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  #11  
Old 04-26-2009, 03:52 PM
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Originally Posted by Inukshuk View Post
<snip> I think its that the ARB metal tab is too thick, so the antenna pin is not making contact. I will need to go buy (or make) a flat metal mount, bolt that to the tab...
Is the ARB powder coated? Why not just grind the tab down and shoot with rustoleum?
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  #12  
Old 04-27-2009, 08:56 AM
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Originally Posted by 80lolux4x4 View Post
Is the ARB powder coated? Why not just grind the tab down and shoot with rustoleum?
That's a lot of grinding.
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  #13  
Old 04-27-2009, 09:28 AM
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Dave, if you had the time I probably could swing by, you are pretty close. Not a big deal though.
Got your message last night, but it was sorta late and I was tired. Sorry for not having my phone outside with me.

BTW, the ARB is powdercoated and you do often need to do a little grinding to make antennas work right. The powdercoat around the antenna mount on the tab needs to be ground off to metal maybe (if the mount needs to be grounded, like do most ham mounts) and the bumper itself needs to have a good ground to the frame. You don't have to grind all the coating off, usually just expose the inside and a little around the hole in the tab.

In my case I made a ground strap that connects the bumper to the frame, but you could also grind a little paint off around a couple of mounting bolts. The powdercoat ARB uses is pretty good and doesn't rub off enough when you mount the bumper to make it a good electrical ground necessarily.
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Old 04-27-2009, 10:08 AM
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It's somewhere in here with the part # but i bought a flat adapter plate for the arb bar. It was a pita to drill it out to the nmo size.
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  #15  
Old 04-28-2009, 08:11 AM
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I bought a flat mounting bracket from SBS Electronics, attached it to the ARB tab, now I can hear a people talking! I still have not talked to anyone, but I'm optimistic.

My bumper is grounded. Good continuity from the outside of the mount (i.e., not the center pin) to battery ground. What I do not understand is why I would get continuity on the antenna element. Anyone?
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  #16  
Old 04-28-2009, 08:12 AM
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Got your message last night, but it was sorta late and I was tired. Sorry for not having my phone outside with me.
No apology needed at all! You have been helpful already!
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  #17  
Old 04-28-2009, 10:11 AM
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Originally Posted by Inukshuk View Post
What I do not understand is why I would get continuity on the antenna element. Anyone?
Are you saying that you get conductivity from the antenna whip itself to ground? That is not necessarily wrong, so don't worry too much if the antenna is new and in good shape.

With a single band antenna that is physically not constrained (i.e., you don't mind whatever the actual length might be within reason), the whip can be just a correct length of metal rod. In this case (like most of our CB antennas), the whip is isolated from ground. This is a good example of a typical monopole vertical whip. If you remember the class, this is electrically half of a dipole and the grounded metal of your truck is the other half of the dipole. With the right length and a good ground the electrical impedance of the antenna will be close to 50 ohms (actually about 36 ohms). FWIW, a true two-lead dipole has a feed-point impedance of about 72 ohms.

The problem with this antenna is that it's only going to work on one range of frequencies, whatever it's tuned for. This is fine for a CB or a single band radio, but poses a problem for multiple bands obviously. Also you are locked into certain lengths for this to work, 1/4 wavelength of the frequency.

So let's say you want to change the length for some reason, one of which is increasing horizontal range to let you talk farther without using more electrical power. This is where antenna gain comes into play. Let's say you increase the length of metal rod to 1/2 wavelength and it's still vertical. This will increase the amount of power the antenna radiates in azimuth (i.e. horizontally if the whip is oriented straight up). This might be a 3dB gain in effective radiated power (which you might have seen, it's abbreviated ERP). The problem is that the impedance of the antenna will no longer be 36 ohms and if it's an end fed 1/2 wavelength whip the impedance will actually be very high, about 2K ohms give or take. This is a major problem if you are feeding the antenna with 50 ohms and will reflect most of the power back without radiating any. IOW, your SWR will be VERY high.

But all is not lost, let's say you build a little circuit that can translate 50 ohms to 2000 ohms. This is called impedance matching and is very common. And I mean very, very, very common, in antenna design. You do lose some efficiency with more parts, but you gain convenience of not needed several antennas.

OK, so that you remember why I am talking about this, whip-to-ground conductivity. The reference for impedance, really everything radio, is conductor to ground. So 50 ohms is the impedance of the center conductor to the shield, the radiating element to ground, radio final transmitter amplifiers to radio ground, etc. That matching circuit (called a reactive network) will be referenced to the ground that the antenna is mounted against, i.e. your truck frame. This network is a collection of capacitors and coils and there is not just one way to make the matching circuit, there are dozens of different configurations of the caps and inductors that will make a high impedance look like a low impedance and vice versa. But all of them are using the center conductor and ground as two of the terminals.

Now remember back to the class on electrical parts. Remember that a cap looks like an open to DC and a coil looks like just a wire to DC, in effect a very low resistance. Depending on your meter, it might look like a complete short even (a low precision meter might see 4.274 ohms at just basically a short, but there is nothing that is really zero ohms). Soooo, if the matching network that your antenna uses has a coil that goes from center to ground, then definitely yes, it might look shorted if you measure it DC-wise. In this case you have to maybe take a few other measurements (like seeing if it's really 8 ohms DC rather than 0.3 ohms, which is a big difference), check the antenna for impedance (feeding it AC) rather than resistance (feeding it DC), look at SWR or ask Groucho to use his fancy antenna analyzer. But the best thing IMO is just find a good meter and measure the DC resistance. If it's something solidly in the single digit range, then you can usually assume that you are just measuring the DC resistance of a matching network.

Just to finish the thought, if you have a multiple band antenna, it must be electrically resonant on more than one band with the same length(s) of rod. So if the antenna is the right length of 2 meters but you want it to work on 70cm, then you must do something to make that happen. You might get lucky and if the bands you pick are multiples of each other, then 1/4 wavelength on one band might happen to be 1/2 on another. Usually you are not that lucky and there is no fundamental relationship. So you can put a network (usually just a coil, but sometimes a LC network) in the system to make the antenna look electrically like it's longer, shorter or maybe multiple elements (this is fairly common now, a 1/2 wavelength 2 meter antenna that looks like two 70 cm antennas connected together, this is called stacked elements).
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  #18  
Old 04-28-2009, 10:45 AM
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Are you saying that you get conductivity from the antenna whip itself to ground? That is not necessarily wrong, so .... [SNIP]
... looks like two 70 cm antennas connected together, this is called stacked elements).


I think I earned college credits just reading that!
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