Transcript Wide Band Folded Dipoles

Wide Band Folded Dipoles
A comparison between Wide Band
Folded Dipole (WBFD) and an Open
Wire Fed Dipole (OWFD)
18 October 2003
By NNN0JQH
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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 (WBFD’s) as a possible
solution.
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By NNN0JQH
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Claimed Advantages of WBFD’s
• Low SWR (<2:1) over their specified
operating range
• Since they are folded they are
approximately one half the length of a
regular 1/2 wave dipole
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By NNN0JQH
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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
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By NNN0JQH
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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
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By NNN0JQH
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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.
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By NNN0JQH
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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
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By NNN0JQH
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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
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By NNN0JQH
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WBFD Issues
• The “matching resistor” satisfies the
transmitter as far as SWR but does nothing
to change the fact there are very high
SWR’s on the antenna and therefore high
currents and voltages.
• The resistor absorbs this “reflected” power
at a cost of 50-90% converted to heat.
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By NNN0JQH
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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.
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By NNN0JQH
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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
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By NNN0JQH
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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
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By NNN0JQH
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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
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By NNN0JQH
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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
18 October 2003
By NNN0JQH
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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
won’t work for you a WBFD may work
with the understanding that the performance
is much poorer.
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By NNN0JQH
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