Results 1 to 7 of 7

Thread: mobile antenna height

  1. #1

    Default mobile antenna height

    I am about to help a friend install a mobile whip antenna on 10m. When I made the one for my truck, for some reason I came to the conclusion the maximum height from ground to the tip was 14ft 6inch. So, on my already reasonably tall truck with the mag-mount whip on the roof (which started out at 102"), I had to cut however much off to make it ground to tip 14'6" and load it appropriately (gotta love the VNA and smith chart software). I am about to go through all this again and cannot figure out where I saw this 14'6" requirement.

    After looking online, I see there are various height requirements for various overhead lines and other obstructions. Does anyone know a general rule of thumb for maximum highway/freeway height limits? I am not so concerned about data/cable tv lines in a town where speed limits are low. At 14'6" off the ground, I hit tree branches and have to be cautious at McDonalds, but so far I do not recall hitting any overhead lines or bridges here in MN.

    Does anyone know a specific mobile antenna height limit that is either regulated by law or recommended by the ARRL? Bumper mount is not an option for good performance, this bugger is going on the roof. 14'6" seems to work good for me around here but I just wanted to check.

  2. #2

    Join Date
    Oct 2018
    Posts
    96

    Default

    Mag Mounts don't work well for 10 / 11 meters.

    The antenna needs to be - to DC ground.

    The Bond between the antenna and the mass of the vehicle is what causes the antenna to bond to the vehicle, creating a dipole effect.

    The skin of the vehicle becomes the missing half of the dipole antenna.

    To get this bond you need to bond the crap out of everything not welded to the body with at least 1 inch wide copper strap.

    Doors to door jams, hood to cowl, engine to body, engine to frame, frame to axles, frame to exhaust, etc etc etc until you make the body appear as one large mass.

    These new all plastic vehicles makes a poor ground plane.

    To get it to radiate properly so you can talk more then local, or trick yourself into believing that an amplifier will help your cause - then all you have is one loud mouth talking to another loud mouth and neither one being able to hear what then becomes a weak signal.

    You need to have a Chevrolet Surburban / Ford Expedition type vehicle, 18 feet long with a roof 6 feet wide and the length of the vehicle to get it to work..

  3. #3

    Default

    This wasn't a question of performance, it was one of maximum legal height off the ground.

    As for the performance aspects you mentioned... allow me to set your mind at ease. The vehicle is a 2004 chevy silverado crew cab with a big tool box in the back. Plenty of counterpoise. The mag mount is a triple magnet mount with metalized foil magnet coverings electrically connected to the antenna base. The capacitive coupling between the mag mount and vehicle body was calculated at 4ohm capacitive reactance, which for RF is essentially a solid connection to the roof just as if I used copper strap like you suggested. The antenna also finds DC ground to prevent static buildup as the custom loading coil I made for it is a tapped coil (essentially coax to shunt coil to series coil to antenna like an L network with the coils linear at the antenna base) which grounds through the coax braid through the radio to the vehicle eliminating static buildup. I have a Sirio827 5/8wave ground plane antenna tuned to 10m on top of my 30' tower and the mobile with the mag mount sounds just as good 25 miles away as the GP antenna running the same power and both shoot skip well. The vehicle I am going to be installing the new antenna on is, in fact, a big SUV old enough to also be made of metal. I will worry about performance, I'm just concerned about maximum height ~ although I am already planning to go ground to tip 14'6" and custom load it like i did mine unless someone has a really good reason not to.

    Also, this friend is only interested in reliable local communication without depending on a repeater. And i had no idea an antenna with less performance than yours made us "loud mouths". Thats excellent. Are you sure you are not one of those coil-spread connex guys with an xforce amp that sits on channel 6? Sometimes you sure sound like one.... I never did catch your call sign, what was it?
    Last edited by brandon lind; Tue 21st Jul 2020 at 15:03.

  4. #4

    Join Date
    Oct 2018
    Posts
    96

    Default

    Im done - you don't know what you are talking about, and a tool box is not a ground plane - thats all cbr talk - g ood b y e..

  5. #5
    Super Moderator m0bov's Avatar
    Join Date
    Nov 2007
    Location
    Surrey
    Posts
    457

    Default

    Sorry to the op, but r2d2 is now banned. Hope someone can help with your question.
    73
    James
    http://www.m0bov.co.uk
    HamRadioForum founding member and moderator

  6. #6

    Default

    I agree with and appreciate your decision, M0BOV. Thank you!

    This post can be laid to rest as my friend is arriving tomorrow and I've already decided to proceed as planned as I have not yet hit anything consequential at that height myself. We are gonna mount that monster dead-center on his SUV with a spring mount, cut it down so its 14'6" ground to tip, zap it with the VNA through a calibrated coax and put the impedance on a smith chart to see what size coil to wind and where to tap it. I'm sure it will work just as well as the one on my truck.

    73! KE0KOY

  7. #7
    Super Moderator pmh's Avatar
    Join Date
    Apr 2014
    Location
    Bury, Lancashire
    Posts
    646

    Default

    16’ 6” is the height of a double decker bus, from memory, and heights of cables over roads, etc, have a minimum height to take this into consideration. I would have to dig the regs out to find exact minimum clearances. That said, our infrastructure varies considerably, thought there are advanced warning signs for low bridges, etc.

    I very much doubt you will experience any issues with overhead power cables or telephone lines. The biggest issue comes from height restriction barriers on car parks (over here anyway) and low bridges.

    Be aware of your local area and take this into consideration when deciding.

    Kind regards,



    Phil

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •