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  #11  
Old 01-11-2008, 01:29 PM
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Hey Dave,

While you're at it, what is the best mounting point for a 2m antenna in an FJ40? I was thinking of putting a tab on the roll cage somewhere. Maybe even a plate big enough for a mag mount. One for the CB on one side and one for the ham antenna on the other.
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  #12  
Old 01-11-2008, 07:13 PM
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Default Don't get hung up on theory

You are looking for a problem that might not be there. Get a good NMO mag mount, slap it on the roof behind the sunroof and try it. If you plan on starting with 2m only, get a Larson NMO 150 and give it a go. I'm willing to bet a 6 pack it will work fine. If it doesn't or you want to optimize your investment and use 70cm, go with a Larson NMO 2/70.

The 2/70 is 1/2 wave end fed on 2m and in "theory" does not require a ground plane (manufactures other than Larson claim their 1/2 waves are ground independent) in practice, however, the ground independent thing only holds true for the exact freq the ant is tuned for. Move off that freq and ground current flows and the outer shield of the coax becomes part of the ant system (not good) But if you place the 1/2 wave over even a marginal ground plane, ground currents are not a problem because of capacitance coupling of the ant and the car body. The motorcycle guys love the 1/2 wave because of the lack of an adequate "ground plane" on their bikes.

In "theory" going from a 5/8 wave (like the NMO 150) to a 1/2 wave you give up some range for elevation, but in the real world you would be hard pressed to tell the difference.

When Groucho and I went up to Mt Bross, for the 14er event, we were using his home brew beam ant. At some point the coax connection came in contact with one of the driven elements of the beam. Had we been using something as cheaply made as a CB, the radio would have been toast but the "fold back" protection in the radio kicked in and cut back the output power (if we were able to measure it, I'm willing to bet it was only a few mw's) So in "theory" we were putting a couple of mw's into a dead short and yet we were able to transmit over 40 miles, go figure

Theory is great and interest in it will help you to advance in the hobby. But if you are using theory to find problems that ain't there, you'll never try or gain anything. Try it, if it don't work apply the theory in your trouble shooting thought process and then try something else.

It's a Hobby, have fun with it, don't get hung up on it.

Last edited by Seldom Seen; 01-11-2008 at 11:28 PM.
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  #13  
Old 01-12-2008, 03:43 PM
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Who, me, hung up?
Just trying for doin' it right the first time.
So whatcha think about just plunking this NMO 150 thingie on my spare tire carrier then?
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  #14  
Old 01-12-2008, 06:16 PM
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It will be fine.
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  #15  
Old 01-12-2008, 06:23 PM
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It will be fine.
yep
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  #16  
Old 01-12-2008, 10:44 PM
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Originally Posted by Red_Chili View Post
So whatcha think about just plunking this NMO 150 thingie on my spare tire carrier then?
What?
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  #17  
Old 01-13-2008, 07:55 AM
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Quote:
Originally Posted by Shark Bait View Post
What?
The NMO150 is a good 2m antenna. Whatcha confused about? It's that Bill's installing a dual band radio, probably. Yeah, I'd suggest putting a NMO2/70 on the tire carrier instead. A single band antenna will work, no problem, but you have to give up UHF transmitting.
Quote:
Originally Posted by Shark Bait View Post
Hey Dave,

