Spiral Dipole Antenna
Figure 1 – A New Twist on Portable Multiband HF Dipoles a compact 40 through 6 Meter spiral dipole is very attractive for portable and restricted CC&R use.
I have always been looking for a better performing HF antenna that would comply with my no antenna CC&R restrictions in my retirement community of single story homes on small size lots. This simply means my antenna solution must be invisible or not look like an antenna. The common solutions are attic dipoles or loops and some flagpoles which each have efficiency problems at HF. My trial and error attempts to build a better antenna only confirmed these limitations. I kept looking and experimenting with small HF antennas then I heard about homebrew and commercial 40M spiral dipoles less than three feet long with great signal reports. I thought it could be a good antenna if you live with poor soil conditions or do not have the space for the radials that a flagpole antenna needs for low angle radiation but I was very skeptical as my past experience with small HF antennas had been bad. The Spiral Dipole is OK, great if you consider the size vs. radiated power. For RF efficiency it is equal to a screwdriver on a car just more wideband and easier to tune. Just want to remind you a Spiral Dipole is NOT a miracle antenna from the National Snake Oil Company. What I have done different is found better matching methods, so more RF is radiated. If you want something that can look like a table lamp or bird cage on your patio the spiral dipole is a good choice. If you have 25 feet of space the best answer is NOT a spiral dipole antenna. I have refined two practical Spiral Dipole antenna solutions;
Figure 2 – Vertical Spiral Dipole with T-Match-Feed
has two quarter wavelength spirals separated by three Feet (1M)
Figure 3 – Vertical Spiral Dipole with Off-Center-Feed has a 1/3 wavelength
spiral over a 1/6 wavelength spiral separated by 3 Feet (1M) and 4:1 BALUN
1) A Single-band Spiral Dipole T-Match antenna solution.
Think of a T-Match as a “balanced Gamma match” as the feed connections are equal distance from the center of the dipole. This particular T-Match antenna consists of a 1/4 wavelength spiral about 27 Inches in diameter over a second 1/4 wavelength spiral separated by about 3 feet in the vertical, see Figure 2. To my surprise I found it to be a simple to build antenna and works great without tuner. Its table lamp size makes it easy to setup next to your tailgate or patio deck yet on the air has the same performance as a full size dipole, flagpole or small loop in the attic of your home. More about signal reports including more details on how to easily test antenna performance accurately are discussed a little further on.
2) A Multi-band Spiral Dipole Off-Center-Feed match (OCF) antenna solution.
The Off-Center-Feed match antenna consists of approximately 1/3 wavelength spiral over a 1/6 wavelength spiral separated by about three feet in the vertical, see Figure 3. Again I found it to be a simple to build antenna and works well with an in the shack tuner. Its table lamp size make it easy to hide yet with a matching feed has the same performance as a 21 Foot flagpole, 5 Foot small loop or full size 40M dipole in a one story attic.
These antennas are a variation on the Petlowany (see note 1) spiral dipole designs. I have seen spiral dipoles flaunt grandiose performance reports and plenty of folklore on how they work just to keep everyone guessing. Sounds like a fairy tale, however the idea of a 40M Dipole I could hold in one hand and would work sitting on top of a wooden fence post was worth a try. I looked but never found the Rosetta Stone for these spiral dipoles so I started experimenting. At first I built a few 20 M spiral dipoles for testing as the smaller size and good daytime propagation was a good starting point. I was surprised that the on air results are equal to a full size dipole in my attic. I found the spiral dipole will compete with the performance of full size HF dipoles that are less than ¼ wavelength above the ground.
Size is nice but how good is it?
If you can’t have a full-size antenna due to size or height restrictions the spiral dipole sitting on a tripod makes a very Omni-Vertical antenna with good low angle radiation compared to a low full-size dipole 1/8 wavelength elevation above ground, see Figure 4. The spiral dipole does more with less by putting more energy at the horizon. The "less" part being the Spiral Dipole is less efficient than a full-size dipole. As elevation above ground increases to 1/4 wavelength the full-size dipole antenna will outperform the tripod mount spiral dipole. Full size antennas always are better in free space, the Spiral Dipole is a better solution for restricted conditions, limited space and one story housing areas.
