QST - September 2010 - (Page 38)
Attic Antenna Experiments
Outside antennas are recommended, but attic antennas can work, too.
Max E. Norman, W2IQE
move to a retirement community with restrictive covenants 7 years ago presented me with problems in continuing a long Amateur Radio career. Some research into indoor antennas in The ARRL Antenna Book provided encouragement about their use and since this house has easy attic access, I thought I’d give an attic antenna a try.1 I decided to go with a 40 through 10 meter trap dipole.
The First Indoor Antenna
The traps were constructed based on details from a QST article using PVC tubing and coax.2,3 A grid dip meter was used to check the resonance of each trap. The length of the antenna when finished was 39 feet. It was mounted in the attic at about 15 feet above ground at the center with the ends at roughly 13 feet. The antenna performed well although running low power (QRP) with an indoor antenna requires patience. In addition to many stateside contacts, the antenna performed well into Central and South America. The only downside of the trap antenna was its limited VSWR frequency range on each band.
Solving the SWR Problem
Recently MFJ introduced the MFJ-927 Remote Automatic Antenna Tuner.4 This tuner looked like the perfect candidate for feeding a remotely tuned multiband attic antenna. A big advantage of this autotuner is that no cabling, other than the coax feed line, is required since the coax is used to supply 12 V power to the unit. Using the house construction drawing, I determined that a center fed 40 meter dipole could be installed though the open truss bays from the center of the house to the front corner and to the opposite rear corner. While the trap antenna could work well with the tuner, a center fed 40 meter size dipole would provide additional gain on higher frequencies and be more efficient. As you can see from the layout drawing (Figure 1), the roof truss vertical supports required the use of several intermediate insulators (made from Plexiglas) as well as a few others to insure better clearance. The antenna is about 20 feet above ground at the center and approximately 11 feet at the ends. The tuner is mounted on one of the vertical truss supports near the center of the antenna with 7 feet of open wire line from the tuner to the antenna. Since the output of the tuner is
38 September 2010
Figure 1 — Attic construction drawings provide a road map to obstructions as well as antenna support possibilities.
unbalanced, a 1:1 balun was installed at the tuner output to provide a balanced output to the feed line.
The Results are In
The auto tuner tunes the antenna to a 1:1 VSWR across all amateur bands from 40 meters though 10 meters. Running 10 W to the antenna on 40 meters, a contact (QSO) was made with CM3GW in Cuba despite the extremely noisy conditions in Florida at night. That same evening, another 40 meter QSO was made with DK3FW near Hanover, Germany. The antenna performs very well on 40 meters. Unfortunately, propagation on 20 meters and higher bands has been very poor preventing a realistic antenna evaluation. Hopefully, this will change in the near future as propagation improves. The bottom line is that if you’re faced with not being able to erect an outside antenna, consider experimenting with an attic antenna. You might be pleasantly surprised.5
no. 9876. Telephone 860-594-0355, or tollfree in the US 888-277-5289; www.arrl.org/ shop; firstname.lastname@example.org. 2R. Sommer, W4UU, “Optimizing CoaxialCable Antenna Traps,” QST, Dec 1984, pp 37-42. 3R. Johns, W3JIP, “Coaxial Cable Antenna Traps,” QST, May 1981, pp 15-17. 4www.mfjenterprises.com 5Note that an inside antenna often results in transmitted RF in proximity to people and animals. Be sure to perform a careful RF Safety assessment as required by the FCC. While low power HF operation generally does not require a detailed assessment, in cases such as this it should be performed to verify compliance.
ARRL Life Member Max Norman, W2IQE, was first licensed as WØUJE in 1948 and now holds an Amateur Extra class license. Max received a BSEE degree from Iowa State College and has attended graduate school at St Louis and Columbia Universities. Max is now retired. You can reach Max at 4624 Turnberry Ln, Lake Wales, FL 33859 or at email@example.com.
Notes D. Straw, Editor, The ARRL Antenna Book, 21st Edition. Available from your ARRL dealer or the ARRL Bookstore, ARRL order
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