High SWR on all HF Bands


Max
 

Hello everyone. I have an Icom 7000 that now shows a high SWR (2:1 - 3:1) depending on band into a dummy load (2 different ones). Here's the catch, I can change the SWR by using different lengths of coax between the radio and my external power/SWR meter. My external meter shows my dummy loads to be a good match but the radio doesn't agree. For a few months now I've had to use a 25Ft RG-58 jumper just to keep the SWR around 2:1 on the radio.

Is this just some failed part in the SWR detection circuitry or do I have bigger problems. I know its just guessing but what are the chances of it being an easy repair and are the parts available?

I'd hate to send it in just to be told the parts are unobtainium.

Max KG4PID


William Kerker
 

I'm wondering if you need a common mode choke on the coax. Sounds like RF on the coax. 

On Tue, Oct 27, 2020 at 8:13 PM Max via groups.io <kg4pid=yahoo.com@groups.io> wrote:
Hello everyone. I have an Icom 7000 that now shows a high SWR (2:1 - 3:1) depending on band into a dummy load (2 different ones). Here's the catch, I can change the SWR by using different lengths of coax between the radio and my external power/SWR meter. My external meter shows my dummy loads to be a good match but the radio doesn't agree. For a few months now I've had to use a 25Ft RG-58 jumper just to keep the SWR around 2:1 on the radio.

Is this just some failed part in the SWR detection circuitry or do I have bigger problems. I know its just guessing but what are the chances of it being an easy repair and are the parts available?

I'd hate to send it in just to be told the parts are unobtainium.

Max KG4PID



--
Bill,  KMØF

Radio Amateurs must be tough, it's a 'Contact Sport'


Dallas N4DDM
 

Max,
If you have ever looked in an SWR meter there are only a handful of components to go bad...

I've attached the SWR ckt of the IC-7000 from the service manual (you can find this on-line in pdf format)...

I was able to find the diodes and the opamp on the internet...  I even found them on ebay...






73 Dallas, KD4HNX
Whatever you do, don’t fall victim to “paralysis by
analysis.” Go ahead and buy/build one and start 
tinkering with it. This is a learn by doing hobby.

No politician or scholar assured your freedoms.
A Soldier, Sailor, Marine, or Airman did!


On Tuesday, October 27, 2020, 09:13:36 PM EDT, Max via groups.io <kg4pid@...> wrote:


Hello everyone. I have an Icom 7000 that now shows a high SWR (2:1 - 3:1) depending on band into a dummy load (2 different ones). Here's the catch, I can change the SWR by using different lengths of coax between the radio and my external power/SWR meter. My external meter shows my dummy loads to be a good match but the radio doesn't agree. For a few months now I've had to use a 25Ft RG-58 jumper just to keep the SWR around 2:1 on the radio.

Is this just some failed part in the SWR detection circuitry or do I have bigger problems. I know its just guessing but what are the chances of it being an easy repair and are the parts available?

I'd hate to send it in just to be told the parts are unobtainium.

Max KG4PID


 

Spot on on the forward and reverse power signals. Get you a known good piece of coax and use the dummy load to make sure that the coax is good. I check them with the antenna allayer I have as well.
After you have made certain that your coax and dummy loar are good and 50 ohms (use a good ohmmeter too) they you can check not the swr, which is a derived signal in the CPU, but the forward and reverse power readings. With a 1:1 swr the numbers should tell the tale. Forward power should be some value (I forger wha) and reverse should be close to zero (0).
These signals go to the ALC circuit to control power output and drive to the finals. I have had problems in the ALC circuit as well as in the JHREF and JH FOR. The JHFOR is used to regulate the power output and when compared to the JHREF reduce the power to protect the finals as well. 
Somewhere here I have posted the relevant sections of the schematic for the ALC circuit.


Dennis Klipa
 

That is a curious problem and I look forward to hearing the resolution. 

