QST - September 2010 - (Page 51)
GETTING ON THE AIR
A Close Look at the Terminated Folded Dipole Antenna
It is an age old saying that every antenna is a compromise. With every antenna design, it is important to know what is being given up as well as what is being gained. There is perhaps no antenna that is the subject of more controversy that the terminated folded dipole. This antenna is also known as a terminated, tilted folded dipole or T2FD. Its major characteristics are similar whether horizontal, vertical or tilted. What is promised is wide SWR bandwidth. What is given up is signal intensity or gain. This article will attempt to put all the cards on the table so an intelligent choice can be made.
So What’s a T2FD and Where Did it Come From?
The T2FD was first introduced to the amateur community in 1949 in a QST article by then Cmdr G. L. Countryman, USN, W1RBK, W3HH.1 He attributed the initial development to the US Navy, but did not provide a reference. Interestingly, in those days of open wire feed rather than coax, he was more interested in providing an antenna with omnidirectional characteristics over a wide frequency range than with a particular SWR, although he got both — the omnidirectionality over a 5:1 frequency range. He used two sizes to cover the HF spectrum as shown in Figure 1. His sizes were optimized for omnidirectionality, not always a priority for an amateur. He fed his antenna with 600 open wire line to a link coupled transmitter output circuit. In today’s world, the common configuration is to use a 12:1 transformer at the feed point and feed the antenna with 50 coax. It has also been observed that while omnidirectionality suffers on the higher frequencies, a reasonable match can be found over the entire HF spectrum using the longer version.
Figure 1 — Details of a tilted terminated folded dipole conﬁguration as proposed by Countryman in a 1949 QST article.
10 5 3
2 1.5 1.1 1.0 3 Frequency (MHz) 30
Figure 2 — 600 SWR of 93 foot terminated folded dipole. The blue plot is tilted, the red horizontal.
How Does it Play?
Countryman reported similar results for his tilted version to those from a similarly tilted 40 meter dipole during comparison
tests. He also provided an indication that it worked on 80 meters, although he did not do any comparisons on that band. A much later EZNEC assessment of the horizontal configuration by ARRL Technical Advisor Jack Belrose, VE2CV, was validated by some comparison testing to a half wave dipole on a number of frequencies.2 I chose to make a
comparison to a nonresonant dipole of the same length as the TFD, since I thought it would be a fairer comparison. The results described below track fairly closely to those of Belrose, taking into consideration the differences in our approach. Belrose used a commercial version of the TFD, still available from B&W.3
Countryman, W1RBK, W3HH, “An Experimental All-Band Nondirectional Transmitting Antenna,” QST, Jun 1949, pp 54-55.
Belrose, VE2CV, “Technical Correspondence — Terminated Folded Dipole,” QST, May 1994, pp 88-89.
Pagel, N1FB, “Product Review — The B&W Model 370-15 Antenna [now BWD-90 — Ed.],” QST, Mar 1981, pp 50-51.
Joel R. Hallas, W1ZR
QST Technical Editor
September 2010 51
Table of Contents for the Digital Edition of QST - September 2010
QST - September 2010
It Seems to Us: Disaster Drills: The Last Word?
This Just In
Guide to ARRL Member Services
Up Front in QST
A Solar Powered Repeater for Emergency Communications
A Portable Antenna Mast and Support for Your RV
One Ham’s DC Power Connector Preference
Attic Antenna Experiments
The Doctor is IN
Getting on the Air
Hints & Kinks
The Flight of PBH-8
Ham Radio Manufacturing: An Inside Look
JOTA 2010 is October 16-17
2009-2010 School Club Roundup Results
ARES®: 75 and Counting
2010 Simulated Emergency Test
This Month in Contesting
2010 ARRL DX Phone Contest Results
2008 ARRL International DX Contest (Phone) Scores
The World Above 50 MHz
Amateur Radio World
At the Foundation
Convention and Hamfest Calendar
ARRL VEC Volunteer Examiner Honor Roll
Field Organization Reports
75, 50 and 25 Years Ago
Index of Advertisers
QST - September 2010