OCF Spiral Dipole
Omni AZ Pattern
Easy to build
-4 dB 15 Deg Radiation vs. +2dB for text book dipole
T-Match Spiral Dipole
Omni AZ Pattern
-4 dB 15 Deg Radiation vs. +2dB for text book dipole
60 KHz SWR BW
6 Ft Mag Loop
Narrow SWR BW
Omni AZ Pattern
Big Yard for Radials
Attic OCF Dipole
Omni AZ Pattern
Easy to build
-8 dB 15 Deg Radiation vs. +2dB for text book dipole
Figure 4 – OCF Spiral Dipole has all the good features and equivalent low angle radiation needed for DX
This chart shows four antenna types 40M predicted patterns over “real ground” with typical but equal conditions. The model inputs used were similar or better than the conditions most stations experience in real life. These results have been checked with on-the-air tests see text for description of testing.
All 59 Reports… Oh Come On! My RST is better than yours!
Yes we have all heard it a million times how someone gets great signal reports with the new wonder antenna from XYZ Corporation. In this case I can say from measured data the spiral OCF dipole will perform as well as a full size OCF dipole. Where did I get a test range? I did not get a test range I found the Reverse Beacon website www.reversebeacon.net which provides SNR reports accurate to 1 dB if you send in CW "CQ de KN9B" on an HF frequency some station will copy you and post your call. The stations are using "CW Skimmer" program (see note 2) and wide band SDR receivers. CW Skimmer sends the information to Reverse Beacon website. First transmit CQ at 10 Watts on the full size dipole (reference antenna) then transmit CQ at 10 Watts on the spiral dipole. Then compare how well the antennas performed by reviewing the website spotter reports from multiple locations at the same time and frequency. One try is not enough due to propagation changes; takeoff angle/hops vary and more so average 100 reports. Each report should be logged within a minute from the same receiving station to keep conditions the same. During good conditions you can get five to ten different receiving station reports from one CQ. The report includes; receiving station ID, time, frequency, SNR and CW speed. After you have gathered the data you have 100 SNR reports for both antennas to compare. Another way to say this is a 100 delta SNRs comparing a full length dipole to the spiral dipole. A pretty good test range for free! The 100 reports should be spread over weeks to cancel any unusual propagation conditions.
Background on the “Spiral Dipole” and why build it!
The spiral dipole is a simple variation on one of the most basic antennas the dipole. Like all dipoles it is piece of wire cut to a half wavelength and fed at the middle with a transmission line. To reduce the size of the dipole, several actions are possible like bending or folding. Think of replacing some of the wire length with coils but not like the slinky dipoles you may have seen. This antenna has a big two inch spacing wire spiral pancakes to reduce physical size with minimal inductance and capacitance effects. Some general truths remain; the smaller the antenna, the lower the radiation resistance. The spiral dipole shows resonance, but low feed impedance nearly 1/10 the normal 72 Ohm dipole resistance at close to 6 Ohms. Another way to think about this antenna is it is just like a dipole too close to ground some matching is needed for a relatively good match to a 50 Ohm feedline/transceiver. After months of experimenting with feed matching networks, tapped matching coils, T-Match and Delta matches I was getting good matching into 50 Ohms but I was only seeing small improvements in VSWR BW. Reviewing my results for clues I could see the balanced variation on the T-Match matching gave the best results in VSWR BW when increasing my match impedance. I found T-Matches at 50, 200 or 300 Ohms with the VSWR bandwidth increasing from 60 KHz, 100 KHz and 160 KHz respectively and showing most attributes of a classic dipole. The T-Match match was providing better results than direct match but I wanted more, the whole band under 2:1 VSWR but the T-Match is the best performance obtained.
Figure 5 – Spiral Dipole Frequently Asked Questions
about the Spiral Dipole covers the common concerns and reservations
Q Is the SP DP better than a Flagpole or attic dipole?
A SP DP has equal RF performance to a Flagpole with poor soil conductivity or dipole 20 Feet above the ground. On air it is equal to a screwdriver on a car .
Q What make this antenna different from other spirals I have seen on the internet?
A SP DP has a impedance matching solution resulting in more radiated RF on a small antenna with more SWR bandwidth.
Q How high should I mount this antenna?
A Mount at least 5 Feet above ground. A camera tripod is a good choice for portable ops. I could not measure a difference between 5 and 15 Feet in testing.
Q What about the attic heating ducts and do I need to remove all the stuff in my attic?
A Keeping a separation of 10 Feet is best, 5 Feet will work if you must near ducts. Do not worry about direction blockage. The pattern is not noticeably degraded by boxes, wood, shingle or tile roofs. Sorry no metal roofs or metal heat reflective sheathing.
Q What kind of wire can I use?
A Wire size is important for mechanical support, not an electrical issue. Use #14 AWG Solid as the minimum needed to be self supporting. #12 AWG is best. #10 AWG is difficult to glue in place. FYI - Heavier guage wire is required holding the shape, not for current capacity.
