mag mount on roof with roof rack w/ mesh floor OK?

[e9999]

a tech question:

tempted to go with mag mount on roof. Except that I have a permanently mounted Kaymar Tradesman roof rack with mesh floor (steel?) which may be an issue.

Would it be OK to put the mag mount on the rack so that either the rack mesh floor or the roof under would be the ground plane for a 1/4 wave -if that's even possible?
Or would it be better if I stick the mount on the roof and stick it through the mesh? Then there would be a chunk of antenna between the mesh and the roof, which may be an issue if it's a short 1/4 wave antenna for the 2m?

 

[rusty_tlc]

As long as the rack is well grounded with respect to the antenna the mesh will look like a solid sheet to RF, depending on the size of the mesh and the frequency in question. For Ham frequencies the expanded steel openings should be < less than one wavelength so it will look solid to RF emmisions.
I would suggest you read up on RF grounding.

 

[e9999]

I don't have a good book on antennae yet. Is the main ARRL one OK?

I did understand that the mag mount establishes a ground plane with a coil in the base. That seems iffy on mesh, but I would need to add a steel plate for the magnet anyway to put it up there, so I guess I'd have to ground the plate very well to the rack, then. Or am I better to stick it on the roof under the rack?

And yes, I could use a fixed mount but the mag mount seems so versatile, it's tempting to try that first.

 

[rusty_tlc]

I don't have a good book on antennae yet. Is the main ARRL one OK?

I did understand that the mag mount establishes a ground plane with a coil in the base. That seems iffy on mesh, but I would need to add a steel plate for the magnet anyway to put it up there, so I guess I'd have to ground the plate very well to the rack, then. Or am I better to stick it on the roof under the rack?

And yes, I could use a fixed mount but the mag mount seems so versatile, it's tempting to try that first.

So far the ARRL book looks OK, I haven't read the theory section much. It's more about base station set-ups than mobile rigs so you might want to shop for a book which concentrates on mobile applications.

The mag mount antennas I am familiar with have nothing more than a permanent magnet in the base. What coil are you talking about?

The mag mount does not make a direct electrical connection to the rack, they share a common ground via the body/chassis. If the rack is properly grounded, and the body and chassis are properly grounded, it will provide an RF ground for the antenna. You could zip tie a square of 18ga sheet metal to the mesh if you are worried about the hold with a mag mount, no need to ground it.

 

[e9999]

I have no idea if this is correct (in fact, seems rather implausible), but I was told that some mag mounts have a coil (?) in the base that does something to set up (?) a correct ground plane in the sheet metal it is stuck to despite no direct electrical contact between the 2.

I'd like to be enlightened as to whether this is true, and if so, how this works.

 

[Pskhaat]

Kind of true. AC/RF and DC ground are different animals. DC needs a direct connection, RF not necessarily so though it's never a bad thing IMO. I won't show my ignorance in EE/RF and will allow someone else to explain the gory details but rustly_tlc's correct in that the roof rack may not need to be DC grounded to the vehicle for proper usage.

 

[rusty_tlc]

I have no idea if this is correct (in fact, seems rather implausible), but I was told that some mag mounts have a coil (?) in the base that does something to set up (?) a correct ground plane in the sheet metal it is stuck to despite no direct electrical contact between the 2.

I'd like to be enlightened as to whether this is true, and if so, how this works.

Pskhaat is kind of right, RF are DC ground are different animals. I did not say that the rack would not need a DC ground to act as an RF ground plane.
I think what you were told about is a loading coil which adjust the antenna to have an effective length equal to a 1/4 or 5/8 or whatever wavelength. Sometimes there is also an adjustable inductor and trim cap to match SWR.

The basic problem here is you are confusing electrical ground with an RF ground plane. I'm lousy at explaining stuff like this but here goes.

Think of the ground plane as a reflector that directs more of the antennas radiated energy outward. If you use a gutter mount the reflector is only reflecting half of the power compared to mounting the antenna in the center of the roof. This really isn't the easiest analogy I could come up with.

You should probably pop for the $45 to get the ARRL book and read the theory section.

 

[spaldam]

I've heard it explained as a kind of mirror:

Think of a simple light bulb in a socket on your ceiling sending light out in all directions. Now put a mirror on your ceiling, and suddenly the light that was absorbed by the ceiling is now reflected back down into the room, increasing the illumination on the rest of the room. You can now use a lower wattage light bulb to get the same illumination on your desk to read your book, or more efficient use of a higher wattage light bulb.

Now translating that to RF energy, the mirror is like the RF ground (reflection) plane, and the light bulb is your antenna, the desk is where your trying to send a message too, and the book is your message that can now be read more clearly because there’s more light on the subject.

