next best place to mount an antenna
Forgive me if this topic has been hashed out a thousand times before.
I'm finally getting around to installing my mobile (Kenwood TM-V71A) after removing it from my last vehicle that I sold 21 months ago. That vehicle was a 1997 Dodge Ram and I used a Larsen NMO 2/70B with the Larsen Dodge front fender mount. That seemed to perform well, though I never did check my SWR, as I didn't know anybody with one and didn't want to buy one. Even if I wasn't afraid to drill the roof, I couldn't because I had a ladder rack and my ladders would have destroyed the antenna.
I now have a 1997 Honda Accord that I use for commuting and a 2002 Dodge Ram for parking in the garage. I don't have a problem drilling the Accord, I'm just deciding on roof or trunk deck. Now the truck is a different story. Where is the second best location other than drilling the roof? The truck is a 2500 with a long bed and extra cab.
I plan on using the Larsen NMO 2/70B on the Accord. It's a 1/2 wave over 1/2 wave co-linear design, and though I live along the Wasatch front in Utah where the repeaters are up high, I think the 1/2 wave will be better suited for simplex. I'm open to suggestions on anennas for the Ram, as long as it's built by Larsen. I'm not a fan of the Diamond or Comet for the vehicle.
After installing a large number of Lasen mobile ants., I won't offer a 2nd best. The NMO 2/70B was designed for mounting on a horizontal ground plane of adequate dimensions. The Larsen cutting chart was created with the ant. on a ground plane representative of an average roof. Since you plan otherwise, the best I can offer is think high, and then a bit higher. For both 2m and UHF, height over ground is important, not only for propagation, but to minimize performance degradation caused by the truck body. Since the cutting chart will be of little use w/o a ground plane under it, I strongly recommend you use a SWR meter, directional wattmeter, or antenna analyzer to tune the antenna this time.
Originally Posted by KE7PMY
GL, Roy and 73.
Originally Posted by NM7G
I don't like what you're saying. Not because I disagree, but because drilling the roof can't happen. Low hanging branches are going to wreak havoc when they catch the whip and/or coil and possibly bend the roof sheet metal. Isn't there sufficient ground plane on a fender mount? What about a bed rail mount, either against the cab or against the the rear. How close to the ground plane does the bottom of the antenna need to be?
There is nothing wrong with mounting it on the trunk lid, and you'd probably never be able to tell the difference. The other issue is, mounting one on the roof places the antenna in harms way if you garage your vehicle. The have a tendency to get caught between the door panels, with predictable results.
Alan Applegate, KØBG
Low hanging branches, garage door overhang, and undergound parking structures may be accomodated by rooftop mounting with creativity. I had a NMO 2/70 on my full-size Ford P/U roof, with a 0.060" or thicker stiffener plate below the NMO nut, and I installed a swivel with wing nut right above the NMO base tapered hex nut. For me, it was worth the effort. I've studied on-vehicle radiation patterns comparing roof-top mount to inferior locations. Police and emergency vehicles mount roof-top on trucks, or as close to top as possible because they require reliable coverage.
On a truck, most folks look forward, to a fender, close to the gap between hood and fender in order to put as much GP under the ant in a near-horizontal plane. It's best to locate the mount well forward of the A-post, the steel pillar at each side of the windshield. I would go as far forward as you can, without reaching the point where body curvature tilts the whip significantly, or the antenna gets in the way of routine service, but even that can be helped with a swivel. You can either drill the large hole for the NMO there, or use a right angle bracket. If you do the latter, a clean connection is important, and the connection should be inspected and cleaned periodically to avoid debris buildup and corrosion. Placing it on a fender avoids disadvantages associated with mounting on a vertical surface with a 90 degree bend on the cab side, or likewise bent on a side of the bed. You really don't want a large piece of steel parallel to the whip! The point I made about tuning and measuring SWR is particularly important with secondary locations, because I promise you the ant. won't resonate as intended by Larsen, nor will the VSWR be what you'd expect on a large flat surface. I never said it can't be done at other locations. You just need to set VSWR and signal report expectations appropriately.
Trunk lid mounting for an automobile is also inferior to roof mount, but not terribly so if the antenna is near centerline of the trunk. If a trunk mount is moved a foot or more left or right from centerline (towards a fender), the more radiation becomes asymetrical, favoring some directions at the expense of others. Not to belabor the point, but do measure SWR. Some extreme mount locations folks try result in really bad SWR that throttles back the radio's protection circuit.
Where you are, a wet noodle should get you into the repeaters in the Wasatch from a big part of the state, so I would not sweat minor issues with antenna efficiency.
I use a trunk-lip mount on my hood, with the antenna directly opposite the broadcast antenna on the other cowling. For 'around town' use, I have a 1/4 wave whip that goes in and out of parking garages with no trouble, and would probably work well for you most of the time, too. I have a 5/8 wave and a taller antenna - I think it's 5/8 over a 1/2 wave - that I use on the open road. I prefer not to use the tallest antenna, though - it puts quite a bit of bending force on the hood. The shorter ones are no worry.
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Gary, while I will agree that offsetting the antenna to one side or the other effects the pattern, it isn't all that significant, and the average operator would be hard pressed to tell.
The same can be said about HF antennas too. Here again, you can model it, and see the change in the plot charts, but the difference in on-the-air results can't be noticed.
Alan Applegate, KØBG
Yes, one can model, Alan. One may also take radiation pattern measurements of vehicles using drive-on test ranges. I've done the latter on three different antenna ranges (Detroit, Chicago, and Vancouver companies). Effects of mounting location aren't simple to model accurately, and vary with body style, vehicle height, test frequency, accessories (e.g., luggage rack), vehicle's nonmetal skin content, and other antennas on the vehicle, to mention a few.
Originally Posted by K0BG
While I agree with you that most hams won't notice an on-the-air sig report difference on HF, because vehicles are electrically so much bigger on VHF/UHF, I don't think your blanket statement is appropriate. Many hams will say "It works" if they make a few QSOs, and they move on. Often, however the installation wouldn't meet the antenna manufacturer's spec. when thoroughly measured. It has been my policy to tell folks the reality as I know it from extensive measurement experience, though they may not like the answers. The course of action they choose is always theirs.
I've taken the time to model three different vehicles with the commercial version of EZNEC, and the difference between center, side, front, and rear. The difference in pattern was about 3 dB on average. It's a bit more if you model rear mounting on a van of course.
I'm always tickled when folks try to prove the point by driving in circles, with little or no thought into the difference in ground losses, or instantaneous changes in propagation.
Alan Applegate, KØBG
here an option that uses a nmo mount and doesn't involve none to minor drilling the truck body, remove the fm antenna and put your nmo here. Apparently the hole is already 11/16 on gm antennas and one of my ham radio mentiors did that on his van, it looks factory stock except for the nmo base. As for restoring the fm radio antenna, simply run a wire under the plastic trim around the window and connect it to the radio harness you unhooked,they sell kits for this but its a fancy wire really with a small fm amplifier that rarely does a whole lot. One thing to watch for is if i recall correctly the engine computers are near that spot, so i'd get a magnetic base antenna first and transmit and check for problems first, its unlikely really.