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Tech237
Moderator
Username: Tech237
Post Number: 1224
Registered: 4-2004
| | Posted on Friday, September 25, 2009 - 12:20 pm: | 
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Installing Radios in Vehicles -part 2
Last time we looked at Roof Mounted antennas, so now let look at Trunk mounting an antenna.
Trunk Lid Mounting
Now trunk lid mounting, including rear hatches, can be both a blessing and problem. The blessing, because you have this large flat metal area for a ground plane, the problem, or problems, come in with mounting the base and feeding the cable to the radio.
Modern cars are usually made with a tight fitting trunk lid that may not leave much of a gap around the edges. This small gap means that many antenna mounts, designed for mounting on the edge of a trunk or hood, will not fit without scrapping the paint every time the trunk is opened or closed. It also means that you may have issues with feeding the coax to the inside of the trunk. The hatchback type of car then adds the problem of the angle of the hatch, that can approach vertical.
There are some brackets that will fit in these smaller gaps, but the thin metal used makes them, to me, suspect on all but the smallest and lightest antennas. You could always use a magnetic mount too.
Our next issue, assuming you have used a bracket that keeps all of the antenna base outside the trunk, is getting the coax into the car. The coax, has to be run so that it cannot get damaged when you open and close the lid, doesn't compromise the waterproof seal along the edges and doesn't prevent the lid from shutting completely – both a waterproof and security hazard. By the way don't forget to seal and waterproof the base and coax using proper coax seal and not just electricians tape.
Cables that, run along the gutter inside the trunk, can be cut, have the insulation damaged or be squashed badly enough to change the impedance of the coax, all of which may eventually damage your transceiver.
Anecdote: OK I'll be honest and admit to NOT following that rule – ONCE. I ran the coax cable from a fender mounted antenna through the front edge of the drivers door (wife's car and Jo had a strict NO HOLES policy). It lasted 3 years before becoming so badly damaged (flattened) that she could no longer reach the local 2m repeater from our driveway. 100mw,and a rubber duck would normally access the repeater. The coax now goes through a neat grommeted hole drilled in the firewall, and I have the damaged cable above my bench as a reminder.
In the past, I have been known to buy a cheap trunk lid from the wreckers, and drill a hole for a through the body type antenna mount in this lid. This gets the coax inside the waterproof (hopefully) environment of the trunk in a way that prevents it from getting damaged.
Now this will work for most antennas and bases up to a limit. Some of you saw photos of the first mounting I used on my 5 band HF antenna on the old Grand AM (edge style mount). Drilling a hole and mounting THIS antenna in the middle of the trunk lid would not of worked. The size/weight of the antenna along with the relatively thin sheet metal used in the more modern cars would have destroyed the trunk lid in short order. I have used this method for a spring based 9ft stainless with no problems.
In the past, if using a lid edge type of mount, I have run the coax along the lid hinge (using good quality cable ties and leaving a little slack at each end) back to the main car body. This technique supports the coax at the point where most flexing will take place, ensures the coax is out of the way of anything being placed into the trunk, and with a little ingenuity ensures the trunk lid has a good ground connection.
How do we do the latter aspect? I will carefully remove a 1” piece of the outer insulation at the point the coax is near one of the bolts holding the hinge to the lid. . A tinned copper saddle clamp is placed over the exposed shield and tightly wrapped to make good electrical contact. The holes in the clamp are now bolted to the lid using the hinges bolt. The same procedure is followed at the body end of the hinge. If you are good at soldering, you may want to solder the clamps to the shield. Either, way before you bolt it up, waterproof and seal the exposed areas.
Another suggestion is, where feasible, mount the bracket as close to the hinge points at possible, without getting too close to other metal surfaces. This will produce a compromise mounting location, but gives you a mounting point far enough away from vertical or semi-vertical metal surfaces and yet close enough to enjoy the strength of the hinges mounting location.
One of the problems that maybe encountered with mounting an antenna on a hatch style trunk lid is that to keep antennas from scraping on garages, car parks etc you have to mount near the rear of the car. This also, generally, is the best esthetically too. This can haves the problem of what to do with the coax. In the case of my Hyundai, I ran them under the trim panel inside the hatch to the hinge, and then down the interior trim panel to the radio mounted in the rear.
As an aside, it is possible that the extra weight of antenna, base and bracket can reduce the life of those hydraulic cylinders that help keep the hatch open.
Now, once inside the trunk, the coax can follow any convenient path into the main cabin of the car and then to the radio. Again try to keep it as far as possible from normal automotive wiring, keep it secured and as protected as possible. Again MY preferred path is to run alongside the rear seat and then under the kick strips to the front of the car. Here I zip tie it to a convenient support that keeps it away from the bulk of the cars wiring, and run it to the radio. There is nothing wrong with running under the rear seat and under the carpet along the transmission tunnel, just keep in mind the carpet will not really protect it.
Fender Mounting.
I am not going to go too much into mounting an antenna on the fender, as most of the issues discussed above or in Part 1 can occur.
I will state, what to me are fairly obvious comments –
1)If mounting it on the side of a fender keep as much of the antenna above the body as possible. This reduces the amount of antenna tuning changes as the antenna moves.
2) Watch the flex and strength of the metal in the fender. The flexing of a large or stiff antenna can bend a body panel pretty quickly.
3)Waterproof and rustproof all holes made in the fender to mount the antenna
4)Watch for a good ground, interior of fenders may have a rust preventative and sound deadening material on them, that does not conduct very well.
5)Seal your coax well, and watch for sharp edges when running it inside the car.
OK all for this part. Next Part 3 – Mounting the Radio and Running Power.
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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Roadbadger
New member
Username: Roadbadger
Post Number: 6
Registered: 12-2009
| | Posted on Thursday, December 17, 2009 - 9:02 pm: |
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I just wanted to make sure I understand this correctly. Because I do not want to mess up my coax if I misunderstood) You are saying to actually remove a 1 inch piece of the outer coax cable shield exposing the inner wire. Then attaching the tinned saddle clamp secure it with tape,waterproof it and bolt it to the hood and also do this again to the body and this will ground your trunk and antenna? I should'nt have to run ground straps to the trunk when following this method? |
Tech237
Moderator
Username: Tech237
Post Number: 1276
Registered: 4-2004
| | Posted on Tuesday, December 29, 2009 - 10:39 am: |
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No. Remove 1 inch of just the outer insulation so that the shield is exposed at that location. If you ground the inner conductor, you'll blow the finals in your radio.
Tech237
N7AUS
Chipmonks roasting on an open fire
Hot sauce dripping from their toes.
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Tech237
Moderator
Username: Tech237
Post Number: 1555
Registered: 4-2004
| | Posted on Monday, March 19, 2012 - 8:42 am: |
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Mmm seems I've been remiss, with setting up training courses, and teaching new hires that I still have not done Part 3 of this series. Keep an eye out it is on the scribble pad.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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