Dual Antennas

Imagine that.

AM op in most cb's doesn't require any more than class c & may as well be considered close to it because the very small amount of forward bias included for the SSB op doesn't really do anything to make the AM any "cleaner". However, that small adjustable bias circuit found in most SSB units makes it easier for unknowledgeable techs to use as a means to bring the AM carrier up or down while ignoring the real reason that circuit is there.

Normally I found the bias cranked all the way up which added a little extra carrier output on AM but overheated the finals very quickly on SSB.

That particular rating is generally for a continuous duty test bed in which the part is subjected to operation over a long period.Naturally the testbed is designed with enough heatsink or cooling capability to support the part during extended test routines.

The part you left out about the ratings is that these 6 watt parts are designed with a PEP output power of at least 4x the average carrier.

In other words, the part is designed with the application in mind & the 6watts becomes the suggested max carrier knowing that it will produce a PEP output of at least 4x the average.

You might also notice in your specs that most of these "final PA" parts are tested & rated under full drive/load/voltage conditions at very wide ranges of phase angle. That means these parts are designed to operate into very, very bad swr's without incurring damage (running at rated spec). It's mostly when you are exceeding the standard ratings in some way & subjecting them to a bad swr, that the chances of destroying the part become real.

On average, I've seen more radios with their AM carrier power turned down rather than up. I believe the intent is to drive a succeeding amplifier to the correct point without overdriving it on AM.

Naturally, the same radio is misadjusted for more than 100% mod which necessitates running the carrier even lower.

In the ideal operation, the radio is adjusted for 100% mod, the amplifier presents a good swr to the radio & the carrier from the radio is raised or lowered to the point where the amplifier begins to distort the fully modulated envelope & then backed off a bit.

I recognize this is off topic but the correct answers to the original post have already been supplied.

I just wanted to say & qualify that most of these finals in use today are not as tender as some think. I can't tell you how many times people have keyed their radios without an antenna & not caused the damage some desperately warn about. Me thinks there's a bit too much of the "chicken little" thinking out there.

 

Sorry about that... I got carried away...

Be careful here as coax is used as a transformer in a co-phase harness. The 75Ω cables are usually 1/4 wave or a multiple of 1/4 wave causing shorts to become opens and opens to shorts... only in the 1/4 wavelength case. Given this, the effect of a missing antenna should be dreadful.

Agreed. Testing is the best. I am off to test this myself...

Sure, but only if they design them that way. The Cobra schematics I looked at did not provide enough bias for AB only B at best. Hats off to the designers though... they do a great job of making things simple and manufacturable.

 

By saying average power I included the PEP case because the transistor does not care a thing about Peak Power (so long as voltage and current limits are not exceeded). That transistor rating is a heat dissipation specification. 4x peak watts is certainly doable and is clearly done, but only because it averages to something much less assuming a typical speech pattern. So we have the CB circuit designers to thank for designing the system to handle PEP in a speech transmission system, not the part itself.

In AM the average power is the carrier (4W?) plus the modulation sidebands which varies with voice.
In SSB the average power is the time average of the PEP which also varies with voice helping to keep the average power low... thus the reason we can peak out with much higher peak power.
In AM if you push the carrier to 6 watts and then add the sporadic modulation power, you are probably exceeding the part rating and shortening its life.

If the CB manufacturers are putting more "beef" in their finals that's great. However, I sense the parts are still right on the edge and won't tolerate a bad SWR for an extended length of time. I have seen it happen over and over where someone keys down repeatedly without an antenna thinking they are getting out, and then fry the final and sometimes the driver due to the reflected power. Being a heat related failure it can take some time for the part to die and it won't if they realize something is wrong and stop keying up. I do recall my days in the CB Shop that most every person who brought in a toasted radio were also less technically aware and just kept keying up even when something was obviously wrong. The technically aware folks go "Hmmmm," stop and check things out, thus saving their radio.

Of course it is possible something else zapped these components like ESD or lightning EMF, but that's another story.

..., but anyway...to get back on the thread topic the moral of the story is you might mess up your radio if you transmit into a cophase harness with only one antenna attached.

