Wide Band Folded Dipoles

1
Wide Band Folded Dipoles

• A comparison between Wide Band Folded Dipole
  (WBFD) and an Open Wire Fed Dipole (OWFD)

2
Introduction

• Finding suitable H.F. antennas with wide
  bandwidths or having enough individual antennas
  to cover the spectrum piece by piece with good
  efficiency is a challenge.
• There has been interest in Wide Band Folded
  Dipoles (WBFDs) as a possible solution.

3
Claimed Advantages of WBFDs

• Low SWR (lt21) over their specified operating
  range
• Since they are folded they are approximately one
  half the length of a regular 1/2 wave dipole

4
Sources

• Most of the data was taken from two articles by
  L.B Cebik W4RNL who is the author of the ARRL
  Antenna Modeling Course. They can be viewed in
  their entirety at http//www.cebik.com/wbfd.html
  and http//www.cebik.com/t2fd.html
• http//www.n1nc.org/Newletters/2003/12101.pdf
• The ARRL Handbook and Antenna Manuals

5
Test Conditions

• WBFD and OWFD of equal overall length (90 ft)
• WBFD design frequency 2-30 MHz
• OWFD fed with open wire from a tuner
• Equal height

6
Problems Common to Both

• SWR varies cyclically in relation to the antennas
  length vs. the frequency in wavelengths.
• At frequency increase and the antenna becomes
  greater than 1.25 wavelengths long it develops
  lobes. The lobes have nulls that can be 20-30 dB
  deep. At 25 MHz there are 8 lobes.
• Height above ground affects both equally.

7
WBFD Considerations

• Wire spacing has little effect comparing 8 and 16
  inch
• SWR for an antenna at 20 feet typically between
  1.2 and 2.2 (lower will be higher)
• Should be within the tuning range of modern
  transceivers
• Certainly in the range of external tuners

8
OWFD Considerations

• For all band operation must be fed with open wire
  feed from an antenna tuner
• Has greater than 5 dB gain over the WBFD over the
  entire frequency range
• Lighter weight and no twisting problems

9
WBFD Issues

• The matching resistor satisfies the transmitter
  as far as SWR but does nothing to change the fact
  there are very high SWRs on the antenna and
  therefore high currents and voltages.
• The resistor absorbs this reflected power at a
  cost of 50-90 converted to heat.

10
WBFD (continued)

• WBFD has a knee in the gain curve that falls off
  dramatically below 7 MHz.
• The knee can be moved lower by lengthening the
  antenna. This worsens the problem of lobes at
  higher frequencies.

11
OWFD

• In the OWFD/tuner combination reflected power is
  kept on the antenna and only the losses are the
  resistive losses of the wire
• Most of the power is radiated

12
The Numbers

• 5 MHz WBFD 7 dB below the OWFD
• 4 MHz WBFD 10 dB below the OWFD
• 3 MHz WBFD 15 dB below the OWFD
• 2 MHz WBFD 24 dB below the OWFD
• These are significant values with a negative
  effect on communications capability

13
2 MHz on-the-air Tests

• Comparing full length 160 meter dipole at 65
  feet, 90 foot WBFD and full length 80 meter
  dipoles at 25 feet
• Logged listening tests on stations across SNE for
  several weeks
• Compared to the 160 meter dipole the WBFD was
  down 5 S-units.
• The 80 meter dipole was down 4 S-units

14
On-the-air Test (Continued)

• In terms of SWR the WBFD was matched to the
  transceiver. The 80 meter dipole was not.
  Inserting a matching circuit between the 80 meter
  dipole and the transceiver would recover much of
  the lost signal making it several S-units better
  than the WBFD

15
Wrap Up

• If you want to be on the low bands and
  circumstances require a minimum length antenna an
  OWFD will give you better performance.
• If frequency hopping is required and a tuner
  wont work for you a WBFD may work with the
  understanding that the performance is much poorer.