Posted: 1/4/2008 2:12:44 PM EDT
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Gents, I'm looking to do an install of a few different radios in my truck. I'll be sure to put a range report on here when I get done. I'm building a center console to house a CB, scanner, Fire/EMS UHF radio, and eventually an amateur radio (see my other thread). Of course, these will all need antennas. I picked up a Wilson 1000 NMO mount antenna for the CB, and the UHF radio takes a Motorola NMO mount kit, for which I got the part number at work. No decision yet on any specific amateur antenna kit, but rest assured I'll find something decent, based on reviews here and elsewhere. The scanner doesn't have an antenna currently (please feel free to make recommendations - it's one of the small uniden bearcat 350's, I believe) The question is, how best should I lay out these antennas on my roof? The vehicle is a 2002 Toyota Tacoma Doublecab (crew cab), so I have a fair amount of space to work with. I understand that for best performance, the antenna SHOULD be in the middle of the roof. Of course, there's no way I can fit 3 transmitting antennas in the same spot. I figured I'd mount all 4 in a row down the centerline of the roof. So, how would you lay them out? Any particular order? Any particular spacing between them? Will they interfere with each other? Please remember, I'm a dummy when it comes to radios. I'm brand new and willing to learn but please don't flame me for my admitted ignorance. All help is greatly appreciated! Pic of roofline - just shy of 60" from front to rear (sorry for the darkness) 
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Go to this most excellent website and read the info: K0BG's Website If you still have questions...you missed smething. 
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I'll do some thinking and post tonight. I would go with a 1/4 wave on 11meters, (not on the roof though  ) but I will play along with the rules. I wish I had that kind of real estate, I only have a standard cab truck so the most I'll ever mount on top is 3 or 4 (band dependant of course).
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I found this years ago. It's for cell phone use but the same ground plane principles hold true - lower frequency antennas need a bigger ground plane. I don't have that many antennas but do have a CB and a pair of ham antennas. I have three mounts and have a maximum of two of the three in use at a time. Normally I don't mount the two biggest antennas - I put them on as I need them. I don't use CB around town and the repeaters are all so very close that I get away with an 18" glass mounted "cell phone" looking 2m/70cm antenna. When I go off road I put up the big antenna on the center of my rood and get 3 dB gain on 2M and a bit more on 70 CM. If someone in my group has CB I'll put the Firestik on it's mount all the way on the back of the truck. What you don't want is any antenna within a 1/4 wave length of the next - and CB is 11 meters - that's 9 feet you'll want to keep the antennas away from each other. If they get too close they'll load each other up cause the RF from the transmitter to appear on the receiving antenna next to it. You'll also see this RF energy on the pre-amps of the receiving radio which might damage it (unlikely) but also might cause it to lose sensitivity. I don't know what the back of your truck looks like but I would go with a bumper mount or a stake-bed-hole mount in the back most passenger side and put a big 4-5" top loaded Firestik KW back there. I would keep the center of the cab's roof for the amateur radio transmitter as if you put the thing off to the side the 440 MHz transmissions will be very asymmetrical. For your receive only antenna I'd look to the edges to mount that - another stake bed mount forward of your CB or even off that diamond plate in the front of your bed. 
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What sort of UHF system do you operate on? Do you know the system coverage, do you ever operate on the fringe of the coverage? You might be able to get away with a 1/4 wave, but I think for a low profile you could use an antenex phantom painted to match the truck. It would still give you 3bd of gain on UHF
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This is what I would do, of course there are many different ways you could do this as well. The CB antenna would be mounted off the roof, either near the toolbox or the rear of the truck. I would use a 1/4 wave, but that's just me. The other antennas would be centered on the roof from front to back, with the antenex phantom for your UHF radio, a dualband antenna in the center for amateur radio use, and a simple VHF 1/4 wave for your scanner. If you have a certain band you prefer to listen to with your scanner, then a single band of your choosing could be used instead of the 1/4 wave VHF whip. They have worked well for me over the years for general scanner use. 
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The Master Chief nailed it: 1/4 wavelength of separation between antennas, so most likely you'll want to locate the CB antenna somewhere other than the roof. His diagram is also GTG. I'd suggest you choose the radio/antenna that's most critical to your needs and place its antenna at the center of the roof, then locate the remaining rooftop mounts in relation to that one. If the Fire/EMS UHF is a transceiver vice a monitor, it sounds like that one might get the center spot (if you go with all NMO mount antennas, you can always swap that one with your future VHF/UHF amateur antenna later. In a previous life, I was a communications officer for a state EMA. On our pickups, Suburbans and larger vehicles with ample rooftop real estate, we used the 1/4 wavelength rule of thumb (based on our lowest frequently-used VHF frequency) and placed NMO hard points on the roof for almost all of our antennas, both along the long axis of the truck and across the roof. HF antennas (long whips on autotuners) went on fender walls, low and high VHF, UHF, cellular and SATCOM went on the roof. Good luck with it. |