 

You're forgetting the basics of Wilkinson power dividers. There's a good 10-15db of return loss/isolation between the line segments when the power divider is properly built. That means the radio will not see but a 10th or less of the forward power returned as reflected power.

I don't know which schematics you looked at. There's a big difference between a SSB rig & a AM only rig.

The SSB rig has a designed-in active & adjustable bias circuit for the final & driver stages which can place the parts in a class A condition if adjusted wrong.

Typical AM only rigs use either a small "contact potential" bias or a fixed active bias & they run somewhere in or near the class B area. There's no need for them to run anyting less than class c because the driver & final are predominantly collector modulated. They will sometimes add some of the modulating energy to the base of the predriver & driver stages just to add a tad more linearity.

The peak power is important as well. The transistor has to be designed to withstand being driven to the peak voltages/currents without damage & without creating more IMD at that peak pwer. The part is responsible for allowing the operation at peak powers, not the circuit designers.

Not generally. Most AM only sets use a final rated at aprox 20-25 watts PEP. If you set the carrier for 6 watts & the max modulation for 100%, your PEP output will be aprox 4X the carrier, or 24 watts...give or take a little. Given that speech has about a 2:1 or 3:1 average, you're only at that peak power for 1/3 to 1/2 of the time you are actually speaking. A transistor with fair heat sinking can handle that easily.

It's not really a question of "beefiness" because the nature of AM is to present 4x the carrier when fully modulated (100%). This is a minimum design point.

Again, most of these rf rated parts are designed to withstand a wide range of phase angles without damage. I'm sure at some point the transistor will breakdown but not under normal intermittant abuse. They usually breakdown from repeated long term abuse that weakens the part over a period of time until it can't take the abuse it could when new.

Heat is generally the long term culprit but over voltage can do it too. Heat causes the materials/metals to migrate across the transistor die. With just a little extra abuse, a short can form.

Over voltages can pierce insulation layers. Heat along with over voltages will exhilarate the breakdown process.

Nahhh!

 

An excellent point, but the Wilkinson design (with the 100 ohm resistor across the outputs) protects each split port from the other, but does not seem to protect the common port from either split port's incoming energy (undesired reflected power or desired received power) other than maybe a 6dB isolation from reflected power (3dB out and 3dB back). This is the case with the 100 ohm resistor (under ideal conditions), but off the shelf CB co-phase harnesses don't have this resistor feature with the 75 ohm cables literally ending at each antenna.

The original poster wants to know the effect to the radio if one of the two antennas is not connected to a typical co-phase harness. The CB co-phase harnesses I have sold and seen on trucks don't have anything other than two multiple-1/4 wave lengths of 75 ohm cables paralleled together at the single radio connector. This lacks the Wilkinson innovation of the additional resistor.

I followed your message about power and such and agree you are correct with the calcs (By the way thanks!). Assuming we agree to disagree about the failure susceptibility of the radio's components to high SWRs (I say "they can overheat and die and have witnessed exactly this" while you say "they are built tough now and not to worry about it") what can we tell our original post author about the SWRs when one of the two antennas connected to an off the shelf non-Wilkinson Y cable co-phase harness is broken and rendered infinite ohms?


Good Antenna
.~50ohms====75ohmCoax====\...............|============|
..........................|==50ohm@XCVR==| SWR@Radio? |
Inf.Ohms====75ohmCoax====/...............|============|
Broken Antenna

If I have missed something please let me know.

You know what? This is an easy test to do so I will go to the test range with a sample of antennas, a co-phase harness and an antenna analyzer to see what happens.
Cheers.

 

Good Antenna
.~50ohms====75ohmCoax====\...............|============|
..........................|==50ohm@XCVR==| SWR@Radio? |
Inf.Ohms====75ohmCoax====/...............|============|
Broken Antenna

What you'll see is only a slight increase in swr.
By slight, I mean from 1.1:1 to aprox 2.?:1.

Exactly how much more will depend on the cable quality(z)/length & the groundplane qualities. The meter accuracy also affects this reading because it introduces an extra line section into the equation that otherwise wouldn't matter much.

 

i would have never asked this question -- iwould have just bought a couple new antennaes and had swr set

 

what do u guy;s think of the firestik Dual 4 foot Antenna Kit??

 

wilson 4' duals work great for me