The Weekly North Texas YL net

Making a difference in the way women connect.

Blog - How to Install an Antenna Through the Roof

A special thank you to our net topic presenter and guest blogger, Bill Galloway, KF5YIV. What follows is a how-to for installing your mobile antenna through the roof. As in drill a hole. In the roof. "The only way to go," said she who paid to have hers installed.



I have been wanting to permanently mount my Kenwood TMV-71A and the Comet sbb5 dual band mag mount antenna for some time since I bought my Altima. Being a guy, I am not caught up on aesthetics so much, but the wires running from the cigarette lighter to the radio and the feedline laying across the roof from the magnet mount just gets annoying. I have to lock out the rear door so the kids are not causing excess stress on the coax where it runs through the door seal with the constant opening and closing.

I recalled the many QSO’s heard since I’ve known KB0DBJ saying: “How about drilling a hole and mounting your antenna.” He also mentioned I should get away from the cigarette lighter port. The ports are usually rated at 10 amps, but are not designed for the power draw of 60 to 100 watts the radios can draw. The permanent mounting process creates an excellent ground plane and you will hear a noticeable improvement in areas of weak signals. Also, your radio will not roll back the output power due to low current availability. So, I began the planning process to get things started.

The checklist I created included:

• 16 gauge wire, sized to power one radio hooked directly from battery to the radio

• Locating a fully coaxial NMO mount that didn’t require pulling the headliner to install. KM5HT – Steve came to the rescue. This one requires a ¾” hole

• LMR-240 Hi-Flex feedline with proper connectors to mate with the NMO mount and the output of the TMV-71a

• Finding the best mounting location for the TMV-71a in the vehicle

• Finding the location in the roof of the car to drill the ¾” hole so as not to hit the brace that runs across between the driver and passenger doors

• Find an ¾” OD hole saw for metal

This process took about 3 weeks to plan, research and acquire the parts and tools. I watched several videos of audio amplifier and antenna installations to find the best route to run all the wires. Viva la YouTube. I even checked with a local auto body technician. He showed me how to simply pull the door gaskets down from the passenger side doors. This exposed a perfect channel for the wire to run and provided access between the headliner and roof to fish the feedline over.

I disconnected the negative lead on the car battery for safety. This is very important when working around “side curtain” airbags. I measured off and the marked the spot for the hole on the roof. I placed a 4 inch border around the hole location with masking tape and then covered the remainder of the roof with towels. This is to keep the hot shavings of metal from melting into your paint. I drilled a 1/8” pilot hole and then inserted a smaller bit by hand through the hole to determine how much space I had between the metal roof and the headliner below. I had 1 inch. This was perfect.

I wrapped masking tape around the hole saw just above the cutting teeth so I would have a visual as not to drill any deeper. ** Just so you know, they make hole saws with this safety collar built on, but they are over $30 and I got mine for $7. ** I drilled through and all went well. I took a small file and removed any small burs inside the hole and removed most of the shavings from the roof with the sticky side of the masking tape and shop vac.

I have a fish tape for running wire in conduit, so I fed it through the hole over to the passenger door. I attached the feedline with electrical tape and pulled it back through the hole. Connected it to the NMO mount and pushed the NMO right anger connector into the hole. I placed a small amount of silicone sealer over the O-ring on the top NMO mount ring and made sure the mount was centered in the hole and snugged it down.

I tested for continuity between the feedline and the NMO mount with my ohm meter and all is good. The shield and center conductor had continuity, and no shorts detected. I finished running the feedline and connected the antenna. I removed all the protection from the roof, replaced the door gaskets.
Now it is time to test. I attached the antenna analyzer and was impressed with a 1:1 VSWR along the lower portion of the 2M band and 1:3 at the top. Similar results for the 70cm band. I shook the car and bent the cables and no drop outs, so I am good to connect to the radio.

I drove to work the next day and within 10 miles of work, the radio signal went dead. My heart sunk. I couldn’t even get the NOAA weather station. This is bad. At lunch time I went out to begin some troubleshooting. I checked to make sure the center pin of the antenna had solid contact with the center of the NMO mount. I then selected a known strong NOAA weather channel and began to pull on some of the excess feed line stuffed behind the floor board. The station came in full signal.

I worked my way up to newly installed PL-259 connector on the back of the radio and by bending and pulling, the signal went away and came back. Yeah! I found the problem. When I got home, I immediately retrieved the solder gun and reflowed the solder at the center conductor of the PL-259. After things cooled, I attached it back to the radio and began bending and pulling at the cable. Not a blip in the signal now. It has been good all week now.

This wraps up my process, and thanks to Murphy’s Law I was presented with a challenge. I feel that if everything goes perfect all the time, we miss out on excellent learning opportunities. Back to net control to field any questions. 73.

Motorola NMO Mobile Antenna 3/4" Hole Mount FME

Dave’s Hobby Shop by W5SWL
721405422630 – Coax Connector FME Female Crimp
LMR-240 2-pk - $2.49