DIY Logper Antenna

Here you will find the describtion and all needed details about the log-periodic antenna (Logper, LPDA) I designed for the 1.0GHz – 2.5GHz frequency range, mainly for the reason to have a single antenna to cover the 23cm frequency range and the lower wireless network range at 2.4GHz.

The basic antenna was designed after a tutorial from Wolfgang Rolke which you can find on his homepage at http://www.wolfgang-rolke.de/antennas/ant_400.htm (in German language only).
He provides a simple tool to calculate all needed construction dimensions for the Logper. Available for download also on his page at http://www.wolfgang-rolke.de/antennas/ant_500.htm. Scroll the listing down to the bottom of the page, the ZIP file contains the QBasic code and an EXE file as well, so no need to deal with QBasic.

I made the antenna after the calculated dimensions from the tool, but as revelead on measurments a lot of mechanical tuning was necessary to obtain the results you will see later on.
The final dimensions of the Logper antenna are given in the drawing in addition to the calculated dimensions.

In the diagrams you will see that the antenna, initially designed for 1.0GHz – 2.5GHz, covers a bigger spectrum especially below 1.0GHz. All frequency ranges for DECT, GSM900, GSM1800 and UMTS are handled as well with an VSWR better than 1:2.0.

The theoretical calcualtion

The given parameters and calculated dimensions from Wolfgang Rolke´s tool:


Antenna from top

Parameters

The dipole length are of one halfe dipole, see the drawing!

The practical result

Drawing with final dimension
Drawing with final dimension
logper_drawing.pdf
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Details

Construction hints

  • Feedlines are made from square 5x5mm brass tube
  • Dipoles are from steel wire
  • To solder the steel wire you need either a solution of Zinc chloride and Salmiac in water or pure Zinc chloride to remove oxides on the steel during solder process
  • A powerfull solder iron is needed as well
  • Length, diameter and distance of dipoles are not to critical, but should be close to the calculation
  • Make the length of the dipole a bit bigger than needed and cut them later when soldered to the feedlines
  • Length of each half dipole is given from the centre of the feedline, so depending on which edge of the feedline you start measuring you have to add or subtract 2.5mm (remember feedline is 5x5mm square) from the calculated dipole length
  • Spacing of the feedlines is critical
  • The cap at the feedpoint is not critical but should not be to much, 1mm is good and easy to maintain
  • Use only washers and screws from plastic to mount the feedlines together
  • Since the coax cable is routed through the lower feedline the srews shouldn´t be to long to prevent the coax to be damaged
  • Don´t tight the screws to much or they will be damaged as well
  • RG316 coax is used with an SMA connector at the end
  • A connection of the feedlines at the lower frequency end (the stub) was foreseen, but I removed it because the measurements showed better results without
  • The antenna must be mounted in a way that both feedlines will be isolated always, for example when mounted on a pole
  • If you have access to appropriate equipment then measure the antenna and tune it for best results

Pictures of the final Logper antenna


Antenna from top

Antenna from top


Antenna from side

Antenna from side


Front view of feedpoint

Front view of feedpoint


Sideview of feedpoint

Side view of feedpoint

Results and Diagrams

Measurements were taken with a Rohde&Schwarz FSH6 and FSH-Z2 VSWR bridge in vector analyzing mode, of course calibrated for each run.
Markers were added on insteresting frequencies to show the results in detail.

VSWR Diagrams measured for verification with an Anritsu Vector Network Analyzer

Results are almost equal to the first measurement results so the design is proofed to work fine on the calculated frequency range and VSWR <2.0:1.

2 thoughts on “DIY Logper Antenna

    • A balun is not needed, the LPDA can be made for 50Ohm match. In case if you don´t need a directional antenna look for the Discone I made. Easier to build in my opinion.

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