Here is a summary of tests a Ham ran on 10meters (CB is 11m, close) and 6 meters:
http://digitalcartography.com/n0eq/AS3900.htm
Use on CB band doesn't look very promising.
I'll summarize, but this info is already available on this forum in other threads. Search on AS-3900.
The AS-3900 was designed to provide an antenna that does not need the switch at the base as it's predecessors required. The reason is that the military upgraded to radios that jump around in frequency and can't wait for tuning. The AS-3900 was designed to "work" across it's entire operating range ("instantaneous bandwidth"), but "work" needs some explanation.
SWR is high, Gain is NEGATIVE (the antenna radiates much less than is put in). Published specs say around 3 to 1 SWR and most CB and ham radios will not like that. A CB (or ham radio) will degrade output power due to high SWR. How much is unpredictable.
Gain of the AS-3900 at 30 MHz is around -7 db (that's with a minus sign). For every 3 db the power gets cut in half. Starting out with 4 Watts from the radio (legal) divided in half is 2 W, and that's only -3 db. 3 more negative db puts power out at 1 Watt. That accounts for 6 db loss, so now add another 1 db (7 - 3 - 3 = 1) loss and output from the antenna is well below 1 W.
Then remember that your radio turned it's power down to below 4W to protect itself. Also consider that 26 to 27 MHz CB freq is outside the designed performance range of the AS-3900 so that -7 db was probably optimistic! Sure the AS-3900 it will work, just not work well.
Another consideration is that the AS-3900 is a dipole antenna. The feed point is not at the base, it's near where the two halves join. That's important because this type antenna cannot be considered a whip antenna that happens to be near the length needed for CB even if the measuring tape says otherwise.
Now for a real world comment (all the above is theory): Anything you hook to your CB might provide enough signal leaked out to be able to use it, as in a convoy, in close proximity. Even a dummy load will do that if it has a few feet of leaky coax in line with it.
As for using two antennas of any kind at the same time, two reasons it's tricky: First, paralleling two antennas designed for 50 Ohm feed will require matching to 25 Ohms. Ignore that and it's a 3 db loss, but that's not so bad that it won't still work. You'll only LOSE half your power by adding an antenna!
Secondly, consider the pattern achieved by two antennas located across from each other on a vehicle. The gain pattern could easily be best perpendicular to the vehicle, sideways to the road. You just lost even more power to the trucks in the convoy.
Bottom line: you are much better off with one antenna on a vehicle unless extra effort is made to match the antenna feeds (build a feed harness to transform impedance back to 50 Ohms) and arrange the antennas for radiation pattern where you actually need it.
Bob WB4ETT
