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mobile set up help???

M0GVZ said:
If the air dams are fibreglass there's no reason why you can't mount on the top of the roof and have the whip protrude through a discreet hole in the air dam.

Back of the cab is OK as long as you can get almost at the top of it so as little of the antenna is shadowed by the body as possible. The hood is a good option if its bonded well and the front fenders as a second choice although you want to be doing it on the drivers side for pedestrian safety.
Click to expand...

Sitting ducks pics of his rig are a good example of how alot of trucks are built here in the U.S. He has a high top sleeper, meaning inside the cab and sleeper are open to the top of the air dam, that are mostly made of
fiberglass. Truck hoods are also made of fiberglass. The only metal in the whole body is the cabin shell, which are aluminum. Some of the "bolt on" sleepers are even fiberglass.

So, if we're not allowed to drill holes in our trucks, and/or everything else is fiberglass, we really have 2 options.
Back of the cab, using existing holes, or the mirror arms.

If one were to mount an antenna on the back of an aluminum cab, in the center, and as close to the top as possible. What would be the difference in the way sitting duck is mounting his on a perch mount, or mirror arm, and bonding everything? Wouldnt it basically be the same? The
antenna is still off the side of the cab, whether it be the side, or the back.

I'm just generally speaking here, for someone just running a barefoot radio, or maybe a small amp. Not a kw of power.

Sorry, not trying to hijack the thread. I've often heard mirror arms aren't the best place to mount antennas, just never knew why. Mounting options are very limited for most of us.
Looking good so far there sitting duck(y)
 
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sitting duck 616 said:
Click to expand...

No amount of bonding to the mount will make up for the lack of a ground plane. Its about how much metal is underneath the antenna, not alongside it. The groundplane starts where the coax feeder braid connects. The idea is to send it over the lowest loss surface we can which is the body of vehicle. The braid straps in your installation are doing nothing because the RF is flowing through that nice meaty mount. Its just a shame its bolted to the door and not the truck body. Those braid straps you made would be better served being put across the hinges of the door.

One mistake people make is assuming because you can put an AVO meter on and it shows no resistance its working well. AVO meters use DC and RF is AC. A trap on a multiband antenna shows perfect continuity with an AVO meter but to RF its a dead short at a specific frequency. Likewise a 4:1 Unun looks like a dead short to an AVO meter but works fine for RF.

Put a MFJ259/269 or other antenna analyser on and see what the value of R is when X=0. The further away from 27 Ohms it is the worse its performing and the more losses you have. Low SWR is not an indicator of efficiency.
 
Sideways said:
Sitting ducks pics of his rig are a good example of how alot of trucks are built here in the U.S. He has a high top sleeper, meaning inside the cab and sleeper are open to the top of the air dam, that are mostly made of
fiberglass. Truck hoods are also made of fiberglass. The only metal in the whole body is the cabin shell, which are aluminum. Some of the "bolt on" sleepers are even fiberglass.
Click to expand...

Here's what I drive, the company I work at. Believe me I know all about it. Everything above the windscreen is fibreglass.

6138609564_30d8593919_z.jpg


If one were to mount an antenna on the back of an aluminum cab, in the center, and as close to the top as possible. What would be the difference in the way sitting duck is mounting his on a perch mount, or mirror arm, and bonding everything? Wouldn't it basically be the same?
Click to expand...

No, it would be better although the side would be better than that depending on how closely you couple trailers if you're using high trailers such as box or curtain siders. Its got a larger surface area and the ground plane starts at where the coax connects to the mount, not alongside it which is what the horizontal bar bolted to the door does. The saving grace on his is that the bar is short so it mitigates it to some extent but bolting to the door reduces the surface area the RF has to flow through unimpeded.

You showed what the SWR is at its lowest point but what is the value of R when X is 0 (where the antenna is actually resonant) or as near as possible? Where X is at its lowest point will be slightly different to where the SWR is and should be at a higher frequency. If they're the same its because of ground losses. If you can't get X to equal zero its probably because of capacitive coupling with the truck body as well as ground losses. You can end up with several points where X can be very low in this case so go for the dip closest but above the frequency where the SWR is the lowest.

Please understand I'm not having a go but trying to make sure you get as efficient an installation as possible which is what you're going to have to have running the power you want or even just the rig barefoot with its 350W unless you want a ton of RF flowing on the coax getting into the rig as you transmit and worse, the vehicle wiring and all the onboard computers. We want the RF to go where we want it to go, out of the antenna and over the outside of the body of the vehicle, not floating around inside.
 
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