W5GI Mystery Antenna
A multi-band wire antenna that performs exceptionally well even though it confounds antenna modeling software
Article by W5GI ( SK )
The design of the Mystery antenna was inspired by an article written by James E. Taylor, W2OZH, in which he described a low profile collinear coaxial array. This antenna covers 80 to 6 meters with low feed point impedance and will work with most radios, with or without an antenna tuner. It is approximately 100 feet long, can handle the legal limit, and is easy and inexpensive to build. It’s similar to a G5RV but a much better performer especially on 20 meters.
The W5GI Mystery antenna, erected at various heights and configurations, is currently being used by thousands of amateurs throughout the world. Feedback from users indicates that the antenna has met or exceeded all performance criteria. The “mystery” part of the antenna comes from the fact that it is difficult, if not impossible, to model and explain why the antenna works as well as it does. The antenna is especially well suited to hams who are unable to erect towers and rotating arrays. All that’s needed is two vertical supports (trees work well) about 130 feet apart to permit installation of wire antennas at about 25 feet above ground.
The W5GI Multi-band Mystery Antenna is a fundamentally a collinear antenna comprising three half waves in-phase on 20 meters with a half-wave 20 meter line transformer. It may sound and look like a G5RV but it is a substantially different antenna on 20 meters. Louis Varney’s antenna, although three half waves long, was an out-of-phase aerial. Mr. Varney had two specific reasons for selecting a 3 half waves on 20… he wanted a four-lobe radiation pattern, at least unity gain and a low feed point impedance. The Mystery antenna, on the other hand, presents a six-lobe pattern on 20 meters, gain broadside to the antenna, and also low feed point impedance to simplify matching the antenna to the rig. Additionally, the Mystery antenna is designed to work at least as well, on the other HF bands as a G5RV. In short, the Mystery antenna is a sky wire that incorporates the advantages of a 3 element collinear and the G5RV antenna.
In its standard configuration, a collinear antenna uses phase reversing stubs added at the ends of a center fed dipole. These stubs put the instantaneous RF current in the end elements in phase with that in the center element. You can make these phase reversing stubs from open wire line or coaxial cable. Normally, a shorted quarter-wave stub is used, but an open-ended half wave stub would also work. The problem is that the dangling stubs are unwieldy and or unsightly.
An article written by James E. Taylor, COCOA- A Collinear Coaxial Array, published in 73 Amateur Radio, August 1989, describes a low profile collinear coaxial array.
According to Taylor, when you apply a RF voltage to the center conductor at the open end, the stub causes a voltage phase lag of 180 degrees at the adjacent coax shield. This happens because the RF is delayed by one quarter-cycle as it passes from left to right, inside the coax to the shorted (opposite) end. There’s another quarter-cycle delay as the wave passes back from right to left inside the coax and emerges on the shield at the open end. Add up the delays and you get a total time delay of one-half cycle, or 180 degrees. In essence, the coax section serves two purposes: it provides the necessary delay and provides part of the radiating element in a collinear array.
The first prototypes of the Mystery antenna used the Taylor formulas, which which called for cutting the wires to a quarter wave length using the formula 234/f(Mhz) and the coax, using the same formula, but applying an appropriate velocity factor. The first version of my antenna worked well on 20 meters but failed as a multi-band antenna.
The second antenna was built with constructed with the coax cut to the same length as the wire. This was done with the belief that perhaps the coax didn’t behave like coax and therefore the velocity factor wasn’t applicable. Surprisingly, the new antenna performed exceptionally well on 20 meters, had low SWR and performed just as well on the other HF bands and 6 meters as my G5RV reference antenna.
The W5GI Multi-band Mystery Antenna looks like a plain dipole (see figure1 and photo A below) and is very simple to build.
Builders of the Mystery antenna will need the following materials:
- 3 wish bone insulators
About 70 feet of wire (14 gauge household electrical wire works well,)
Sufficient twin lead or open wire to make a half wave section on 20 meters. Window-type 18 gauge 300 ohm ribbon works best. The Wireman is an excellent source for antenna wire and 300 ohm line.
