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Tech237
Moderator
Username: Tech237
Post Number: 1216
Registered: 4-2004
| | Posted on Monday, August 31, 2009 - 1:50 pm: | 
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Installing Radios in Vehicles
This is the first part in a series of articles aimed at installing radios in vehicles. During these articles we will take a look at antennas – location and mounting, radio location and mounting and running power and speakers.
Antenna Location
When I worked installing commercial two-way radios, I was often asked about why some antennas were located on the vehicles roof, some on the trunk lid and upper fender area and others mounted down on the bumper.
Common sense should tell us that some antennas cannot be mounted on the roof of a vehicle due to size and safety constraints. For example mounting a full length ¼ wave antenna for 28Mhz or lower is going to cause problems with low cables, covered car parks, bridges and drive-thru. (By the way the covered car park I learnt from personal experience – it cost me one Hustler mast and 30 fluorescent tubes.) The other reason some cannot be roof mounted is simply they are too big for the thickness of metal in a cars roof and will cause the roof to bend.
If mounting an antenna on the car’s roof can cause problems why don’t we then mount all antennas on the trunk lid or lower? The answer to this is efficiency. To radiate the best it can, any antenna needs to be in clear surroundings, and as far away as possible from any parallel conductive surfaces.
Mount a ¼ or ½ wave 2m antenna on the trunk lid and because the bulk of the antenna is below the metal roof of the car and close to the rear door pillars, it will not be as efficient as the same antenna mounted on the car’s roof. Also, as the antenna sways due to vehicle movement, the gap between it and any nearby conductive surface changes. This, in turn, can cause the resonance, SWR and radiation patterns to change.
Now mount the same antenna on a bumper
mount, and as you can guess, things get even
worse, because more of the antenna is near a metal surface. However, some antenna can really only be mounted on a bumper level mount, so like most things, mounting is a compromise between efficiency, safety and looks.
Now Up to this point I haven’t mentioned On Glass antennas, and that was purely due to the fact I have not had any need use one. Now an On Glass antenna will not be as effective as a direct connected antenna of the same style and size, due to several factors.
With the antenna being mounted on a window we have little if any ground plane to work with, so how do they work?
The signal is capacitive coupled to and from the antenna with the glass being the dielectric of the capacitor. Now as the glass can be of varying thickness, this can change the value of the capacitor, which in turn, can affect how well the signal is transferred. Now add things such as window tinting (some of which can completely stop the capacitive coupling), the thin wires of the rear window demister and we can have even more trouble with an on-glass antenna.
Do they work? Yes, in fact a lot of cell phone antennas used this principle, but personally, I’d only use one as a last resort when no other type of antenna can be installed, but that is a personal decision.
In short, always try to mount the antenna as high on the vehicle as possible contingent with safety and other important factors (“you’re not drilling a hole in the roof of MY car”, or not having a roof on the car).
Anecdote: One of the funniest looks I have ever seen was a CEO who brought a brand new BMW to have a commercial two-way installed. His face went pale as I walked towards his car with a drill fitted with a 5/8” drill bit and promptly drilled a hole in the roof. He had not seen me take the precautions that I'll mention a little later on.
Antenna Mounts
In the last section I made a comment about not being able to drill a hole to mount an antenna, there are several ways that you can work around this restriction by choosing different type of antenna mount. By this I mean consider using a magnetic mount on the roof instead of drilling a hole, or use an L bracket or trunk lip bracket instead of drilling thru the lid or fender.
Now I’ll be honest and admit, while magnetic mount may work I have never been really happy with them. Over the years I have been involved with radio, the majority of times I have had problems with electrical noise in a radio I have been able to reduce or eliminate the problem by replacing a magnetic mount antenna with a properly grounded style. The one occasion where this did not work was an early 60s VW Beetle, that no matter what was done, had unbearable ignition noise.
The other reason that I try to avoid magnetic mounts is that the coax has to enter the car via a window or door, and this can lead to wind noise, water leakages and possible even damage to the coax as the door shuts. Remember that a crimp in the coax not only changes the impedance of the coax, but also creates a weak spot that may fail later.
Other people swear by magnetic mount antennas, so as with most things caveat emptor.
Now trunk lid mounts have their own pros and cons, with the pros including such things as not needing to drill any holes, mounts are available for just about every style of antenna base, and the mounts adjust in all three planes so that the trunk does not have to flat in order to get a vertical antenna.
Conversely the two biggest cons, that I have found, are not all cars have a big enough clearance around the trunk lid to use a trunk lip mount, and the ground on a trunk lid may not be the best as, in most cars, it relies on the hinges. Neither of these are insurmountable issues.
