AIS VSWR troubleshooting help please!

[S/V Illusion]

Originally Posted by Cpt Pat

It's also useful to connect the dummy load to the antenna end of the cable and then measure SWR of the antenna itself. Even a cheap SWR and power meter, such as this one, will be very valuable for troubleshooting: https://www.gigaparts.com/mfj-822.ht...ABEgKEGfD_BwE#

To measure cable attenuation, place the SWR meter at the antenna end of the coax, terminating into the dummy load instead of the antenna. Then compare the power output at the radio into the dummy load to the power measured at the antenna end of the cable.

A 50 ohm dummy load is an important tool to simulate a "perfect antenna" so you can confirm/eliminate problems elsewhere.

Patrick
N8QH

You can verify the precision of this cheap SWR meter online but I suspect it isn't a very reliable way of measuring power at the 25 W level or lower precisely enough to differentiate power by insertion at both ends of the coax to measure loss.

Parenthetically, simple attenuation along the coax run wouldn't cause the symptoms described.

[stimpsonjcat]

I would use my AIM4170 VNA. ;P

But you can get cheap antenna anaylzers these days!
https://www.ebay.com/itm/Nanovna-50K...r/163804773031

[Paul L]

Originally Posted by stimpsonjcat

I would use my AIM4170 VNA. ;P

But you can get cheap antenna anaylzers these days!
https://www.ebay.com/itm/Nanovna-50K...r/163804773031

Have you tried this usd$66 network analyzer? I've never used a similar unit. What is it going to tell me about my cable and antenna from a practical sense? More accurate SWR, or ???

[Wotname]

Originally Posted by S/V Illusion

You can verify the precision of this cheap SWR meter online but I suspect it isn't a very reliable way of measuring power at the 25 W level or lower precisely enough to differentiate power by insertion at both ends of the coax to measure loss.
.........

I'm curious - why wouldn't it be suitable for measuring coax insertion loss at a 25W level?

[stimpsonjcat]

Originally Posted by Paul L

Have you tried this usd$66 network analyzer? I've never used a similar unit. What is it going to tell me about my cable and antenna from a practical sense? More accurate SWR, or ???

Full disclosure...NO...I have not used this exact analyzer. But I have trusted ham friends who have.

The analyzer has two features helpful for troubleshooting not just AIS, but also VHF and HF (SSB) antenna installations.

First useful feature - SWR frequency sweep chart

This is a graph with SWR on the vertical axis and frequency on the horizontal. Here you can see where an antenna (well, usually an antenna AND it's feedline) are resonant. The graph is better than just knowing the SWR number the radio sees because it tells you if the antenna is electrically too long or electrically too short. Which allows you to lengthen or shorten the antenna to get the best SWR.

I should stop for a moment and point out that SWR is an offset from the ideal resistive load of 50 ohms in this case. This is important because folks often think that you can 'fix' SWR with feedline, but that is not true. As you move down a feedline the SWR simply rotates in a circle around the ideal based on the offset from the ideal it started with.

Sorry...that probably didn't help.

Second useful feature - TDR (time-domain reflectometer)

So to understand a TDR, you need to understand that any transmission cable is designed to be a consistent impedance all along its length. People get confused about this because they think of it in terms of DC impedance. i.e. what you would see if you attached a volt-ohm meter to the cable. When we talk about impedance in a transmission line we are talking about it's impedance at RF frequencies...not DC.

So, backing up, your antenna is supposed to be a 50ohm load. It is supposed to be connected to coax that has a 'characteristic impedance' of 50 ohms. And the cable is supposed to be connected to a radio whose output impedance is...you guessed it...50 ohms. If all of that happens then the SWR is 1:1. Yay!

Remember, SWR is the ratio of FORWARD to REFLECTED power. So to have an SWR of 1 means ALL of the power is going OUT and none is coming BACK. The ideal.

So the TDR feature allows this device to do something very useful. It can send a signal down the coax, at the appropriate frequency, and watch for reflections of power coming back. If it sees any, it measures the TIME it took for that reflection to return and it then knows how far it was to the impedance mismatch that caused the reflection.

So, if you know how many feet of coax you have to the antenna, you can disconnect the antenna, connect a 'dummy load' (comes with the analyzer) and run a test to see if the impedance of your coax (including any connectors along the way) matches the 50 ohm spec and if the analyzer notes a 'problem' it will tell you EXACTLY how many feet/meters away that mismatch is!

I have a cabin in the north Georgia mountains that is about 800 feet from the ridgeline of a mountain at 1800 ft elevation. I keep a ham radio rig at the cabin and enjoy talking on it from there because there is no local noise and the antenna I have on the ridgeline can hear EVERYTHING. Yes, it is 800 feet of feedline, nearly 3.6dB of loss at 14MHz...it's still amazing.

BUT, every now and then the possums find the feedline and gnaw a hole in it. All I have to do is hook up the VNA, run the TDR feature, and it tells me to the foot where the damage is and I can go fix it.

