Force 12 Why They Work

We have been requested to place on the web site at least a generic answer to a question that has continued to move to the front row regarding antennas. This question has shown itself to be a main topic after more than 12 years of designing, manufacturing and selling antennas, plus speaking at dozens of forums and club meetings. The question is, "Why do Force 12 antennas work so well?" Besides being an interesting question, it is an important one, because you should be able to ask a manufacturer to explain the theory behind their designs and to support the claimed performance figures of their antenna(s). I answer this every week for people who call or write and anyone who takes an open-minded look at the claims will find an unfortunate fact: there is misinformation designed to guide the potential customer towards an antenna purchase, not to accurately represent the product.

Only one established company has provided accurate specifications since day #1 - Force 12, Inc. Ask the manufacturer about their designs, how they get their gain figures and how they otherwise measure performance! As an example, one manufacturer was claiming 4dBd for a short 10 meter mobile vertical antenna, plus rating it at 5KW. I asked him face to face, noting that the "dBd" means the gain is compared to a dipole. His response was that the figure was what he got when he compared the new product to a mobile whip. I then asked about the 5KW rating, noting that it would become a roman candle and he said it was OK, since nobody would ever try to run that much power mobile. When a manufacturer answers your questions, the answers should be reasonable and, in this day of computer models, the claims should be backed up with a practical model. It is your money - you have a right to know the answer.

There are three (3) sections to this page: 1) why Force 12 antennas work so well, Part A; 2) questions and answers about other manufacturer's products; and, 3) why Force 12 antennas work so well, Part B, including more detail on Force 12 designs, including the Sigma^© series, which are rapidly becoming the hottest HF verticals in the market.

The opening question of why do Force 12 antennas work so well makes a statement from our customers: Force 12 antennas outperform whatever was being used before. The practical result of this performance is that customers are having more enjoyment from amateur radio because they are using an effective antenna. One fundamental building block of Force 12 designs is the absence of traps, "dual driven" and log periodic cells. If we thought these techniques were the best choice, we would have used them in some manner; however, they are not the best choice. We use original designs that have proven themselves to be more efficient and the majority were invented from the ground up. In my book Array of Light, there is a history of the C-3 antenna and it notes that before beginning the C-3 design, we tested many trapped antennas to determine the actual gain they had. We did not depend on the printed specifications, because they were unreal - absolutely impossible. The testing showed that to be significantly ahead of all trapped antennas, it only required a moderate 2 element Yagi. The trapped specifications do not show this, but the facts do. If you read my article in July 2000, QST entitled "Everything Works", it covers more detail on the importance of having an efficient antenna.

I should also mention that Force 12, Inc. does not copy designs from other companies. Force 12 is the original and the company that has led the way to new designs, beginning in 1991. Unfortunately, there are now some companies that are copying our designs, both electrically and mechanically - taking the short cut and not going through the expensive and time-consuming development cycles. Anyone who purchases from them does amateur radio a disservice - encouraging plagiarism. In literature and journalism, people who copy another's work are essentially drummed out and banished from the field. Another point is that a copy company does not know why the designs were done, so they cannot put anything back into amateur radio in the way of new products and innovation - they lack the design foundation to do so. They are also unable to provide customer support and otherwise answer related questions.

Efficiency is always the key to antennas. Efficient antennas do not have significant loss, either through a poor design or having components that cause loss. Many times we have read or heard people make a statement like, "A few dB is nothing, because when you are 20 over S9, what's the difference if you are only 15dB over S9?" The answer is: A LOT! If you want to compare an antenna with another one, use comparisons with stations that are being received at the margin of copy. It is at this point you will realize that an efficient antenna makes all the difference in enjoying radio - you will work stations others cannot hear. How much makes a significant difference? 2dB. How much difference between a typical trapped triband Yagi and a multi-monobander such as the C-3? After hundreds of these in the field, the answer is 4-10dB. (Independent testing a few years ago showed these figures to be realistic.) This does not mean the C-3 has some miracle design with gain that is out of this world. All it means is that the other antenna(s) do not have the claimed gain. A recent test in real life performed with our large C-49XR and the largest trapped tribander ever built atop a separate tower 40' higher than the '49 shows a difference that runs 2-4 S-units in favor of the C-49XR.

