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I am looking to mount this antenna on a 4 door sedan, without harming the vehicle. I dont have the specifications of the vehicles, but it will need to be sturdy enough to withstand a cross-country road trip. This is the antenna for the telemetry system of the chase car. Could anyone point me in the right direction of where to find something that might accomplish this, or something that could be modified to accomplish this task. I dont even know where to begin, all I can find is roof mount antennas, and that is not what I want.

Also, the antenna has to be vertically oriented, and is 4ft tall.

Thanks!

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  • \$\begingroup\$ Have you checked out the ARRL Antenna Book ISBN: 978-0-87259-694-8? It has a good section on mounting mobile antennas. \$\endgroup\$ – JonnyBoats Mar 14 '12 at 20:22
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While one may be a wee bit hard pressed to find many ohms, volts and amps in here, this is the sort of question that arises as part of electrical engineering and is the sort that self respecting electrical engineers should be able to roughly address themselves without having to ask for help from their lesser brethren [tm].

So, that said:

A good way to mount this type of antenna is to use "roof rack" parts as the basis for mounting. If you want an exceptionally even radiation pattern you can consider mounting a ground plane sheet horizontally above the vehicle roof. Wind forces on the added ground plane need to be considered. Ground plane can be covered with copper sheet.

The calculations below (E&OE) suggest that it may be enough to use a strong magnet mount using say a number of large rare earth magnets attached to a plate which magnetically attached to the roof and which provides a mounting base for the antenna.

  • The magnets should have more attachment force that the large ferrite magnets on the blue and red flashing light plus siren unit that I found lying on our local motorway at 1am on new years morning on 1/1/2000. [Returned to me by police after 3 months - red/blue is illegal to use except by police and the police may have wanted to discuss it having been dropped on the motorway at speed.]

Windage on this antenna will be noticeable but acceptable.

As a guide, wind force is roughly given by

$$ F = 0.5 \cdot \rho \cdot C_{d} \cdot A \cdot V^{2} \, \, {}^{(*)}$$

F in Newton, \$ \rho \$ = air density = 1.3 kg/m^3, Cd = drag coefficient \$ \leq \$1, A = area in m^2, V= velocity in m/s Approximate this as F = 0.6 x A x V^2. Your A ~~= 1.2 x 0.04 =~~ 0.05 m^2, so at 100 kph ~= 30 m/s force will be ~~~ $$ F = 0.6 \cdot 0.05 \cdot 900 = 27 \, Newton \simeq 3 kg $$

Consider that to act at the top - although it wont.
Fit 2 or 3 roof rack cross bars, tie them longwise or with a diagonal brace and try exerting sudden 3kg x 1.2m ~= 5 kg/m forces at the centre point.


*This drag formula is surprisingly accurate and exceptionally useful. It provides good enough results for raindrops, skydivers, parachutes, Field mice - and antennas.

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  • \$\begingroup\$ "It provides good enough results for ... Field mice ..." And you know this, how? :) \$\endgroup\$ – JRobert Mar 15 '12 at 14:44
  • \$\begingroup\$ @JRobert - Note that this is NOT the US meadow Vole which is called a FM in the US. This is en.wikipedia.org/wiki/Apodemus and more paricularly the A sylvaticus or wood mouse en.wikipedia.org/wiki/Wood_Mouse found all overurope. It is generally known [tm] that a FM can be dropped from any height and survive, This is because with their extremely small size the cubed squared law provides enough area per mass that the drag equation balances (Force from air = mass x g) at low enough terminal velocities to ensure survival - as log as they do not fall on a cat. \$\endgroup\$ – Russell McMahon Mar 15 '12 at 15:14
  • \$\begingroup\$ @JRobert - figures now long gone but long ago I had the fall velocity and used mass and size to calculate terminal velocity - which matched. Cargo parachutes give an excellent match. Skydivers both in normal mode and also in tucked in speed descent also work. Raindrops seemed to fall far too slow - until you see slomo video of them and realise that they fall very flattened and NOT as all pictures show them. Motor vehicle drag also works well - if Cd can be deduced. In many cases using Cd=1 makes sense. (Flat plate drag). \$\endgroup\$ – Russell McMahon Mar 15 '12 at 15:17
  • \$\begingroup\$ Thanks, Russell. I've sat in chairs supported on Cd.1/2.Rho.V^2.S, so I'm fine with that part - but "a FM can be dropped from any height and survive" is generally known? The mind boggles at the image! :) \$\endgroup\$ – JRobert Mar 15 '12 at 16:03
  • \$\begingroup\$ @JRobert - Gargoyle "Field Mouse" "terminal velocity" or just ...velocity and you'll get many hits. Alas, none I could see at a quick skim seemed overly authoritative or treated the subject as their primary point. Quite a few discuss dropping elephants and field mice - but are usually teaching Physics and not biology. It's claimed that cats take about 7 floors to reach terminal velocity and may well survive falls from buildings from any height - albeit with injuries. \$\endgroup\$ – Russell McMahon Mar 15 '12 at 16:18

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