DIY No-Solder, Cheap, Stupidly Easy & Simple Dipole Antenna (And By The Way, What Is An Antenna And How Does It Work?)

Are you a no-code, appliance operator type of ham who memorized the general class amateur radio test questions in order to be able to talk on the high frequency bands? Well, me too. Or at least I was. And then I got sucked into this weird 3+ year long vortex of RF lovefest. And, oh boy, the antenna, in its deceiving simplicity, has captivated me.

Want to just get started with the build and skip all this antenna theory talk? Click here!

What exactly is an antenna? I am not kidding you here. Think about it. Can you define what an antenna is? Ok big shot, then explain how it works. I do not mean point me to a handful of mathematical equations (ahem, Jimmy Maxwell…) I mean, how does it work???? In words. Truth is, no one really knows. And I do not care if a bunch of electrical engineers and physicists just cringed.

An antenna is a transducer. This simply means that it converts one form of electricity into another. An antenna changes the alternating current produced by a transmitter into “radiofrequency”, i.e., the electromagnetic waves that hop off an antenna and travel into the so-called “aether”. See, we already reached our first problem. There is supposedly no aether. That invisible, weightless, matterless substance that permeates everything, and allows for the propagation of electomagnetic waves. Just like waves in the ocean, just without the ocean, or really without even the water. Something is there. We can measure and use RF. And it propagates. Just we’ve decided it “self-propagates”, meaning, I guess that it moves by itself. At the speed of light, and just like light really, because light is an electromagnetic wave too. Except when light is a packet of energy called a photon. And we know all of these theories and (non-)explanations are true because of mathematics. Just don’t ask anyone to put it into words…well, quite frankly it doesn’t make sense.

I have heard RF called “black magic”. I was much more intrigued when I heard it called God. Electromagnetism is mysterious, no matter what you think or believe. The antenna of course also transduces RF from the non-existing aether back into alternating current that can be translated (meaning demodulated) by a receiver.

So, what exactly is an antenna??? The best set of criteria I can find that defines an antenna are from https://learnemc.com/electromagnetic-radiation . They are 1) The antenna must have two parts; 2) both parts must not be electrically small; 3) something must induce a voltage between the 2 parts. I am going to rewrite these to say what I think they mean: 1) The antenna needs a positive side and a negative side, and these sides need to be able to flip back and forth in polarity (i.e., positivity and negativity) creating an oscillation of current; 2) both sides of the antenna need to be large enough that the wavelength corresponding to the frequency of interest can electrically “fit” on them; and 3) I kinda touched on #3 in #1, but… some force needs to cause there to be a difference in the amount of electricity stored on one part versus the amount of electricity stored on the other part, i.e., a voltage difference must exist between the two parts.

So, to put this all together, a receive antenna is a device that when in the presence of ambient radio waves propagating at that antenna’s resonant frequency, a voltage difference is induced between the antenna’s two electrical poles leading to current flow first in one direction, and then due to the antenna’s innate oscillator properties, back in the other direction, thereby creating alternating current. Thus the antenna has tranduced electromagnetic waves into alternating electrical current.

Likewise, a transmit antenna accepts alternating current from a transceiver, and if at the proper resonant frequency, also develops a bipolar oscillation consisting of phase shifted (i.e., one goes one direction at a certain time, the other goes another direction at another time) electric current and magnetic forces, thereby establishing electromagnetic waves (i.e., radio waves). It is unknown, as far as I know, why the electromagnetic waves dissociate from the antenna or how they “self-propagate” through the non-existent aether. I have read several theories, but I believe them to be just that: theories.

So where is this leading us? On how to make arguably the most important antenna in all of ham radio. A halfwave dipole. The antenna that largely serves as a (real) reference point for all other antennas. And the antenna that is the backbone of nearly every other antenna, even if it may not seem like it at first. For the record, the other important reference antenna is the isotropic antenna, which simply does not exist.

This schematic above represents a cut section of coaxial cable. From outside to in, the black outer insulation surrounds a squiggly yellow metal braided shield. Then there is white dielectric, or insulating material. And finally, there is a brown center conductor. In the drawing, you can see that I have attached a black wire to the braid and another black wire to the center pin. If those black wires were each 1/4 of a wavelength of a given frequency, then this would be a half-wave (those two quarters added together) dipole. And it would be resonant on that frequency it is a half-wave length (or two 1/4 wavelengths) of. The braided shield and the center conductor should not touch.

Here’s an example: the frequency of interest is 14.250 mHz. The speed of light is 300,000,000 m/s. If you divide 300 by 14.250, you will get 21.05 meters. So, the wavelength of the frequency 14.250 mHz is 21.05 meters. One half of 21.05 is 10.53 meters. And one quarter of 21.05 is 5.26 meters. If you cut two wires that are 1/4 of a wavelength long for the frequency of 14.250 mHz, and attach one of these to the braided shield of a coax cable, and the other to the center conductor of coax cable, you have made a half-wave dipole antenna. Convert it to feet, and you’ll find each leg is 17.25′ in length. And if you use a dipole calculator, you will get a different answer all together due to correction factors.

Regardless, start with legs that are definitely too long, and cut them back a bit at a time until you have the correct length. For this 20M dipole build, I started with each leg at 18 feet.

