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Hole placement for NMO mount on my car

Discussion in 'Mobile Radio' started by KK6MYS, May 21, 2015.

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  1. KK6MYS

    KK6MYS Ham Member QRZ Page

    I want to drill a hole in my 2009 Toyota Corolla for an NMO mount. My understanding is that the center of the roof is the best place. It is easier to drill the hole in the trunk lid, how much worse would the antenna on the trunk lid be?
  2. K5GHS

    K5GHS Ham Member QRZ Page

    Depending on the size of the lid, it may not be too bad. The main thing you have to look at is being able to open the trunk lid without your antenna smashing into the back window. :)
  3. KD2IAT

    KD2IAT Ham Member QRZ Page

    This may help you decide where to locate your antenna (click on the pic for a larger view):

    Antenna Placement 640x396.JPG
  4. K8JD

    K8JD Ham Member QRZ Page

    I was going to guess about 3 dB down going from center roof to trunk lid due to half the height above ground. On VHF and UHF height is the importaint factor !

  5. KD2IAT

    KD2IAT Ham Member QRZ Page

    I think there are multiple factors at work here. As you say, at VHF and UHF, height comes into play, but I suspect centering on the vehicle matters, too.

    At lower frequencies, I know for sure that centering the spike on the vehicle is achieves the best possible radiation and reception in all directions. The most useful example I can think of is the old CB radio bunny hunts. With the antenna mounted the roof, it was virtually impossible to detect variations in received signal strength based on which direction the car is pointed in. No matter which way the car is pointed, the strongest received signal is equal off the front and back of the car, and nearly so off the sides. Move the antenna to the rear of the vehicle and you'll see a noticeable change in the reading as you drive the car in a circle. When the meter reads the strongest, the nose of the car is pointed right at the bunny.

    I have both VHF and UHF antennas on the roof of my truck. Some installation obstacles placed them in the rear half of the roof, but both have adequate ground plane for good radiation in all directions.
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    You only get one chance.

    I would drill from the Inside out, to make sure you do not hit a support beam, or other wiring.

    All needs done after Spouse approval. Sometimes Insurance company too, So your eye don't get poked out.

    Good Luck. Disconnect the Battery. Wear PPE.
    N7MLW likes this.
  7. K8JD

    K8JD Ham Member QRZ Page

    Roof mount, extra effort but worth it

    DRILLING FROM THE INSIDE WILL TEAR UP THE HEADLINER FOR SURE, iF YOU HAVE BEEN ABLE TO GET HANDS ON A ROOF MOUNT HOLE SAW, IT LIMITS THE CUT DEPTH SO YOU CAN make the 3/4 inch hole without damaging anything else and then fish the coax between the roof metal and headliner, down to a corner column to get it to the dashboard, and then to the radio. Running the coax is the hardest part.
    Before you start, tap along the roof, front to back, to see if there is a strut inside the roof, usually is there to stiffen the roof but may not be in the middle. The tapping sound changes as you get to any inside struts. DON'T drill into the strut !

  8. N5MJ

    N5MJ QRZ Lifetime Member #98 Platinum Subscriber Life Member QRZ Page

    Over the last 35 years of installations, a 3/4" greenlee chassis punch has always made a straight, flat hole. One is shown below.

    Center roof mounting involves pulling the headliner down to get to your desired location. That's done simply:
    - remove the dome light and anything else screwed to the headliner. You may need to remove sunvisors.
    - pull the black rubber door deal down from the door opening. If you have a 4-door, that would be the rear driver side door.
    - if the headliner doesn't drop down enough, remove the driver side "B" pillar panel, that's the plastic cover near the driver shoulder belt loop
    - look between the headliner and the roof metal for a clear, flat span of metal near the center of the roof
    - gage the car centerline for the chassis punch pilot hole
    - drill pilot hole, thread chassis punch pilot bolt from the top of the roof, fit the cutter onto the pilot bolt from the headliner side, turn the cutter until flush with roof metal
    - crank pilot bolt with 1/2" open end wrench until a nice 3/4" hole exists in the roof
    - scrape primer/paint from *inside* about 1/8" around the hole. Paint on the outside leave in place.
    - remove ring from and mount NMO mount/coax from the headliner side. reach in and guide the mount and center it in the hole. it'll be obvious when it's right, hold mount in place.
    - thread the ring onto the mount from the roof side. tighten enough to slightly compress the o-ring. Gorilla torque not required.
    - thread the antenna onto the mount.
    - connectorize the other end of the coax, check vswr, trim as necessary if using a quarter wave spike
    - connect to radio.

    Haven't had any mount leak or cause issues in >35 years doing these installs. Trading the car in? Look online for an NMO rain cap. Remove antenna, screw rain cap on in place of antenna, done.

    You'll enjoy performance and range superior to any similar antenna on a mag mount, and rain won't wick down the coax and leave moldy puddles on your car's carpet.

    Mike N5MJ IMG_2108[1].JPG
    KE0RG likes this.
  9. K0BG

    K0BG Ham Member QRZ Page

    This "drill" of sorts is called a rotobroach. They're great for drilling holes for an NMO when you can't get into the back of the sheet metal to use a chassis punch. Home Disappointment sells them in the tool aisle.

    And one more thing. Note the term CHASSIS punch. This is NOT the same as a 3/4 inch electrical conduit punch. If you use the latter, the hole will be too big!

    KE0RG likes this.
  10. K0BG

    K0BG Ham Member QRZ Page

    One more thing, albeit a bit off topic.

    All late model Fiat Chrysler products, and many other makes too, utilize one piece headliners. They're inserted before the windshield is set into place. You mess one up, and it isn't just the cost of the headliner you have to worry about.

    And, I can't say for sure what all of the headliners cost, but the one for the Jeep Cherokee is over $1,000 plus labor. Obviously, care is required whatever tool you use to drill the hole.

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