Toothbrush Holder Dipole – A Portable HF Antenna

Background

antenna_finished_round_small_comp
The toothbrush holder makes a nice neat package for all that wire.

If you’re new to Ham radio, the term Dipole Antenna should be fresh in your brain from the exams you took to get your license. These antennas are really easy to make. All you need are two pieces of conductive material of the proper length. Really, the only complexity involved is that “proper length” part, but that’s what this guide is here to help you with. Typically the conductive material involved in the antenna is some kind of wire or thin pole, but there are other possibilities. I will walk you through building one out of wire, with the addition of a toothbrush holder simply for ease of portability.

I first learned how to build one of these from a gentleman in our local club (Atlanta Radio Club), Bill Perkins (KB4KFT), who runs a build class every so often.  He always points out, what I previously mentioned, that you can make an antenna from any conductive material. In fact he’s successfully tuned in trash cans, a picnic table, and even an aluminum extension ladder. There’s even an occasional competition, here in Atlanta, for “crazy antenna” builds where participants build antennas out of strange objects like that.

But enough jabber, lets get to the build…

Goal

Build a Dipole Antenna with the following qualities:

  • Cheap as possible.
  • Portable (can be packed in a backpack or small bag – typically along with your radio).
  • Tuned to transmit and receive on 20 meter band (around 14MHz).
  • Containment device to help avoid tangling of wire.

Materials

  • 100ft of Stranded Copper Wire – $0 to $50 [Ideally you have some lying around or find some on the side of the road – length will vary depending on the band you are targeting]
  • UHF SO239 Female Connector – ~$5 Single or ~$1 each in bulk [I prefer to buy a pack of them, 10 for $10 on Amazon]
  • Solder – ~$2
  • Small Zip Ties – ~$2
  • Plastic Toothbrush Holder w/ Removable Lid – $1 [Found some good ones at Dollar Tree]

Optional/Nice to Have

Implementation

I’m about to go into detail on how I calculate the length and why. If you’re not particularly interested in that part scroll down or just click the Skip link below:

Skip to Instructions

Basic Calculations Explained

The chief characteristic of a Dipole is that it’s roughly half the wavelength of whatever frequency you’re trying to tune to. You can calculate the wavelength if you know how fast the wave is traveling and at what frequency it oscillates. In a vacuum – take a mental note of that prerequisite – radio waves travel at the speed of light, so we’ll start there.

I built my first antenna for the 20 meter band, which starts at 14MHz (Mega Hertz). Hertz is a unit of measurement representing a single, full cycle per second. The length of the wave is how far it travels during that cycle. So if our target wave cycles 14,000,000 times per second and light travels at roughly 983,600,000 feet per second[1], it becomes a simple matter of dividing the speed of light by the frequency to get the total wavelength in feet. By my calculations, that comes out to 70.25 feet for this example. But I only need to account for half the wave, so divide that length by 2, giving 35.13 feet.

L=\frac{C}{F}=\frac{983\:MFt/S}{14\:MHz}=35.13\:Ft

Sounds simple enough, right?

Well, unfortunately, if you were to continue building with that measurement, you’d find you’re not quite on the mark. In fact, you’re probably pretty far off the mark when measuring the SWR (Standing Wave Ratio).

So what’s gone wrong?

Remember earlier when I asked you to make a mental note about the speed of radio waves. Radio waves travel at the speed of light in a vacuum, but they are slowed by atmosphere and proximity to objects with capacitance. In this context, it’s the insulation surrounding the conductive medium (wire) that most affects it’s speed.  Since many factors can affect the actual speed of a wave, it’s hard to predict exactly. However, you can get really close with a slight adjustment to the calculation.

Velocity Factor: Precision Calculations Explained

The slow down of radio waves through a conductive medium is generally referred to as the Velocity Factor (VF). It is the ratio between the actual speed and full speed (speed of light). In other words, a smaller percent of the speed of light. Though external factors can influence the velocity factor too, we generally know how different mediums will behave in an outdoor environment. You can either Google the VF for a particular medium (wire) or find out from the manufacturer. If you happen to get really close, though, you can reverse calculate the VF. More on that towards the end, though.

The Ham exams require you to memorize a particular number (Velocity Constant) for calculating a dipole length. That number is 468 and it comes from old ARRL documentation for wire commonly used back when it was written – bare copper wire. Bare copper wire has a velocity factor of 0.95 or 95% the speed of light. If you go by that it reduces the speed I used earlier by 5%. This will get you even closer, definitely close enough to tune easily enough by hand. However, most people use insulated wire these days, which slows things down a little more. Insulated wire is more like 0.90 to 0.92 velocity factor.