In my popular book, ARRAY OF LIGHT, there is a chapter on manufacturer's specifications. It includes some simple models of typical Yagis: reasonable models of trapped Yagis and monoband Yagis where gain was optimized at 100%. Two pages of listings compare manufacturer's claims to the real-life and maximum theoretical values. The results should not surprise anyone - just like the independent testing showed a few years ago, Force 12 is the only one that tracks specs with the real world.

The following answers to questions have been provided for thousands of people at meetings ranging from large forums of several hundred to small, intimate club groups. As a speaker, I am always keenly aware the information must be correct. The scheduled forums remaining this year will offer more opportunities to listen to questions, share experiences and clarify issues. They are not lectures - I take ALL questions from the audience!

How do our antennas compare to LPDA's (log periodic dipole arrays)? We also manufacure LPDA's, mostly for NATO applications. For a given boom length, the LPDA will have about 2dB less gain than a multi-monobander like an XR series Yagi. An example is a classic LPDA design using 7 elements on a 20' boom for 14-30 MHz. The actual model shows gain compared to a full size dipole at the same height above ground to be 3dBd, or 5dBi (compared to the isotropic source). A smaller C-3 and C-3S will run about 4.5dBd and it is an overlay design of 2 element Yagis, so the 3dBd figure for the LPDA is actually a reasonable number - the LPDA does not have 2 elements dedicated to each amateur band. Please research the extensive work performed by L.B Cebik (W4RNL) to understand the gain figures of LPDA's. Where do the gain figures of 5-6dBd for LPDA's come from that some manufacturer's claim? Ask them! Ask for the real-life model. This is nothing a person cannot check out for themselves. You wouldn't take a salesman's word that a 1-ton truck would get 35 mpg, so why take their word that a typical LPDA gets equal or more gain than a 3 element monoband Yagi? LPDA's can be designed to do this, but the boom lengths are much, much longer and the element count higher than those found in the amateur marketplace.

How do your verticals compare to the GAP verticals? Experience in using these antennas has shown them to perform reasonably well on the middle frequencies (with proper installation); however, at the lower frequencies, they are quite poor. This is to be expected, as an antenna can be designed to efficiently cover an octave (doubling or halving the frequency range), but more than that makes the antenna ineffective. There are multi-element designs that can do this, such as a large LPDA; however, these are obviously not related to the single-element verticals. (See also the write-up below on broadband dipoles.) Reaching out for the second (i.e. 7-30 MHz) or the third octave (i.e. 3.5-30 MHz) is more than difficult. What is the theory behind the design(s)? Ask them. Force 12 verticals are mostly true half wave vertical dipoles, with some 1/4 wave verticals in the line-up. I am always glad to explain how they work and the Sigma^© series description is included below. Mechanically - ask someone who has had a GAP up for any length of time. On the air performance will show the Force 12 verticals to be the highest performing verticals available.

How about compared to something like an R-7000? This has been done in various ways and the conclusion is that the Sigma-5^© is several dB ahead on 20 meters and about equal on 10 meters. The Force 12 V-3 is the best triband vertical available, with taller elements than the Sigma-5^© and only working on 20-15-10 meters.

What do you think of the new motorized Yagi (the SteppIR)? An educated response is that because of dealing with more than 10,000 antennas in the field in all sorts of locations and weather, the last thing I would want is a mechanical antenna up on the tower. Another educated response is that one can purchase a time-proven multi-band Yagi of at least equal performance, with no moving parts, for less money. A related question is whether or not the company will be around when there is a problem with the mechanical and/or software. There are some additional questions that should be prompted by the ad copy. On all the claims, ask for the back-up information. Some make no sense at all. For example, the feed has a fixed value (e.g. 22 ohms and stepped up to 50 ohms through a matching device). This means the feed point must be 22 ohms for the claimed "1:1 SWR - on every frequency." Anyone can run various software to see how this affects the designs on the bands, which will be a compromise due to the fixed element spacing and fixed feed point. Given this situation, how can the product meet the other claim that "Our 3 element yagi on a 16' boom outperforms much larger arrays"? The main contributor to gain on Yagis is boom length (and number of elements on larger arays), so to think a 16' boom 3 element 20 meter Yagi can outperform a 28' boom 3 element Yagi (with no feed point impedance, nor element spacing restrictions) is not reasonable. On the higher end of the frequency range, try modeling a 16' boom 3 element 10 meter Yagi and see how it does - this is very wide spacing (almost a half wave long boom) and nothing you will find on any 10 meter monobander for only 3 elements. The last claim that the "2 element version rivals most 3 element yagis in performance - on a 57" boom!" cries out to be clarified in writing by the manufacturer, but maybe it depends on what the definition of "rivals" is. Another issue is that the loss figures and tracking to modeling software is not clear. The antenna necessarily has small (flexible) conductors, so it will have more loss than tubing elements. Some recent comments on the Tower Talk Reflector now note the power limit has been set at 1500 watts PEP with a time limit of 5 minutes, which makes one wonder what is heating up inside the antenna and also if it is less than 5 minutes in hot climates. There has been too much misinformation in the amateur antenna market for way too long - let's get it cleaned up.

