Invisible Mobile Antennas††† †††††††††††††††††by Gary Watson - ZL3SV
So you have purchased a new or used modern vehicle that has no visible antennas and you want to keep it that way.
The challenge is to still use your Amateur radio equipment but with invisible vehicle antennas.
Well there is an option that I use that may not suit everyone but I find it works fine for me so I am happy to share my experience.
This article describes the use of Fractal antennas that can be hidden behind the plastic bumpers of most modern cars.
Recently I purchased a new Lexus hybrid that has no visible antennas despite having NZ GPS, AM Ė FM radio, Wi-Fi, hands free,
Bluetooth phone, remote and keyless entry, Voice control, parking sensors, anti collision radar and many other electronics devices
including 3 computers.
Location of some of the existing vehicle built in Antennas†† -† Photo shows where the Fractal antenna sits behind plastic bumper.
Installing Amateur radio transmitters needs caution to avoid causing problems to all the vehicle electronics and you should check with
the vehicle manufacturer to ensure this wonít affect the warranty or after sales service. I take no responsibility for anyone trying these
antennas or any events arising.
I wanted to operate 160 metres to 70 cm, use my APRS (Vehicle Tracking system) unit plus monitor many other frequencies. 95% of the time
I am within 60kms of home so I had a few options.
Fractal antennas have been around a long time and are widely used in cellphones, military aircraft and many other applications so I
thought why not use them behind the plastic front and rear bumpers in my new car. I have 2 Lexus so I thought I would try it out on
the non hybrid first. It worked very well so I installed the fractal antennas in the Hybrid as well.
To achieve as much of an Omni-directional radiation pattern as possible I installed two dual band fractal antennas under the right and
left rear side plastic bumper and behind the front bumper for the APRS.
The APRS electronics are installed in a small box in the engine compartment. The fractal antenna is a copper plated piece of steel
wire soldered into a PL259 Plug. A socket was installed onto the metal work of the vehicle with an L bracket with good
bonding to the vehicle metal chassis and 1 metre of 50 ohm coax to the APRS TX.
Outside view showing no sign of an antenna††††††††††††††††††††††††††††††††††††††††††† Fractal antenna behind the rear wheel plastic bumper
The GPS has a magnetic base and it is also installed behind the plastic bumper and plugs into a TT4 ( Vehicle Tracking Radio modem).
The APRS is only for 144.575 MHz so tuning was easy. I took a piece of copper plated steel wire that was about 10% longer than
a ľ wave and shaped it to fit downwards well away from anything metal and tensioned it against the plastic bumper.
I made sure to clean the area well as once it is tuned for low SWR, I put some black silicon spots to hold it in position.
It is important that the ends of the antennas are well away from anything metal in order to keep the SWR low and achieve
The area below the line from the top of the front and rear wheels downwards is plastic and itís an ideal space for Fractal antennas.
The photo (above right) shows the 50 ohm coax, mounting of the feed point and the Fractal antenna against the inside of the
plastic bumper. Once it is tuned a few small spots of black silicon ensures its there to stay. SWR is 1:02. I silicone the
PL259 once it was all tuned to weather proof it all.
The antennas are dual band but have many low SWR spots outside of Amateur bands but I made sure I had 1:02 on 2 metres
(147 MHz) and 1:03 on 70 cmís (437 MHz) using a stub. This enables me to use 2 meters and 70 cmís mobile with good results
and monitor most of the VHF and UHF spectrum.
The reason the APRS is at the front is to provide as much isolation as possible to the rear antennas.
Also if there is any engine noise it is not so important for the APRS. Fortunately both the Lexus vehicles have no noise on any
frequency but this may not always be the case with some vehicles.
For the operating TX / RX I use a Kenwood TM-D710E dual band transceiver into two fractal antennas on the left and right corners
of the vehicle behind the rear bumper. The photo (below right) is the 2 Metre SWR.
I can operate all HF bands using the SKY command to my TS2000 at home. I live on a hill so the range is over 50 kmís.
The SKY command means I can operate my TS2000 remotely on all HF bands from the TM-D710 in my car or I can also operate my
ANAN 100 watt SDR via my smart phone if I am out of range of the SKY command system.
The dual band fractal antennas were about 10% longer than a ľ wave and I shaped it to fit downwards and well away from anything
metal and tension it against the plastic bumper. It also has a u shaped stub for 70cm as shown in the photo.
It does require a good antenna analyzer (I have a Rig-expert 1000 Antenna Analyzer) and some experimenting to get the SWR low.
Itís important the coax feed point has good bonding to a metal part of the vehicle and the antenna can point downwards with many
twists and bends. I tried to keep as much vertical component as possible.
The two antennas are phased at the transceiver. I mounted the transceiver at the rear near the car battery and installed the transceiver
head end in the compartment between the seats. It has panels that conceal it so there is no sign of any antennas on the vehicle and
no sign of any rigs inside the vehicle. I also have a Diamond Screw driver HF antenna that plugs into both vehicle tow bar sockets
and a mag-mount dual band antenna (2M & 70CM)† so all options are covered if ever needed.
A fractal antenna is generally a rough or fragmented geometric shape that can be split into parts, each of which is approximately a
reduced-size copy of the full
I have not included all the theory to design these antennas as itís much easier to start with a ľ wave length of copper plated steel wire
that is about 10% longer than theory and tune it with an antenna analyzer. NB I tuned the 70 CM stub on the dual band fractal first.
There are plenty of sites on line with the theory but this article is designed to make it as quick and easy as possible.
For those of you who want every possible frequency, every mode and the highest gain vehicle antennas possible, then a Fractal
antenna may not be the way for you. I hope to have more time for the hobby as I am semi retiring sometime when I sell or lease out some
of my companies here in New Zealand. see www.mainlandtv.nz
Best regards - Gary Watson - Amateur callsign ZL3SV