Tuning Shortened-element Yagis

(which can be applied to verticals and other antennas, as well)

and

Tuning Elevated Radials follows the Yagi discussion

Shortened-element Yagis ("SE Yagis") have been in service for decades. Their performance has advocates who rave about them and adversaries who argue against them. The usual measuring yardstick is the front-to-back ("F/B") ratio. This has been the determining factor as to whether or not the SE Yagi is "working"; however, this is an inaccurate assumption. Just because an antenna has a well-defined pattern does not mean it has any gain.

A basic understanding of antennas shows that pattern is not necessarily a useful indication of gain. It is commonly thought that if an antenna has a nicely defined pattern, it must have gain; however, the popular Beverage receiving antenna has a fine pattern, but it has no gain at all. The Beverage is a very lossy antenna with a minus gain figure, which is why it "works" as it does. The key to an antenna having gain is mainly in the design and construction. The most obvious source of losses is in trapped Yagis, where the elements are not only in less-than-optimum locations, but also the traps that have loss and do not necessarily place the element on the proper frequency. Over time, what gain might be there when the trapped antenna is new can drift out of band, or the elements can drift in opposite directions. Some people have reported traps on the same element tuned to different frequencies, which certainly will not help the antenna performance. For this discussion, we will only address monoband SE Yagis.

SE Yagis have three advantages to full size:

1). They are physically smaller, so the tower and rotator do not have to be as large and powerful as with full size elements;

2). The shorter elements do not interact as much with other antennas. A full-size 40 meter Yagi will destructively interact with just about everything on the same mast.;

3) A 2 element SE Yagi can achieve a higher F/B ratio than using full size elements. This is a unique circumstance that is shown in real-time performance and also using a computer model by L.B. Cebik, W4RNL.

There are three (s) electrical consderations for Yagi antennas: gain, pattern (F/B or front to rear) and operating bandwidth (VSWR bandwidth). In Yagis with a minimal number of elements, all three cannot be achieved. Designs with additional elements (such as the direct feed 50 ohm designs developed by myself back in the early 1990's and first produced commercially as the Force 12 Magnum 620 in 1993) can provide all three optimized conditions. This 20 mtr Yagi has 6 elements on a 44' boom. The same peak gain can be achieved with 4 elements, but over 13.950-14.400 MHz, the 6 element design maintains forward gain within about 0.1dB from the peak, F/B +/- 2dB and the VSWR is less than 1.3:1. For low band antennas, mechanical considerations begin to be more important and decisions need be made to prioritize the characteristics, because the number of elements is usually minimal. In these cases, the F/B and gain curves can go in opposite directions, so it is important to understand the trade-offs available.

The operating bandwidth is usually the 2:1 VSWR bandwidth. For some amplifiers (i.e. auto-tune Alpha 87A) this is too wide and the 1.5:1 VSWR bandwidth is the design goal. As the parasitic reflector element is coupled tighter (closer in frequency) to the driver frequency (working towards maximum F/B), the feedpoint impedance goes down, which increases the current in the elements and might lower the efficiency because of more loss in the components (cannot handle the increased current efficiently). The VSWR bandwidth also becomes narrower. This means the pattern might be wonderful, but the operating bandwidth will be unacceptable and the efficiency less than desired (forward gain will be less). An operating factor enters in, too, as when the reflector is tuned very close to the driver frequency, it becomes a director when the frequency of operation drops below the reflector's frequency, thereby reversing the direction of the Yagi. In this case, the operating frequency range must be selected beforehand so that the antenna will not reverse direction in the desired range. Devices are available to allow frequency agility of the Yagi for extended operating ranges, such as the relay boxes in service for many years by Force 12.

Setting an SE Yagi for best F/B generally ensures the forward gain is within about 85-90% of the maximum, as the pattern indicates the elements are coupling to each other. This maximum gain is the maximum for THIS design only with whatever loading system is employed, not 85-90% of what might be theoretically possible. A general rule is that if there is no discernable F/B (zero), the parasitic element is improperly tuned. The F/B and forward gain curves for 2 element Yagis do not overlay, meaning the best F/B and highest gain do not occur on the smae frequency. Oftentimes, the best gain is achieved above the frequency of best F/B ratio; therefore, it is quite possible to have good gain and not have the best F/B, provided the Yagi is efficient in its basic design. On the low bands, the most common focus is in pattern, as it creates the best conditions to hear the DX stations. In this case, gain takes secondary importance (especially if there is no power limit and the lack of gain can be compensated for by increasing the transmit power!). Besides proper tuning, the most important factors in maintaining antenna gain are the conductor size, the quality (efficiency) of the loading system and doing the best to have the element length as physically long as possible.

