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Author Topic: Semi with forward canted vertical?  (Read 8003 times)
KD0ZGW
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Posts: 158




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« on: June 25, 2014, 06:58:34 AM »

New ham here.  I find I now am looking around for antennas as I drive around.

Saw a semi yesterday that had a vertical element (maybe 40") with a large base loading coil.  Antenna was canted forward about 35 degrees.

Was wondering why it would be mounted that way.  I assumed it wasn't just loose as wind loading would've caused it to lean back.

Thx. in advance for help.

KD0ZGW
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K9YLI
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« Reply #1 on: June 25, 2014, 08:03:03 AM »

The radiation pattern around any antenna element is in the shape of a  donut with the  element running  through the  hole.
So an antenna canted 35 degrees  will allow communication most  strongly  on the road surface
directly  in front of the truck and also  air planes behind it.
On either side there would not be  much difference in  signal strangth.

I have seen plenty of  CB antennas  slanted back in a rackish look.  mostly to look 'cool' I suppose.  Definately  not   efficient.
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K0BG
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« Reply #2 on: June 25, 2014, 09:31:55 AM »

The canting is used typically to keep stray capacitance low. You see these types of installs on motor homes more often the OTR trucks. Canting does distort the pattern, but most folks would be hard pressed to tell any difference.
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K5LXP
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« Reply #3 on: June 25, 2014, 09:48:27 AM »


I always thought they did that to keep the whip from banging into the cab or air dam while underway.


Mark K5LXP
Albuquerque, NM
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G8YMW
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« Reply #4 on: June 25, 2014, 10:56:48 AM »

Also the whip would bend  towards vertical when travelling at highway speeds
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73 de Tony
Sent by WW2 Royal Navy signal lamp
K1CJS
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Posts: 6055




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« Reply #5 on: June 25, 2014, 12:26:22 PM »

Also the whip would bend  towards vertical when travelling at highway speeds

Bingo.  That's what I was told by a CB shop once, a long time ago.  You've got to remember that those trucks--and motorhomes, for that matter--mostly use CB while on the road, and at highway speeds.  Also that some of those spring/antenna combinations aren't as stiff as you may think.  
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K1CJS
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« Reply #6 on: June 26, 2014, 07:39:35 AM »

I should have also said that it doesn't make much difference, and that it's just another 'gimmick' that those CB shops have sold to unsuspecting truck drivers!
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W9MMS
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Posts: 121




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« Reply #7 on: June 26, 2014, 08:26:47 PM »

95% of the times, Truckers will tilt their Antennas forward for one of two reasons.
And, sometimes for both of the them.
Reason #1)
The height of the Antenna is above 13' 6" and will be hitting those overpass
that are exactly 13' 6", and the occasional low hanging tree limbs that hit the Antenna on the
passenger side out on the secondary roads.

Reason #2)
Style! It looks stylish leaning forward.


((((73)))) Milverton.

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KA5PIU
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« Reply #8 on: June 27, 2014, 04:49:51 AM »

Hello.

Kenworth came out with the factory CB antenna mount, just above the sleeper, and it did not work that well.
2 antennas looks nicer, but if only one is connected, that is fine as well.
The antennas with the base load are, as a rule, not with springs.
Forward rake or straight up, from a performance standpoint, it does not matter.
I have mounted antennas on the back, and they work just fine.
Past the top of the door, that is all fiberglass.
In fact, the qualcomm dome can be mounted under the air dam and work just fine.
I am a ham, but as a rule, when I am on the road I stay away from other hams.
A CB is used primarily to get advance warning of problems ahead and at shippers or receivers for the door numbers, etc.
The reason I avoid hams when on the road is that I am driving or sleeping.
I do NOT want to meet someone, I have a job to do.
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W2RSA
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« Reply #9 on: June 28, 2014, 05:38:05 AM »

OK, mines slanted to about 35 degrees to make 13'6". I'll throw a question out as well. I've recently returned to OTR after three years on the couch recovering from a traumatic brain injury (I still suffer some memory trouble, the reason for this question) I have a mirror mount run to separate ground and I use my 10/11 meter antenna and my outbacker as well. The Volvo has factory mounted antennas (substandard in my opinion) but what I would like to do is cut them down and used them for 2 meters. Forgetting most of what I've learned over the years due to my injury my question is will the 75 ohm coax they have installed to the dual whips be a problem matching for 144.2 ? I dont mean to but in on your post but there seems to be a few here with mobile knowledge.
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K1CJS
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« Reply #10 on: June 28, 2014, 07:02:21 AM »

You would be much better off replacing the co-ax and running only one antenna for 2 meters.  The only real reason for using dual antennas is so that the trucker could 'dress up' their rig and make it look stylish.

Also, if you replaced both lengths of coax, you could run one antenna for 2 meters while still running 11 meters for your CB on the other one--for loading dock assignments and so on.

BTW, canting the antennas to make 13'6"?  Either you're running some long antennas (5 or 6 foot whips) or you're mistaken on the measurements.  The mounts--even on the Volvos--are usually around the 9 foot level, plenty low enough for standard 4 foot truckers antennas for CB.
« Last Edit: June 28, 2014, 07:07:43 AM by K1CJS » Logged
W2RSA
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« Reply #11 on: June 28, 2014, 09:43:53 AM »

I'm using a mm-9 and its a little long, no trouble with the outbacker Perth lol.
Removing the coax is close to imposable same as feeding new thru the small access they've given. Keep in mind this is a company truck and I'm only allowed to do so much in the way of modifications to it. I'm lucky enough to have been umm allowed to install the 857 to begin with. The shop manager got a little twitchy when he heard ham radio, it took me quiet some time to convince him we were not referring to an amped up CB. That's why I was curious as to how difficult it would be to try and match the Francis whips I put on. Easy enough to cut them but I'm not sure of the impedance mismatch etc. As I stated before most of what I was required to learn to pass my General (yes even the 13 wpm) is long gone (I know some of the older guys are shaking their heads about now but thru my recovery I was more focused on being productive and returning to work, I promise to pick up the books again once this initial whirlwind settles down lolol) I do understand the old theory of the twins, they do look coo,l also know vertical is not gonna work to good on SSB just figured I have the antennas why not experiment (I just dint want to fry the radio lol)
Thanks for taking the time to answer me.
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K1CJS
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« Reply #12 on: June 28, 2014, 11:01:38 AM »

If that's the case, just leave the original co-ax in where it is and run some RG58 from your radios to the antenna mounts.  I believe there's a cover on the mirror arm that allows access to the antenna mount.  Sure, it won't look as nice, but you ought to be able to do it without 'angering' the mechanic or the shop (IOW, the owners) too much. 

Heck, when I asked about 'revising' the co-ax in my company owned tractor, all I was told is that as long as the basic driveability and the 'look' of the rig wasn't affected, I do pretty much do as I wanted to because they weren't concerned with CB radios, but that was a few years back, and with the way things are going, (and in probably because it's a different company than you drive for) I can see why that might have changed.  73!
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