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dipole on a big rig swrs

jbullard85

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Nov 26, 2015
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I've been bouncing around the internet and have found that vertical dipoles seem to be a great idea to use on a big rig due to lack of metal in big rigs today. however I wonder wouldnt what little bit of metal that is on a big rig effects swrs of said dipole? I'm thinking of using a 108 whip as the main element and the Radio Shack with the fiberglass base load and 50 something inch stinger as the counter poise. Thanks in advance for answers and comments.
 

however I wonder wouldnt what little bit of metal that is on a big rig effects swrs of said dipole? I'm thinking of using a 108 whip as the main element and the Radio Shack with the fiberglass base load and 50 something inch stinger as the counter poise. Thanks in advance for answers and comments.

You're right it would have an effect and what you are thinking of isn't a dipole as both sides of the antenna aren't equal. You would also find that it would have a high-ish SWR but you'd also end up with a lot of common mode RFI on the coax.

As stupid as it may sound, if you can get to the underside of the panel that you can mount an antenna to and its not metal, even using some aluminium foil sheeting or mesh can make up for the fact there's not a lot of metal there.
 
You're right it would have an effect and what you are thinking of isn't a dipole as both sides of the antenna aren't equal. You would also find that it would have a high-ish SWR but you'd also end up with a lot of common mode RFI on the coax.

As stupid as it may sound, if you can get to the underside of the panel that you can mount an antenna to and its not metal, even using some aluminium foil sheeting or mesh can make up for the fact there's not a lot of metal there.

That was the reason I asked where it was going to be mounted.

From what I read both sides of the dipole has to be the same length. When adjusting for SWR both sides are trimed the same amount at the same time.

You could use two 3ft firesticks from any truckstop. Connect the top one to the center of the coax. Connect the bottom one to the shield side. Then attach the antenna system to truck. Just make sure that the two antennas are isolated from each other and from the truck.

Then check and adjust the SWR. Just be sure to make SMALL adjustments. And remember to adjust both at the same time.

Do not take the above advice as being 100% or for that matter even 10% correct. I'm just starting out myself and learning as I go.

Saw somewhere a guy did this in his attic for a base station.
 
Okay I appreciate the information. I was intending to mount it on the large rubber flap off of the fairing on the drivers side so as they would not ground out against any metal. And I'm not 100% for sure but I don't think that the fire sticks would work very well being as they are 5/8 wave and not quarter wave. I guess I just have some experimenting to do
 
I got a flat match on this one.
20150114_093335_zpsm2rat14r.jpg
The whole body of the truck was metal.
I've been bouncing around the internet and have found that vertical dipoles seem to be a great idea to use on a big rig due to lack of metal in big rigs today. however I wonder wouldnt what little bit of metal that is on a big rig effects swrs of said dipole? I'm thinking of using a 108 whip as the main element and the Radio Shack with the fiberglass base load and 50 something inch stinger as the counter poise. Thanks in advance for answers and comments.
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I once mounted a shortened (used coils on each half) dipole on the rear drivers side quarter panel of my SUV. It worked, and the SWR was good. The bandwidth was narrow, but okay for the CB band. Left it on the vehicle only 2 - 3 days and moved on to something else.
 
I agree with Homer and Rabbiporkchop's ideas for this setup. According to my Eznec models a 1/2 wave dipole can possibly work well when used on a big enough vehicle or a big truck. I used a bigger than usual model of a Bronco I had for this project.

I also know this from practical experience in my real world experiences. Years ago I had two old big rig radio buddies from up north near the Canadian border out West running some setups similar to the image Rabbiporkchop posted and the idea that Homer noted...a full dipole mounted of to the side of a big truck.

These two operators also use to build and sell a quad beam called the Sky King...back in the day in case any older guys might remember. One of the guy's was named Ted and he visited me at my home on a trip he made to Houston and I saw his setup for real and it too was mounted on his drivers side mirror mount.