Can you make a broadband dipole (like the B&W) to cover 80 through 10 meters with an SWR of less than 2:1?
The simple answer is, "Yes"; however, to construct a single-element antenna (vs. a multi-element array) to do this will have substantial loss to achieve the goal. Using components like stainless steel wire (or other high-loss types) and one or more resistors (or other high loss devices) will enable a low VSWR across this very wide frequency range, which spans 3 octaves - 3.5 to 30 MHz. Yes, the antenna will "work" to some degree, so: 1) if a low VSWR is the focus, and 2) if a very marginal performance antenna is acceptable, and 3) if the majority of the communications path can be made up from the other end of the circuit, then this is a good selection.
As an example of the high loss of stainless wire, the photo is of a hairpin made of #12 stainless steel wire. A hairpin (also known as a beta match in a different mechanical implementation) is a matching device placed across the feed point of the driven element in an antenna that is presently less than 50 ohms. The hairpin is used to transform the impedance up to 50 ohms. The feed point (i.e. center of a dipole in a Yagi) is a high current location and, therefore, it is important to utilize low loss components. As a test, the usual hairpin made of 1/8" copper tubing was replaced with this one of stainless steel in a 3 element Yagi. It was run in casual operating for about a month, with the visual result of extreme heating (the blackened area) at the center. Not only was there a lot of power going up in heat (obviously a lot of loss!), but the same loss was evident on receive. Placing the original hairpin of copper back in the circuit made the antenna immediately come to life. The VSWR bandwidth with the stainless hairpin was wider, but that was hardly a fair trade for the loss in performance.

What is it about the RAI Beam that is unique in their patent? This antenna is basically a trio of 2 element yagis overlaid on the same boom - just like the older Force 12 C-3S design. The difference is that the RAI beam is fed using a phased feed and the Force 12 C-3S uses a simpler open sleeve feed system (invented and patented by Force 12). The RAI beam design is said to be patented and the implication is that there is something unique about it that yields the performance claims. Everyone can access the U.S. Patent and Copyright records on line. If one looks at the RAI beam patent, it addresses the element mounting which "reduces the non-radiative losses in the antenna." I have been asked many times to explain this claim - your turn.

Is it reasonable to claim that traps in the directors are in the low current part of the Yagi? This claim was made on the new CushCraft series a few years ago, apparently working towards a better case for traps. Readily available modeling software will answer the question. For typical full size 3 element Yagis, the current in the director will be around 65-70% of the driver and the reflector will have about 35-40%. If the design is changed for lower gain (and higher feed point impedance), the currents will be lower, between 55-60% and the reflector will be closer to 35%. These figures can be changed by variations in the design (i.e. spacing and tuning), but the item of note is that 2/3 of the current in the driver is not all that low. It would be better to put traps in the reflector(s). For reference, on full size 2 element Yagis, the reflector will usually have 75-80% of the maximum current in the driver.

Does the KLM KT-34XA and newer KT-36XA have traps? Of course, but many times I have been told the KLM design does not have traps, because it is "linear loaded", as the literature says. Looking back at the printed material, it is written very carefully to avoid the word, "trap"; however, that is what enables it to operate on the different bands. The traps are only a different design than the "coil in a can" type. A related question is how the new '36 design (and upgrade kit) can claim to have substantially more gain than the original. This is definitely most curious, as the original gain figures are astronomical and the '36 has the same length boom (32'). Converting an existing model to the new one using the kit to achieve more gain and better pattern should be a clue that the original antenna was no where near the claimed performance. Remember that the manufacturer is actually the same: the KT-34XA was originally designed and produced at KLM by the same people who are now M². Compare the numbers for both.

