SWR before linear

[jpachla]

Ok, experts! Situation: president richard 10/11 meter has a swr of 1.0 to antenna capable of 500watts. The monent I put dx350 inline. SWR between radio and linear is 2.6-3.0. SWR between linear and antenna is 1.3

Of course internet is full of answers. Most say the Swr between radio and linear is not important as long as it’s not like 10.

If that is the case, is there anything I can do to get it down other then adding coax which is just a mask. Or just not worry about it

I did notice the connectors on linear are a bit oxidized from age, I was going to clean those up thinking it may be messing with a good solid ground. And yes the linear is grounded to multiple scrubbed and cleaned seat post bolts

Thanks yall

 

[BC Coyote]

That SWR between the radio and amp is much to high for sure. It should be below 2.
I run a Palomar TX200 and the input SWR is 1.1

There are some amp experts on here and hopefully one of them will chime in with an idea of what the issue could be. You definately want to make sure all connections are clean and shiny. Also make sure the PL-259's are fitting into the 239 plugs snuggly....I had one issue recently where the center pin of the 259 was wiggling around in the very loose centre of an old worn out plug. Check the SO-239's on the amp aren't pooched.

 

[sp5it]

SWR rise when amp is ON or OFF?

 

[jpachla]

SWR rise when amp is ON or OFF?

I’ll need to check that tomorrow, I didn’t check for that

 

[Old Barney]

Acquire a 50ohm load that will handle your out put from radio. install it at the end of your jumper at the amp input end and read SWR there. If you get high reading there throw it in the trash. Get another JUMPER made with NEW 50 ohm coax and new PL259 connectors. You said that the connectors showed some OXYIDATION same as CORROSION. Which will make a bad connection and spurious signal going into the AMP. The Amp may amplify the SPURS on MANY frequencies. You will have a DIRTY signal and upset MANY other folks on a few frequencies + - away from your transmit center frequency.

 

[fourstringburn]

Ok, experts! Situation: president richard 10/11 meter has a swr of 1.0 to antenna capable of 500watts. The monent I put dx350 inline. SWR between radio and linear is 2.6-3.0. SWR between linear and antenna is 1.3

Of course internet is full of answers. Most say the Swr between radio and linear is not important as long as it’s not like 10.

If that is the case, is there anything I can do to get it down other then adding coax which is just a mask. Or just not worry about it

I did notice the connectors on linear are a bit oxidized from age, I was going to clean those up thinking it may be messing with a good solid ground. And yes the linear is grounded to multiple scrubbed and cleaned seat post bolts

Thanks yall

Yes, clean the connectors for a better electrical connection but I doubt that's causing the problem.

I'm not an expert on these amps but it has been well known that many amplifiers built for CB use do not have tuned 50 ohm inputs which can make the SWR read much higher than a 52 ohm 1.1 match. This may explain why you have a high SWR with the amp inline.

Quality amplifiers like ones used in the Amateur radio community do not have this problem. Texas Stars and other Class C amp builders seem to neglect having a tuned 50 ohm input which is dumb IMHO.

Many people swear by using longer jumper cables between the amplifier and radio to compensate which essentially creates a 50 ohm matching stub with coax. There are some YouTube videos showing the difference between using a short 3 ft jumper and one that is a half wavelength long with the coax type's velocity factor calculated in and this did drop the SWR to near flat.

If you don't have an antenna analyzer, then use a 7 to 9 ft length of coax and note any differences.

If you have an antenna analyzer and know how to use it's features, you can trim a piece of 10ft coax down until you see 50 to 52 ohms for the most common frequency you use.

Here is a YouTube video on how this guy did it.

 

[nomadradio]

So, if the SWR indicated by the radio only rises when the amplifier is keyed and putting out power, this means that the amplifier's input circuit is not imitating an antenna with a low SWR. The amplifier's input circuit should fool the radio into thinking that it's keyed into a properly-performing antenna. The amplifier's input impedance can usually be brought back down to 1.5 or so most of the time. Does lead me to wonder if the amplifier has been "tweakerized" by someone looking to push the wattmeter higher. The factors that can affect it this way are the value of capacitors in the input circuit. The zero-signal "bias" current being drawn by the RF transistors. Even the output circuit component values can change the amplifier's input-side SWR.

No single tweak covers every case.

Any chance the coax jumper between the radio and amplifier isn't quite right? Most such faults occur in the connection between the coax braid and the connector. We check for this using a dummy load. Normally you won't hear any receiver noise connected to it. We flex the coax behind every connector, listening to the noise level. If it rises, or fluctuates in any way when the coax is molested, that connector is suspect. It might check ok on a multitester's ohm scale, but the true test is how much shielding it has. This is a back-door kind of test, but it's more sensitive than the DC ohmmeter.

It's a little like "the motor stumbles when I accelerate. Sometimes." More than one possible culprit.

73

 

[jpachla]

So, if the SWR indicated by the radio only rises when the amplifier is keyed and putting out power, this means that the amplifier's input circuit is not imitating an antenna with a low SWR. The amplifier's input circuit should fool the radio into thinking that it's keyed into a properly-performing antenna. The amplifier's input impedance can usually be brought back down to 1.5 or so most of the time. Does lead me to wonder if the amplifier has been "tweakerized" by someone looking to push the wattmeter higher. The factors that can affect it this way are the value of capacitors in the input circuit. The zero-signal "bias" current being drawn by the RF transistors. Even the output circuit component values can change the amplifier's input-side SWR.

No single tweak covers every case.

