This paper summarizes the results and presents the significant data for making the modification. While a three element trap Yagi isn't going to be a top performer in relation to other more complex beams... it does have very useful gain along with reasonable directivty and front to back ratio.

    Tri-band beam coverage seems to remain the traditional 20, 15 and 10 meter bands. If you want the WARC band you have to settle for an add-on dipole element. Of course there are many very sound technical and business reasons for this. If you would like a gain producing antenna on 17 Meters and are willing to sacrifice 20 Meters this modification to the old Mosley TA-33M work horse will do the trick. Of course you can add a 20 Meter dipole on if you wish. The procedure for the latter is covered in another paper. As a bonus, without the 20 Meter dipole, the turning radius of the antenna is reduced by approximately 2ft.

    Note that the flat olive green color of the beam was applied to comply with my policy... "No metal shall show in the air". While this is not meant as camouflage, it does not attract one's attention, especially when mounted within a group of trees. My tower and rotator are also lightly spray painted the same color.

    The modification of the TA-33M (shown after modification, in the photograph above) is simply the replacement of the stingers (outboard ends of each element) so as to resonate the TA-33's lowest band in the 18.130 mHz range instead of the factory setting in the low 14 mHz range. I worked out the new length by measuring resonance of the driven element with the factory stingers removed... leaving the 4.5" outboard stub of the trap element. It was then a simple calculation to determine the amount of length required. Very little correction was required from the initial calculations. I also carefully looked for any changes caused to the two upper bands by this modification. No significant changes occurred. I then calculated the lengths of the reflector and director element stingers based on the percentage of decrease of the driven element. The clip (to right) shows the measured pattern @ 18.140 mHz using a signal generator located 200 ft. from the antenna. The asymmetry of the side nulls is no doubt due to the imperfect balun/feed scheme. This is not a practical problem but does make one curious.