I tend to agree with William.  SWR is independent of where you measure it along a length of coax.  There are two exceptions to this rule.  One is when the coax is very lossy and power is consumed in the line during the trip to the antenna and back resulting in less reflected power and an apparently better SWR and antenna match than is reality.  I once used some new Radio Shack RG58 to connect to a 2 meter vertical.  I had a perfect 1:1 SWR.  The only problem was that no rf was actually making it to the antenna and so nothing was reflected!  The second exception is when you have rf voltage on the outside of the coax.  In that case the SWR reading will vary with position along the coax, but you are not reading true SWR.  This situation usually arises with unbalanced antennas or systems.  As William suggested, common mode chokes can rectify that problem.  

I do have a question.  Do you always have the external power/SWR meter in the line or was it there just to explore the problem?  Does the problem exist without the external meter?  If it is always in-line it could be the source of the imbalance.  Does the IC7K SWR reading change if you replace the external meter with a barrel connector?

If you don't have rf on the shield of the coax, and assuming that a defective part in the IC7K has caused a calibration issue of the SWR meter, I would not expect to see the SWR vary with coax length.  But, I have been wrong before.

Happy hunting!

Best Regards,
Dennis, N8ERF


On Wed, Oct 28, 2020, 2:04 AM William Kerker <km0f@...> wrote:
I'm wondering if you need a common mode choke on the coax. Sounds like RF on the coax. 

On Tue, Oct 27, 2020 at 8:13 PM Max via groups.io <kg4pid=yahoo.com@groups.io> wrote:
Hello everyone. I have an Icom 7000 that now shows a high SWR (2:1 - 3:1) depending on band into a dummy load (2 different ones). Here's the catch, I can change the SWR by using different lengths of coax between the radio and my external power/SWR meter. My external meter shows my dummy loads to be a good match but the radio doesn't agree. For a few months now I've had to use a 25Ft RG-58 jumper just to keep the SWR around 2:1 on the radio.

Is this just some failed part in the SWR detection circuitry or do I have bigger problems. I know its just guessing but what are the chances of it being an easy repair and are the parts available?

I'd hate to send it in just to be told the parts are unobtainium.

Max KG4PID



--
Bill,  KMØF

Radio Amateurs must be tough, it's a 'Contact Sport'


WA8Y Steven
 

Can you test with different coax?

If not, at least do a visual inspection of the shield to PL-259 connection at each end.   lf the shield has come lose, or the connection is oxidized,  that could cause your problem.

Steven  WA8Y


Steve W3AHL
 

Dennis,

The length of coax will affect the SWR seen at the radio.  Using a simple program like Transmission Lines for Windows, supplied with the ARRL Antenna Book, will show how much it varies.  Transmission lines are often used for matching purposes in antenna designs for single band antennas.  A quarter wave (electrical length) transmission line is an impedance inverter and can dramatically change the measured SWR, compared to a half wave line.  

But an unbalanced antenna with no common mode choke at the feed point will also exhibit the same behavior, along with distorted audio or other problems due to high RF in the shack.

Steve, W3AHL


don ---------
 

So   a  Swr isssue caused by  the  suspect length of  coax
will change how it responds on diffrent bands.

so if with a dummy load on the end of coax  line 

What is the  SWR on  diffrent bands ?

Ka7qqv
Ka7qqv@...

On 10/28/2020 5:58 AM Steve W3AHL <w3ahl@...> wrote:


Dennis,

The length of coax will affect the SWR seen at the radio.  Using a simple program like Transmission Lines for Windows, supplied with the ARRL Antenna Book, will show how much it varies.  Transmission lines are often used for matching purposes in antenna designs for single band antennas.  A quarter wave (electrical length) transmission line is an impedance inverter and can dramatically change the measured SWR, compared to a half wave line.  

But an unbalanced antenna with no common mode choke at the feed point will also exhibit the same behavior, along with distorted audio or other problems due to high RF in the shack.

Steve, W3AHL


Max
 

From the radio straight to a dummy load. I hope not. The dummy load checked good on my 259B.

