The DK3 Screwdriver Mobile Antenna
Midi playing is "OblaDiOblaDa"
The Screwdriver antenna was invented by Don Johnson, W6AAQ of Esparto, California back in 1991. I am not going into any detail on the antenna as all the information is available from Don for five dollars US, however, I will discuss common problems encountered when checking to see why the particular antenna does not perform as well as it should.
I constructed my first DK3 in early 1993 and immediately realized that it was superior to any antenna that I had used since 1960 while mobiling. I had over 250 requests for information on the antenna as I mobiled all over Ontario due to the outstanding signal it was putting out. I had hams say I was running an amplifier, that 100 watts would not produce the signal they were hearing. I also had some hams follow the coax from the antenna back to the transceiver looking for a "boot" ! It was incredible to be able to carry on
conversations for hours at a time as I traveled throughout Ontario from Thunder Bay, Sault Ste Marie, Timmins and Kirkland Lake to Sudbury, North Bay and Ottawa, from Kingston to Windsor ..... literally every part of the province.
My first DK3 installed on a 1986 Delta 88 Oldsmobile. This version had the capacitors for matching on 75 and 40 meters installed in the trunk right next to the mount. 2 switches on the control box were necessary to operate the antenna if one was changing bands.
I can recall while talking on the Sandbox Net on 40 meters, many foreign stations breaking in and reporting that my mobile signal was exceptional at their QTH. I have worked 15 or so countries on 40 meters without trying, they all asked for signal reports from me when their turn came up in the roundtable. Also earned the Heritage Award as a mobile station with Chris logging the calls for me in only four days of driving
I switched between 40 and 75 meters as circumstances required without stopping the vehicle to change resonators , etc as had been the case with other antennas over the years. I was never one to chase DX but have had my share of it with the DK3 on 20 and 15 meters including contacts into Australia and Japan while traveling at 100 klicks or more on our highways.
I have had a DK3 mounted on three different vehicles, the Delta 88, Chevy S10 Pickup and a 1993 Dodge Caravan which has traveled in excess of 335,000 kms with the same antenna mounted on it. The transceiver is a Kenwood TS50 running 100 watts although I did run a ICOM 740 and a 735 in the Oldsmobile.
This photo shows the DK3 mounted, using an L bracket that is grounded to the body of the van. A short piece of ABS pipe mounted on a wooden block that is in turned fastened to the top of the L forms the upper support for the antenna. To remove the antenna, a three conductor plug and socket is pulled apart, then a 1/2 turn on the antenna mast and a pull upwards frees it in less than 30 seconds. An insulated tapered brass stud feeds power to the antenna, a toroid matching unit is installed in the base of the antenna.
There are some DK3 antennas out there that are not performing the way they should and the fault lies with the installation itself normally.
a) The ground connection from the toroid must be as short as possible, mine is about 4 inches or so.
b) The portion of the antenna where the coil is exposed should be at least 18 inches above the highest part of the vehicle and clear of any metal.
c) The mast portion may be as close as 4 to 5 inches from the body with no detrimental effects.
d) Do not use Junk Box coax, buy a new piece, it only costs a few bucks and will save you all kinds of headaches.
e) Use heavy wire as bonding between frame and the point you are using as ground. Also you may need to bond various parts of your vehicle together due to the construction practices used in modern vehicle assembly.
f) The transceiver is not grounded . Every transceiver has a grounding post on the back! It is there for reason, use the shortest piece of at least # 10 wire to go from that post to the firewall or to the metal floor. Make sure that there is no resistance readable from transceiver to vehicle body.
NOTE! Apparently transceiver installation guides for new vehicles say that the transceiver is NOT TO BE GROUNDED, but should be left floating to prevent damage to the various Electronic Control Modules (ECMs) in new vehicles. Improperly installed ham gear has caused ECMs to fail and resulted in very costly repairs to the units and are not covered by warranty. For more information on this go to http://www.arrl.org/tis/info/carproblems.html . There you will be able to get the numbers for Installation Guides that pertain to your vehicle. They can be ordered from your dealer.
Be advised that I have had no problems in any of the vehicles, in which I have had transceivers installed ( and Grounded) and I will not change the grounding at this stage of the game.
Hope I have been able to get across the above basic points for getting exceptional signal reports from your mobile instead of mediocre ones.
Fingerstock, Contacts , etc
One of the most frequently asked questions about the DK3 Screwdriver antenna is
about the contact arrangement or "fingerstock" used.
Here we see the collar that contains the contacts on my DK3 removed from over the coil and the mast of the antenna.
As you can see the contacts are resting in a groove that has been machined into the collar. The later versions of this antenna just have a section of the mast near the top enlarged by about 1/4 inch or so and the new contact arrangement fits between the groove so created and the coil form
This photo shows the fingerstock removed from the machined groove. This fingerstock is actually RF Shielding Strip with the clip portion removed along back of the strip. The part number is Instrument Specialties Catalogue # 97-605-02
This one has been in use since 1994 with little or no wear showing!
Feel free to ask questions in the Guest Book about the DK3 Screwdriver Antenna. I will answer any questions there and if you leave your email address or telephone number we can discuss it further.
When time permits, I am available to do a Power Point presentation of the DK3 Screwdriver Antenna at Ham Radio Club meetings. The presentation averages about 45 minutes depending on your time available.