A DC ground wouldn't help in this situation, unless you want to shock the guy reading the book.

As for running the antenna threw the metal grate/rack, I think you'll find that it will cause interference with your antenna, kind of like putting a shroud over the light bulb.

 

[e9999]

talked to some more folks in stores. They all seem to think that putting it on the roof rack should be fine. On the roof through the rack does appear more questionable.
Got a mag mount to experiment a bit with, will see.

I guess I should ground the roof rack?

 

[spaldam]

It certainly wont hurt. It's probably already grouned anyway if it's bolted to a metal roof. I simple ohm test between the rack and the body of the vehcile will tell you for sure. A good ground will show less then 1ohm.

 

[e9999]

no way it's grounded. It's painted, and there are rubber shims under everything in contact with the roof. Is there something magical about braid or would any sturdy piece of wire do?

 

[Pskhaat]

Again, DC ground and RF ground are different, though they are not DC connected, RF will still at some level use your roof (or rack) for it's counterpoise. I say give it a try with an SWR meter both at the antenna and at the radio and see.

 

[groucho]

A little help on the theory...

This was brought up on Paradisecruisers group. Great information for someone who wants to know about the questions being asked. A little technical at times, but if ya read through the info more than once, yur sure to become familiar with the basics.

http://www.k0bg.com/

Make sure to browse the page on "bonding" and the one on "antenna mounts". For the antenna mounts page, check out what is said about mag-mount antennas, about 1/4 the way down the page. Don't be shy and read the page on "Grounds, RF & DC" as well.

HTH.

 

[spaldam]

no way it's grounded. It's painted, and there are rubber shims under everything in contact with the roof. Is there something magical about braid or would any sturdy piece of wire do?

Like Pakhaat said the DC grounding isn't that important, but if you’re worried about it a sturdy piece of wire will do. The braided stuff is just a more flexible and durable way of getting a high capacity ground. Plus they look better then a comparable but bulky 4 gage wire.

 

[e9999]

This was brought up on Paradisecruisers group. Great information for someone who wants to know about the questions being asked. A little technical at times, but if ya read through the info more than once, yur sure to become familiar with the basics.

http://www.k0bg.com/

Make sure to browse the page on "bonding" and the one on "antenna mounts". For the antenna mounts page, check out what is said about mag-mount antennas, about 1/4 the way down the page. Don't be shy and read the page on "Grounds, RF & DC" as well.

HTH.

that site is good, thanks

 

[sandcruiser]

'course you could always just slap the mag mount onto the roofrack and have a buddy sit in his truck (with a cellphone) a 1/4 mile away or so.... see how it works.
The cellphone would be for you two to talk about how the antenna *isn't* working, if that were the case.

 

[e9999]

'course you could always just slap the mag mount onto the roofrack and have a buddy sit in his truck (with a cellphone) a 1/4 mile away or so.... see how it works.
The cellphone would be for you two to talk about how the antenna *isn't* working, if that were the case.

good idea in principle, but I'd be rather disappointed if I couldn't do 1/4 mile.
I'd probably pick a repeater and drive towards or away from it while testing to get a feel for range.

Only played a bit with the antenna (1/2 wave) on the rack so far, so no good idea of the range yet. I was able to hear airports from well over 50 miles away, but these may be more powerful than normal repeaters, I suspect...

 

[groucho]

Like Pakhaat said the DC grounding isn't that important, but if you’re worried about it a sturdy piece of wire will do. The braided stuff is just a more flexible and durable way of getting a high capacity ground. Plus they look better then a comparable but bulky 4 gage wire.

Good points. Just keep in mind that in order to optimize the system, the wire should be flat and as short a run as possible because at the right length at some frequency that same ground strap will make a perfect antenna, making your mag mount antenna less effecient.

"There are a few things we can do to assure both a good RF and DC ground. One of these is to use braided wire. Not just any braided wire mind you, but one which is flat and wide. RF flows at the surface rather than through the wire, and flat braid has more surface area for any given current carrying capacity. Thus it provides less resistance to RF than an equivalent round wire. It also has more capacitive reactance which increases the self resonant point. Flat braid is also much more flexible and less likely to fail due to repeated flexing."

One other thing to the benefit of your setup is that the wire mesh actually will act as a reasdonable ground plane in itself, separate from the vehicle body. When setting up a HF vertical, if someone can't copper plate their backyard, it has been suggested that chicken wire around the base of the vertical would do in place of a good ground radial system. Try using the antenna on the mesh without strapping it to the car, check your results and then bond the wire mesh to the roof and test again. You may get the same results.