WELCOME TO THE VE3FHM SHORT ANTENNA PAGE
Hi. My handle is Graham and here are some short antenna designs.
The first antenna is a combination of a folded dipole and loops. I started with a normal folded dipole and compressed it from the ends. The result is an antenna that is one third the length of a normal dipole. The driving impedance is 50 ohms. No matching network is required except that a one to one balun is recommended since the antenna is balanced. A diagram of the antenna follows. A half wave dipole is shown for comparison. Note that the dimensions are 0.158, 0.154, 0.138, and 0.051 wavelengths. The feedpoint is in the middle of the folded dipole portion as in a normal folded dipole.
This diagram shows how the antenna compares in size to a quad antenna cut for the same frequency. Note that all dimensions are in wavelengths.
This is the theory of operation. A normal folded dipole has a driving impedance of about 300 ohms and is 1/2 wavelength long. The current is a maximum in the middle and a minimum at the ends. Since maximum current is proportional to maximum magnetic field, most of the useful radiation occurs at the middle of the antenna. The 1/2 wavelength is needed to make the antenna resonant. In my design, the overall length of conductor is similar to a folded dipole so the antenna is still resonant but the ends are folded up in such a way that the physical length is shortened to 1/3 the length of the original folded dipole. The centre portion of the antenna that does most of the radiating is retained. Cancellations in the magnetic field occur in the folded up portions but the current is low there anyway so the efficiency of the antenna does not suffer much. The impedance drops from 300 ohms to 50 ohms but this is a benefit because the antenna becomes a match for 50 ohm coax. Because of the restricted length, the bandwidth of the antenna is about 1/3 that of a normal folded dipole.
Next follows a vertical version of the antenna for 20 m
This was built on a frame of 1"x2"x8' lumber. The vertical section was sharpened and driven into the ground and an 8' cross piece was attached at the top. The wire was lamp cord split in two. The dimensions were altered until a 50 ohm match was obtained. Note that you can't just cut the dipole version in half for this because the impedance would only be 25 ohms. The vertical section must be extended to increase the impedance. Don't expect miracles with this antenna but it has a takeoff angle of 30 degrees. If you can use more radials, this will drop.
For a 2 el beam design go to this site.
For a 160 meter antenna design go to this site.
For a three element 20 m. beam go to:
For a vertical dipole without radials go to:
Vertical loop dipole antenna for 20 meters
Another version of the vertical folded dipole loop antenna
For a multisection ground mounted vertical go to:
Multisection ground mounted vertical
These pages were started May 19, 1999 and were last updated on Apr. 24, 2003.
My email address is gstratford1@cogeco.ca should you wish to contact me.