SIGMA^ã Verticals

Frequently Asked Questions

These antennas are new Force 12, Inc. designs to provide high performance on the HF bands. Sigma^© verticals are true vertical dipoles, so they do not need radials and the real estate requirement is minimal. Our antennas are the product of more than 4 years of research and development to provide the best vertical antennas in the world. The development includes testing of many types, such as empirical to validate the computer model, comparative tests with several other antennas and competitive testing: Team Vertical set their first World Record in the 1998 CQWW CW competition winning the multi-multi class using early-generation Sigma^© designs. In the past two years, they have set and hold all 6 single band World Records in maybe the toughest class - QRP (5 watts), using Sigma^© verticals on 40 and 20 meters. These efforts have re-written the book on verticals and vertical antenna performance.

To assist in answering questions about these verticals, their installation and performance, the following FAQ's have been prepared:

1. Do I need to read the manual that comes with the antenna?

Certainly! The Sigma^© vertical manual should be read completely, as it contains detailed information on the design, operation, factory settings and installation suggestions, plus photos of assembly.

2. How high or close to the ground should the Sigma^© vertical be mounted?

This is an excellent question and the answer has been clouded by the apparent observation that trapped HF verticals continued to "work" better as they were raised higher and higher above ground. A real HF vertical (including the radial system required) located a foot or two above ground will emit energy in a single lobe centered at an angle of approximately 20 degrees when the ground is rated "good." As the entire vertical (and its radial system) are raised, the lobe will become slightly compressed due to the feed point being elevated. This lobe compression results in a small increase in energy in the lobe, which equates to a small increase in gain. A vertical that is close to 100% efficient will have a gain figure of about -0.36dBi (-0.36dB to the isotropic source) above good ground. Yes, the gain figure is minus and is expected. A vertical antenna does not receive enhancement from the ground like a horizontal antenna does ("ground reflection gain"). The following plots are for a full size, 1/4 wave 20 meter vertical with full length, elevated radials. These plots are called "elevation" plots and are from the side of the antenna, slicing through the pattern:

(note - the resolution is better if you print these plots)

Full Size 1/4 wave 20 meter vertical with 2 elevated radials at several heights above average ground: upper left is 1', upper right is 10', lower left is 20' and lower right is 27' (3/8 wavelength).

The above plots show how the lobe changes as the vertical antenna is raised above ground. This is on 20 meters, so on higher bands, such as 10 meters, the relative heights are half as high (e.g. 20' on 20 meters is the same as 10' on 10 meters).

Looking at a Sigma^© vertical dipole on the same 20 meter band, here are the plots:

Sigma-5© vertical on 20 meters at 3 heights above average ground are below: upper left is the standard height, lower left is 10' higher and lower right is 15' higher (as on a roof top). The upper right plot is the Sigma-5© vertical on 20 meters compared to the full size 1/4 wave with elevated radial vertical in the plots shown above. The comparative plot shows the Sigma-5© vertical has more energy at a lower angle than the full size 1/4 wave vertical. This is due to the Sigma-5© being a vertical dipole with the feed point elevated.

Elevating a vertical will also change the feed point impedance. As it is raised, the feed point impedance is lowered and, depending on the type of vertical, the resonant frequency can also move due to its proximity to ground. Sigma-5^© verticals are tested with the base on the ground (lower T-bar 3-4' above ground) and this has usually been acceptable even when the antenna is mounted elevated on a deck or roof top. It can be that the vertical "sees" the roof top as the immediate "ground", which can be a good thing.

The quality of the ground is also important for a vertical. The better the ground, the less loss there is due to the ground characteristics in the area extending outward from the base of the antenna.

Sigma-5© vertical on 20 meters over 3 types of ground: upper left is average ground, upper right is good ground, lower left is salt water. Notice how the energy at the lower angles improves with increased quality of ground, with salt water being the absolute best. Adding a ground screen under a vertical that is over ground will improve the lower angle energy. The lower right shows the pattern looking down on the antenna from above. It is omni-directional - equal in all directions.

