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SWR gone up after replacing components in this transceiver

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yo2550

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https://www.electro-tech-online.com...iving-sensitivity-of-this-transceiver.128590/

I replaced D8, Q6, D13, D7, D1, R11.

Before replacing the components, the SWR read 1.13 but after replacing the components, the SWR read 1.4 using the same antenna.

The cause would be that I accidentally pulled out small circuit path on the board between R11 to Q26 and D8 to L15. I soldered the path to connect them already, but I am not sure if this would change the output impedance of the transmit signal, but I think it's unlikely.

I guess that the SWR goes up from 1.13 to 1.4 because the output impedance changed from 50ohm after I repaired the radio.

I hi-lighted the transmit signal path in the picture(red square). So, please give me a suggestion how to match the output impedance to come back to 50ohm. I have not put the metal cover over the transmit path as it was originally placed there. I'm not sure if it affects the output impedance.

View attachment 66016
 
D7 and D13 are in the transmit path, did you use the exact replacements from the manufacturer?.

Are any of the coils/capacitors in the path adjustable?, usually in a transmitter you connect a 50 ohm dummy load and adjust the transmitter to match it.
 
A VSWR below 1.5 is usually classed as good, but it does seem strange.

Have nothing else to add apart from that - Nigel hit a lot of nails on the head.
 
Typing this for the second time!
Did I screw-up or was it the interwebs thingy?

Anyway,

SWR, or to give it its full name VSWR (Voltage Standing Wave Ratio) is a function of the load, ie the antenna and its associated feeder cable.
It is not a function of the transmitter.

Assuming that you are using an external SWR meter rather than something built in to the transmitter, any repairs and modifications to the transmitter will not make any difference to the SWR.

Having said that, it is possible that harmonics or other spurii from the transmitter will affect the SWR reading.
If changes are made to the low-pass filter between the PA stage and the antenna connector, the level of harminics/spurii could change and so change the SWR reading.

JimB
 
SWR, or to give it its full name VSWR (Voltage Standing Wave Ratio) is a function of the load, ie the antenna and its associated feeder cable.
It is not a function of the transmitter.

It's a function of the transmitter just as much - the output of the transmitter needs to be 50 ohms, the cable needs to be 50 ohms, and the aerial needs to be 50 ohms.

Part of the calibration of a tgransmitter is to set it's output to 50 ohms - which is easily done with a 50 ohm dummy load and an SWR meter, it's basic radio ham stuff.
 
It's a function of the transmitter just as much
No it is not.

the output of the transmitter needs to be 50 ohms,
Depending on the design of the transmitter, it will produce maximum output when matched to a 50 ohm load.
That does not necessarily mean that the output impedance of the transmitter is 50 ohm.

the cable needs to be 50 ohms, and the aerial needs to be 50 ohms.
Quite correct, if the transmitter is designed for a 50 ohm load, most are, but not all.

Part of the calibration of a tgransmitter is to set it's output to 50 ohms - which is easily done with a 50 ohm dummy load and an SWR meter, it's basic radio ham stuff.
Adjusting the tuning of the output stages of the transmitter will not change the VSWR of the antenna system.
The apparent VSWR as displayed by most (cheap) SWR meters will change because they tend to be non-linear and display a lower SWR at lower transmitter power levels.
This is due to the non-linearity of the diodes used in the forward/reverse power measuring circuits in the SWR meter.

JimB
 
No it is not.

According to your personal opinion.

Depending on the design of the transmitter, it will produce maximum output when matched to a 50 ohm load.
That does not necessarily mean that the output impedance of the transmitter is 50 ohm.

I believe that's exactly what it means - maximum power transfer.

Quite correct, if the transmitter is designed for a 50 ohm load, most are, but not all.

But most are (75 is the next most common), the point was that ALL items need to be matched

Adjusting the tuning of the output stages of the transmitter will not change the VSWR of the antenna system.

Not of the aerial system, but of the entire system - if the transmitter isn't matched, then the VSWR will be wrong.
 
All of the diodes and resistor I replaced are from the dealer in my country and I think it is exactly the same spec as the original.

For more info, I connect the digital SWR meter as this:

Transmitter-->SWR meter-->5DFB--->Antenna

Transmitter to SWR meter is directly connected by N-type to SMA, no cable here.

The change I noticed is considerable after repairing the transceiver. To be more precise, I compare the SWR reading with another same model of the transceiver and it reads around 1.13-1.2 whereas the repaired one shows 1.4.

One of the cause I could think now is the transmit path was covered by a thin metal plate which is connected to ground. I guess it was put there to reduce interference or harmonic coming out when in transmit mode.

The part of the metal cover I removed is only in the yellow box in the picture here.

Note: The picture is from someone else's radio which is the same model as mine.

View attachment 66028
 
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One of the cause I could think now is the transmit path was covered by a thin metal plate which is connected to ground. I guess it was put there to reduce interference or harmonic coming out when in transmit mode.

Put it back - removing the plate will most likely affect the tuning of the transmitter, and could be the cause of your problem.
 
More info: The output power comes to around 3 watts from 5-6 watts before repairing. I'm not sure whether the PA is distorted or the new PIN diodes reduce the output power.
 
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D7 and D13 are part of the T/R switching and likely most critical to the Tx path. They get forward conducted during Tx and their lead inductance and Rs are very critical to match. Lead dressing is very critical because the PA output path is relatively low impedance. A couple of extra nH's in series of these diodes will significantly change the match for 220 MHz operation.

If the replacement diodes have higher ON resistance it will add to losses. The fact there are two diodes in parallel shows the Rs-ON series resistance is critical.
 
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ok, I have put the plate back and soldered it to the left and right side. The SWR came down to 1.3 now. I have not soldered and put the lead to cover all the gaps on the left and right of the cut plate.

Thanks RCinFLA, I totally agree with your post.

Another factor which could cause the higher SWR I can think of is I cut 1-2mm. of the wire, that connected from PCB to ANT jack. It is a regular small wire. If it is shorter, it could affect the SWR I guess.

Note: I put newly charged battery and measure the output power. I got 4 watts now. So, I think the PA hasn't been deteriorated. At least, not much after I have been used the radio.
 
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