Wide Band Folded Dipoles

1. Wide Band Folded Dipoles A comparison between Wide Band Folded Dipole (WBFD) and an Open Wire Fed Dipole (OWFD) By NNN0JQH

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 (WBFD’s) as a possible solution. By NNN0JQH

3. 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 By NNN0JQH

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 By NNN0JQH

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 By NNN0JQH

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. By NNN0JQH

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 By NNN0JQH

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 By NNN0JQH

9. 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. By NNN0JQH

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. By NNN0JQH

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 By NNN0JQH

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 By NNN0JQH

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 By NNN0JQH

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 By NNN0JQH

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 won’t work for you a WBFD may work with the understanding that the performance is much poorer. By NNN0JQH