How does a 2 element quad compare to your smaller antennas (i.e. C-3S)? Both are basically 2 element arrays; however, we have heard and read for decades that a quad of the same boom length outperforms a Yagi. This is not necessarily correct. If we think back, the popular quad was the triband quad (20-15-10). It was compared to the triband Yagis, which were all trapped designs. The results were reasonable - the triband quad seemed to have more gain than a trapped Yagi, but it was not that the quad was a miracle antenna, it was because the trapped Yagi was sub-par. Tests were performed at VHF to full size Yagis and a similar false conclusion in favor of the quad was reached. Wayne Overbeck later showed that the problem at 440 MHz was that the gamma match on the Yagi was inefficient and he moved forward to develop the excellent Quagi (VHF & UHF), which uses a quad element for the reflector and driver, with Yagi-type directors. We have also been told that the non-trapped Yagi is observed as being quieter than a quad. Why quieter? Have not done exact testing here, but it could be that the quad loops are not polarized, except for the driver. This might allow for reception of noise that is most often vertically polarized. A trapped antenna has also been noticed to have more noise (and more RFI) than a non-trapped Yagi, so perhaps the traps are emitting and receiving energy in directions and polarizations other than what one assumes. The argument about quads and Yagis will likely continue for more decades. One thing is for certain - the quad is a 3-dimensional structure, more difficult to build, erect and maintain than a Yagi.

There are often trade-offs that need to be made, such as available space for the antenna, the bands covered and the price. Solutions can be rendered easy if VSWR is the focus and performance is ignored. Force 12, however, has on the air performance as the "prime directive", because this is what brings you enjoyment of radio. Performance is accomplished by utilizing designs that are efficient and do not have loss in the design. If we had found traps to be a good selection,we would have used them, but they are not. If we had found the LPDA to be the best for multi-band Yagis, we would have used it. The same holds for dual-driven feed cells - they are not the best choice. The Force 12 Yagi antennas are mostly built using full size elements and feed systems with the highest efficiency and a minimum of parts. The main feed systems used are the direct 50 ohm feed and the open / closed sleeve feed.

The 50 ohm direct feed was first used in 1993 on the Magnum 620, the 44' boom 6 element 20 meter Yagi. It was the result of earlier modeling at 2.4 GHz performed in the late '80s. This design is sometimes looked at as having too many elements for the boom length; however, the elements are there for several reasons. For example, the 620 maintains the forward gain within 0.1dB, the F/B within 1dB and the VSWR less than 1.4:1 across the entire band. A Yagi with less elements can reach the same peak gain somewhere in the band, but cannot achieve this wideband performance in all aspects. This design and the other principles behind it have been used on Yagis up to 5.8 GHz. All Force 12 6 meter, VHF and UHF use this design and Yagis for 20 through 10 meters are available with direct 50 ohm feed as well.

The open sleeve was developed for the C-3, which is the world's first non-trapped tribander. It has re-written the books on triband antennas. The open sleeve had been used before to feed a tribander, the HyGain Explorer 14, but it had traps in the driver and was less efficient that Force 12's performance mandate. After much design and experimentation, a 3-band open sleeve without traps was designed, built, tested and patented. It has since been used on many other Force 12 antennas and copied by some other unscrupulous companies. The feed was then changed slightly using parallel feed between the elements for the C-3SS. The parallel feed 3-band open sleeve still follows the basic patented approach, as the elements remain in close proximity. The feed was further developed for use on the XR series, where it is a combination of "closed" and "open" sleeve feed, where all the driver elements remain in close proximity, with 2 being parallel fed and 2 open sleeve fed. Again, the basic design principles remain as in the original implementation.

Multi-band Yagis were achieved using overlay and forward stagger designs. There had been a couple overlay designs in years-past that used full size elements, but they were not implemented properly and performance was weak. Force 12 utilized computer modeling to "make it work." It should be noted that Force 12 antennas are all designed manually, not using optimizers, as there are factors Force 12 deems are of prime importance and the optimizers do not take them into account. The ultimate 5-band Yagi, the 5BA, was first built after printing and analyzing over 5,000 models - all done one at a time. It uses the forward stagger design that has since been used on dozens of Yagis.