20M HALF-WAVE DIPOLE INSTRUCTIONS

MATERIALS:

• BNC female to double binding posts connector
• 18awg speaker wire (18 feet length, double conductor)
• Optional: spade or ring terminals (crimp)
• Cable ties
• flexible 50 ohm coaxial cable (like RG8X)
• wire cutters, wire stripper, and crimp connector (optional)
• antenna analyzer (I use a RigExpert AA-55 Zoom, but a nanoVNA is a cheap option).
• tape measurer
• Various RF connectors and adapters as needed

STEPS:

1. Measure and cut 18 feet of 18awg speaker wire.
2. Separate the two conductors of the speaker wire by pulling them apart.
3. Strip one of the ends of each wire and add a spade or ring connector, or alternatively just attach the bare wire to the binding post. Take a look at your binding posts and see what works best. I crimped on spade connectors mainly because I had them available.
4. Tie a bowline knot at the end near the spade connector of each wire. Refer to picture below for bowline knot configuration.
5. Use cable ties to secure the bowline knot to the holes in the BNC to binding posts connector. This is for strain relief so that the there is no tension placed on the spade connectors when the antenna is erected. See picture.
6. Feed another cable tie through the strain relief cable ties (in step #5). This can be used to hang the antenna.
7. Loop coaxial cable six times and secure with two cable ties. The diameter of the loops will be between 5 and 6 inches. This is a common mode current choke that will hopefully make the antenna less noisy by keeping stray RF signals from reaching your transceiver. Alternatively, you can use a 1:1 balun, such as the one by LDG RU-1:1 balun shown in the picture above (blue box; I did not end up using it).
8. Connect the coaxial cable at the end closest to the loops to the BNC connector. I needed to use a BNC male to so239 (female uhf) adapter, and then attach the pl259 connector of my coax cable to that. Use what you have, although ideally, you would use coax cable terminated at least at one end with a male BNC connectors.
9. Hang up the antenna in an inverted “V” configuration, with the BNC to binding post connector about 20 feet up in the air. Spread out each “leg” of the antenna until the ends are approximately 10 feet off of the ground.
10. Measure the SWR of the antenna. It is easiest to do this with an antenna analyzer with a graph display of SWR and frequency like the RigExpert series or the nanoVNA. If the SWR “dip” is at a lower frequency than the one you wanted, then your antenna is too long, and you need to remove some of the wire. For instance, when I initially measured this antenna, the lowest SWR reading was at 13.33 mHz, I needed to clip an equal amount of wire from each side a couple inches at a time until I was able to get a lowest SWR reading of 1.06 at 14.230 mHz. There are pictures of these readings below. If however, the lowest SWR reading is at a higher frequency than your intended frequency, your wire is now too short. Clip very small amounts of wire from each side, and take a lot of SWR readings!
11. That’s it! Get on the air!

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This picture below is a bowline knot. It is very useful for creating a non-slip loop in a wire. I highly recommend learning this for its utility in providing strain relief in antenna radiator wires.

This is the “front” side of the BNC to double binding posts.

This is the “back” side of the BNC to binding post.

TUNING AND TESTING THE ANTENNA:

Here is Limey (my van) heading out to Tully Mountain in Orange MA for a Summits-On-The-Air activation (W1/CR-012).

We ducked into the woods up at the peak and set up shop. I was using my Yaesu FT-818ND for its inaugural SOTA activation. If you look carefully, you can see I brought the DIY homemade roll-up Slim Jim antenna for vhf/uhf in case SSB did not work out.

This is how the antenna looks hanging by the cable tie “hook”.

The ends of the antenna are attached with another bowline knot in the wire around a cable tie. The cable tie is then secured with paracord to a tree trunk. Any clipping of wire that I had to do, I just used the free end of the wire hanging from the bowline knot. When I got too close to the knot, I just undid it, and made a new one farther back. This way I only needed to lower and raise the antenna. I did not have to redo the configuration of the ends each time.

This is the first SWR sweep that I performed. You can see that the blue vertical band is the frequency range for the 20M ham band. The SWR “dip” is shown at its lowest point at 13.33 mHz. This frequency is lower than the intended 20M band, therefore the antenna is too long. I would have to clip the antenna ends equally until the SWR dip was in the blue band.

This gives an idea of the length of wire I would clip at a time. Each time I cut, I would make sure to take an equal amount off of each leg of the antenna and retest.

After raising, lowering, and clipping each half of the antenna five times, I managed to get an excellent SWR of <2 across the entire band, and most of the band under 1.5 SWR.

The SWR is not trivial. Perhaps not a perfect measurement, but it gives us hams something to look at to assess how much power from our transmitter is actually making it into the antenna. A “standing wave ratio” is exactly that, a ratio. So a 1:1 ratio indicates that 100% of the power from your radio is getting put into your antenna. Whereas any other ratio, such as let’s say an SWR of 2 (i.e., 2:1), means that some fraction of the power that your transmitter is putting out is now hitting an impedance mismatch wall when it hits your antenna, and is now bouncing (reflecting) back toward your transmitter. You can use an antenna tuner to deal with these high SWRs, and it’s ok when you are running 100 watts. But when operating qrp like I was today, I needed an antenna that was resonant on 20M with a low SWR, so that every bit of the the power I transmitted made it into the antenna.

Here is the final position of the half-wave dipole hoisted up in a tree.

AA1F, using his typical 20M EFHW and QRO station made his activation while I was tuning my antenna.

My station was a lot more luxurious! If you look carefully, you can see a tiny part of our dog Nellie bundled up in that sleeping bag. I had just completed my activation, all QRP at 6 whole watts!!! My first serious venture into low power…N4TIZ was my first contact on this rig and antenna, and he happened to mention that he reads this blog. So THANK YOU for getting me started today!

Well that’s that! Hopefully this gets you new hams pointed in the right direction when it comes to constructing antennas. This one is easy enough to figure out just by looking at the pictures, I think. But that humble dipole makes up the backbone of many (most?) other antennas. I know too that I took liberties in my explanation of what an antenna is and how an antenna works. I am no expert in the subject, and if I am way off base, or missing the point, drop me a line.

And revel in the fact that you are harnessing perhaps the greatest force in the universe.

Always true,
KM1NDY