It is just that you need enough elements to ensure sufficient inter-element coupling to achieve the benefits of the chosen boom length. Actually it is the boom length that is the critical parameter. It is an unfortunate truth that with yagis there are rapidly-diminishing returns with increasing size (elements and boom length). The additional gain attainable with one additional element is not a lot, even with the longer boom. Similarly the cable supporting the wire elements should be mounted no closer than 5 meters below the top of the towers, and preferably more if the directors are positioned near either tower. This is to reduce interactions that could distort the patterns of one or more antennas. Ideally the antenna supports, if they are towers carrying yagis, should be further apart than this antennas "boom" length of ~30 meters. Performance at low angles will also be reduced due to the lower height of average current. Vertical supports can be used to reduce wing span and increase safety for anyone walking in the area.Ī smaller interior angle reduces the wing span but will require longer elements and retuning due to lower feed point impedance. Wing span scales linearly with increased height. The "wing span" (or width) of the antenna including tie ropes anchored to the ground is 70 meters, when the apex is 20 meters. So not only is the antenna long it is also wide. The antenna elements have an interior angle of 120°. A height of 30 or more meters is even better. I recommend a minimum height of 20 meters (λ/2). This is especially so with inverted vee elements since the ends can come close to ground. As with any antenna for 40 meters you want this antenna high. This is not a small antenna! It is 29.6 meters long and requires two tall supports, such as towers supporting rotatable yagis, preferably guyed to deal with tension in the cable "boom". In modelling I found that the position of this element needs no adjustment to compensate for its mutual coupling. The amount varies with frequency - in fact all element currents substantially vary with frequency. In the above current plot you can see that there is only a small induced current on the 2 nd director when the adjacent switched element is configured as a reflector. If it seems odd that a director could be placed behind a reflector do keep in mind that the action of the reflector pretty much isolates the antenna from any resonant conductor behind the reflector. For this reason a similar element must be placed on the other side of the yagi for the 2 nd director in the reverse direction. Wire yagis work as well as his tubing-based designs but with higher element loss and somewhat reduced bandwidth.Īs mentioned above the 2 nd director is not switched it is a continuous length of wire. This approach is in keeping with W2PV's modified NBS designs that can achieve optimum results for gain, F/B and bandwidth. I made it the same length as the 1 st director used the same spacing as that for the driven-to-director. I started by adding a 2 nd director to the 3-element model.
1 Comment
David Sturm
1/28/2024 03:05:48 am
Good day, I was trying to download your 4 element 20m yagi,,but I can't open your file, What type of file is this?
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