Max KG4PID

On Wednesday, October 28, 2020, 01:04:16 AM CDT, William Kerker <km0f@...> wrote:


I'm wondering if you need a common mode choke on the coax. Sounds like RF on the coax. 

On Tue, Oct 27, 2020 at 8:13 PM Max via groups.io <kg4pid=yahoo.com@groups.io> wrote:
Hello everyone. I have an Icom 7000 that now shows a high SWR (2:1 - 3:1) depending on band into a dummy load (2 different ones). Here's the catch, I can change the SWR by using different lengths of coax between the radio and my external power/SWR meter. My external meter shows my dummy loads to be a good match but the radio doesn't agree. For a few months now I've had to use a 25Ft RG-58 jumper just to keep the SWR around 2:1 on the radio.

Is this just some failed part in the SWR detection circuitry or do I have bigger problems. I know its just guessing but what are the chances of it being an easy repair and are the parts available?

I'd hate to send it in just to be told the parts are unobtainium.

Max KG4PID



--
Bill,  KMØF

Radio Amateurs must be tough, it's a 'Contact Sport'


Max
 

Good to know that the parts are available. Hopefully that will help reduce the repair costs.

Max KG4PID

On Wednesday, October 28, 2020, 05:12:35 AM CDT, Dallas KD4HNX via groups.io <mylastname@...> wrote:


Max,
If you have ever looked in an SWR meter there are only a handful of components to go bad...

I've attached the SWR ckt of the IC-7000 from the service manual (you can find this on-line in pdf format)...

I was able to find the diodes and the opamp on the internet...  I even found them on ebay...






73 Dallas, KD4HNX
Whatever you do, don’t fall victim to “paralysis by
analysis.” Go ahead and buy/build one and start 
tinkering with it. This is a learn by doing hobby.

No politician or scholar assured your freedoms.
A Soldier, Sailor, Marine, or Airman did!


On Tuesday, October 27, 2020, 09:13:36 PM EDT, Max via groups.io <kg4pid@...> wrote:


Hello everyone. I have an Icom 7000 that now shows a high SWR (2:1 - 3:1) depending on band into a dummy load (2 different ones). Here's the catch, I can change the SWR by using different lengths of coax between the radio and my external power/SWR meter. My external meter shows my dummy loads to be a good match but the radio doesn't agree. For a few months now I've had to use a 25Ft RG-58 jumper just to keep the SWR around 2:1 on the radio.

Is this just some failed part in the SWR detection circuitry or do I have bigger problems. I know its just guessing but what are the chances of it being an easy repair and are the parts available?

I'd hate to send it in just to be told the parts are unobtainium.

Max KG4PID


Max
 

The external meter was always in the circuit but has been checked with my 259B. In fact everything has been checked, from all my coax jumpers, both dummy loads and my other power/swr meters. 

Since my eyesight is failing and I have no SMD rework equipment I'll be forced to send it out for repair. Any suggestions on who to send it to?

Thanks, Max KG4PID 

On Wednesday, October 28, 2020, 06:00:10 AM CDT, Dennis Klipa <klipadk@...> wrote:


That is a curious problem and I look forward to hearing the resolution. 

I tend to agree with William.  SWR is independent of where you measure it along a length of coax.  There are two exceptions to this rule.  One is when the coax is very lossy and power is consumed in the line during the trip to the antenna and back resulting in less reflected power and an apparently better SWR and antenna match than is reality.  I once used some new Radio Shack RG58 to connect to a 2 meter vertical.  I had a perfect 1:1 SWR.  The only problem was that no rf was actually making it to the antenna and so nothing was reflected!  The second exception is when you have rf voltage on the outside of the coax.  In that case the SWR reading will vary with position along the coax, but you are not reading true SWR.  This situation usually arises with unbalanced antennas or systems.  As William suggested, common mode chokes can rectify that problem.  