While you're at it, what is the best mounting point for a 2m antenna in an FJ40? I was thinking of putting a tab on the roll cage somewhere. Maybe even a plate big enough for a mag mount. One for the CB on one side and one for the ham antenna on the other.
With VHF and UHF, there are two things you are thinking about, a decent ground plane and height. Of the two, height is probably the more important. A decent sized disc in the middle of the roll bar will be good. If you can swing it, 18" or bigger, but if it's a little bigger than a good mag mount it should work fine.
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  #18  
Old 01-13-2008, 09:19 AM
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Thanks Dave. I'm trying to figure out where to put antennas on my FJ40 that will work the best with the least amount of hassle. I'm thinking now for my Ham antenna, a mount on the top of the spare tire carrier is how I am going to go. Can I get away with a CB antenna next to it? I also have a nice Wilson mag mount CB antenna I could stick in the back of the tub on one of the wheel wells, or something. The trick, for me, is to find a place to mount stuff that will work with the soft top either on or off.
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  #19  
Old 01-13-2008, 09:39 AM
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Originally Posted by Shark Bait View Post
Thanks Dave. I'm trying to figure out where to put antennas on my FJ40 that will work the best with the least amount of hassle. I'm thinking now for my Ham antenna, a mount on the top of the spare tire carrier is how I am going to go. Can I get away with a CB antenna next to it? I also have a nice Wilson mag mount CB antenna I could stick in the back of the tub on one of the wheel wells, or something. The trick, for me, is to find a place to mount stuff that will work with the soft top either on or off.
Chris
I have my Ham and CB antenna's mounted on the spare tire carrier 6 to 8 inches apart and have seen no problems. I know a lot of people say not to mount Ham antenna's on the bumpers but I did not want to cut a hole in the roof. I have talked 30 to 40 miles on simplex rolling through canyons with the current setup.
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  #20  
Old 01-13-2008, 11:27 AM
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With the interchangeable tops that does make things more complicated. You could do dual mounts, a tab on the roll bar and NMO in the hard top. Put a SO239 someplace and have pigtails on both mounts. So when you put the hard top on, you hook it up, soft top, vice versa. You could get real fancy and put in an antenna switch, even.

You can mount antennas for two different bands in close proximity, won't hurt anything RF-wise. As long as there's some spacing and both transmitters are not generating horrible out-of-band noise (keep 'em tuned!), the radios won't care that much. They might care if the whips were like an inch apart because no front end is perfect. That's not a realistic situation because...the whips must never touch.

During both receiving it might not hurt anything if they touched, I'd have to think about that. But if you happen to be transmitting when that happens you stand a good chance of killing the receiving one or maybe both radios. This would be particularly true if you were talking at 50W on the ham and the whips touched. You could say bye-bye to the CB receiver for certain.


Some informational geeky junk:

You can generalize your power and field strengths by using:
Attachment 7414
S = field power flux density (would be W/m^2 using unit below)
P = transmit power in watts
G = antenna gain
R = distance from center of antenna in m^2

Where gain G is found from your antenna gain in dBi:
Attachment 7415

To find the E (voltage) field strength:
Attachment 7416

At 1 foot, a 50W TX on 145MHz through a 1/4 wave (0dBd gain, i.e. 2.2dBi) antenna produces a power density of about 70.9 W/m^2, which would be an approximate field strength of 164 V/m.

Consider that modern (as in made since about 1940) radios can detect fields in the tenths of microvolts per meter (a FT-8800R can detect a signal as low as 0.2uV/m) and so you can see being a couple of feet away means the field will be many thousands or millions of times stronger than the weakest detectable signal. I'd think most any decent front end filter will be able to tolerate this sort out of band interference and still not too significantly affect the sensitivity of the radio for its in-band frequencies. However, if the whips touch then there's no free space and you get the full current (practically an infinitely strong field) going straight into the front end. Not a good situation pretty much no matter if it's 1W or 100W. No matter how good the rejection in on the radio, grossly overloading it will be bad. Also this is only true of wildly out of band signals. The front end of our mobiles is designed for wide band RX, which means they are looking for a signal from about 108MHz up to 1GHz. So doing this with a CB is OK since it's quite a bit below the range of the FT-8800R and so the designers (it's an assumption, but should be a safe assumption on our ham gear) will design the front end to just attenuate it and it never even makes it to the RF amp. If you put a pair of 2m radio antennas this close the field would be too strong for the RX and it might hurt it. You want to keep a lot more separation between same band TX and RX antennas.
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Last edited by DaveInDenver; 08-26-2010 at 10:16 AM.
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