Homebrew antennas

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Antennas are tuned circuits designed to resonate at a particular frequency or band. Homebrew antennas can be quite simple to construct out of materials you may find laying about. Most of the information on this page will assume you have little to no knowledge on how to make an antenna, and will try to shy away from a lot of technical jargon and formulas, instead relying on basic easy to follow instructions.

Contents

Intro to Antennas

There are two basic classes of antennas. Receiving only antennas and transmitting antennas. While you can receive on a transmitting antenna, you should never ever transmit on a receiving antenna. You can damage the radio. The antenna on your car for your radio is probably a receiving only antenna, while the antenna on your mobile phone is a transmitting antenna.

Wavelengths

In order to understand antenna systems you have to understand a little bit about wavelengths. Radio signals are sine waves, this means that they oscillate up and down like waves in the ocean. The period of one cycle on that sinewave is a wavelength. Historically the greek lambda symbol (Λ) is the symbol used for wavelength. To calculate it you can use the following algorithm:

Λ = C/F

  • C is the speed of light, which can be rounded to 300,000,000 meters/sec or 984,000,000 feet/sec.
  • F is the frequency of the transmission.

If you express the frequency in MHz you can divide the speed of light by 1,000,000 and make it even easier. To find the wavelength of 150MHz you would do:

  • Λ = 300/150 = 2 meters
  • Λ = 984/150 = 6.5 feet

(I omit using the square root of the dieelectric constant of the propagation medium for simplicity, in general this is very near 1 so it can safely be omited for practical measurements)

Propagation

You should be familiar on a basic level with propagation of radio signals. In basic terms, with lower frequencies the closer the antenna is to the ground the more it will interact with it. This can be an advantage depending on what you want to do.

NVIS (near vertical incidence skywave) antennas are good for shorter range communications on HF. The military defines NVIS as antennas that cover about 500 miles, or what a battalion may cover. Ham radio people generally consider it NVIS if its 700-1000 miles or less coverage.

If you have a horizontal dipole antenna which is less than 1/2 wavelength from the ground it will have a high angle of radiation. What this means is that the angle that the radio signal travels off the antenna may travel off at 75-80 degrees, or almost straight up. When it hits the ionosphere it will be reflected almost straight down.

If that same antenna is raised up higher, then the angle of radiation will be lower, perhaps 20-25 degrees. This will cause it to travel further across the planet before it hits the ionosphere and bounces back to the surface. This gives you much more range.

The beauty is that you can use the same antenna for both of these types of communications. Additionally if your antenna is capable of operating on multiple bands (frequency ranges), you may be able to mount it such that one band is NVIS and the other is DX (long distance).

Antenna FAQ

Won't a bigger antenna work better?

This may seem logical, but there are limits. If the antenna is too large, you will see decreased performance. Additionally the length of an antenna can affect how it radiates RF energy if it is used for transmitting. This may make the antenna perform worse if it is too long. In general though, for reception, the bigger the better.

Can't I just use a coat hanger?

For radio reception, any antenna that gets results is fine. A coat hanger or wet string may work, but other things would be better. Generally, the better a conductor the metal is, the better it will perform. Aluminum foil works well, especially compared to a wet string.

Can I replace the antenna on my radio?

Maybe. Some radios it is illegal to change the antenna. This is to prevent people from pushing out too much power when they transmit. Other radios have special antennas that the radio will rely on when it transmits, if you change the antenna you may harm the radio when you transmit. In general as long as you replace the antenna with one of a similar type it will work, especially if it is a receiver and not a transmitter that you are connecting it to. With transmitters you need to match the resonant frequency and RF impedance (more on this later).

What is the best antenna?

This question is impossible to answer by itself. Antennas are tools, it depends on the radio you have, how you are going to use it, and what materials and space are available to you. A 10 foot antenna may work well if you are at a base location, however it will be difficult to carry around and use in a portable situation.

Are antennas dangerous?

YES! If you are transmitting through an antenna with any amount of power you can get a burn or start a fire with an antenna. For receiving, the only danger is poking your eye out because you did not see the antenna. Those foam antenna balls for cars are great for home scanners to prevent this. Mobile phones and most handheld transmitters do not transmit enough power to cause problems, however larger mobile or base units can. The tips of the antenna are generally hotter than the place where the feedline connects.

Terms

  • Bandwidth - this is the frequency range that an antenna is suitable for. This is usually expressed as the range (300khz for example) and not the specific frequencies.

Antenna Plans

Roll up J-pole

wire antenna suitable for transmitting. It can be made for any frequency, however it is more practical for higher frequencies. FRS, GMRS, 2m/70cm Ham, MURS even CB are all suitable for this antenna.

Roll up Dipole

wire antenna better suited for lower frequency transmitting. This antenna can be used in a variety of different ways to provide a variety of different solutions to the problem of how do you get a signal from your radio to someone elses. this can be made for all frequencies.

coaxial collinear

vertical antenna made out of coax cable. With high gain properties this antenna is idea for base operations and due to size it is not as portable. This antenna is really only practical for FRS, GMRS, 2m and above Ham, and MURS due to the resulting size that would exist.

tiger tail

improve the performance of your hand held with a simple strand of wire

tie an antenna

How to tie a roll up dipole or similar types of wire antennas

Protecting your HT Antenna

How to help prevent your flexible HT (hand held transceiver) antenna from breaking

See Also

References

External Links

Personal tools