- 34 feet of RG8X mini-coax
- An electrical connector, available from most electrical parts stores, to connect the twin lead and coax
- Shrink tubing to cover the exposed coax joints
The antenna can be built in less than an hour when you have the above materials. When you’re ready to proceed, perform the following steps:
- Cut the electrical wire into four equal lengths of 17 feet.
- Cut the two lengths of coax to 16’6 each.
- Cut a 20 meter half-wave section of twin lead. This piece needs to be adjusted by its velocity factor. If 300 ohm window type line is used with a VF of .91, the total length will be 30 ft. Alternatively, 450 ohm, solid 300 ohm or homemade open-wire line can be used provided the electrical length is on-half wave on 20 meters. Actual length will vary, typically between 27 and 35 ft., depending on type and velocity factor.
- Trim two inches of braid from one end of both lengths of coax (Item A).
- Trim one inch of braid and center insulator from the opposite end of both coax sections (Item B).
- Build a 20-meter dipole without end insulators. Note: The next two steps 7 and 8 of the construction process involve connecting only the “inner” end section of the coax section to one end of the dipole; the shield is not connected to anything here. At the other end of the coax section both the coax shield and second wire section are connected to the coax center conductor.
- Connect one end of the dipole to the center conductor of the coax (Item A) and cover with shrink tubing as shown in photo B below.
- Connect the opposite end of the coax (Item B) to braid AND quarter wave wire section, cover with shrink tubing, and connect to end insulator as shown in Photo C below.
- Install the twin lead through the holes of the center insulator (you may have to enlarge the holes) and solder to antenna wire as shown in photo D below.
- Connect the opposite side of the twin lead to the coax as shown in photo E below. Almost any type of connection will work provided the connection is stable and sealed properly.
- Install the antenna with the center conductor at least 25 feet high. Mine is installed in a horizontal plane; however, others have installed the W5GI antenna as an inverted-vee and are getting excellent results.
On 20 meters, you should expect 3-6 dB gain over a dipole and a 6-lobe radiation pattern with an elongated figure 8 pattern perpendicular to the plane of the antenna. This is typical of a 3 element collinear array. For a simple explanation of collinear arrays read “Troubleshooting Antennas and Feed lines” by Ralph Tyrrell, W1TF. On all other bands the antenna performs like a G5RV, which is really a random length dipole on all but 20 meters. M. Walter Maxwell, in “Reflections II, Transmission Lines and Antennas”, aptly describes this phenomenon. Several users report it is possible to use the antenna on 160 meters but you will need to connect the twin lead together at the point where it connects to the coax. On 160, the antenna performs like a Marconi. Those who have used the antenna on 160 say the W5GI Mystery antenna is a quieter receiving aerial compared to other 160-meter antennas.
As for the theory of operation, it remains a mystery. At least three experts tried computer modeling the antenna. All three rendered completely different findings.
You will enjoy building a W5GI Multi-band Mystery Antenna! Many hams has done so and find it to have been a fun project and an excellent performer.
(1) Information on this page has been taken from an article published in the July, 2003 issue of CQ magazine. You can download a copy of the article in Adobe Acrobat format by clicking HERE.
(2) W5GI will build an antenna for a nominal fee. Discount prices start at $65.00, plus shipping, for the W5Gi multi-bander. Mono band antennas cost more because a 4:1 balun is used.
(3) For additional information, or to order an antenna, please call or send an mail.
(4) Dimensions for the mono-band antenna:
|AND||Inside wire||Coax||Outside wire||Overall length|
|10.1||23′ 10″||23′ 4″||23′ 6″||141 ft 4 inches|
|14.18||17′ 2″||16′ 8″||16′ 10″||101 ft 4 inches|
|18.13||13′ 7″||13′ 1″||13′ 3″||79 ft 10 inches|
|21.25||11′ 9″||11′ 3″||11′ 5″||68 ft 10 inches|
|24.9||10′ 1″||9′ 7″||9′ 9″||58 ft 10 inches|
|28.5||8′ 11″||8′ 5″||8′ 7″||51 ft 10 inches|
|50.125||7′ 10 “||7′ 4″||7′ 6″||45 ft 4 inches|
(4) Dimensions for the multi-band antenna:
|Inside wire||Coax||Outside wire||Overall length|
|17′ 2″||16′ 8″||16′ 10″||101 ft 4″|
This antenna uses a twin lead matching stub instead of a 4:1 balun.
