How do I tune twin CB antennas?

[Radshooter]

Going to try to finish my CB install this weekend. Never tuned twins before. They will be on stake pocket mounts right behind the cab.

When I make adjustments for SWR, do I adjust both of them at the same time, or adjust one for lowest SWR then start on the other antenna?

Thanks..... Steve

 

[Duluth Diesel]

It is well known among radio nuts that dual antennas is more of a hinderance than a benefit. Single whips work better. With duals, you are just cutting your wattage in half.

 

[Pawpaw]

You'll probably get lots of opinions on this, but here is what has worked for me in the past.



First, your antennas must be installed so that the radiating part of the antennas have an unobstructed view of each other. By unobstructed, I basically mean no metal. If you have a fiberglass cap, that won't cause much problem unless the paint has a high metalflake content. Also, your antennas need to be taller than the truck cab.



Second, it is best to tune the antennas in a place with no buildings or other vehicles close by. An open field works best.



Third, when taking all SWR measurements, do it from inside the truck with the doors closed. It may not seem important, but it is.



I start with both antennas set to the middle of their adjustment. Take the SWR reading, then make a small adjustment on one antenna (let's say the left one for simplicity) and re-take the SWR reading.



If your reading went down, your adjustment was in the right direction. Keep making small adjustments on the left antenna, until you get your lowest SWR reading.



Follow the same procedure on the right antenna. After you've go the lowest SWR that way, go back to the left antenna and see if you can improve on that reading.



Only adjust one antenna at a time, but work back and forth.



If your SWR is under 1. 5, you've done a good job. If you're really picky, you might get it down to 1. 2 or 1. 3, depending on the quality of your antennas, coax, and installation.



Hope this helps.



Bob

 

[Pawpaw]

It is well known among radio nuts that dual antennas is more of a hinderance than a benefit. Single whips work better. With duals, you are just cutting your wattage in half.

It also used to be "well known" that the world is flat.



Regardless of antenna theory, what will perform best in the real world is far more dependent on 1. intended use and 2. quality of the setup.



Cophased antennas will indeed reduce the radiation pattern to the sides. In a mobile CB applicaiton, most people want to talk to the front or back. This is where properly setup cophased antennas shine.



I used to have a pair of little 18" Hustlers mounted on a station wagon that would consistently out-talk my friend with a 108" SS whip and 100-watt linear.



The bottom line is that you can't tell until you try it. HF radio transmissions frequently don't follow either conventional wisdom OR antenna theory.



Next, we'll discuss making a base-station antenna out of 12 AWG wire attached to the garage ceiling.





Bob

 

[RDusch]

Co Phased antenas will only radiat for and aft if the distance between antenas is at least a 1/4 wave lenth, 102" i think, the closer they are the less for and aft radiation and greater the side to side radiation.

You also need to use 75 ohm RG8au or Rg59au cable and not the more standard 50 ohm RG58.



Rick

 

[The patriot]

The twins aren't far enough apart on a big rig to get them tuned properly.

In a PU truck they'll be too close to each other.

Eric

 

[Highfinance97]

The twins aren't far enough apart on a big rig to get them tuned properly.
In a PU truck they'll be too close to each other.
Eric

The antennas need to be atleast 12 FT apart, otherwise interfearance acurres
IMO
Michael

 

[TowPro]

Best bet it to wire one, and leave the other one on for looks.



I remember asking some trucker "why do you guys bend the antenna down at an angle with the top pointing forward like that" he replied "because it looks neat"!