W5TOM Subaru Impreza

Yaesu 857D Mobile Installation

Dealing with HF, VHF, and UHF in a 2011 Vehicle in a new way.

This is an unorthodox installation for the following reasons:

*100Watt radio connected to battery with a single 22ga. wire.

*Only one ground for the whole station, no ground loops.

*16ft. all band mobile? antenna.

This is the business end of the 857 installation with Vibroplex paddles for CW.


It has been over 30 years since my last mobile installation and things have changed tremendously.  The cars are full of computers, the firewalls are impenetrable, the AM/FM broadcast radio antenna is 8 inches long with a high gain preamp, but a good thing, the mobile radios have detachable heads.

I saw the possibility that 100 Watts of HF might cause a very expensive problem with the modern electronics and void my new car warranty.  Several attempts to research the problem turned up nothing.  I could not find any info on the internet for an installation in a car newer than the nineties.  I anonymously called a couple of dealers and they had not heard of Ham radio, much less how to install one.  It appeared that I was on my own.  I thought it would be a good idea to plug the 857 into the cigarette lighter outlet, put an antenna on the roof, and load it up on all bands with full power and see what happened.  Well I discovered the new cig receptacles are only good for 10 amps, so I put a 7Ah gel cell on the seat.  Full power on all bands caused no problems.  If it had, I had full deniability since no evidence would be left behind.  Hey, I tried to get their help.  If the computers had crashed it could not have been a Ham radio...they haven't heard of Ham radio anyway. 

This shows the header battery, the Schottky diode, fuses, Yaesu power cord, and single wire to cig plug


We still had a power problem.  After a trip up on a grease rack and extensive investigation, a way through the firewall was deemed impossible.  Then I thought of keeping the 7Ah battery as a voltage header.  After calculation and experimentation, I came up with the design shown here.  The coil of 22 gauge wire adds ¼ Ohm resistance to prevent the initial amperage from rising too high when the voltage on the header battery is low and the ignition is activated with 13V or better.  The Schottky diode was added to prevent the header battery from running the accessories when the ignition is off.  A Schottky was used since they only drop about half as much voltage as an ordinary diode.  The single wire diode combination created a high impedance connection to the car electrics, minimizing any exchange between the radio and the car. 

Testing made sure that the duty cycles of operation and charging were compatible with a full charged battery at a max charge rate of about 5 amps.  The installation is completely trouble free, and probably responsible for the fact that there is absolutely no QRN from the car in the receiver or problems with the built in cell phone, radio, garage door opener or computers.  The Yaesu FT-857D is best suited because it has a supply voltage readout on the display, and uses the least current of any mobile radio.  It Draws 0.6A in RX and 1.5A in SSB TX with no audio.  A timer automatically shuts the radio off in one hour if you forget to shut it down when you leave the car. The famous IC-706 draws 1.5A in RX and 4.5A in TX with no audio.  If you want to install a 706, it would be best to use a 12 mAh header battery.



The car is shown with the roadside crappy pole antenna deployed. A flag pole is in background with a Texas Flag. I am too lazy to reshoot the pix.


My original idea was to have 2 meters while on the road, but have HF capability when pulled over.  I bought the clamps that held accessories on the luggage rack, and made a fitting to hold a 16ft. crappy pole with wire wrapped around it.  I would use my LDG Z11 Pro tuner with internal batteries to load it.  This worked very well, but I had to add a toroid inductor to load on 80 meters.  Later I added a mag mount with MFJ Short Ham Sticks for HF while driving, but the 16ft. antenna still worked best when deployable. 

Shown here is the mount for the crappy pole and the auto tuner.  There is no ground connection here.  Coax to tuner and a single wire to antenna.


The radio and the header battery are Velcroed in place. The ground strap, and wire that goes to the speaker under the other seat are viable.


This shows the only ground in the whole installation connected to the body of the car.  The 857, mic extension, and header can also be seen.


Radio Shack adhesive Velcro tape is used to mount the control head.  


This auxiliary "S" meter was added later.  It is also attached with RS Velcro tape.


L to R: 2 meter/440 antenna, broadcast antenna, and HF Ham Stick antenna.  The crappy pole mount is on the front rack..


Here is a schematic showing the installation without the Ham Stick and mag mount.  It is switched separately to the radio in place of the tuner.


At this time, this system has been duplicated in three cars with the same outstanding  results. The fact that there is only one ground in the whole system eliminates any possible ground loops which can cause all the usual RX noise problems, and interference with the cars computers. "What about the return path for the antenna", you say.  Well, my first reaction, being in Texas, is to say, "We don't need no stinkin' return path".  In fact  the return path is through the ground strap connected to the body of the car.  "Yea, what about the capacitive path made by the mag mounts, that's your real return path!"  Not so fast, my friend, the area and spacing under those mag mounts don't even come close to providing enough capacitance to couple HF.  It would take a mag mount the size of a rented space in a trailer park to do that.  Your real hit is in the capacitive path to real earth ground under those toroidal insulators on those pretty rims and the concrete road bead.  I hope I gave you some ideas for your new installation in your new car.  CU on the road and 17meters, de W5TOM SK   This web page edited and approved by KT4QW and W5RH.