- Use only 300 ribbon line for the matching stub. Start with 34 ft 7″, trim as necessary to obtain lowest SWR.
- Mono-banders with either a voltage or current (preferred) 4:1 balun.
- This antenna exhibit significant gain only on 20 meters. On all other bands the antenna performs like a G5RV.
Can RG-58 [or any other coax] be substituted for RG-8X (mini)?
Theoretically it should work, at least electrically. The main concern is the strength of the smaller wire. A portable Mystery Antenna using RG-174 is in the planning stage. If anyone has tried using different cables, please pass along your thoughts and observations for posting on this website.
[Note: disregard velocity factor (VF) of coax-it is not applicable in my design]
Can I use twin lead or open wire line directly to an antenna tuner in lieu of coax and a twin lead stub?
Absolutely. The twin lead stub is only necessary to give the antenna its multi-band characteristics.
How can I determine the twin lead Velocity Factor?
The best way to determine velocity factor is to ask the distributor or manufacturer. It is very important that the correct VF be used in calculating the length of the twin lead. 300 ohm ribbon is a good choice. There are numerous types available. The VF range from 80 to 90 percent-for 300 ohm line, which means the half wave line could be anywhere from 26.4Ft to 29.7FT. 450 ohm ribbon line typically has a VF of 95%, which means the length should be 31.35 FT. The ARRL Antenna book list VF for different lines.
[Note: Use 33 feet as the 20 m half wavelength for the above calculations.]
Should I use a balun?
A balun is unnecessary and will only degrade performance. A line isolator between the end of the twin lead and the coax is recommended. It helps minimize feed line radiation and stray RF, which can cause problems in the shack.
[Note: The original article recommends the use of a 4:1 balun for a mono-band version of the Mystery Antenna; however, a balun is no longer recommended. Instead, try a quarter wave 75 Ohm matching transformer. RG59 (Beldon 8241) is a good choice. A section of 75 ohm cable cut for 20 meters 10.89 feet can be used with the the multiband antenna. It will lower the SWR on 20 m to close to 1:1 and is transparent on the others bands.]
I get higher SWR readings than appeared in your article, am I doing something wrong?
Probably not. SWR is a function of several variables to include wire and cable type, height above ground, etc. The best measure to ensure the antenna is constructed correctly is the 20m SWR. It should be under 2:1 across the entire 20 m band, and if you use a 75- OHM match transformer (see above) closer to 1:1.
Can I install the Mystery Antenna as an Inverted V?
Absolutely. There are thousands of Mystery Antennas in use in many different configurations. Feedback from builders indicates that the antenna is very forgiving and will work under almost any condition to include indoors and with the ends bend.
Where can I buy a Mystery Antenna?
The W5GI Mystery antenna is available as a kit from the Wireman or, if you prefer, direct from W5GI ready to install for $100 shipped within the U.S. W5GI can also build mono-band antennas for any band 40 to meters. Call or email for a cost estimate.
PS be informed that article, text and pictures are Copy Right by W5GI now SK
I’m not sure how I would use 75 ohm coax as a transformer on W5GI antenna for 20 meters as I feed my inverted vees with 75 ohm coax per ARRL antenna book. They claim that an inverted vee feed point is closer to 70 ohms than 50 ohms. Explain please! I would rather use the 75 ohm for a transformer because of the shorter length and lower SWR. It seems to me that my whole feed line would be a 20 meter antenna but that can’t be because it wouldn’t be tuned for 20 meters.