3. What about interaction with other antennas?

There are two, general types of interaction: destructive and constructive. Both apply to all kinds of antennas (i.e. horizontal, vertical, circular). Constructive interaction is used in Force 12 multi-monoband Yagis, where the elements for several bands occupy the same boom and assist each other, increasing gain and improving the pattern. The 5-band 5BA is an excellent example. In this case, the interaction is planned and included in the design.

Destructive interaction is more common. This is where other antennas or metal structures (i.e. guy wires) can affect the desired antenna in a negative manner. Remembering that antennas can couple effectively out to 1/4 wavelength and beyond, everything within a 3-dimensional length of 1/4 wavelength can affect an antenna. On 20 meters, this is about 18' and on 10 meters, it is about 9'. A common example of this type of destructive interaction is having an inverted V wire dipole for 40 or 80 meters underneath a tribander: the inverted V will interfere with the tribander. A good indicator is if the VSWR changes as the tribander is rotated; however, even if the antenna is not visibly detuned, destructive interference can be affecting the field in some way, such as skewing the pattern and/or shielding the energy in a particular direction. Remember that an antenna receives as well as transmits, so the influence on the pattern affects both receive and transmit.

Verticals can also couple into nearby objects and the same, basic rule of proximity applies: anything within 1/4 wavelength can be expected to interact and objects over 1/4 wavelength away might interact. Another resonant antenna within 1/2 wavelength will certainly be a strong candidate for destructive interaction. The Sigma-5^© verticals have been tested within 10-12' of a typical stucco house with no de-tuning noticed. Stucco has chicken wire backing, so it can be a problem and might tend to shield the energy in that direction, even if the antenna is not detuned. If a vertical is placed above a triband Yagi, there will be no interaction, as the two antennas are cross-polarized (the Yagi is horizontal and the vertical is vertical). Other vertical-type antennas and towers can obviously couple with a vertical antenna. It is always a good idea to see what is near-by. Metal fences might also be a problem, of course, so testing the antenna in more than one location is a good idea. Trees and shrubs might couple to an antenna - not always known.

4. How can I change the point of lowest VSWR on 20 meters on my Sigma-5^© vertical?

The 20 meter frequency is determined by the total of all the center loading coils. On each side of center (the feed point), there are 4 air-core coils. The first pair (closest to the center) are for 12 meters, then 15, then 17 and then 20. To change the frequency of 20 meters, carefully remove the cap from the bottom end of the styrene cover. Next, slide the cover carefully upward to expose the PC board. If you want to move the frequency UP, spread the upper and lower 20 meter coils apart some more. To move the 20 meter frequency DOWN, squeeze both coils closer together. Please note the manual covers adjustment of the 20 meter coils only! Field adjustment of any band other than 20 is not recommended.

If you want to completely retune a Sigma-5^© vertical due to accidental adjustment of other than the 20 meter coils, the sequence is this:

It is best if the VSWR meter on your transceiver is used to take VSWR measurements. Handheld antenna analyzers sold for amateur (i.e. MFJ, Autek) use have a common problem: a small amount of RF received from any other source (such as AM broadcast, paging and other commercial transmitters) can cause incorrect VSWR readings. This is should be noted in the device's instruction manual. If these instruments are used, please use them with the following caveats: compare the analyzer VSWR reading with your transceiver VSWR meter or a Bird wattmeter to see if the readings are nearly the same; and, only consider the VSWR reading and not the values and components of impedance. On an MFJ this means use only the left-hand VSWR meter, not the right-hand "resistance" one: adjust for a dip on the VSWR meter, not for a reading of 50 ohms on the right-hand meter (it is unknown what it is actually reading).

