i have only seen the 5% rule applied when building yagi antennas. the reflector is normally 5% longer and all subsequent directors are are 5% shorter thn the preceding element!! now , as for the DB-201 ground plane, It is a 1/2 wave folded loop ground plne which exhibits no gain. the radials are considerbly longer than the total height because it is a 1/2 wave length . doot shoten he radials to the same length as the vertical element. you will be sorry. i convert these ntenas to 6m all the time and have even lengthened a couple of 31 mhz versions to the 10 fm portion of the band. my total height dimension for the 6m version is 54 inches which is right at 50 mhz and i can lengthen it to as much as 55 inches if necessary( 50 inches normally puts them on 54 mhz. ) basically, 1 inch equals 1 mhz wth this antenna on 6 meters. I normally cut the antenna about 1 inch above where it slides into the the antenna base and drill and install a 10-32 SS machine screw and self-locking nut. then i find aluminum tubing that fits snugly over the stub, cut it aprox. 12 inches(1 ft.) long and attach it to the stub via he 10-32 screw. I then slit the end of the new tubing an lay the antenna radiator next to it with the looped end inserted into the grouned clamp end and slide the antenna own to 50 inches(measured rom the outer-most bend of the radiator, down to the top of the ground radial flange this is my highest freq(54 mhz) i then cut the radiator so that is will bottom out inside the newly installed tube at that freq. if i want the freq. higher, then i trim off more of the radiator (i have at least 10 inches of adjustment to play with).I thn slide the antenna radiator out to the lowest freq. i will use and then trim the grounded end of the radiator to about 1 inch longer than it is sticking out. debur al the cuts add a SS hose clamp to the new tubing with the slit in it and you have a fully adjustable DB-201. as for ground radial length, i use the cutting chart for the DB-201 and cut them for the lowest frequency(50 mhz). From 49 to 55 mhz this length does not seem to affect the swr enough to worry about. like all db-201s "0" gain so the more height the better. they work just fine in repeater operation. I have side-mounted all low-band versions of these antennas, a minimum of 4 ft. from the tower with no noticable swr increase.
well, that my to cents worth on the subject, I just finished up converting 4 of these antennas for a few emergency operating locations for our ham club, so i figured i would do a write-up on it. this now maks @ 65 of these conversions so far. If i can get more antennas, i'll do up a few more and maybe even a couple more for 10m fm. 73 es gud luck with your projects!!! Tyrone N5XES
