Please watch for improvements. Last edited August 11, 2004.
First: if you find this site helpful, I would love to hear from you your comments and how you found this site. And if you build an antenna please let me know. I plan to post pictures and graphs here soon. Many Thanks, John zbtaz@yahoo.com March, 2001
WHAT IS THIS SITE?: This site provides info on making your own highly directional, high gain FM Antenna for the Lower FM band ONLY. If you want an antenna for the entire FM band, then I suggest you purchase either an omni-directional antenna for about $20 for local stations only --- or a directional LOG-PERIODIC design from either Radio Shack (cheap) or Antenna Performance Specialist (expensive) antenna, mount it on a motorized rotor as high as you can. But if you want a superb and inexpensive FM antenna to pick up more NPR and educational stations of the lower FM band only, read on....
What kind of antenna do I need?
For local FM stations you only need an omni, as mentioned. But for more distant stations you want three things: GAIN, DIRECTIONALITY, and GOOD ANTENNA CHARACTERISTICS including Front to Rear directionality and SWR (Standing Wave ratio) of less than 2, a constant impedance, etc.
A log-periodic design has much less gain than a YAGI. But log periodics are more forgiving over a wide frequency band. So if you want to listen to the whole FM band, buy a LOG-PERIODIC antenna, as mentioned above.
A YAGI design for the lower FM band has excellent gain, directionality when rotated, suppression of unwanted signals, and other antenna characteristics, (such as standing wave ratio and Q factor). No log periodic design of the same boom length can approach a good YAGI design. Any antenna does better if mounted as high as possible and outdoors, away from other objects.
Still with me?
WHY BUILD YOUR OWN LOWER FM BAND ANTENNA?
1.To receive more educational stations with greater program diversity, 2. To listen to the same program twice or more 3. To receive a cleaner and more recordable signal.
I used to receive only one FM station, my local one. I now listen to 4 in good mono to excellent stereo 90% of the time, and these are from 75 to 150 miles distant; and to another 12 stations in good mono to good stereo from 30% to 80% of the time, (from 100 to 175 miles distant. My listening choice is much wider and once you have a good antenna and tuner, THE MUSIC AND PROGRAMMING IS FREE!
SAFETY DISCLAIMER:
First - this website does not include complete or even adequate partial information on antenna safety. Please consult the ARRL (American Radio Relay League) ANTENNA BOOK, a local ham radio person, or an electrician -antenna installer if you have any questions on safety regarding working at heights, electricity, or lightning protection. Always rope up your self to a safety harness and disconnect all wiring to the antenna before working, this includes the antenna wire to the tuner. Please consult CODES for more info. Also - I disconnect the antenna at the tuner whenever there is a lightning storm or I am away. Most of this information comes from The Antenna Book, published by the American Radio Relay League. Most public libraries have a copy.
INTRODUCTION:
Though mounting the antenna requires some expertise, it is easy and inexpensive to build your own antenna. Virtually all commercial antennas greatly sacrifice signal characteristics (gain, front to back ratio, and Q factor),for more bandwidth, so that the commercial antenna, typically using a LOG-Periodic design can pick up all the FM band. However, for educational programming (classical, jazz, folk and cultural programming), 95% of all stations are in the low FM region, and are sufficiently concentrated in frequency range for a good Yagi design. This bandwidth is perfect for the far superior YAGI antenna design. And until about 10 years ago,(1989), computer designs were not widely available and antenna design was a black art. No longer is it! And broader band Yagi designs can now be built. This antenna design was constructed using ARRL software which is designed to very high engineering standards.
And if you wish to receive the rest of the FM band, you can use a $15 omni FM for your local stations higher in the band and just switch antenna wires at the tuner.
COSTS:
I have spent over 200 hours researching antennas and have a design free for you. This design is fairly large, 7 elements and about 15 feet long. A 5 element design of about 7 feet in length is also posted here. The cost is about $35 for building the boom and elements, and will take you about 3-5 hours. Current retail antennas are not as good and cost at least $150 and up. I bought the square tubular boom (Aluminum only)and elements (Aluminum solid rods) from the "cuts" pile at a local metal company for about $25. Hardware (stainless steel only) cost another $10. The mast should be at least 1.5" square tubing. Attach elements using the method shown in the ARRL ANTENNA BOOK (again available at most public libraries, AND WHICH YOU WILL NEED), which is in most libraries, using insulating hypalon rubber (more UV resistant than neoprene) and U bolts going through flat metal pieces.
HERE ARE THE ANTENNA CHARACTERISTICS FOR THE 15 FOOT ANTENNA:
Greater than 9.5 dBd, SWR (Standing wave ratio) less than 2, greater than 28 dB front to rear ratio, a bandwith from 87.7 to 92.3 Mgthz. A fairly constant resistance of about 22 ohms.
