There has been much discussion about the mounting of a 2720H and the
visibility of the control panel. I have two of these rigs, one
mounted in a pickup truck with the head turned slightly toward me and
one mounted in a Liberty with the head mounted low and in the center.
The one in the Jeep is MUCH more visible than the one that is
tilted toward me but neither one of them is bad. Here's how I
installed the rig in the
Liberty including the antenna mount and the key-off control circuit for
The antenna mount was another
problem altogether. The Liberty has no surfaces that lend
themselves to mounting an antenna except the roof. My wife has to park
in a parking garage that limited the available space on the roof to the
point that that wouldn't work. I contacted Larson about a glass
mount and they admitted it was a compromise at best so back to the
drawing board. What I came up with was the idea of
mounting the antenna to the top surface of the bulkhead in front of the
windshield and letting it protrude through the plastic intake air vent,
below the windshield.
I have always Liked Larson antennas and because of the height problem, went with the short 2/70 NMO that they make. In addition, the base for this 2/70 is very short so only the antenna itself needed to protrude through the plastic cover. The mount for the NMO kit is made of two pieces of stainless steel. You can see one screw in the rear of the overlapping section of the mount. After the antenna was mounted and the angular position was determined, a second hole was drilled in the front edge of the mount to hold the vertical position. Next I needed to determine the position of the hole through the plastic to clear the antenna mast.
To do this, I used a Larson NMO cap and chucked it in a lathe to determine the exact center of the antenna mount. I drilled a hole through the cap and screwed in a sharp pointed sheet metal screw from the inside out. I then installed the plastic air intake back on the car and pushed down on it to mark the underside to determine exactly where the hole for the antenna needed to be. This actually worked very well and a 5/8 hole was carefully drilled for the antenna to clear the plastic.
As you can see, the installation came out very nice indeed. The short antenna does not use a loading coil at the bottom so there was clearance (just barely!) under the plastic cover for the mount. The antenna loads well and the SWR is below 2:1 across both the 2 meter and 440 bands. The antenna is well off to the side of the line of sight so it does not interfere with driving at all. I installed the base of the antenna the the fabricated mount, then the plastic air intake over it and then used an allen wrench inserted in one of the set screws to tighten the mast to the lower mount.
The last issue was the electrical work to make the radio do what I
wanted it to do. I wanted it to come on with the key in either
the RUN or ACC position. I also wanted to be able to use it
without the key if I was doing an event and had the rig set up to
cross-band repeat and was away from the car. I contacted Icom to
see if there was any issue leaving the radio itself turned on and
shutting it on and off by controlling the power to the whole radio.
They indicated there was no problem at all doing that. I
used a 30 amp relay to control the radio. The initial control of
the relay was from one of the circuits under the dash that is at
12 volts with the key in the RUN or ACC position. The other part
the control circuit was a bit tougher. I wanted to be able to
trigger the radio on and have it stay on without having to leave any
other circuits on in the vehicle.
To provide the additional control
of the relay, I used an
SCR that is triggered by the running lamp circuit. In that way,
I can momentarily flick on the running lights and that latches the SCR
and it then stays latched (powering the relay) until the key is turned
either the RUN or ACC position. Doing that raises the cathode of
SCR to one diode drop below line voltage, back biasing the SCR and
it off. The 100 ohm resistor was necessary to desensitize the
so that it wouldn't be turned on by things like the remote door opener.
me how I know...... The components were soldered directly to the
of the SCR and then heat shrink was put over the individual diodes and
larger piece of heat shrink was used to cover the entire device.
snipped the heat sink tab off the SCR (so the assembly was smaller)
it is handling virtually no current and doesn't even get warm.
you can use any momentarily energized circuit for turning the SCR on.
is no magic to the circuit I chose. It would appear that there
a couple of unnecessary diodes in the circuit on both sides of the SCR.
turned out to be necessary to achieve reliable turn off of the SCR.
Without them, it would occasionally remain on with a turn off
The protection diode (stops
the pulse from the relay coil when it turns off) and the filter cap
were mounted right to the bottom of the relay and soldered directly to
the relay terminals. I used a 30 amp 12 volt relay from AutoZone
for the radio control. That way, no high current is handled by
any of the original Jeep wiring or switches. The high current
supply to the relay comes from a fused line directly connected to the
power distribution panel under the hood. I mounted the relay up
under the dash (inside the car) and then drilled a hole and lined it
with a grommet for the antenna lead and the +12 volt lead to come
into the passenger compartment.
the schematic of the circuit. None of the parts are
particularly critical. The diodes need to be about 100 volt parts
and the resistors are 1/4 watt. Although the schematic shows the
coming from the left front turn signal, I had to change that since the
flashes the turn signals when you unlock the doors with the remote.
ended up using the parking lamp circuit. That way, I can just
on the parking lamps for a second and it latches the SCR.
E-mail me at ultrasport(REMOVE)@mail.com
© Copyright 2002, 2003, 2004 Steve Das