No1PC: A Collection of Mobile Installation DC and RF Tips

A Collection of Mobile Installation DC and RF Tips

(Updated 10 Dec 16)

I dedicate what I try to share about any technology to my beginnings - my parents, their practical interests, the mentors they exposed me to and allowed me to work with, most of my teachers, many peers and great friends - some who have taught me well and some who have provided information that just didn't work out, causing me to seek good information.

I spent the first 30 years of my amateur radio adventure as a code-bearing Technican class licensee in the obvious VHF and UHF systems and operations. The recent past 15 years I discovered and tried to devour as much as I could about HF operation - fixed and mobile. They are quite different and quite the same all at once. Good, no, excellent RF practice trumps everything.

I thought I knew a little about RF until I was granted General class privileges as a pre-1978 code-qualifying Tech. I was quickly proven wrong on several fronts, not the least of which is that even though my house is a bit older than the one I grew up in, we have a LOT more gadgets in our homes than back in 1955 and how my dad was able to facilitate a ham shack.

You really get to learn about RF, antennas, grounding, shielding, etc. when you experience interference, sub-optimal antenna performance, and then try to make a mobile HF station work. That's where my good friend Doren and his friend Don Johnson (W6AAQ,sk) take hold of my RF-life and school me. Don was one of the first of a very few hams who pursued running HF mobile back in 1948.

Amateur radio operators have it a LOT easier today (with the Internet) than the likes of mobile HF operation pioneer Don Johnson and others. We also have it a lot more difficult (with the Internet) as bad information spreads as fast or faster than good information. As well, beware, vehicle manufacturing materials, techniques and electrical systems are constantly changing.

To quote a friend of Don's, Russ Farnsworth W9SUV (sk 1977):

"Many people like to display knowledge without understanding."

"It is easy to limit your knowledge to what someone has written or said."

What I offer to share with you will hopefully defy both points, and if I don't think I have, I will say so and usually point to other reference work to support what's being shared.

These are but outlines and thinking points as I work to compile a modern-day work about mobile radio installations and operation, mostly, seemingly about HF (at or below 50MHz) but absolutely worthwhile implementing at and above 50 MHz.

The links below are more or less as indicated - several vehicle manufacturer's guides to their electrical systems, electrical system options, primary and secondary power take-off points. Not fully covered, and I'm researching and will document appropriately, are the nuances of some very 'smart' and tricky vehicle electrical system components and functions that you MUST know about in modern vehicles. As of this date I consider 'modern' as most any vehicle built after 2000 - when more and more computer systems, diagnostics and vehicle use data were mandated.

The common thread through all of these is:

* DO NOT wire your radio "direct to battery" - despite what your radio manual says - they are ignorant several critical battery/alternative/energy control functions and intimate knowledge of modern automotive electronics.

* Always run BOTH the positive (+) and negative (-) wires from supply-point-to-radio/radio-to-supply-point

* Your RF system should have its own stout, brute force, highly effective "CHASSIS GROUND", best if all body and frame surfaces are well-bonded/connected to each other

* Never ever under-estimate the 'power' of (stray) RF to get into anything and everything in your vehicle and those around you.

*** What goes on in a vehicle, and the best practices are not just mine - I merely follow them and find that they work nearly 100% of the time (if they don't vehicle problems are found and fixed) - the vehicle is the product of design and manufacture of engineers who know what they put into a car/truck and how to make it work - and this is documented.

*** What happens with RF voltages and currents if not properly contained, terminated, matched, efficiently directed to a radiator and balanced with a decent counterpoise is anyone's guess. We can't always measure it, certainly cannot see it, but we can understand basic circuits, comprehend stray energy/radiation, and certainly take advantage of what we do know about the science and proven practices to do things right/better - the point is maximum RF in and out of the antenna, minimum noise/interference. Tough to accomplish with most plastic cars today, but we can try to work around the compromises.

*** To "pick a fight" about these things at least undermines the vehicle design and manufacturing engineers.

On with the sharing of information...

MUST-reads for any Mobile Installation

DC Power References

Due to the varied use of different vehicle models in public safety applications, the Big Three U.S. auto manufacturers have produced significant resources for mobile installations. They designed the vehicles and systems, they certainly ought to know how they work and where they intend auxillary devices to obtain power. Similarly the Society of Automotive Engineers published vehicle electrical system guidelines.