I do have a question.  Do you always have the external power/SWR meter in the line or was it there just to explore the problem?  Does the problem exist without the external meter?  If it is always in-line it could be the source of the imbalance.  Does the IC7K SWR reading change if you replace the external meter with a barrel connector?

If you don't have rf on the shield of the coax, and assuming that a defective part in the IC7K has caused a calibration issue of the SWR meter, I would not expect to see the SWR vary with coax length.  But, I have been wrong before.

Happy hunting!

Best Regards,
Dennis, N8ERF

On Wed, Oct 28, 2020, 2:04 AM William Kerker <km0f@...> wrote:
I'm wondering if you need a common mode choke on the coax. Sounds like RF on the coax. 

On Tue, Oct 27, 2020 at 8:13 PM Max via groups.io <kg4pid=yahoo.com@groups.io> wrote:
Hello everyone. I have an Icom 7000 that now shows a high SWR (2:1 - 3:1) depending on band into a dummy load (2 different ones). Here's the catch, I can change the SWR by using different lengths of coax between the radio and my external power/SWR meter. My external meter shows my dummy loads to be a good match but the radio doesn't agree. For a few months now I've had to use a 25Ft RG-58 jumper just to keep the SWR around 2:1 on the radio.

Is this just some failed part in the SWR detection circuitry or do I have bigger problems. I know its just guessing but what are the chances of it being an easy repair and are the parts available?

I'd hate to send it in just to be told the parts are unobtainium.

Max KG4PID



--
Bill,  KMØF

Radio Amateurs must be tough, it's a 'Contact Sport'


Dallas N4DDM
 

Max,
I agree with the other folks that suggest you swap out your SWR meter, jumpers, and dummy load from those of a nearby ham...  

Prove beyond a shadow of drought that your radio has a problem and not your jumper, SWR meter, or dummy load...

Case in point:
I have an SWR meter from my CB days and it works OK at 2m to give me a ballpark test of SWR...  It is NOT accurate as my Bird Wattmeter but it's easy to use, if I drop it, who cares...  If it says the SWR is low, it's low...  If it says the SWR is high, it's high...  If I ask for an audio test on the local repeater it sounds like I'm holding an electric shaver next to the microphone...  It took a while to figure out what was going on...  To find the problem you have to swap things out 1 at a time...


73 Dallas, KD4HNX
Whatever you do, don’t fall victim to “paralysis by
analysis.” Go ahead and buy/build one and start 
tinkering with it. This is a learn by doing hobby.

No politician or scholar assured your freedoms.
A Soldier, Sailor, Marine, or Airman did!


On Wednesday, October 28, 2020, 02:33:13 PM EDT, Max via groups.io <kg4pid@...> wrote:


Good to know that the parts are available. Hopefully that will help reduce the repair costs.

Max KG4PID

On Wednesday, October 28, 2020, 05:12:35 AM CDT, Dallas KD4HNX via groups.io <mylastname@...> wrote:


Max,
If you have ever looked in an SWR meter there are only a handful of components to go bad...

I've attached the SWR ckt of the IC-7000 from the service manual (you can find this on-line in pdf format)...

I was able to find the diodes and the opamp on the internet...  I even found them on ebay...






73 Dallas, KD4HNX
Whatever you do, don’t fall victim to “paralysis by
analysis.” Go ahead and buy/build one and start 
tinkering with it. This is a learn by doing hobby.

No politician or scholar assured your freedoms.
A Soldier, Sailor, Marine, or Airman did!


On Tuesday, October 27, 2020, 09:13:36 PM EDT, Max via groups.io <kg4pid@...> wrote:


Hello everyone. I have an Icom 7000 that now shows a high SWR (2:1 - 3:1) depending on band into a dummy load (2 different ones). Here's the catch, I can change the SWR by using different lengths of coax between the radio and my external power/SWR meter. My external meter shows my dummy loads to be a good match but the radio doesn't agree. For a few months now I've had to use a 25Ft RG-58 jumper just to keep the SWR around 2:1 on the radio.