Q Which spiral should be on the top?
A Just like a conventional vertical dipole it does not matter which end is up.
Figure 6 – The 40/60 OCF Spiral Dipole shown in my tiny backyard on a camera tripod. Surrounded by walls and houses on air results show I still have an omni directional pattern and low angle radiation for DX
Not knowing I had seen the best VSWR BW I kept looking. Observing the spiral dipole had classic dipole characteristics in the feed matching experiments I decided to try an Off-Center-Feed, OCF or Windom (see note 3), match by feeding from the 1/3 – 2/3 point on the half-wavelength dipole shown in Figure 5. I expected 30 Ohm impedance as the center feed dipole was 6 ohms but found the OCF was close to the traditional 200 Ohms. The OCF gave me 170 KHz VSWR bandwidth on the first try using 200 Ohms (a 4:1 BALUN with 50 Ohm Coax). Just for fun I looked for the 2nd and 4th harmonics normally on an OCF but found surprises however that’s a lengthy story. The main point is the 2nd harmonic is very high impedance and the 4th harmonic is strong with a wide VSWR bandwidth with the 33% to 67% ratio. This was only the beginning of a series of spiral experiments, shifting the OCF ratio to from 15% all the way up to 45%. I settled at the 40% as the best compromise. As the 2nd and 3rd harmonics improved and the 4th harmonic suffered so the answer was finding the middle ground. Wow this little antenna is a multiband OCF spiral dipole! I made many test antennas after that discovery.
As time passed and many variations of the antenna were built I never found a perfect multiband solution without the help of a tuner. I never found the perfect ratio for all bands and wide VSWR bandwidth any improvement came at the cost of degrading another band. I did find the best compromise OCF ratio to be 37% to 63% for the 40M to 6M antenna. This compromise gives good multi-band operation with a small antenna tuner but sacrifices having wide VSWR BW on lower order harmonics.
The OCF Spiral Dipole can be matched with a small tuner. This antenna is unlike most compact antennas with elements running parallel to one another and in close proximity making the elements are tightly coupled, while the completed antenna uses less real estate, there is a great deal of interaction between adjacent elements. Such an antenna requires a great deal of careful trimming to achieve resonance on multiple bands and then with narrow bandwidths. I am happy to say the OCF is a build it and use it, NO TRIMMING. Although you still need a tuner for multi-band operation. I think this is great as most of us run with a tuner anyway.
The spiral dipole conclusion I found is you have a choice of a single band solution without a tuner or a multiband with a tuner. Whichever solution is better for your situation read on I will show you how to build both. Next I will share with you the step by step of how to build a single and a multiple band antenna with wire, glue, PVC pipe and NO analyzer. I call this approach the 98% solution because 98 of 100 HAMs can get-r-done!
20 M T-Match Feed Spiral Dipole building Instructions
A good starting point is the 20 M spiral dipole, it is simple, small, portable and no turner is needed. The mechanical details of this antenna are must be followed to yield a ready to use antenna. A SWR meter will provide enough information for trim tuning or feed point adjustments if you follow the plan. You MUST be exact (1/2 Inch) in cutting the dipole wire elements for center frequency to be correct. For now let’s stay with the known configuration. This configuration provides good results on 20 Meters with a 2:1 VSWR Bandwidth of 60 KHz that can be moved around the band at will in the field without tools or test equipment.
Click on the link for the step by step instructions used by my radio club members to build the 20M spiral dipole. This antenna consists of two spirals, T-Match match and feed line. 20 M Spiral Dipole DIY Instructions
40 M to 6 M OCF Spiral Dipole building Instructions
The mechanical details of this antenna are must be followed to yield a ready to use antenna. A SWR meter will not provide enough information for trim tuning or feed point adjustments. You MUST be exact in cutting the dipole wire elements for primary frequency (and harmonics) to be correct. For now let’s stay with the known configuration. This configuration provides good results on 40, 20, 15, 10 and 6 meters, lesser output but very usable on 30, 17 and 12 meters.
Click on the link for the step by step instructions used by my radio club members to build the 40M to 6M spiral dipole. This antenna consists of two spirals, BALUN and feed line. 40 M OCF Spiral Dipole DIY Instructions
Notes and references;
1) Bill Petlowany, K6NO, in World Radio Magazine, March, 1998.
2) CW Skimmer program, Afreet Software, Inc.
3) Loren Windom, W8GZ, in QST, September 1929.