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next best place to mount an antenna

Discussion in 'Mobile Radio Systems' started by KE7PMY, Nov 3, 2011.

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  1. KE7PMY

    KE7PMY Ham Member QRZ Page

    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.

    Thanks.

    Roy
     
  2. NM7G

    NM7G Ham Member QRZ Page

    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.

    GL, Roy and 73.

    Gary
     
  3. KE7PMY

    KE7PMY Ham Member QRZ Page


    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?
     
  4. K0BG

    K0BG Platinum Subscriber QRZ Page

    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.
     
  5. NM7G

    NM7G Ham Member QRZ Page

    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.

    73, Gary
     
  6. K0RGR

    K0RGR Premium Subscriber QRZ Page

    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.
     
  7. K0BG

    K0BG Platinum Subscriber QRZ Page

    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.
     
  8. NM7G

    NM7G Ham Member QRZ Page

    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.

    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.
     
  9. K0BG

    K0BG Platinum Subscriber QRZ Page

    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.
     
  10. KC9TLP

    KC9TLP Ham Member QRZ Page

    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.
     
  11. JOSHALBERT38

    JOSHALBERT38 QRZ Member

    Mounting antenna on top is not a wise as per my opinion because there are many chanced of its getting entangled.
     
  12. G7DIE

    G7DIE Ham Member QRZ Page

    Up until last month I'd have agreed with every word, but consistent on air tests with a friend of mine, proved that his trunk mounted 10m antenna has a significant bias towards the front of the car in the order of 6dB, this wasn't a scientific test, and had it been just the once then I'd have put it down to coincidence.

    I was working overseas on the Island of Crete, SV9, for the month of October, and was in regular contact with a friend in the UK, I was only using very simple equipment and maximum 5W and in order to help me work some DX my friend would 'direct' his car toward the NW, the opposite direction to me, and on every single occasion his signal dropped with me and increased with any of the North American contacts we were working, and conversely my signal dropped with him, I accept the anecdotal nature of this and the none scientific approach, and I also accept there could be a myriad other reasons for this, but on first pass it looks very like the antenna / car combination has significant directivity towards the front.

    Regards

    Stephen
     
    Last edited: Nov 6, 2011
  13. K9STH

    K9STH Ham Member QRZ Page

    It has been known for decades that the pattern is "pulled" by the mass of the metal in the vehicle. That is, the more metal the more that the pattern is "pulled" in that direction.

    When mounted in the center of the roof of an automobile, the signal will be stronger to the front and back than it is to either side. When mounted in the center of the trunk, the signal will be stronger to the front of the vehicle than it will be to either side or the rear. When the antenna is mounted on the left-hand side at the rear of the vehicle then the signal will be stronger towards the right-hand front and when the antenna is mounted on the right-hand side at the rear of the vehicle then the signal will be stronger towards the left-hand front of the vehicle.

    These patterns have been measured on true antenna ranges by the various manufacturers of mobile antennas.

    Glen, K9STH
     
  14. KE7PMY

    KE7PMY Ham Member QRZ Page

    I'm wondering where the perfect spot on the roof of a full size extra cab truck with a long bed would be. Dead center of roof, or as close to center of truck as possible? I guess the same could be asked about a car.
     
  15. G7DIE

    G7DIE Ham Member QRZ Page

    I'm with you all the way Glen, I knew this to be the case, what I didn't expect was how pronounced it would be, I said it appeared to have a consistent 6dB benefit, however on a number of occasions the difference was a couple of S points, a bigger difference than him using 400W or 100W, but a minimum of one S point every time, with an appreciable increase in audio.

    We tried this technique regularly and a couple of times we were DXing together for over three hours, during that time he 'directed' the car often, with the same repeatable results, neither of us expected such a large measurable difference, I'd really like to be in a position to use an antenna test range, so as to put some scientific fact behind our findings.

    Regards

    Stephen
     
  16. M0GVZ

    M0GVZ XML Subscriber QRZ Page

    I hope not - it must be really thin metal if it does. I have a Little Tarheel II with a 6ft whip on mounted on the middle of the roof of my Ford Mondeo car via a 3/8 body mount. I do 70+MPH into headwinds. To get out of my street I have to do a 3 point turn in a turning area with overhanging tree branches which hit the whip every time as do the ones at the bottom of the street I have to drive past. I even drive under the max height bars at McDonalds with the antenna still on. A year and a half later and there's no sign of bending even though the antenna is on virtually all the time.

    A dual band antenna has nothing like the loading on the mount point that my Tarheel has.
     
  17. KF5FEI

    KF5FEI Ham Member QRZ Page

    Well, you could do the roof mount and run a smaller 1/4 wave dual-band around town and install the Larsen out on the road. Works for us and there is very little difference in town -- the gain antenna does provide some additional range when we are out further from the repeaters.
     
  18. KE7PMY

    KE7PMY Ham Member QRZ Page

    I've realized the best thing to do is also drill the roof of my truck. So, as of now, I plan on drilling both my 97 Accord and 02 Ram (2500 2nd Generation). I've already checked out the Accord, and there doesn't appear to be any obstacles for the coax. I have yet to check out the Ram.

    I like the idea of the 1/4 wave 2m antenna around town. That's about a 3/4 wave for 440. I wonder what kind of pattern that would be?
     
    Last edited: Nov 7, 2011
  19. KE7PMY

    KE7PMY Ham Member QRZ Page

    One more thing...I have a Lenox hole saw set, and would rather buy a 3/4" hole saw and use an arbor that I already have, than to buy a Larsen or AnteneX holesaw. Is there any reason a Lenox wouldn't work well?
     
  20. M0GVZ

    M0GVZ XML Subscriber QRZ Page

    A hole is a hole. As long as it is the right size it doesn't matter what made it.
     
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