Two types of loading are most common: linear loading and coil loading. Various theories are around as to the effectiveness of each. Whichever technique is used, or even a combination of both, the element tuning is much more critical than when using full size elements for best F/B ratio.

Linear loading usually has variables in assembly, where the desired, exact frequency of the parasitic element(s) is not necessarily "plug and play." The result is the expected F/B ratio is not always achieved where desired. The basic linear loading technique is essentially a shorted transmission line stub and the Q is very high. The loading has classically been placed parallel to the element and then going towards the center of the element. In these linear loading structures, the frequency is set by adjusting the tuning jumpers across the linear loading, which changes the amount of loading. Recent developments by Force 12 on the "N" series of 40 meter elements uses a much different design, wherein the loading is pre-set and goes out towards the element tip. These "N" elements are also longer than typical, being 80-85% of full size and modeling on NEC showing they are within 0.1dB of full size, while having impressive mechanical and electrical advantages over full size. The frequency setting on these elements is excellent and predictable. The "N" series elements have a fixed linear loading structure and the tuning is accomplished by adjusting the tip length.

Coil loading has been popular, as it is easier to assemble and erect. If the coils are made with reasonable accuracy on 40 meter SE Yagis, the F/B performance will be good and the antenna will be deemed to be "working." It will have a pattern, but the gain is determined by the efficiency of the antenna. The small coils on the popular 40 meter SE Yagis are not in the "high Q" category, but if the coil loaded Yagi has any amount of gain, it will appear to be a tremendous antenna. We need to remember that a horizontal, rotatable dipole is a fine antenna, so anything that has even 2dB over a dipole (2dBd) will be perceived as very effective and will "work" well in the pile-ups.

Short elements that are efficient will have a narrow operating bandwidth. This is the bandwidth of acceptable VSWR, which is usually accepted as being where the VSWR reaches 2:1. (For reference, the 3dB points is where the VSWR reaches 2.7:1.) Within the 2:1 VSWR bandwidth, the performance of the shortened element is fairly constant, but does drop off at the edges of the bandwidth curve. SE Yagis for 80/75 meters is where the tuning is most critical,, because the operational bandwidth of the individual elements is quite narrow, on the order of 45-70 kHz. Fortunately, the operating window on 75 meters is also quite narrow. In the USA, it is usually centered on 3.790-3.800 MHz and an operational bandwidth of 25-30 kHz is probably sufficient. Shortened elements also require tighter coupling, which means the reflector element must be tuned much closer to the driver than in a full size Yagi. Accuracy when using fixed inductors (coils) is most important and a simple example that anyone can model is to use coils in a 68' element. With mid-point loading, it will require about 27uH in each coil. How accurate can you make a coil? If the coil is off by 1% (0.2uH), the frequency of the reflector element (and the F/B) will shift at least 10 kHz. When trying to optimize in the DX portion of the band at 3.790, 10 khz is a lot. The same accuracy holds for linear loading structures. A real time technique is necessary for optimum F/B on SE Yagis. Whether or not the antenna has gain is left to some other test!

The above example referenced the frequency of the reflector element and not that of the driver. This is because the F/B ratio is determined by the frequency of operation of the Yagi, not the frequency to which the driver element is tuned, or even matched. It is surely useful to have the driver near to the final frequency, but the reflector setting is the key. The reflector will also pull the driver frequency down in frequency, closer to the refelctor frequency. If one experiments a little, in a well-tuned linear loaded SE Yagi, the reflector will be set to about 3.777 MHz for a desired (matched) driver frequency of 3.790 MHz. In this case, the driver will be pulled about 12 kHz lower in frequency than when the reflector is not in the circuit (i.e. center of the reflector element is open). This tuning will produce a F/B ratio in excess of 20dB at the driver frequency and be >15dB over a range of about 20 kHz, possibly more, depending on element length, boom length and the loading system.

The first step in tuning an SE Yagi is to have the parasitic element (i.e. reflector) at approximately the proper frequency. This can be done by feeding it by itself as a dipole through a 1:1 balun and elevating it as high as possible. In many cases, 20-25' will be sufficient for this step. If an antenna analyzer is used (recommended), the point of the lowest VSWR dip on the VSWR meter is the frequency of the element. If using an MFJ meter with the right-hand meter reading in ohms, do not use it for any indication at all - only use the VSWR meter. The "ohms" value makes no difference. It is the frequency of lowest VSWR that is important. What frequency should be used?

