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Author Topic: Tuning an Alpha Delta DX-LB Plus Antenna  (Read 9766 times)

Posts: 74

« on: April 22, 2012, 07:19:02 AM »

I need to move the resonance point on this antenna on 80M and 160M. I know you change the length of the sections between the two coils to tune 80M, and you change the length of the end sections to tune 160M. But, by how much? Is there a rule of thumb for kHz per inch? The instructions from Alpha Delta just says change it a few inches at a time. It's rather tedious to lower and raise my antenna, so I want to minimize the number of adjustments.

BTW, are the coils on this antenna doing double duty as both loading coils and traps? Quite a trick.

Bobby Dipole ND9B

Posts: 3160

« Reply #1 on: April 22, 2012, 08:08:52 PM »

Quote from: Alpha-Delta instructions
Low Band Dipole Covers 160-80-40 meters with an overall length of only 100 ft. Antenna performance and 2:1 VSWR bandwidth is site dependent and varies with height above ground and surrounding objects. The typical bandwidth is:

160M – 20kHz • 80M – 40 kHz • 40M – 300 kHz

Full band coverage can be accomplished with an antenna tuner.

Model DX-LB PLUS, same as Model DX-LB but adds parallel wires for operation on 20 thru 10 meters.
Bobby -

As you can read, from the instructions, this antenna has NARROW bandwidth on 80 meters (40 kHz) and 160 meters (20 kHz).  Since you have the PLUS model, there will be some interaction between the wires in this fan dipole configuration.

FIRST, determine where these 2:1 points are on 80 and 160 meters.
SECOND, decide where you are going (SSB/phone portions of band are shorter wavelengths)
 to move the 80 and 160 meter segments.
THIRD, it is easier (and cheaper) to shorten elements, than trying to lengthen.
FOURTH, Use the old standby formula: 468 / Frequency (in MHz) = Total Length (in Feet)

The ISO-RES coils eliminate a specific length of wire --
you should be able to determine this for each coil with simple math
based on antenna's current frequent dip point and wire lengths.

BTW, are the coils on this antenna doing double duty as both loading coils and traps? Quite a trick.
NO.  A trap requires Inductance (L) and Capacitance (C) .... your ISO-RES coils are not L-C traps, just Inductance (L) / loading coil.

K7MEM calculator for shortened antennas
« Last Edit: April 22, 2012, 08:37:22 PM by W9GB » Logged

Posts: 763

« Reply #2 on: April 23, 2012, 06:59:53 AM »

Im W9INN dipole one would have tuning flexibility by having some extra wires before each coil and the end, and then wrapping back those wires. Wrapping more increased the resonant frequency. With wrapping one did not have to cut.

At best wrapping would be symmetrical to avoid the loss of symmetry.

Posts: 57


« Reply #3 on: April 25, 2012, 08:15:37 AM »

The coils do provide a trapping action on some bands and a loading action on others, due to the way they are designed. For example, on 40 meters the first coil (on each side of the antenna) is a "trap" and uses the L of the coil for the inductance and the interwinding capacitance between the turns for the C, thus making an LC "trap". It takes careful design to do that. Then, for 80 meters, that first coil is simply a loading coil. The second coil is the LC "trap" for 80 meters, as previously explained, and then just a loading coil for 160 meters. That's why we call them "ISO-RES" coils. Isolator-Resonator, depending on the band.

Randomly winding a loading coil for an antenna won't produce the same results. Due to the effect of the coils, adjusting the wire lengths don't follow the regular 468 formula. The wires can be pulled through and twisted back on themselves and don't have to be cut.

Hope this clears up some of the questions.

Don, W8AD, Alpha Delta Communications
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