Below are my two models with a CB 1/2 wave dipole mounted on the side of my 18' foot long model entitled as a Bronco. One model is offset to the side 1' foot and the other is offset only 6" inches. The dipole antenna is mounted with a 1/4 wave vertical radiator and with a slanted down 1/4 wave radiator as the other end of the dipole. The only other difference is the location of the antennas and its offset support along the drivers side of one model to simulate the setup to the rear of the truck. The other one is located to simulate at the drivers door window and is attached where a mirror mount would be mounted.

There is a little difference in the length of the radiators as noted, 102" and 105" inches to each side of the dipole due to location differences. Take note of the differences in pattern and gain also due to the differences in location. Take notice of the match at resonance with these models as indicated when the elements are slanted a little. These antennas produce about the same results if the are standing alone without the vehicle included. IMO this suggest the vehicle is transparent to the antenna. That surprised me, but this is what Homer and Rabbiporkchop reported too.

Click on attachment below to open model results.
 

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@Marconi I wondered if you had the SWR plot, too.
I think it might be an answer for a big rig that has otherwise poor antenna performance, but I am not a fan of antennas on the back corner of a vehicle so I didn't run it long.
I still think of trying the roof mounted dipole in the U shape once made for RVs. I have materials for it except the 8" standoffs (the distance the original was above the roof), but I can easily enough make them. I'm way too busy with too little time. . .
 
I've been bouncing around the internet and have found that vertical dipoles seem to be a great idea to use on a big rig due to lack of metal in big rigs today. however I wonder wouldnt what little bit of metal that is on a big rig effects swrs of said dipole? I'm thinking of using a 108 whip as the main element and the Radio Shack with the fiberglass base load and 50 something inch stinger as the counter poise. Thanks in advance for answers and comments.


With the dipole, one side is the HOT side and the other is the RF ground side. Obviously you would use the HOT side as the top vertical section of the dipole. You can use MFJ -347 dipole mount or make your own as long as the HOT antenna uses the insulator bushing and the RF ground does not.

As long as you weren't mounting the antenna on the exhaust stack grab handle on a Pete, I wouldn't worry about what little metal is left around the antenna.

You SHOULD use equal lengths antennas to do this correctly. However 2 full size 1/4 wave whips will be too long at being over 18 ft with mounts. Even using just one 1/4 wave whip mounted to the lower mirror bracket arm on a Freightliner or Volvo or any truck with upper and lower mirror posts will exceed 14 ft.

You would be better off using loaded antennas of the same brand and lengths, either 4 or 5 ft. fiberglass or steel ones. Most fiberglass antennas these days are 5/8 wave but that doesn't really matter. I believe the Francis antennas are still 1/4 wave so I would use two 4 1/2 ft antennas for this project.

There is one point to consider, Since a dipole is a balanced antenna, it is not 50 ohms. Even using 2 full size 1/4/ wave whips in this manner will have an feed-point impedance around 75 ohms, not 50 ohms, so the SWR will not be flat if that's what you were hoping for.

With 2 loaded antennas matched for 50 ohms and even using 50 ohms coax, I would expect your feed-point impedance to be even higher around 100 ohms. This can be verified easily if you have an antenna analyzer one or know a real CB shop that has one.

This is a easy fix using a 2:1 balun. A balun is a transformer device used to balance an unbalanced feed-line for feeding a balanced antenna, Hence the name BAL-UN. At a 2:1 ratio, this means it will cut the feed-point impedance down at 100 ohms in half to 50 ohms which should give you a nice flat SWR match. 50 ohm coax cable is an unbalanced feed-line.

You could also use a mobile antenna tuner which will be easier. But antennas tuners don't really tune an antenna, they just match your radio to a mismatched antenna.Your antenna will still have a higher SWR from the impedance mismatch, but at least your radio will see a near perfect match.

Good luck with your project!
 
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