I augment these with some generic information breaking down the vehicle power system into discrete sections, and go from there. There is one author/site and a related article in QST about mobile power I am hesitant to share until I can 'qualify' some of the material from a lot of yadda/verbiage to what some of it really means, or not, because the information presented never once acknowledges the vehicle manufacturers specifics.

Ford 2012 Upfitting Guide
Local copy of 2010 Police Interceptor guide

General Motors Upfitting Guide
Local copy of 2013 'municipal' guide

Chrysler/Dodge Upfitting Guide
Local copy of 2011-2016 upfitter guide

NOTE: The web pages/URLs and organization of each maker's web site can change at any time. Helpful search terms to use to find these guides: modder, upfitter, fleet, municipal

Society of Automotive Engineers - Electrical Info

Unique Antennas for Your Quad-Band Radio

A Generic Mobile Power System Schematic

Big-A DC Filter

My Silverado 2500HD Install
- A step up from a prior GMC Sierra 1500 install, but all according to GM and Don Johnson best-practices. No whine. No voltage sag. No ignition/injector/fuel pump noise. No charge system or other vehicle issues. No stray RF.

While ideally a correct vehicle DC power connection should NOT present alternator whine to and through your rigs on Rx or Tx audio, be prepared for it. As well, there is a lot to be said for presenting very clean and 'stout'/'stiff' DC to the rigs, especially for peak loads like 100w SSB operation. For this I lucked out extracting a huge iron-core choke from an old surplus DC supply, and typically have a few 50-100k ufd caps around. You can ADD a 'stiffening' or "super-cap" typical of high-power mobile audio installations, but 100k ufd should hold things pretty well by itself.

I have used this filter in all of my mobile installations so there is little or no question the rigs get DC. I use one of the generic Radio Shack filters for the low-draw DC feeding my "glove box" installs of scanner, CB, and USB-Buddy for device charging.

Mobile Antenna References

Along with the DC power issues and I am working on a compilation of mobile installation/antenna/counterpoise/ground-effect references beginning with the first-ever HF mobile operations circa 1948 - RF is RF than as now - it's just a more complex issue in modern glued-and-crimped vehicles versus hand-welded chassis of old with better contiguous ground/chassis conductivity.

The vehicle cabin, chassis and frame are critical elements for optimizing mobile HF operation, and best-practices in this realm also compliment non-HF (> 30MHz) operations.

Obviously you can't easily go with an alternative of a full, balanced, half-wave dipole on a mobile. We're more or less stuck with verticals, which on the mobile scale are a significant compromise from the at-home 43-foot vertical and ample field of radials.

Consider that last example carefully... what it takes to be an adequate vertical antenna for fixed station operation... got that?

Where in a mobile environment are you getting the required (or barely adequate) radial field from ??? Think about that...

Mobiles Have No Ground!

Ground implies Earth. Supported by 2, 3, 4, or more rubber tires, no vehicle has any contact with ground. A vehicle chassis and frame act as coupling for ground-effect.

At best... at least on VHF and UHF, perhaps low-VHF if the vehicle is big enough, you have a less than ideal counterpoise for a classic vertical antenna.

Few vehicles are large enough to be an adequate counterpoise for any frequency below 28 MHz, and even that is dubious with mid-size and compact cars.

The best we have at 28 MHz and below is a rolling coupling device to obtain ground-effect. When you see how current motor vehicles are put together, that capability is highly questionable - a vehicle body is seldom a contiguous piece of metal with electrical conductivity from roof to axles.

To get RF counterpoise currents to a counterpoise and begin to establish ground-effect coupling requires very low-impedance conductivity from the antenna feedpoint to the lowest components of the vehicle - the frame, axles, lower body pan.

You are simply NOT going to get a good RF ground with a clip-on/clamp-on window or door mount alone... NOT POSSIBLE. You need a LOT of FAT CONDUCTORS!

Imagine every surface of your car/truck is an isolated sheet of poorly conductive sheet metal - NOT electrically connected to anything next to it.

From that expectation... figure out how to get as much conductivity from the base/feedpoint of your antenna to everything under your vehicle. THAT is how you begin to establish ground-effect coupling so that your mobile antenna has some form of, admittedly compromised (high-impedance) counterpoise.