Well I tried to build me one of these mystery antennas and it dont work and has a very high SWR. I have built many diapoles and this does not work like the design I have says so. The design I have says as followed: Cut 4 sections of 14 guage electrical wire at 16’6″ and 2 sections of RG8X coax. Then solder the inner section of 14 guage wire to the center conductor of the RG8X coax wire. This end “does not” get the shield soldered to the 14 guage wire. The solder the outside end of the shield and center conductor to the 14 guage wire. The as the design claims to use a piece of 450 Ohm ladder line at 31′ 10″. and solder to the inner ends of the 14 guage wire (using a center insulator of course). And then solder your coax directly to the 450 Ohm ladder line. And according to the chart for this I should have an SWR on 20 meters of about 1.5. My SWR is 3.5. I went over this antenna many times to figure this out and its got me stumped. And yes I do have it mounted right and away from all metal objects and its about 35 feet or so up at the apex. If anyone has any sugestions feel free to write me at my call sign at Yahoo.com….Thx…Mark (KD4QHG)
complimenti per articolo e per l’analisi che fai … ma una cosa ti devo chiedere ,
per favore posso leggere larticolo in ITALIANO , sai il mio inglese Ã¨ veramente scarso.
Ho bisogno di chiarirmi qualche idea .
73 de ik3ery
Curious if the is a minimum length for the coax line from end of ladder line to the radio ?
I am interested in buying a w4gi mystery antenna ready to install . Can you send details on how much and how to pay ?
i would like to do one for 75/80 has this been done yet
HAVE BUILT A MYSTERY ANTENNA,HAVE NOT PUT IT UP YET,WOULD YOU PUT A RF CHOKE ON IT WHERE THE WINDOW LINE GOES TO COAX? SAY 10 INCH x 10 COILS OF COAX.?
Not having a coaxial choke and “crossing fingers” is why this outperforms a G5RV on lower bands. No mystery at all – use your coax as the antenna. Enjoy lots of noise pickup and RF in your shack!
I saw the article in a magazine years ago and built and have been using this design ever since. No baluns, no chokes. I have new cut lengths of wire ready to be soldered together to replace my original antenna as it has been re-soldered a couple of times due to mother nature and branches that have reached out for it over time. Time to refresh. No new mods, building it right from the original article. I actually had the pleasure to make on air contact with W5GI a couple years before he went SK.
Follow the instructions and you will enjoy the performance of this design for years to come.
Simone, I have successfully used a home made antenna called “coax antenna”. I received the design from WB5CPO (SK) many years ago. I can send you a single sheet copy of the design. Its simple and uses only 50 ohm coax in its entirety.
Let me know how to send you a copy if you care to see it.
Robert, I’ll send you an email.
I built one of these back in 2007 and I have used it VERY successfully on 80/40 virtually every day since. The apex is suspended by HB mast made from 18′ of galvanized Schedule 80 pipe through bolted to a 6″x6″ fence post (concrete base), followed by another 18′ of fiberglass tubing (tapered sections nested into each other). Total height AGL is 36′. The ladder line feedline is fed inside the fiberglass tubing and out through a slot near where the fiberglass tube base (where it fits into the galvanized pipe) then out to the 1:1 balun mounted onto the main mast and the other side fed with 50 ohm coax back to my radio. It is VERY important NOT to keep the ladder line away from any conducting mast. It is in a more or less flat top configuration with only a slight drop at the ends. Works very well and it has been very durable.
I have built this mystery antenna years back, I have used this with my I COM 746, it works very good. Especially I get very good reports from our Local hams working on 40 ! I have also worked some DX with the other bands! But on six meters I haven’t heard any one!
I am a Ham operator from Srilanka, my Call sign, 4S7LF.
Handle. Leonard Fernando.
Kind Regards and Best of 73!
Can the co-ax sections be wound onto a 40 m/m diam pvc tube.???
There are 2 different sets of lengths for the wire / coax to build this antenna in the article above:
initially, it says to cut 4 pieces of wire to 17′ – but at the end under “Dimensions for the multi-band antenna”, there is a diagram showing 2 pieces at 16′ 10″ and 2 pieces at 17′ 2″ – and also different lengths of coax as well.
Which is the correct set of lengths for the multi-band antenna ?
Thanks and 73,
Hi my Ham friends,
I am from Srilanka with a call sign, 4S7LF, I have constructed this Mistry antenna years back and had great results on all HF bands! Presently I use this on Forty meters
( 7060) daily in the morning at about 0820 hours local time and the results I get is excellent!
Kindest Regards! ………73!
You should cut 4 pieces at same length, then you should adapt 2 of 4.