The computer model can aid in this regard for coil loaded elements. In the finished model, remove the driver and use the reflector as a dipole. Adjust the frequency in the model until there is zero (0) reactance and this will give you the frequency of the modeled reflector. In typical 75 mtr SE Yagis using coils, this will be in the range of 3.780-3.785 MHz for operation in the DX window 3.790-3.800 MHz. If the actual antenna is a linear loaded design (such as the Force 12 EF-280B), the reflector target will be slightly lower, such as 3.775-3.777 MHz. After performing this initial reflector tuning, remove the balun and short across the reflector center to make it a continuous element - a functional reflector element. Set the driver for its initial setting at 3.795 MHz and match it to an acceptable level (1:1 is not necessary now).

Initial suggested frequencies for reflector elements are as follows:

VERY IMPORTANT - When the frequency of operation moves lower than the reflector frequency, the Yagi will REVERSE direction. When setting up a 40 mtr Yagi for best F/B high in the band (i.e. USA SSB portion at 7.150-7.300MHz), it will reverse direction in the CW portion (low end) of the band. If both parts of the band are needed, the Yagi should be set for CW and operated up in the phone part of the band with less F/B, but still with forward gain. A relay box (such as available from Force 12, Inc.) can be used to maintain optimum operation in both portions of the band.

Please refer to the drawing for the technique of tuning an SE Yagi in real-time. The basic principle is this:

A) The actual frequency of best F/B is found by measurement;

B) the reflector frequency is adjusted to move the best F/B into the desired operating range;

C) the driver is set on frequency and matched.

D) the antenna can be rotated around to the front to measure the F/B in dB on the receiver's S-mtr. The actual F/B in operation will not necessarily follow this measurement, as incoming signals will arrive at different angles.

This is a simple procedure and will give excellent results. Go slowly and be accurate in the measurements and adjustments. The procedure has been used on 2 element 75/80 mtr Yagis, 75 mtr 2 element vertical, 40 mtr Yagis and 30 mtr Yagis. The average time for each antenna was less than two hours.

Please let us know of your questions and results. The purpose is to give the best performance and most enjoyment using these shortened antennas.

Tuning Elevated Radials for 1/4 Wavelength Verticals

Verticals for the low bands have classically been an electrical 1/4 wavelength long (tall) and require a radial system for the current return. The exception is the vertical dipole, which is now more commonplace since the development of the SVDA antenna system (see the write-up on K5K) and the new SIGMA series vertical dipoles from Force 12, Inc. When using a 1/4 wave vertical, laying 120 buried radials (as indicated in texts for decades) is rarely practical for most of us; therefore, a different, but efficient approach is needed. The original concept was discovered in an I.E.E.E. article some years ago where the buried radial systems on commercial AM broadcast antennas were disintegrating over time. Adding elevated radials was shown to be extremely efficient and a practical solution for the current return. The concept was put into practice on our first trip to Jamaica for the A.R.R.L. competition as 6Y4A. It has been used on every installation since that time and on many other installations world-wide.

We had carefully selected the 6Y4A operating site, which was right on the ocean with several hundred feet of beach available for vertical antennas. One of the first antennas we installed was the 160 mtr vertical, followed by the 80 mtr verticals. Since we were on the ocean, we laid the radials on the "ground." This consisted of a combination of rock, salt water and grass, all right adjacent to the ocean. Conventional "wisdom" would say we had done a wonderful job - hardly. We were unable to have anything close to a 1:1 VSWR, nothing less than 2:1 at best. The team looked to me for an overnight solution (didn't get much sleep) and my memory was working well enough to recall the I.E.E.E. article. After not much sleep, we got up early and did an important test: when we added the hairpin match to increase the feedpoint up to 50 ohms, the VSWR got worse. The feedpoint for a full size 1/4 wavelength vertical should be in the low 30 ohm range. Shortened verticals are lower, often in the 12-20 ohm range. We were using physically short verticals with efficient linear loading to load them to an electrical 1/4 wavelength, with an expected feedpoint of less than 20 ohms. When we added the hairpin to transform the feedpoint impedance higher towards 50 ohms for an acceptable match, the VSWR got worse. This meant the feedpoint was already above 50 ohms, which was caused by the added loss (resistance) of the ground. We now knew the problem and we quickly worked out the "gull wing" elevation technique for lifting the radials above ground. As soon as the radials were in the air, the feedpoint impedances became acceptable. It should be noted that even with the poor match (and high losses) we did use the verticals that night, with excellent performance, due to the proximity of salt water. After reducing the losses by elevating the radials, they were even better.

A procedure was developed over the next year to elevate and tune radials. The drawing below shows both the basic technique for elevating radials, as well as the height above ground for effectively de-coupling the radials from the ground.

Any questions or comments, please zip off an e-mail to us at force12info@fix.net.

73, Tom, N6BT

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