I have a Hi-Q 5-160 RT installed on my 2006 Toyota Tacoma truck. I have used two different approaches to tune this antenna.
Initally, tried using a homebrewed remote controlled L-network I had laying around from an old homebrew 160 meter mobile project. I found that there were just too many degrees of freedom to make tuning the antenna to resonance an easy job.
I elected to use a commercial UNUN product from CWS Bytemark, using a design based on the work of W2FMI. The unit I chose has two different impedance taps available on the output side. I actually have two physically identical UNUNs available here. The one I use exclusively now has either 32 or 18 ohm outputs from the transformer to the antenna. I ordered the UNUN from CWS with N connectors instead of the standard PL259s. The N connectors are infinitely more watertight than PL259s. In addition, I had the transformer potted at the factory. This serves to make the UNUN waterproof and shockproof.
After designing a cantilever beam bracket to hold the antenna, I designed a vacuum latching relay system based on a Kilovac K43R SPDT to remotely switch taps. The relay assembly was placed in a cast aluminum box with N connectors. The vacuum relay assembly was potted under vacuum with GE RTV 11 silicone potting resin. This makes the relay assembly waterproof and shockproof as well. The relay attaches to the outputs of the transformer with two N barrel connectors.
The UNUN-Relay assembly is mounted on a aluminum plate secured to the antenna mount right at the base of the Giant quick disconnect. A short unshielded jumper runs from the relay output to the antenna feedpoint.
The control system for the vacuum latching relay uses a couple of LEDs to indicate high or low impedance selection. This is accomplished by using a micro DPDT latching relay slaved to the (On)OFF(ON) DPDT micro toggle switch at the operator control point. This means the control head "remembers" the impedance setting at the vacuum relay, even after power cycling the system. I also had to build a 12 to 24 volt DC/DC converter to provide the proper voltage pulse to the Kilovac K43R.
Before the RG-213 coax reaches the UNUN, it turns into an air core choke of about 6" diameter and about 20? turns long on a plastic sewer pipe form. This choke is tucked up above the tow bar assembly beneath the bed. Previous experience has taught me that choking the coax can make a BIG difference for a number of factors of system performance.
My version of the 5-160 RT from Hi-Q has a custom lengthened lower mast section. It used to be 6 feet to the coil base, but I shortened it to 5 feet when I moved the antenna from my old Tahoe to the Tacoma. This allows the dismounted antenna to just fit inside the camper shell covered short bed of the Tacoma.
I mention the length of the Hi-Q antenna here because I want you to keep my experience in perspective. With the setup I have, I get a perfect match on 160, 80, 40 and 15 meters using only the proper transformer tap and careful positioning of the antenna coil contactor. 20 meters is a little off at about a 1.4 best match. I have found that the 18 Ohm tap on the transformer is only needed for 160 meter operation. Everthing else works fine on the 32 ohm tap, although sometimes, 80 meters likes the low tap. Environmental factors can strongly affect resonance of the antenna, particularly on the low bands. Nearby metal structures (ie gas station pump roofs and passing semi trucks or freeway overpasses) will cause rapid VSWR excursions on 160 meters.
I use various size hats available from Hi-Q above the 5" coil for differnet bands. For 160 meters, I use Charley's largest cap hat and a four foot whip above the hat (the hat itself is two feet above the coil). This puts me legal and safe under any overpass in WA state, but the occasional tree limb does snag me now and then!
I use the Ameritron SDC-100 digital controller for coil position telemetry. I found the stock design of the SDC-100 control head to be susceptable to RFI induced dropout of the microprocessor. This device was re-engineered into a double-shielded enclosure with massive filtering of the I/O lines with mutiple stages of ceramic capacitors and ferrite chokes. In addition, a PWM circuit was added to the Ameritron design to allow for a "Slow" setting for the coil contactor.
It was found during system testing that the internal magnetic reed switch installed at the Hi-Q factory inside the 5-160 RT antenna was not the proper choice of reed switch for this design. Charley used a 1/4" diameter end triggered reed switch as delivered in my antenna. It did not perform (trigger) properly in the rotating magnetic field provided in the coupling bushing by two neodynium magnets between the motor and the contactor all-thread rod. I replaced the factory reed switch with a proper side-actuated magnetic reed switch. Even with the proper magnetic reed switch installed, you must be careful to index the reed switch properly for it to work reliably. I also found upon disassembly of the Hi-Q antenna that one of the set screws that holds the DC motor in place had contacted and damaged the wire bus to the motor. Charley was sent a picture of the problem and also a note about the sensor problem. I believe he has updated the manufacturing to reflect my findings.
Now, when I run the contactor up and down inside the 5" coil, I get perfect postional data EVERYTIME with no system dropout due to RFI scrambling of the Ameritron SDC-100 microprocessor, even when running 600 watts output power. This makes for very quick, easy, and reliable tuning of this system. For what it is worth, I placed first in the 2006 WA State Salmon Run as SO Mobile HP Mixed class with this design.
Making a vacuum latching relay system to interface with the UNUN transformers from CWS Bytemark is not hard, and is a good project for those who like to roll their own RF equipment. I highly recomend the UNUN approach with Charley's wonderful Hi-Q antennas.
Pictures of my installation are available to some degree on K0BG's mobile radio web site in the gallery linked to his "Other Installations" subpage.http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12686