[aprssig] Re: I'm gonna drill....

[Stephen H. Smith]

yahoo at davidarush.com wrote:
> I need to do an antenna install for a tracker (see, it's 
> APRS-related), and I'm tired of mucking around.  I've ordered an NMO 
> mount and a hole saw, and the target vehicle has a nice, big, flat 
> roof (and it's even made of metal, steel even).
>
> Any tips on installing it?  I'm particularly concerned about doing 
> whatever possible to ward off rust, including from the inside, so I 
> don't want to leave any unprotected bare steel, if possible, even on 
> the inside.
>
> Tips?  Suggestions?
>


The NMO mount has to have intimate metallic contact with the sheet metal 
for most antennas attached to it to work.  Standard NMO mounts contact 
the sheet metal on the underside of the mount.  You can't completely 
paint the underside or you won't have a ground connection.   The locking 
ring that screws down over the inserted mount, from above, has an o-ring 
seal on it's underside that should, if installed correctly, make a 
waterproof seal between the roof's sheet metal and the mount.   The key 
is to very thoroughly clean the top surface  of the roof after drilling 
or cutting the hole, and before inserting the mount.   

It also helps to have a specialized "Motorola antenna cutter" instead of 
a generic 3/4" hole saw.   This specialized hole saw has a very stubby 
(about 5/8" long) 1/4" drill bit that steps down to a 1/8" anti-wander 
starter bit, in front of the cutter so you you don't drill through the 
headliner below the roof.

The mandrel that holds the actual hole saw also has a couple of flanges 
that scrape the paint away in a perfect 1/8"-wide ring around the hole 
so the mount gets a perfect metal-to-metal ground.   A single tiny ding 
made with a center punch, before drilling,  will guarantee a perfectly 
placed smooth hole, with the paint removed in exactly the right place 
every time.   

The back end of this cutter has a hex shaft meant to fit neatly in a 
3/8" drill chuck without slipping.






--

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com


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