably not critical. I used #6 ones from our local hardware store. Smaller would have been nice but they didn't have any. You'll need one for each band. You could substitute a small screw or something. They need to be small enough to pierce but not destroy the stranded wire and long enough to allow you to clip the alligator clip to it.
- Electrical tape and shrink wrap tubing. The tape will be used to secure the winding's temporarily and perhaps permanently! That raises the point that this is probably not a for-ever antenna. It won't be particularly rugged. For that you should use stronger wire than speaker wire and something more permanent than electrical tape. The small shrink wrap tubing is for covering mistakes in trimming the elements, securing a loop at the end of the top element, strengthening any connectors you solder to the elements, etc. Another very handy thing to have is Liquid Tape. I used it to put a dab around the screw eyes where they pierced the wire to keep moisture out and slow corrosion.
- A small alligator clip. This will be soldered to the pigtail that connects the lower element to the appropriate tap on the coil for the band you select. That allows easy band changing!
- Gold-Plated Brass Bullet Banana plugs and jacks. Depending on how you make your antenna you may only need two of these or even none at all. I used four pairs to connect my upper and lower elements to the coil, to connect the lower element to the feedline and to connect the radials to the feedline.
- Soldering iron/gun and solder. Straight pins. An antenna analyzer or some way to measure SWR.
Time to Build
- Begin by drilling a small hole about an inch from the top end of your coil form. Make the hole just large enough for the wire to pass through.
- Next pull about 12-1/2 feet or a bit more of your wire through the hole from the outside and tie a knot in the wire it at that point. The knot should be inside the coil form.
- Now cut a couple pieces of electrical tape about an inch or more long and keep them handy. And have a felt tip pen handy.
You should now have the coil form with about 100 turns on it and with a twelve foot wire protruding from inside the top and a four or so foot wire protruding from inside the bottom and a one foot pigtail coming out the side through the bottom hole. You have completed the simplest and easiest part!
- Decide whether you want to trim the top element or the bottom element to tune the antenna. It's easier to trim the bottom one but either or both will work. The exact length of each is not critical, it is the combined length that is important.
- Form a loop in the top end of the 12 foot wire by bending it back on itself a couple inches and tape it in place leaving a half inch or so loop for tying the end to a rope. This will make it easy to suspend the antenna during tuning. After you have finished trimming the length to resonate on 20 meters, you can replace the tape with heat shrink tubing, making the loop pernanent.
- Slip a 1-1/4 inch piece of heat shrink tubing over the the one-foot pigtail coming out of the side of the form, and then trim the pigtail so it easily reaches the top of the coil form with a couple inches to spare. Now strip the wire and solder your alligator clip to the end. Slide the piece of heat shrink tubing over the back of the alligator clip and shrink it into place.
- If you plan to tune the antenna by trimming the bottom element, strip an inch of insulation from the bottom end of the lower 4 foot wire and tin it. You will fasten this under the red screw top of the BNC to banana jack adapter during tuning. Once it is tuned you'll put a banana plug on the end with a 1-1/4 inch piece of heat shrink tubing over the shoulder of the banana jack and up the wire. That protects the solder joint from stress and helps with plugging and unplugging the banana plug.
- It's time to prepare the three radials. Cut a 33 foot length, a 23 foot length and a 16 foot length of wire. Strip 1/2 inch from one end of each and twist the three stripped ends together. Now solder the twisted ends of the radials to a gold plated bullet banana plug, and shrink a 1-1/4 inch or so piece of heat shrink tube over the plug shoulder and up the wires a ways to protect the wires and give you a handle to unplug the radials without breaking them.
Hanging and tuning the antenna
Now we are coming to the fun part, hanging and tuning up the antenna. You need to decide how you are going to hang it. If you have a tree branch that is easy to get a light weight rope over, 25 or so feet up and near enough to your shack to connect a coax to the bottom of the suspended antenna, that's a good option. Another option is to suspend it from a collapsible fiberglass fishing pole or mast around 20 feet long.
- With the antenna suspended so the bottom is two or three feet off the ground, grab your BNC Banana jack connector and tighten the bottom wire onto the red banana jack. Plug the radials into the black banana jack. Then connect your coax to the BNC jack and to your antenna analyzer or rig.
- Fan the three radials out approximately evenly from the bottom of the antenna and tie them to nearby bushes, or stakes in the ground so they stay a foot or so off the ground and keep the center point more-or-less directly below the top of the antenna.
- Grab your straight pin and carefully press it through the insulation and into the top winding of the coil a half inch or so back from the hole in the form and gently clip the alligator clip to the pin being careful not to pull the pin out of the winding.
- Turn on your antenna analyzer or rig and find where the antenna is currently resonant. Hopefully it is below 14.00 mhz. It may be way below depending on the combined length of the top element, pigtail and bottom element. Once you find the resonant frequency, trim the the bottom or top section (whichever you choose) an inch or so at a time until it resonates near the center of the 20 meter band, about 14.20 mhz. If by some chance the resonant frequency is above that frequency, you'll need to add wire to either the top element until it is resonant at 14.2 mhz. Mark the location of the straight pin with the felt tip marker as you'll need to put a screw eye there later.
- Next, push a straight pin through the 10th or 12th winding from the top of the coil. Clip the alligator clip to that pin and find where it is resonant. If it is below about 10.12mhz, move the pin up a winding and check again. If it is below 10.12mhz, move the pin down a turn. You may need to move the pin part of a turn around to form, adding or removing a half turn or so, to find the magic spot. Mark that spot with your felt tip marker as above. DO NOT CHANGE THE LENGTH OF THE WIRE ELEMENTS AT THIS POINT or you'll have to start all over. The antenna, with the coil bypassed is a 1/4 wavelength resonant vertical on 20 meters. The coil taps change the effective length to resonate at lower frequencies.
- Repeat this process for each band for which you plan to use the antenna. It's a good idea to double check your work as now is the easiest time to make changes in tap locations. I made mine with taps for 60 and 80 meters in addition to 30 and 40 meters. I don't think the antenna is very efficient, particularly at 80 meters but it's nice to have the option.
Finalizing the antenna
It's time to finalize the antenna by adding the screw eyes, removing unneeded windings, trimming the coil form and making it all somewhat robust.
- Solder a banana bullet to the bottom end of the lower element and add shrink tubing so you can simply plug it into the red banana jack on the BNC to Banana adapter rather than having to screw it under the cap.
Put it on the air!
You will likely discover other things you want to do to make the antenna truly road worthy. I decided to alter mine so I could remove the top and bottom elements by installing banana jacks to the coil form and banana plugs to the ends of the elements. That makes setting up and taking down a bit easier. I mount mine on a fiberglass collapsible fishing pole and use 1/8" bungee cordage and cord locks to hold the coil and the BNC connector to the pole. I also use a short piece of bungee cord with a cord lock to attach the top of the antenna to the fishing pole. That too makes it easy to set up and gives everything a little flexibility. Whatever you do I hope you find the antenna fun to build and have success with it. I really like mine.