Multi-band verticals presented a serious challenge. Several years of asking us to make an effective antenna for restricted locations has brought about the Force 12 CC&R Friendly^© antennas. Research showed us that the requests were correct - there was nothing available in the market! Further research into restrictions pointed us in the right direction and, after several years of development, we introduced two products: the Sigma^© type antenna and a flag pole that is both a real flag pole and also an antenna. These CC&R Friendly^© antennas are all verticals and have omni-directional patterns. This means they emit and receive energy in all directions, not intentionally favoring one direction over another.

The Sigma^© series is available in three (3) 5-band models: the Sigma-5^© , Sigma-SV5^© and Sigma-GT5^© . They can be mounted on the ground, slightly elevated, or on the roof. Raising the vertical with the feed point greater than 3/8 wavelength above ground creates a secondary high angle lobe in the pattern at about 55 degrees. Sometimes a tall roof is the only place available, so this second lobe becomes a small trade-off for the ability for getting on the air!

The Sigma^© series verticals (multi-band or single band) are all true vertical dipoles. The multi-band models cover 20-17-15-12-10 meters and are already tuned at the factory before shipment. The Sigma-5^© is the original model, being 9' tall (plus the base), weighing only 7 pounds and comes in 2' sections. It was designed for easy assembly and disassembly without any tools. The Sigma-5^© comes with a standard 18" tall base post. An outer sleeve for the base post is available for installation in a 5-gallon bucket of cement and there is also a portable X-base (like the one you have seen in our booth at the conventions).

Further requests moved us to develop a Sigma^© series antenna that could conform to the FCC OTARD (Over-the-Air-Reception-Devices) Rule, which overrides all restrictions to install an outdoor antenna for receiving local TV broadcasts. It was requested to be able to attach the antenna to a chimney, which meant the Sigma-5^© was too tall, as the OTARD Rule has a 12' height limit. The Sigma-5^© is 9' and we needed a 4' base for the chimney; 1' too high. To lose this one foot, the new model received larger diameter tubing, making it possible to mechanically reduce the height, but keep it electrically the same height. We also increased the wall thickness for chimney mounting and used 4' lengths for more strength (realizing this antenna was typically going to stay in place). We then went to the local Radio Shack store, purchased a UHF-to-F coax adapter, plugged the coax into the TV set and proceeded to watch local TV broadcasts. Now we needed to make the antenna look like like a typical TV antenna. Having a full aluminum welding facility at Force 12 is always useful, so we added cosmetic TV elements to the upper T-bar. The new model is the Sigma-SV5^©, named after my good friend George Varvitsiotes, K6SV, who politely pressed us for this product.

The Sigma-SV5^© was then used as the basis for the third Sigma^© series multi-band antenna, the Sigma-GT5^© . This has the strength of the Sigma-SV5^© without the TV elements and is an excellent selection for placing in the yard. The larger and thicker wall tubing can support a trellis of line between the T-bars for ivy, or also support lightweight flower pots using non-conductive hangers. As with all Sigma^© antennas, it can be painted to blend in with the surroundings.

Sigma^© verticals are re-writing the book on vertical performance. They were developed through years of real-life testing and using many designs for contests. Special thanks goes to Kenny Silverman, K2KW, who is the Team Leader and co-founder of Team Vertical. This contest team is best known for all the records set from Jamaica using verticals, including 9 in one weekend in the CQWW CW last November using QRP and another multi-multi (high power) North American record in the CQ WPX CW this past May. Another result of these long-term efforts is the SVDA, the most popular DXpedition antenna for island operations. Please see the K5K write-up on this web site and also the article in QST that covered the DXpedition.

New Sigma^© products are being released this year to meet customer requests. These include the Sigma-40XK^©, which is a 16' tall kit type antenna that can easily be set for operation on any band 40 through 10, all 7 bands. This is the same design we have used to set many records from Jamaica, especially on 40 meters. Horizontal Sigma^© antennas are also being released. Several select customers have been using single and 2 element Sigma^© 75 and 80 meter antennas and the Sigma-80^© vertical has been very successful and recently further enhanced for easier tuning.

Appreciate your time to read through all of this and let's continue to make progress by challenging claims, asking questions and then accepting accurate information. If you want to see me in person, I am speaking at least once a month in various places through November - call for the schedule.