The first can be overcome by making your own thin lip mount out of steel or aluminium, and the second by using ground straps from the lid to the chassis. More on these later on, when we look at mounting an antenna on the trunk lid.
Roof Mounting and Antenna
Roof mounted antennas, where feasible, are nearly always the best option. Just look at commercial two –way radio installs – nearly all have a roof mounted (or at least as high as possible mounted antenna). Installing an antenna in the car’s roof is not the daunting task it may seem – especially if a few precautions are observed. Note the roof mounted VHF antenna on the photos of the Pontiac earlier in this article.
Whenever I am going to drill into a vehicle’s roof for an antenna, I start by laying down a square of blue masking tape in the general location of where the antenna is wanted. This does two main jobs. Firstly it gives me a surface that I can mark on without worrying about the vehicles paint. Secondly, when it comes time to drill the tape helps to reduce the chances of the drill bit wandering and scratching the paint.
So how do I then locate where I am going to drill to mount the antenna? I use several methods to ensure the antenna is as near as possible to the center of the roof.
Firstly, I make a chalk line on the roof of the car (front to back) using body contours, inside rear view mirror and any other indications of the center line as aids. Now I repeat this only going from side to side. On most 4 door cars, this line generally is based on the leading edge of the door pillar, as this seems to be close to the mid-line, but also misses any internal bracing on the roof. Another good aid is to remove the roof mounted interior light and visually check. Once I have the location worked out, I'll transfer the lines to the tape with marker pens.
Now, I will drop the roof lining down and insert a length of wood under the spot where I'll be drilling. This piece of wood does two tasks – 1) it makes sure the roof lining stays away from the roof and reduces the chance of it being damaged by the drill, 2) once I see wood shavings coming up, I know I am through the roof. After drilling I clean the edge of the hole to remove any sharp edges.
Here is a couple of tips for when you install the antenna base through the hole you just drilled. On the outside you will normally have a rubber washer, I normally smear a THIN layer of Petroleum Jelly on both sides of this washer. This thin layer helps prevent water from creeping under the washer and into the car. By the way, if you base doesn't come with a rubber seal for the outside, it may pay off to make your own. On the inside I also try to use a layer of mastic type sealant on the roof and base after installing the base. DO NOT use a sealer that smells of vinegar.
Now that we have the antenna base mounted, it is time to run the coax to where you will have the radio. The route you run the coax will depend on several factors, such as where the radio is to be mounted, what access/trim panels can be removed and what space is behind them. I have a preference to use the door pillar, as most, have plenty of room behind the seat belt to run a coax cable.
Now the cable can either run under the kick strip by the door, or under the carpet to the center console and then under the console to the radio. Both methods work, and while I have used both, again MY preference is to run it under the kick strip, as it seems to have less chance of getting damaged there, than just laying under the carpet. Use the one that works best for where you intend to mount the radio. Which ever method you use, for longevity, make sure the cable is secured every 18” or closer, and that should be covered where ever possible. This is not only for neatness, but also protects the cable.
One word of advice here, where ever possible, try to run all radio cables (power and coax) as far away from normal car electrical wiring as possible. This reduces the chance of inducing RFI into the cars electronics. I have seen cars where this was not done, that have had issues with engines dying, indicators changing flash rate in step with modulation and a whole host of other weird issues.
Next part – Trunk, Fender and Bumper Mounting.
Simon
Tech237
N7AUS
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I thought he said, "there was no rust for the wicked, and I own an MGB"
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Milkman21218
Intermediate Member
Username: Milkman21218
Post Number: 388
Registered: 1-2004
| | Posted on Friday, September 18, 2009 - 6:55 pm: |
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Very good Simon |
Starface
Senior Member
Username: Starface
Post Number: 2574
Registered: 1-2005
| | Posted on Saturday, September 19, 2009 - 3:25 am: |
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Simon, Thanks for the Articles
George/Starface/KI4NBE
CEF 476 CVC 014 HAM 181
SOUTHEAST CEF NET CONTROL
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Tech808
Moderator
Username: Tech808
Post Number: 17115
Registered: 8-2002
| | Posted on Saturday, September 19, 2009 - 6:33 am: |
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Simon ~ Tech237,
Very Good & Helpful / Informative Article for the Newer & Older radio operator's. 
With the newer models of vehicles it is getting harder & harder to install mobile radio's.
Lon~Tech808
N9CEF
CEF#808~CVC#002
Radio Enthusiast!
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