Is this just some failed part in the SWR detection circuitry or do I have bigger problems. I know its just guessing but what are the chances of it being an easy repair and are the parts available?

I'd hate to send it in just to be told the parts are unobtainium.

Max KG4PID


 

You evidently have at least two patch cables and the one from the SWR meter to the dummy load must be good. Try hooking the IC-7000 directly to the dummy load using that cable. Then for the heck of it try the other one too.


Max
 

I've connected the dummy load directly to the radio. I still have a 2:1 to 3:1 depending on band. I'm gonna have to send it out for repairs.

Max KG4PID

On Wednesday, October 28, 2020, 03:01:38 PM CDT, J.D. Barron <jeter.d.barron@...> wrote:


You evidently have at least two patch cables and the one from the SWR meter to the dummy load must be good. Try hooking the IC-7000 directly to the dummy load using that cable. Then for the heck of it try the other one too.


Steve W3AHL
 

With a pure resistive 50 ohm dummy load the impedance seen at the SWR meter or radio will not change significantly with different lengths of coax, assuming the coax is close to 50 ohm Z.  

It will change when the  load impedance is different than the coax.    Of course many 50 ohm loads are not really 50 ohms at RF, even if an multimeter says they are OK.  If the 259B measures the dummy load as 50 ohms non-reactive, then it is probably OK.

Steve, W3AHL


 

Maybe the grounding screws on the SO-239 on the radio are loose? or the bonding inside the radio?


Rick Robinson
 

Center pin on pl259 may not be a snug fit. Possible undersized pin or so239 leaves need tightened.

On Wed, Oct 28, 2020 at 4:52 PM J.D. Barron <jeter.d.barron@...> wrote:
Maybe the grounding screws on the SO-239 on the radio are loose? or the bonding inside the radio?

--
Rick Genesis 1-29


Max
 

Thanks for that suggestion since I hadn't thought of that. I just checked and it seems to be making good contact. A FULL reset also didn't change anything.

Thanks, Max KG4PID

On Wednesday, October 28, 2020, 07:45:45 PM CDT, Rick Robinson <rickw8zt@...> wrote:


Center pin on pl259 may not be a snug fit. Possible undersized pin or so239 leaves need tightened.

On Wed, Oct 28, 2020 at 4:52 PM J.D. Barron <jeter.d.barron@...> wrote:
Maybe the grounding screws on the SO-239 on the radio are loose? or the bonding inside the radio?

--
Rick Genesis 1-29


Dennis Klipa
 

Steve,

I assume that you know a lot of what I am about to share but I include it for others reading the thread who may not know  A lot of what you said is true about the transmission lines acting as impedance transformers and matching sections.  However, I respectfully submit that the statement that the SWR, in a lossless feedline terminated in a balanced load will change with cable length is not true.  In a feedline of at least 1/4 wavelength, attached to a balanced load the SWR does not change with cable length. 

According to many sources, including the 2014 edition of the ARRL Handbook, p 20.4, the Standing Wave Ratio, SWR, is a measure of the relative amounts of forward and reflected power.  It is often described in terms of voltage and is thus a VSWR or current (ISWR) and is the ratio of the highest voltage (current) anywhere along the line to the lowest voltage (current) anywhere along the line.  Thus it is a characteristic of the system, not a characteristic of a point on the line.  Measuring the voltage minima and maxima along the line is difficult for coaxial cable but forward power and reflected power are easily measured by a directional coupler or reflectometer.   SWR is dependent on the complex reflection coefficient, p, which is calculated from the characteristic impedance of the feed line (Zc) and the impedance of the load (ZL) as p =(ZL - Zc)/(ZL + Zc).  In terms of Forward Power (Pf) and Reflected Power (Pr) the reflection coefficient is defined as p = SQRT (Pr/Pf).  Regardless of how you do the calculation you get the same answer.  SWR is defined as  SWR = (1 + p)/(1 - p).  Any losses in a transmission line would only reduce the measured SWR.  There is no mention or implication in these expressions that the SWR varies with line length.