I use 0.90 and I’ve had great success in builds using that for my calculations. With that adjustment in hand, I’ll rerun the calculations from earlier. The speed of light is 983MFt/S (I converted to Mega Feet to line up with my Mega Hertz frequency, it’s easier to work with) and I’m still going to target 14MHz. Adjusting the speed to 90% gives 885MFt/S, then I cut that in half since it’s a half wave, giving 443MFt/S. Then divide that by 14MHz giving 31.63Ft – almost 4 feet shorter!

L=\frac{C*V}{F}*\frac{1}{2}=\frac{983\:MFt/S*0.90}{14\:MHz}*\frac{1}{2}=\frac{443\:MFt/S}{14\:MHz}=31.63\:Ft

Instructions

1. Pick a Band & Center Frequency

Choose your band: 10 meter, 20 meter, 30 meter, etc. Once you have it, look up it’s frequency range. If you only want to work with voice, your range may be slightly smaller than the band plan states. For example, the 20 Meter band’s full range is 14.00 MHz to 14.35 MHz, so the center frequency would be 14.175 MHz. But the voice portion goes from 14.15 MHz to 14.35 MHz, centering on 14.225 MHz.
Here’s a chart the ARRL put together, which helps me figure this out:

2. Plug Your Center Frequency Into the Formula

Length(in Feet)=\frac{443}{Center Freq.}

3. Cut Wire to Length

Measure the wire out to match the length you calculated. I like to give it a few extra inches than that because I like to tie a loop on each end of the wire for hanging.

4. Cut Wire in Half

Cut the wire into two equal halves. I usually just line up the two ends and pull the wire through my hand, keeping the ends together, until I get a bend. That will be the center and I cut the wire there.

5. Tie Wire to Lid

antenna_lid_bottom_square_small_compNow you should have two wires of equal length. Take one end of the first wire and, going up from the bottom of the lid, tie the wire to a separator between two of the four holes in the lid. Do this in such a way as to have an inch or two of the end dangling. Then move to the separator just to the left or right and tie the other wire in the same manner.

6. Fasten Wire to UHF Connector

Strip the insulation off the short dangling ends of each wire (about a quarter of an inch or so). Then solder one stripped end to the center peg of the UHF connector. Lastly, solder the other stripped end to the flat base of the UHF connector (on the back side, the peg side). This can be the trickier one to do, you may find it easier to strip a little more off and run it through one of the holes in the base and then kinda wrap it around itself before applying solder. I also found that the UHF connector absorbs heat well, so I ended up using a small butane torch/lighter to heat the area before applying solder.

7. Fasten UHF Connector to Lid

antenna_lid_top_round_small_compNow push the connecting end through the one hole that has no wire tied to its separators so that the connecting end is facing out the top. Then use zip ties to loop around the two bordering separators and through a hole in the UHF connector’s base.

8. Tie End Loops

antenna_knot_small_compAt the end of each long dangling wire, tie a small loop, maybe an inch in diameter. You can use these loops to hang the wire later. Keep in mind that the knot you form in the loop will become the effective End of the antenna. If you follow the tuning step at the end of this article, you’ll want to measure from the knots to the center (or knot to knot for full length).

9. Analyse the Antenna

radio_swr_meter
Tuned a little under the target center frequency. This required moving the knots in further.

Extend the antenna out and hang it from the end loops on a couple of trees, poles, or what have you. It needs to be about 3 to 10 feet above the ground. You may want to use string or twine to help attach between objects that are a little too far apart – which is usually the case.

Hook the new antenna up to an Antenna Analyser and measure the SWR. Your radio may have one built-in, mine did, but read the manual to make very sure it does. Typically you set the analyser for the center frequency and it does a sweep below and above, taking readings every so many MHz. You should end up with a U shaped curve where the valley is at your center frequency and at that point the ratio is 1 or very close.

If you find that the bottom of the curve falls somewhere before or after your center frequency, you can make adjustments to the length by moving the knots in or out. Make sure you move both knots the same distance to preserve equal length on both wires. Move knots in if the bottom is below your center frequency and move the knots out if it’s above.

Reverse Calculating the Velocity Constant

If the bottom of the SWR curve doesn’t line up with your center frequency – it wasn’t quite there for me either – then you can use your readings to reverse engineer the velocity constant. Basically, you’ll take the frequency that is at the bottom of the curve, the length of your antenna from knot to knot, and plug those into the equation, and solve for VC instead.

Length(in Feet)=\frac{VC}{True Center Freq.}

or

VC=Length(in Feet) * True Center Freq.

Once the new VC is calculated, plug it back , in place of 443, in the original equation with the center frequency. This will calculate the most accurate length for the conditions you’re in. I like to take the old length and subtract the new, or vise versa depending on which is longer. Take that difference and divide by 2. Now you have the distance to adjust the knot on each side. Remember, if the bottom of the SWR curve was below the center frequency, move the knots closer to the center. If the bottom was above, move the knots out and away.