HERE ARE THE COMPUTER GENERATED DIMENSIONS FOR THE 15' BOOOM, 7 ELEMENT YAGI ANTENNA:
( Note :Elements are solid aluminum rods, perpendicular to the square or round hollow aluminum boom, the boom holds the rods in position, and the boom is attached to a mast, (a vertical support.) The reflector reflects radio energy back to the split driven elements which are separated by about 3/4 inch and directors direct additional energy to the driven element:
Element 1. Reflector...length..65.6", diameter 0.25" (apx), position 0.00"
Element 2. Driven..............30.3 each,2 pieces, diam 0.25, position, 11.57"
Element 3. Director 1..........60.2", diam 0.25, position, 23.1 "
Element 4. Director 2..........58.4", diam 0.25, position 53.1"
Element 5. Director 3..........56.7" , diam 0.25, position, 93.8"
Element 6. Director 4..........56.9", diam 0.25, position 141.9"
Element 7. Director 5..........55.0", diam. 0.25", position 178.2"
Note that Element #6 is longer than element #5. This is against the old rule of constantly decreasing element lengths toward the last director, and is an improvement which comes through with the new ARRL computer antenna design program.
HERE ARE THE DIMENSIONS FOR A SMALLER, 7' , 5 ELEMENT ANTENNA:
Element 1. Reflector..........Length 65.7, position 0.00"
Element 2. Driven..........30.2" (each of 2 pieces),position 11.2"
Element 3. Director 1 .....59.8, position 22.5
Element 4. Director 2 .....58.4, position 53.4
Element 5. Director 3 .....55.5, position 88.0
HERE ARE THE ANTENNA CHARACTERISTICS FOR THIS 7 FOOT ANTENNA:
This antenna has total length of 7' 4" , a gain of 7.5 to 8.3 dBd over the design frequencies of 88 to 93 Mghrtz, a SWR of less than 2 except above 92.5 mghtz where it reaches 4 at 93mghtz, and a Front to Rear ratio of 15 to 30 over the span from 88 to 93Mghtz. The characteristics of this antenna down to 87 mghtz are only slightly compromised.
FURTHER NOTES:
The element lengths should be built within 0.1 inch tolerance, the diameter can be from smaller or bigger stock but 1/4 inch works well. The position should be within 0.5 inch tolerance. The driven element is the only element cut in two. Drill small holes about 1/2 inch from the inner ends and screw stainless tap screws with washers to hold your RG-6 coax leads in place. Use rubberized goo to pack the coax and protect it from water. I bought this from Radio Works located in Portsmouth Va.
Elements must be electrically isolated. I use 1/16" thick Hypalon rubber, which may be purchased from your local rubber supply house, (possibly as a scrap piece for free). See the Antenna Book at your local library for designs on how to mount the elements to the boom.
Elements are individual antenna rods. Position is the position from the Reflector element (the back of the antenna).(Tolerance is +/- 1/4 inch. But try for 1/10 inch.) The only element which is cut into two pieces is the Driven Element. Attach RG-6 Coax to this element.
Parts Suppliers: Radio Works has Kevlar rope with Dacron outer sheath fo rUV protection. ARRL can sell you the Antenna Book and Physical Design of Yagi Antennas by D. Leeson.
The Antenna Book, ed 18, 1997 has the requisite software to design any antenna of Yagi type in a fairly broad band fashion. Many libraries carry this book, but their copy may be an older one which may show Yagi designs which are quite different and inferior.
Please see the following pages in the Antenna Book: (These same pages are in most editions although the exact page may be different and require a little searching.):
1. Mast guying : page 22-5
2. Height gain for distances to station: page 23-10
3. Common mode balun (advanced balancing aid to improve antenna): page 26-19
4. Impedance Step up Balun (I have not yet perfected the use of this balun which should increase signal strength.) page 26-22
5. Cable attenuation (RG-6 is not shown but is just below RG-8 Page 24-16
In Neeson's book, Physical design of Yagi Antennas (Harder to find by far and I plan to post my own drawings soon.) :
1. Triple boom guying (recommended for this 15' antenna) page 6-38
2. Torque compensator ( for wind compensation which will save the rotor)
3. Mounting elements to boom: page 5-20
4. Drilling a straight hole in a round tube page 6-42.
Position your antenna as high as you can. Mine is about 50' off the ground using a 25' Radio Shack mast attached to my attic dormer. Guy the antenna as shown in Physical Design of Yagi Antennas page, with a very shallow guy angle. That is the Guy lines should go well away from the mast at an angle (from the horizontal) of no more than 45 degrees. 6-38.
The yagi design should be mounted on a rotor so that you can aim to your station.
I highly recommend a good tuner such as those made by Magnum Dynalab. Analog tuners are best, and the Dyanlab's sound quality is excellent.
In the future, I will try to include some photos and design graphs here. Feel free to contact me at my web address. I had over 1000 hits from April to December, 2000 and hope this is useful to radio enthusiasts. If you know of ways to promote this web site I would be very interested. Comments welcome.
Links to other sites on the Web
American Radio Relay League
Radio Works - supplier for guying, etc
Rotor Doc - rotators supplier
Another antenna website
© 2000 zbtaz@yahoo.com