Any chance the coax jumper between the radio and amplifier isn't quite right? Most such faults occur in the connection between the coax braid and the connector. We check for this using a dummy load. Normally you won't hear any receiver noise connected to it. We flex the coax behind every connector, listening to the noise level. If it rises, or fluctuates in any way when the coax is molested, that connector is suspect. It might check ok on a multitester's ohm scale, but the true test is how much shielding it has. This is a back-door kind of test, but it's more sensitive than the DC ohmmeter.

It's a little like "the motor stumbles when I accelerate. Sometimes." More than one possible culprit.

73

When I checked the Swr between the radio and antenna only, I use a standard butt 259 connector between the two ends of coax and swr is 1.0. Later today I will check swr with linear off. I didn’t make that test the first go around.

Will a low pass filter BEFORE linear help bring it in line?

 

[jpachla]

When I checked the Swr between the radio and antenna only, I use a standard butt 259 connector between the two ends of coax and swr is 1.0. Later today I will check swr with linear off. I didn’t make that test the first go around.

Will a low pass filter BEFORE linear help bring it in line?

Note. Swr after amp is still very good , below 1.5

 

[9C1Driver]

Will a low pass filter BEFORE linear help bring it in line?

No. It appears you either have a jumper issue or your amp needs some work or both. It is common for Palomar and Texas Star amplifiers to not be properly matched for a 50ohm load.

 

[Shadetree Mechanic]

Unfortunately cb amps sometimes are not optimized and get put together with what is on hand. I have had this problem with my 350 when I first got it, the fix is to change the input tune capacitor.

Thread 'Texas Star DX 350 HDV Input SWR' https://www.worldwidedx.com/threads/texas-star-dx-350-hdv-input-swr.230089/

 

[Eldorado828]

Like mentioned above, it's unfortunate that some builders don't put forth the effort to correctly tune the input on the amps they sell. I've even witnessed the amp working well when powered on but the pass through not being right when powered off.

I don't believe the job is completed if it doesn't work as they advertise it.

 

[BC Coyote]

Like mentioned above, it's unfortunate that some builders don't put forth the effort to correctly tune the input on the amps they sell. I've even witnessed the amp working well when powered on but the pass through not being right when powered off.

The vast majority of "CB" amps are total garbage, including Texas Star.

Like someone else said earlier, it's probably a better plan to just buy a proper ham amp. I run an old Palomar TX200 pill amp built in 1975, but I modified it heavily to make it work properly and cleanly. I rebuilt the input stage so it has a 1.1 SWR input. I built proper bias circuits for both the driver and main pills and I added a high quality commercially made LPF to the output. I also installed a fan so it would run cooler. Been using it for years now and it sounds fantastic.

Prior to making all those improvements it sounded like crap on sideband, had a very high input SWR, and had wildly fluctuating output SWR due to harmonics that were being internally generated......in other words a pretty normal CB amp!

 

[Monk]

In my experience I am with Nomadradio.
I have tuned the input on many SS amps. Usually there is a standard value silver mica cap of around 100pf from ground to the input wire of the input transformer. This is on a two transistor unit. If you look at a schematic of an older Palomar tx100 amp you will see a variable compression mica cap instead of a fixed value cap. That cap would tune your SWR between your radio keyed up and the amp keyed up. The schematic shows a 600pf. On the DX350 it shows for C8 a 470pf so if you put a variable compression cap of 600pf in place of it you would be able to tune your SWR.
When a person changes the length of the input coax I believe what is happening is all coax has a certain amount of Pf per foot, between the center and the shield. When the coax between the radio is changed, shorter or longer, it changes the Pf from the input transformer input to ground thus changing the SWR.

Monkradio on ebay

Looking to clean out 40 years of CB (constantly buying) equipment and parts this spring, Keep checking it will take time to insert everything..................

 

[AudioShockwav]

The vast majority of "[CB]" amps are total garbage, including Texas Star.

I wasn't always that way.
Texas Star started long ago, back with the outcom amps using Motorola transistors, designed for 23 channel radios that really did struggle to make 3.5 watts.
Even early 40 channel radios were making 14-16 watts on SSB.
And as lacking as the bias network is in the TS is, at least it has one.
When the amps were ran as described in the sheet that came with the amp they were decent, no more the 20 watts peak input, keep volts below 15 put the option fan kit on it.
Then
Starting several years ago the quality started going away, soldering looked like some kid was putting them together, Toshibas became extremely expensive and then Ed the owner passed.
Unfortunately the new 350 mini is a flop, I see nothing good reported about them so far.
I don't know if Mike is going to redesign the amps or whats going to happen.

In CB's glory days there's a handful of builders that built decent amps.
Some of the Magnum amps and Cobra clones actually had electronic regulated bias and some filtering.
Palamor TX 5300 series amps even had switched band pass filters in them, yes it was a dodge to get around the FCC, but it proved that they "could" build a decent amp.
The hardcore AM CB guys didn't like them.
Take the magnum 6000, thermal tracking, adjustable regulated bias, filtering.
4x2879 Toshiba transistor amp
And when they sold, the flyers listed them as a 400 watt amp, not 500 not 600, 400 watts peak.
Around that time Dave started building class c comp style amps optimized for running higher voltage, then, suddenly they were going mainstream.
4 x 2879 amps now were looked at like " hey that should make 750 watts at 18 volts " and if it doesn't you aren't driving it hard enough.
Peek would build very clean nicely biased, well isolated, stable amps and got greif for it at times because they would not show outrageous peak output power when driving the snot out of them as a comparable class c comp amp running on 18 volts.
Am I rambling again?

73
Jeff