Short Linear Antennas

Short Linear Antennas
Table of Contents
  • Building Linear Antennas to fit restricted space requirements
  • Applying the linear design to a Flagpole Antenna
  • Why you need a good Ground System

Building “linear” (short) antennas to fit restricted space requirements

Co-Linear loading is folding apportion of the antenna back against itself, in order to reduce the physical length of an antenna to the restrictions imposed on the design. This technique has been used for decades in dipoles antennas; linear dipole and double bazooka dipole are examples. Generally the physical length can be 60% to 70% of a conventional quarter wavelength size without RF performance changing. As an example case; consider a 22 Foot Fiberglass hollow flagpole you want to use as a 40M vertical antenna.

Linear loading Schematic

Electrically, a 22 Foot linear antenna looks like a 33 Foot vertical (¼ wavelength) radiator with 80 Ohms impedance at the feed point instead of 36 Ohms and works the same as a full size radiator over the same ground plane. This makes the radiator resonant on 40M but a physical smaller size. The linear is better than a coil load that would be 40% the efficiency of the linear in this case. If you have an aluminum 22 Foot flagpole and us a load coil or remote tuner at the base feed point as a multi-band antenna the solid aluminum pole will provide 96% efficiency compared to the linear antenna 99% efficiency.

Each results in a compromise between the band, feed length and the location. You should carefully consider your tuner performance to see the predicted capacitance and inductance is exceeded with margin before making a decision Remember an “Antenna System” is the radiating element(s), matching network (tuner) and feed working together.

Demonstration Case

Test Setup for Linear Antenna

As test  antenna was measured at 22 foot linear using a piece of triple #22 AWG conductor zip cord commonly used for older home intercom by Nutone. The base feed was run 11 feet up on an outside conductor, then returns 11 feet down on the center conductor back to the base and is connected to the other outside conductor that is the full 22 feet long. Resonance was at 8.1 MHz or the 22 foot antenna "looked" like a 29 foot antenna element. The higher resonance frequency was attributed to the capacitance of the zip cord.

Measuring a Linear Antenna

Applying the linear design to a Flagpole Antenna

The Villages, Florida has restrictions against erecting any sort of antenna so we must camouflage or hide our antennas. The local HAM club members decided the interest level in using flagpole antennas was so great we needed to pool our resources and information here. Everyone wanted multiband capability 80M to 10M that looked good and performed great; VSWR less than 2, better than 90% efficieny and a high quality Ground Plane for DX reach. The group was divided on using PVC or Aluminum as the pole so both are addressed. The next video covers the basic and most popular type of antenna--the quarter wavelength long vertical operated against a ground-plane constructed in the form of a flagpole. The antenna is usually made from metal or plastic tubing and the radials are wire. The design is for operation on 80M, 40M, 20M, 17M, 15M, 12M and 10M bands with first priority to 40M and 20M bands.  The feed-point impedance of quarter-wave ground plane is on the order of 36 ohms. The actual impedance depends on the number of radials, soil conditions and proximity to buildings. Note, if the "linear" vertical is selected, the impedance is doubled. The quarter-wave ground plane is essentially a single-band antenna. However, a quarter-wave vertical can also be used as a three-quarter wave vertical. Luckily a tuner or matching coil will provide a relatively low-impedance feed from 80M to 10M. An ideal ground plane would be saltwater, but 32 wire radials with a radius of one-quarter wavelength or more will do. However an antenna with only a few radials will work and provide reasonable performance. The serious DX operator should consider 32 or more radials.

Flagpole Antenna