6m Project Antenna Plans
A 9DB Gain Omni Antenna
Currently we hope to use four (4) DB products grounded folded dipole antennas that have been cut down from 43-48mhz. We plan to tune two antennas to the input frequency and two to the output frequency to acheive a better match for the intended 1 mhz split, if we are unsuccesful at duplicating the DB phasing harness. The antennas will be fed with 1/2" andrews heliax. This antenna configuration would provide an antenna gain of about 9 db in an omni-directional pattern.
Antenna Picture 1. Completed DB Products antenna - converted to 52.5mhz.
The DB Products Harness
DB Products uses a special phasing harness that does not use 75 ohm coax. Gary, NZ5V, has duplicated it once before, making a DB-224 copy, and hopes to use his vector voltmeter to make a phasing harness that is more broad-banded than one made using the conventional 75 ohm coax design.
A call to Andrews, where an Ex-Db Products antenna specialist works in tech support confirmed that the harnesses are 1/2 wave of 50 ohm, followed by an odd 1/4 wave multiple of 35 ohm coax. The reason for this unusual approach is that it produces a much wider bandwidth antenna. Instead of two sucessive quarter wave transformers, we only have one, in the case of a 4 bay (four antenna elements).
Design details will be posted on this page if it's sucessful. Yes, it is the phasing harness that makes those DB repeater antennas 10mhz wide on 2 meters!
One alternative we are investigating is the use of the Wilkinson Power Divider, which is a simple box with two runs of 1/4 wave 75 ohm coax in a "Y", but with a 100 ohm power resistor across the two output center conductors. The resistor acts to lower the "Q" of the circuit and makes the power divider very broadbanded. On 6 meters, the power divider is 10-12 mhz wide at the 1.2:1 swr points. The network analyzer shows that the resistor makes a considerable difference in the frequency response and SWR of the device.
The power divider prototype works well, and the decision is made to use it rather than making a coaxial phasing harness that has doubled 75 ohm coax. We don't have the stuff to make the harness last up on a commerical tower site with so many non-standard connections.
Steve- KG5QH, Matt- KE4DXE, Charles- KD4TKR, and Wayne- KB0JOS have all helped to modify the DB Products dipoles up to 52-53mhz from 48mhz. Although not yet completed, they are shaping up nicely.
Here is Steve, KG5QH working on a section of tubing to make the 10m remote base antenna. It was some 2 1/2 feet too short, so we took parts from a broken DB-212 and used them. Here, Steve is using a concrete expansion anchor as a tool to grab the inner piece of tubing and remove it from the inside of a section of antenna.