No mobile will ever be perfect for HF operation, but with a little bit of work you can get it to be better than a horribly poor and inefficient RF termination.

Mobile RF Challenges and Solutions

Anticipate that NONE of the metal parts are truly connected to each other. Hinges and latches and grease do not make good electrical connections. Every door and hood will be at a different potential level than the chassis/cab than the frame than the engine. These differences simply MUST be corrected. The illustration below shows what the standard vehicle looks like electrically, off the lot:

The typical method of correction is with ample use of 1/2-1" braided strapping to create a low-impedance connection for DC and RF potentials. This method reduces RF and DC potential (voltage) differences significantly, helping form more robust shielding against RF interference and increasing the ground-effect capacity of the entire vehicle:

With a 'tighter' vehicle cabin/chassis/frame connection you are half-way to maximizing the efficiency of the HF RF system. The tricky part is finding the right place to mount the antenna, ensure MINIMAL feedpoint loss and maximum ground-effect coupling of the entire chassis/antenna system. Remember that the vehicle body is NOT large enough to be a counterpoise at HF, so we need to couple the vehicle to the earth below to effect a more efficient HF radiation scheme.

POOR coupling of the antenna feedpoint to the vehicle increases the feed impedance and thus reduces the RF current path from the antenna radiator, around the vehicle to the feedline return. The result is a LOT of RF could be lost in the coax to vehicle connection at the antenna. This is bad.

If we do the best we can to ensure that the point where feedline from the radio meets the antenna system counterpoise/connection to chassis is as LOW an impedance as physically possible, then most of the transmitted energy flows to the radiator and less is lost in the wire connecting to the chassis.

'Moral' of the story is that EVERYTHING, EVERY connection in and around the vehicle to all vehicle components MUST be as low-resistance as possible, by whatever means possible (short of welding the doors and hoods shut!) This means that BOLT ON mounts are preferred and clip/clamp/magnetic mounts are 'FORBIDDEN' - NONE of the latter have any where near the appropriate and necessary RF coupling to support HF operations at or below 50 MHz - PERIOD.

Mobile Installation Examples

To start off, I created an album of my current (2013+) mobile installation. Some aspects, at least placement of the HF antenna, are a non-optimal compromise - partially to spread out the counter-poise/ground-effect placement, although undesireably elevated, but mostly due to the fact that most of my HF mobile operation coincides with towing a 31 foot Airstream - so hitch mount is not available and bumper/corner fender mount is just not esthetically pleasing or practical for towing and hauling.

I will detail more of the GM-specific power connections and all of the cab-chassis-frame bonding to increase ground-effect and mitigate any common-mode RF intrusion to vehicle management, and audio and video systems. (Watch this page for updates!)

Not the Way to Get Seat Warmers...

An associate, who's designed many of the mobile install 'templates' for large state/public safety fleets decided to figure out just how bad taking 'ground' via vehicle chassis can be.

He connected a load bank via #4 to the (+) tie point in the engine compartment on a new Tahoe, and the (-) of the bank to the floor pan at the rear deck. (about 18 feet of 'tin' resistor)

It took less than 100A for the floor/deck pan to start warming appreciably - felt through the carpet. Clearly spot welded/glued thin sheet metal is a pretty good resistor, making for bad 'ground' and an obvious safety hazard.

If you get a peek inside the trunk of a fully optioned Ford Crown Vic Police Interceptor - look at the right rear pillar. Ford runs both (+) and (-) back to the deck for equipment installations. Neither lead runs direct to battery at the engine - instead to where Ford engineers say to take power.

The Dodge upfitter manual has some of the best vehicle electrical information of them all. They are all different, owing to their specific alternator/regulator and battery/power management techniques.

Alternator whine and inadequate power capacity are not issues if you know where to get the power from, and don't try to outsmart, outguess or one-up the guys that designed these systems.

RF counterpoise and preventing RFI are different animals. CHP and I have documentation for that.

Recent GM Install

Generic Mobile Tuner and Whip

A Motorola App Note to Make a Dual-Band Diplexer

See also the data collection reviews of various popular VHF/UHF mobile antennas at:

NO1PC Station References


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© 2015 de Jim Aspinwall, No1PC
Rights of referenced and linked material up to their respective authors/publishers/owners.