New truck antenna problem help

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darticus

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Had a Ham magnet mount antenna and a CB magnet mount antenna on my Jeep Liberty. Just got a new Liberty but it has the new 80% open top. A small antenna on the back of the top for AM FM SAT but no metal to bounce the signal off of. Suggestions needed for antenna placement. If I put IT on the last 4 inches of metal there's no metal in front of it. What antenna to get? What to do??? Thanks Ron
 

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LtDoc

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If there's enough metal on that roof for a mag-mount antenna to 'stick' then more than likely there's enough for it to work. May not be the 'best' performing spot in the world, but it'll work. And then you always have the option of putting the antenna on a fender/hood.
If this antenna is for HF then it has to 'put up' with that sort of asymmetrical "groundplane" on any typical vehicle (A Greyhound bus outgh'a work pretty well thought!), nothing unusual about that.
- 'Doc
 

LtDoc

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The thing to be aware of with any 'lip' mount is the size of the antenna mounted to them. It's a mechanical thing, not electrical.
The other 'problem' with those kind of mounts is getting the feed line inside the vehicle. The tiny coax (RG-174 type) commonly used is the worst kind 'electrically', lots of losses. If it's kept as short as possible and you don't over power it, it can certainly be 'usable' and mechanically nice. 'RF' wise, it's the worst stuff you can use, lots of losses.
- 'Doc
 

HDMechanic

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You can also look at getting special fender mount brackets that form to the contours of your hood. I know they make them for ford, Chev, and dodge.
 

dksac2

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How about a quarter wave vertical with one of those multi leg radials you see on e bay at the bottom of the quarter wave or make one yourself and raise the antenna up off the car a little bit with a heavy strap or piece of solid rod. Mount it at the rear, on top attaching it between the lift gate and body, it just might work. Make a NMO mount if possible or an "N" mount, put a piece of welding rod where the wire is usually soldered as your radiator, the four radials in the 4 holes in the "N" mount and solder your coax to the center and one of the 4 legs at the mount solder the coax shield. This will of course be just a one band antenna, I have seen in some publications a piece of metal attached to the main rod that go out and then is bent down to make it a duel band. You also want to keep the feed as close to 50 ohms as possible. That may involve bending the radials downward some to get the antenna itself as close to 50 ohms as you can.
Get some of the flexable LMR 240 for coax, it will have the least loss in a coax that is the right size to run into the car.
The radials will give you your ground. It may not be the best set up, but your vehicle was not made for mounting antennas on it.
Other than that, adapt a front fender mount. Last thought, make or buy a roof rack that connects to the rain rails (I hope it has them), put a piece of metal across the roof mount, as big as possible and mount your antenna in the middle of the piece of metal using an NMO mount. If you do that, you can run an antenna with lots of gain. I'd ground the metal to the vehicle using some wire, cad welded on the metal or screw the wire down with star washers and cover the connection with black permitex so water will not make the connection go bad in time.
Get down to the metal where the other end of the wire is hooked to the car, use star washers to cut into the metal, that should give you a little more ground.
Either will be fine for local work or hitting repeaters that are not too far away, and the idea of metal on a roof rack will get you a lot more distance.

I just think it would be fun to see if you could build a quarter wave that would work on the back, it's going to need the radials back there as there is so little metal for a decent ground.

Next time buy a 1990 Chevy Suburban, there is a vehicle you can put an antenna farm on the roof. hi hi
I know some of the ideas are a little crazy, but they would work, main thing is look for the best way to mount the antenna that will get you a good signal and not hurt the vehicle too badly.

Your going to have to think outside of the box on this one if you want a very good signal; it can be done without messing up your vehicle.

Good Luck, John
 
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LtDoc

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A 'groundplane' is the 'other half' of an antenna. It's size is frequency dependent, and more is typically better than less. With most vehicles once that frequency is above something like 25 Mhz, the metal in the vehicle's body can provide all the 'groundplane' needed. If not, then capacitive coupling comes into play and whatever that vehicle is sitting over also becomes part of the 'groundplane'. That's fairly easy to demonstrate, ever listened while driving over something that has a lot of metal in it, a bridge for instance? Does the receiver seem to 'hear' better for a second or two? That's from that bridge becoming part of the antenna system.
For the VHF/UHF bands a typical vehicle provides more than enough metal for an antenna to work reasonably well. And since the vehicle's metal body is part of the antenna, it's shape does affect how well the antenna will work, just like any other antenna. Antennas tend to not like being -beside- metal, or -under- metal, as much as they like being -above- metal. So, putting them on the roof is a fairly good idea. But there are times when that's just not practical, and in those circumstances an antenna can be 'cootchy-cooed' (technical term meaning made to work adequately) into accepting being mounted somewhere else. That tends to skew the radiation pattern a bit, but that's not always terrible by any means. It happens with any mobile antenna's radiation pattern.
Nothing new in any of this, just a different way of thinking about it...
- 'Doc
 

AgentCOPP1

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Quite honestly you don't need a huge amount of metal for your antenna to be effective. People have made 10 meter DX verticals with a tuna can ground plane that performed reasonably well. Plus when it comes to VHF and UHF, the little nuances in your antenna don't matter that much unless you live right on the edge of your repeater.
 
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