5. Where is the highest voltage on a vertical?

The highest voltages are at the ends of a dipole. The highest current is at the feed point (center). A 1/4 wvelength vertical will have the highest voltage at the top of the vertical portion and at the ends of the radials - this is why the radials need effective insulators at the ends, as they can start a fire. Sigma^© verticals are vertical dipoles and the T-bar ends are the ends of the dipole; therefore, the ends of the T-bars are where the highest voltage is located. As with any antenna, if there is the possibility of contact with people or animals, be sure the high voltage points are out of reach.

6. How much information can I expect from Force 12, Inc.?

We do our best to provide relevant information on all of our antenna products. We answer all installation questions and offer suggestions. We speak with several hundred people every year over the phone and maybe a few thousand at conventions and club meetings. Last year we also replied to more than 10,000 e-mails, plus having the Force 12 Internet Reflector. We are the most active and customer-oriented antenna manufacturer in the country! At the face-to-face meetings, we answer all questions, regardless if they pertain to Force 12 products or not. We provide limited design assistance and are available to do complete design, but not free of charge. Although we would like to be, we are not able to be an endless supplier of tutoring or teaching services - this is better handled through the A.R.R.L. and various publications. We have written a useful book entitled, ARRAY OF LIGHT that has been very popular and discusses antennas in non-technical terms, as well as subjects not found in other publications (such as manufacturer's gain claims). ARRAY OF LIGHT is available directly from Force 12 and the current revision is called the "2003 Enhanced", which has an additional 20 pages added to the original.

If your club would like an interesting program, contact Force 12 and see if Tom, N6BT, can make it to your club. Force 12 is the only antenna company where its President is available on the phone for discussions, is out at many conventions in the USA and in other countries presenting open forums, and travels across the country doing programs at club meetings. Questions via e-mail and FAX are answered all the time. Force 12 can do these things because its specifications have been accurate since day #1 and everything is correct - you get what the specs say!

7. How can I debug an antenna?

An excellent write-up has been provided for everyone. It was first presented to the P.V.R.C. in 2000 when Tom, N6BT did 3 presentations for this excellent club. You can read it on-line and download it for local printing. To do so, click on the following hyperlink: Debugging an Antenna System

8. What is the difference between an RF ground and a DC ground?

Antennas "work" with RF, not DC. An example of this is the matching coil across the feed point on a Sigma-5^© vertical. It makes a DC short across the feed point, but at RF, it raises the feed point impedance to 50 ohms from a lower value. A ground rod at the base of a vertical is a good DC ground, but not effective for providing a current return on a 1/4 wave vertical. A 1/4 wave vertical needs the rest of the antenna, unlike a half-wave vertical dipole where the other half is physically present. On 1/4 wavelength verticals, radials are used. Elevated radials are most effective, as they are far away from the ground. For more information on elevating and tuning radials, please check out: Tuning Short Element Yagis & Radials

Vertical antennas impart current into the ground in the near-by area of the antenna. Adding a ground screen of many radials, or a screen under the antenna, extending for at least 1/4 wavelength will assist in reducing the losses from the ground. Note that the efficiency of the vertical antenna itself remains the same - it is the ground that is being improved (less loss). For vertical dipoles, the radials or screen are not attached to the vertical. For verticals requiring radials, the radials and screen are commonly attached all together. The improvement from adding an extensive ground screen under a vertical dipole or vertical with elevated radials is 2-3dB, which is a lot. Use the antenna first and see how it does, as adding a ground screen is not an easy task.

Salt water is the best under the vertical, but if you are not able to have the vertical next to salt water, a ground screen will certainly help. Verticals need to be close to the salt water, such as within 1/4 wavelength and the improvement is in the direction of the water (back the other way over ground is like being over ground). Force 12 and Team Vertical have written lots of material on this subject, including about the real-time research. It has also been presented at several forums and has been well-received.

For additional questions, please e-mail Natan, W6XR/2 at: force12e@lightlink.com .

The above information can be printed for personal use.

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