Dual Antennas

[KENMAR]

I have a T-800 that has dual 4 ft. mirror mounted fiberglass antennas. I'm guessing Firestik?

I had one snap off when a bird hit it of all things...

Since I have a dual configuration, do I need to choose a replacement with a certain load? Or will any old 4 ft. fiberglass antenna work?

Thanks,

Ken

[BobC]

You can most likely get away with just about any similar looking antenna. There might be some fine tuning involved so checking the match might be a good idea.

[tjgosurf]

I was told by a CB shop, so who knows how true it is. But you need two antennas because you will burn up your radio. How true that is I am unsure.

[Pete_379X]

I dunno, I run Wilsons. But I also run big radios and I make sure my SWR's are set just right. More than likely if it was a stock antenna and a smaller radio then anything should work with it. You can get a good pair of Wilsons for about 40 bucks.

On burning up the radio, I'm not sure about it. The guy that I had build my old Cobra29 with a 350w kicker told me to keep my SWRs right or it would fry the kicker. Who knows. I run a Galaxy Big Rig Series DX48T now. Best radio I've had yet without a kicker on it.

[BobC]

Originally Posted by tjgosurf

I was told by a CB shop, so who knows how true it is. But you need two antennas because you will burn up your radio. How true that is I am unsure.

If the swr was good with 2 antennas, you're unlikely to hurt the radio with 1 missing.

The match won't be that far off from original. You might see an swr of say 2:1 or slightly higher. This assumes it was below that to start with.

In the next post about running an amplifier: Yes, it' possible to damage a high power amplifier with a poor match.

It's unlikely to damage an amp of that power level with an swr of say 2.5:1 or less. The amp would need to be operating at its highest power output into the bad load before it pops.

Most of the time, people usually keep the amp on a lower setting so it doesn't really put out it's max until you fully modulate the radio. Even then, it's not really putting out at constant full power because voices are not constantly high in power. Voices are low in average power but can have high peaks for very short periods.

[ee_john]

Originally Posted by KENMAR

I have a T-800 that has dual 4 ft. mirror mounted fiberglass antennas. I'm guessing Firestik?

I had one snap off when a bird hit it of all things...

Since I have a dual configuration, do I need to choose a replacement with a certain load? Or will any old 4 ft. fiberglass antenna work?

Thanks,

Ken

Here is the deal...

You hopefully have a co-phase harness as part of your antenna system. It expects a good load at both ends of the harness. It makes sense to replace the broken one, but spend the time to get it tuned right. By the way you should tune these things together as each whip interacts with the other in the EM field. All CB antennas are made to present the same load to the 50 ohm cable so mixing brands is OK. All this assumes the shield of each coax is firmly attached to the ground of the vehicle somehow.

There were some comments in other replies about high SWR possibly burning out the finals of CBs. That is quite true of legal and illegal CBs. Legal CBs use a very small final and driver transistor(s) so are very sensitive to the overheating caused by reflected power. High power finals and drivers have the same problem just scaled up: more power and devices designed to just barely deliver it.

The only way to not burn up finals and drivers in high SWR environments is to use a radio designed to deliver, say, 100 watts, to deliver something much less, say, 10 watts. Hams do this by dialing down their 100 Watt radios to 10 watts or whatever.

This is akin to why an audiophile picks 500 Watt stereo equipment. It is not to drive his speakers at the 500 Watts, but to drive 50 Watts very very accurately with zero concerns about overheating (and resulting rise in distortion). This comes at a cost of efficiency which the stereo freak does not care about.

CB Radios, like most consumer electronics, care greatly about efficiency with the least cost possible. They are designed to work very well close to their limits into a near perfect system. Being a Class A amp, it is already inefficient and added SWR can push it over the limit. I have replaced the finals and drivers many times in CBs for this reason. CB Linears are probably made to even worse standards so don't expect them to function long if driving them to full power into a less than perfect load. Being illegal means they are not made to anyone's good standard.

Anyway, you aren't using a linear so just get that second whip tuned in concert with the first and you will be good to go. Replacing both whips is not a bad plan either. Whatever you do please tune them together.

[BobC]

What cb final do you know of that's running class A?

[ee_john]

Originally Posted by BobC

What cb final do you know of that's running class A?

I was going to answer this quick, but decided your question is very good and looked into it a bit more than just my memory.

I believe the final output circuitry of all CB radios (US ones with AM and SSB) is class A. Why? A review of amplifier class characteristics may help...

Class A - The amplifier conducts the entire 360°, has good linearity, but low efficiency. This class' high linearity makes it excellent for modulation methods which utilize amplitude variations to convey information such as AM, DSB and SSB.

Class B - The amplifier conducts half of the sine wave at about 180°, has low linearity, but improved efficiency. However, the drive needed by the Class B can be higher. Without a tank circuit to complete the missing sine wave, one popular way to take advantage of this method is in what's called a Push-Pull circuit where two transistors or tubes are arranged to each amplify the part the other does not.

Class AB - Something in-between Class A and B in an attempt to find compromise between the two extremes. Again push-pull topologies use this technique when higher fidelity is desired.

...Push-Pull might be a way to generate AM, DSB or SSB with enough linearity...

Class C - The amplifier conducts less than half of the sine wave at about 120° with pretty good efficiency and horrible linearity. It is generally accepted that Class C amplifiers need to feed an oscillating tank circuit that will complete the missing sine wave. Class C is appropriate for continuous wave (morse) and frequency modulation (FM). Class C is not appropriate for AM, DSB and SSB.

When I was a CB repair person exactly 100% of the CBs I fixed, tuned or otherwise looked at had one driver and one final transistor for the transmitter. Since this is not a push-pull circuit and since this is an AM and possibly SSB radio, this leaves us with Class A as the only likely amplification method used in US CB Radios.

It is possible the non-US CB radios with FM only might use something other than Class A amplifiers.

So that is what I think at the moment, but your question has now made me want to know the full answer. I am going to research this much more throughly to make sure I have not missed something.

For now, though, my original point in mentioning this was to suggest that the components used in CB radio, whatever the amplifier class, are not designed with much margin. In other words they are very easy to burn up with a mismatched antenna.

[ee_john]

Just as a follow-up to my previous promise of more details about what class of amplification the driver and final transistor amplifiers in CBs use...

I have found some schematics and in each case the final chain of amplifiers all work as Class AB or B amplifiers with tank circuits between each section.

It is certainly not Class C.

I lookup up a few of the transistor used and they are rated for about 6W average when properly heat sinked. For those of you who "tweak" your radios to get more power out of them, know you might be pushing your final to its extreme limit just by keying up.

John

[kd5drx]

Ok after a class in type circuits used in CB here is the answer to your question about your antenna situation if you still need it. Generally speaking in the average situation RF seeks the path of lease resistance like water so if you break an antenna in a dual antenna system your system will probably not even notice it since you still have 1 antenna that is there and assuming it was tuned properly in the first place it will work fine since the open on the other end will not be seen by the radio. True test put and swr meter on the system and see what it is if it is still good your good to go. The radio only will only see what is there not what is not. As for the class of the final amplifier section of most CB radios they are generally considered type AB by most engineers that design them.