Editor’s Note: Over the next several months, OnAllBands will be featuring a series of articles from Harry “Trippy” Brown, AC8S, longtime amateur operator and antenna builder/tester who’s never let his visual impairment stop him from enjoying the hobby he loves. As the title of this series states, we hope these articles provide you with invaluable insights as you pursue your own successful antenna projects.
Last week in our examination of dipole/inverted-V combo antennas and end-fed antennas, we looked at some of the antenna lengths that have been tested by hams. I’ll be answering a few more frequently asked questions in today’s blog post. To read the first seven entries in this series, just enter “Trippy” in the OnAllBands search box above.
Will a half-wave dipole/inverted-V combo or end-fed antenna cut for one ham band work on other ham bands?
Yes. My friend, Brian, N8MNX, has a half-wave dipole/inverted-V combo up right now. It’s a half wave on the lowest frequency band he uses, 80 meters, and it works on 40, 20, 15, 10, 6, and 2. You can tune it using an antenna tuner. It works for me.
What if I have to cut the antenna to get the SWR down? At what inch intervals should I cut the wire?
Yours truly just tried it, and two inches works great, so you can cut the wire two inches at a time.
How long can my antenna be?
Your dipole/inverted-V combo or end-fed antenna can be as long as you want it to be because of coiling. Remember, the longer the antenna wire, the better you will get out and the louder your signal will be! In the book 73 Dipole and Long-Wire Antennas, it says, “With every half wave added to the antenna in length, you add more dB gain.”
NOTE: The problem is, if you make the dipole, dipole/inverted-V combo,
or end-fed longer than a half wave, the antenna will get out directionally, which we don’t want. We want our antenna to be omnidirectional, meaning transmitting in all directions.
However, there is a limit on how short your antenna can be. Do NOT build any antenna that is less than a quarter wave long for a particular band or all the power will dissipate into heat, and you’ll have no signal being transmitted out of your antenna. This warning does not apply to snake antennas or loops because I’ve used them on bands where the antenna was not at least a quarter wave (for example, on 20 meters, where the antenna was only 12 feet, 9 inches) and I made contacts with it on CW.
What formulas do you use during antenna construction?
Formula 1: If you don’t know what meter band a frequency is on, convert frequency to meters by taking 300 and dividing by the frequency in megahertz. For example, 300 divided by 3.750 = 80 meters. If the band is a gigahertz band, take the frequency divided by 1,000. For example, 2,400 MHz divided by 1,000 = 2.4 GHz.
Formula 2: If you don’t know what frequency is on a certain band, take 300 million and divide that by the meter band you’re on. For example, 300 million divided by 80 meters = 3.750 million, or 3.750 kHz. For gigahertz, take the number of gigahertz and multiply it by 1,000. For example, 2.4 gigahertz times 1,000 = 2,400 MHz.
Formula 3: Here’s how to figure out the frequency at which a length of wire will transmit with a low SWR, or find out how long or tall an antenna is if you know the wavelength but not the frequency: Take the antenna formula number, for example, the half-wave length number, 492. Divide that by the length of the wire or the height of the vertical. For example, if an antenna company advertises a 12 foot half-wave vertical which works on 40 meters, take the number for the wavelength (in this case 492) and divide by 7.150 to get 68.8112 feet, rounded up to 68.9 feet. So it’s not a half-wave vertical on 40 (not even a quarter-wave-length vertical on 40). NOTE: The new number for a half wave is 492, not 468, per an article by Ward Silver, N0AX, “Where Does 468 Come From?”
Here’s the most important line from the article explaining why the ham community uses 492 for a half wave, 246 for a quarter wave, and 984 for a 1 wave, also called a “whole wave”:
“We’ve all seen this number over and over again—the ‘magic number’ that gives us the length of a half-wavelength dipole in feet from the dipole’s resonant frequency: L = 468/f. In free-space the wavelength in feet is 492/f, but a practical half-wavelength antenna is shorter so the constant is smaller. The number 468 is on the license exams and in the literature. It’s been there ever since I started reading about ham radio in the mid-1960s. It’s a pillar of amateur antenna theory. Every ham is expected to memorize it. And it’s wrong.”
Formula 4: To find out how long the antenna has to be in feet (for the entire length from end to end), take the wavelength number divided by the frequency to get the entire length in feet for a vertical. But if you’re building a dipole/inverted-V combo or an end-fed, divide the resulting number above by 2 to get how long each leg of the dipole/inverted-V combo or end-fed has to be. For example, take the half-wave length number, 492, and divide by 3.750 to get 131.2 feet for a vertical. If you’re using a dipole/inverted-V combo, add 3 inches to each leg at the tie-off end to get 131 feet, 8 inches for the entire antenna. Divide that by 2 to get 65.9, meaning 65 feet, 9 inches per leg. If it’s an end-fed, take 492 and divide by 3.750 to get 131.2 feet. Add 3 inches at the end of the wire leg to get 131 feet, 5 inches for the entire antenna. Divide by 2, because it’s going to be an end-fed with a coax leg and a wire leg, to get 65.7 and ½ inches per leg.
Formula 5: If you don’t know the number you want to work with for the wavelength number, take the 1 wave number (984) and multiply it by the lower number of the fraction. Then divide by the highest number of the fraction to get the wavelength number. For example, to find the number to work with for a 5/8ths wave antenna, take 984 times 5 and divide by 8 to get 615.
Formula 6: To get the antenna length in inches, take the wavelength number, divide by the frequency, and multiply by 12 inches.
Formula 7: If you do not know the frequency an antenna will transmit on, take the number for the wavelength you’re using and divide by the length you have available for your antenna. For example, 492, the number for a half-wave antenna, divided by a 24 foot dipole/inverted-V combo = 20.5, or 20.5 MHz, which is not in any ham band.
Formula 8: If you want to know how efficient your antenna is, this is how to figure it out. Take the length in feet that you have available and divide that number by the footage for the wavelength you want (at the middle of the band you want to operate) and you will get your antenna efficiency.
For a vertical, 131.2 feet of available space for a half-wave divided by the number of feet needed for a half-wave at 3.750 (the exact middle of 80 meters, which is 131 feet, 2 inches) = 100% efficiency.
For a dipole/inverted-V combo, 131.8 feet of available space for a half-wave (which includes the 3 inches on each end for the insulators), divided by the number of feet needed for a halfwave at 3.750 = 100% efficiency.
Next week I’ll be taking a deeper look at antenna efficiency. Stay tuned.
