Technical Topics



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As a convenience to builders of the log periodic antennas described on this site, permission to download the copyrighted software for mechanical and electrical design of antenna elements (in IBM PC format) has been granted by the ARRL.

The associated book is by David B. Leeson, W6QHS (now W6NL), "Physical Design of Yagi Antennas". This text and copyrighted software are available from the ARRL. The book is ARRL # 3819. The software on floppy disk is available separately as #3827 IBM PC or #3843 Macintosh.

Spreadsheet Update
Sept. 17, 2003

Dave Leeson, W6NL contributed these comments along with new spreadsheets.

As you may know, when I wrote the Yagi book the current revision of the EIA tower standard at that time was EIA-222C. Subsequently, they upgraded their treatment of forces on cylinders in yaw, and the current revision EIA-222F has a more accurate model for wind forces (it also has a slightly different treatment of cylinder drag coefficient and gust statistics, but the effect of this is minimal).

Essentially, the new spec takes account of the issue that almost all the force from air on a cylinder is due to pressure variation, not viscous force. So no matter what the distribution of pressure around the a mast, boom or element almost all the force is perpendicular to the surface. This would affect the calculation of boom force and equivalent area (it would be the greater of element or boom area rather than the square root of the sum of the squares), which would modify the mast calculations. This has no effect on the element calculations.

I've done some experiments with PVC elements and booms stuck out the sunroof of a moving car that satisfy me that this is a reasonably accurate model (I caution my grad students not to confuse a model with real life!). Kurt Andress, K7NV, author of YagiStress, concurs with the use of the new model for boom and mast calculations, and suggests a simple supporting experiment holding a PVC pipe cylinder out the side window of a car.

Under the EIA-222 model, all wind force on cylindrical parts is perpendicular to their axis, so that in an oversimpified picture all element force is directed along the boom. However, there is vertical gusting that must be considered (most boom failures I've seen are from the boom blowing up and breaking at the center). I use an estimate of 15 degree angle above or below horizontal for winds over flat ground and 30 degrees for hilly sites. I've revised the mast and boom spreadsheets to reflect the slightly lower forces from the EIA-222F model. The new ones are MastF.wk1, Mast2F.wk1 and BoomF.wk1.

If you set the vertical gusting angle to zero on the BoomF.wk1, you get the "boom force only" situation that ignores the potential effect of the elements.


From the readme file on the original distribution diskette:

The accompanying spreadsheets are in the Lotus 1-2-3 WK1 format. They are also available in Macintosh Excel format. Be sure to make a backup copy before you change anything, as these files are not protected. The following spreadsheet files are included:

AREA.WK1 Calculates element areas from section dimensions
ELEMENT.WK1 Calculates element wind and ice survival
BOOM.WK1 Calculates boom survival and torque balance
MAST.WK1 Calculates mast survival
MAST2.WK1 Calculates multisection mast survival
SLOPE.WK1 Calculates sloping foreground reflection gain

Start with your Yagi antenna dimensions (the manufacturer's manual is a good place to start, but you may have difficulty finding wall thicknesses). Open AREA.WK1 and enter the element dimension information. This will calculate the element areas.

To use ELEMENT.WK1, enter the same information for any element into that spreadsheet. It will calculate the wind and ice survival speeds. You can experiment with different section lengths, and you can add additional internally reinforced sections (increased wall thickness). You will find that the best design is a balance between wind and ice survival; for the strongest element of a given length and weight, work to make the survival speeds the same for all sections.

ELEMENT.WK1 also contains the W6QHS taper algorithm, which can be used to convert a cylindrical-element Yagi design (as generated by several popular Yagi-design computer programs) to the mechanically stronger tapered format. This algorithm can also be used to adjust dimensions of strengthened elements so they provide the same reactance as the original tapered elements on which they are based.

To use BOOM.WK1, enter the element area data from AREA, and enter the element spacing and boom dimensions. BOOM will calculate the boom survival speed for both unguyed and guyed configurations. You will need to become familiar with the required dimensions to enter, such as the boom guy attachment points and brace offset. Use this spreadsheet to maximize boom survival by experimenting with different section diameters and wall thicknesses. You can enter up to five boom sections, and you can make the reflector and director sides asymmetrical.

BOOM.WK1 also calculates the element and boom torque balance, and suggests dimensions for the compensating element. As you become familiar with BOOM, you will see how to use boom compensating sleeves to compensate for balun and coax moments. For simplicity, BOOM does not consider ice loading.

MAST.WK1 calculates mast survival from mast dimensions and antenna areas. Enter the dimensions and yield strength of the mast. For each antenna, enter the total element effective area, the boom effective area and the location on the mast. MAST2.WK1 uses the same techniques as ELEMENT and BOOM to calculate survival of multisection masts.

SLOPE.WK1 calculates ground-reflection gain for an antenna above a sloping foreground. Enter the frequency in MHz and the height of the antenna above the intersection of the slope line extended to the tower. Alternatively you can calculate the effective height for a tower set back from the edge of the slope. This worksheet also calculates, for the angle of maximum radiation, the center and limits of the ground reflection (Fresnel) zone.

These spreadsheets are quite rudimentary. If you are familiar with spreadsheet development, you can link them and expand their usefulness. For more information, see the book "Physical Design of Yagi Antennas," published by the ARRL (ARRL order no. 3819). Copyright © 1992 The American Radio Relay League.