It is true that if you were to measure the impedance that you see at various lengths along the transmission line, the impedance would vary.  This is clearly seen on a Smith Chart in an article in the files section; "Matching Networks.pdf" by Terry Rogers, WA4BVY (https://groups.io/g/ic7000/files/Matching%20Networks.pdf), page 32, Figure 19, which you uploaded on February 7th.  The Figure shows two circles centered on the center of the Smith Chart which represent SWR ratios of 5:1 and 3:1. At all points along each circle the SWR is the same.   Moving around the Smith Chart in a clockwise direction starting at the intersection of the horizontal axis on the right side of the circle represents moving along the length of the coax.  Any point on the constant SWR circles represents the impedance you would measure at that point along the feedline.  So, yes, the impedance that you see along the feedline changes, while the SWR remains constant.  A changing impedance does not imply a changing SWR.

For a balanced load and a feedline greater than 1/4 wavelength (Reflections III, W. Maxwell, page 2-4, 2010):
"(22) The SWR in a feedline cannot be changed, adjusted, or controlled in any practical manner by varying or adjusting the line length."
"(23) If SWR readings change significantly when moving the SWR bridge a few feet one way or the other in the line, it indicates either"antenna" current flowing on the outside of the coax, or else an unreliable instrument, or both, or even a reliable bridge incorrectly adjusted to the line impedance, but it is not because the SWR is varying with line length.  Some suggest that the bridge must be placed and a 'lambda'/2 interval from the load to obtain a correct reading.  This is incorrect. All readings are invalid if they change significantly along the line, even though they may repeat at 'lambda'/2 intervals."

Byron Goodman in his April, 1977 QST, pg 40-42 article; "My Feedline Tunes My Antenna", says the same thing, although a little less delicately and more succinctly.

So with that I choose to stand by my original comment in an earlier post.

Best Regards,
Dennis Klipa, N8ERF

On Wed, Oct 28, 2020 at 8:58 AM Steve W3AHL <w3ahl@...> wrote:
Dennis,

The length of coax will affect the SWR seen at the radio.  Using a simple program like Transmission Lines for Windows, supplied with the ARRL Antenna Book, will show how much it varies.  Transmission lines are often used for matching purposes in antenna designs for single band antennas.  A quarter wave (electrical length) transmission line is an impedance inverter and can dramatically change the measured SWR, compared to a half wave line.  

But an unbalanced antenna with no common mode choke at the feed point will also exhibit the same behavior, along with distorted audio or other problems due to high RF in the shack.

Steve, W3AHL



--
Best Regards,
Dennis Klipa


Steve W3AHL
 
Edited

Dennis,

In a theoretical world with a balanced load and lossless transmission line, you are correct.  However, in a real world application other factors apply.  In the 22nd Edition of the ARRL Antenna book (circa 2011) in section 23.2.6 Input Impedance the effect of line loss on input impedance is calculated, which agrees with the Transmission Line Program for Windows program V 3.24 by N6BV supplied with the Antenna Book.  A load of 40+j30 with 50' of RG-213 has an input Z of 65.8+j32 and the SWR at the load of 1.84:1 is increased to 1.96:1 at the input.  This can be confirmed with an AIM 4170C analyzer (using properly constructed common-mode chokes) to reduce the inherent unbalanced current in a real dipole).

Many SWR meters are sensitive to current flowing on the outside of the shield and that is indeed the more typical cause of changing SWR with line length, which can be reduced with a CM choke at the feed point of the antenna.

The effects of line length and loss are much more dramatic at VHF and above.  Having spent hours fine tuning transmission lines between duplexer cavities and then between the duplexer TX output  and the repeater.  Losses are much greater as frequencies increase.

For HF bands line length would not account for the 2:1 to 3:1 variation the original poster was reporting, but it can be significant above 20 MHz, especially with solid dielectric coax.

Steve, W3AHL