First the background. I live in an area along the coast of California where the terrain is very diverse, hills, canyons, valleys, and coastal mountains. As such the performance of an antenna becomes critical. I tried using a standard dual band magnetic mount antenna on my vehicles, I ended up the majority of the time skipping in and out of the repeaters. I needed an antenna with a lot more performance, a lower radiation pattern, a broader bandwidth, and higher gain than I could obtain at a low cost and still be mobile.
Solution; you design your own.
I read an article featuring the Arrow dual band J-Pole in the Jan.
98 edition of CQ magazine. I was intriguied, so I started a little
research. With some good internet hunting I found some valuable information.
I designed a similar antenna, and using a borrowed MFJ-259 analyzer
played with the changeable parameters to learn what was what, and fine
tune the antenna for high-performance dual band operation.
I took extensive notes. I remember some of the theory from
my Navy Electronics Technician radio days. "J"
Pole Specs
The other thing that showed up in that article was the "Toss up in a tree J Pole" made with 300 ohm twinlead. Twinlead "J" Pole
I thought I could use the same type of technology to produce a high-performance mobile antenna. The beauty of this beast is it can be made for about 6 cents, fine tuned to your exact needs and is dirt simple. I built the prototype using a Uniden mag-mount CB antenna, sans loading coil, (the one that comes with their handheld kit), 3- #12 1/4" lugs and 24" of #12 copper wire, fine tuned it with the MFJ and tried it out. I found it would out perform a friend's Brand Name 5/8 wave single band NMO mount mobile antenna, at a fraction of the cost. So I bought a dual band mag-mount, (the one with the coil midway up) to try on my radio, a borrowed FT-530, I couldn't hold a repeater worth a darn. The coastal hills caused the radio to drop the repeater. I added the TRIDENT mod to this antenna. The performance picked up amazingly, to the point I was holding my primary repeater all of the time except when I got into isolated canyons, even in the fringe areas where other antennas would drop. Look on a map of central California, trace a path along 101 from King City to Buelton, I can hit Cuesta Ridge 444.525 and 146.800 from every point along that path, AT 5 WATTS OR LESS. Keep in mind there are a lot of coastal hills and valleys in that area.
I made a trip to Kings Canyon, in Grants Grove Campground using an ICOM W-32A, and a FT-50, I hit the 444.525 repeater on 5 Watts, about 200 miles as the crow flies, not too shabby, I could also hit repeaters up and down the San Joaquin Valley. A recent test was made using the 147.180 machine in Oakhurst, from Kettleman City to Wawona in Yosemitte, with nary a missed beat, this test also had a side-by-side comparison with a professionally installed TM-7A and a NMO mount dual band antenna. I could get in, he couldn't!!
These are the specs I know, using a dual band 1/4 wave mag-mount.
Bandwidth 2M------135-155 Mhz SWR < 1.2:1 130-162 Mhz
< 1.3:1 @ 50 ohms Z
440------439-451 Mhz SWR < 1.2:1 437-455 Mhz < 1.3:1
@ 52 ohms Z
Radiation centered angle approx. 15 deg above horizon. This is theory
based only.
Pattern is 3 primary lobes with numerous secondary lobes.
I have used the antenna over 250 miles at 5 watts, in the middle of
the day on repeaters and 20 miles using simplex over fairly flat terrain.
A friend (WB6DPG) said he thought the mod provided a capacitance hat to the bottom of the antenna, widening and flattening bandwidth.
Here is the mod: Click on the thumbnail for a diagram.
Using suitable material cut three lengths of 3/32-1/8" rod approx. 8.5 inches long. Solder a lug with an appropriate size hole for the base of your vertical radiator to each rod. Measure 1.12-1.3" (depending on primary radiator diameter) from the center of the lug to the outer edge of the bend, bend the rod 90 deg with the lug opening parallel to the radiator, such that you end up with another vertical, repeat this 3 times. Attach to the base of the primary radiator each secondary 120 deg from the last in the horizontal plane forming a rotisserie skewer. Using a MFJ-259 or other suitable means to tune, trim the secondary verticals to the appropriate length, I find 6.25-6.4" works depending on the presence of a loading coil or not. You may also need to shorten the length of the primary 1/4 inch to center the frequency. A stainless steel fender washer with stainless rod crimped or silver brazed to it works nicely as a final product once you determine your exact measurements.
If you desire a ready made SS crimped version, you may order them below, price is $15 for the Triad blank or $40 for a tested mag-mount Trident Antenna. Donations to the ICOM 706 MkII G radio fund will be greatly appreciated.
If you use this mod please send me feedback. mailto://goody@surfari.net