I paid over $400 for my VNA. $50 for that functionality is peanuts.

[stimpsonjcat]

Here's a TDR plot of the functioning antenna on the ridgeline:



Here's a TDR plot of the same antenna and feedline after a possum eated the feedline!:



Aforementioned eated feedline!:

[Paul L]

Originally Posted by stimpsonjcat

Full disclosure...NO...I have not used this exact analyzer. But I have trusted ham friends who have.

The analyzer has two features helpful for troubleshooting not just AIS, but also VHF and HF (SSB) antenna installations.

First useful feature - SWR frequency sweep chart

This is a graph with SWR on the vertical axis and frequency on the horizontal. Here you can see where an antenna (well, usually an antenna AND it's feedline) are resonant. The graph is better than just knowing the SWR number the radio sees because it tells you if the antenna is electrically too long or electrically too short. Which allows you to lengthen or shorten the antenna to get the best SWR.

I should stop for a moment and point out that SWR is an offset from the ideal resistive load of 50 ohms in this case. This is important because folks often think that you can 'fix' SWR with feedline, but that is not true. As you move down a feedline the SWR simply rotates in a circle around the ideal based on the offset from the ideal it started with.

Sorry...that probably didn't help.

Second useful feature - TDR (time-domain reflectometer)

So to understand a TDR, you need to understand that any transmission cable is designed to be a consistent impedance all along its length. People get confused about this because they think of it in terms of DC impedance. i.e. what you would see if you attached a volt-ohm meter to the cable. When we talk about impedance in a transmission line we are talking about it's impedance at RF frequencies...not DC.

So, backing up, your antenna is supposed to be a 50ohm load. It is supposed to be connected to coax that has a 'characteristic impedance' of 50 ohms. And the cable is supposed to be connected to a radio whose output impedance is...you guessed it...50 ohms. If all of that happens then the SWR is 1:1. Yay!

Remember, SWR is the ratio of FORWARD to REFLECTED power. So to have an SWR of 1 means ALL of the power is going OUT and none is coming BACK. The ideal.

So the TDR feature allows this device to do something very useful. It can send a signal down the coax, at the appropriate frequency, and watch for reflections of power coming back. If it sees any, it measures the TIME it took for that reflection to return and it then knows how far it was to the impedance mismatch that caused the reflection.

So, if you know how many feet of coax you have to the antenna, you can disconnect the antenna, connect a 'dummy load' (comes with the analyzer) and run a test to see if the impedance of your coax (including any connectors along the way) matches the 50 ohm spec and if the analyzer notes a 'problem' it will tell you EXACTLY how many feet/meters away that mismatch is!

I have a cabin in the north Georgia mountains that is about 800 feet from the ridgeline of a mountain at 1800 ft elevation. I keep a ham radio rig at the cabin and enjoy talking on it from there because there is no local noise and the antenna I have on the ridgeline can hear EVERYTHING. Yes, it is 800 feet of feedline, nearly 3.6dB of loss at 14MHz...it's still amazing.

BUT, every now and then the possums find the feedline and gnaw a hole in it. All I have to do is hook up the VNA, run the TDR feature, and it tells me to the foot where the damage is and I can go fix it.

I paid over $400 for my VNA. $50 for that functionality is peanuts.

Thanks for the writeup. Next time I'm back the states I'm going to pick one up.

[S/V Illusion]

Originally Posted by Wotname

I'm curious - why wouldn't it be suitable for measuring coax insertion loss at a 25W level?

The primary reason is the expected loss along a 50+/- ft run of coax at VHF would be only a few watts which is not within the level of accuracy/precision of this (and most) SWR meters.

[Wotname]

Originally Posted by S/V Illusion

The primary reason is the expected loss along a 50+/- ft run of coax at VHF would be only a few watts which is not within the level of accuracy/precision of this (and most) SWR meters.

Well I accept that it wouldn't be as good as certified / calibrated RF meters but I reckon it is good enough to chase down most VHF defects VHF onboard. One doesn't need pin point precision and with a scale division of 2 W, relative power differences of that order can be observed with this meter.

However you may prefer to use better equipment and it isn't my intention to convince you otherwise.

[S/V Illusion]

Originally Posted by Wotname

Well I accept that it wouldn't be as good as certified / calibrated RF meters but I reckon it is good enough to chase down most VHF defects VHF onboard. One doesn't need pin point precision and with a scale division of 2 W, relative power differences of that order can be observed with this meter.

However you may prefer to use better equipment and it isn't my intention to convince you otherwise.

Not sure you understand what I wrote. It’s not about “pinpoint precision”.

Given the lack of accuracy of this meter, it’s equally possible to obtain measurements which are misleading, e.g., the lack of accuracy may appear as more power at the antenna than at the RF output of the transceiver for example. There is no way to be confident in the measurement.

If someone wants to measure loss due to attenuation or fault, all I suggest is there are better ways which can provide reliable information.

[Silverfox]

Guys, I wanted to update you with the outcome following your help and advice.

On the marina VHF net yesterday I put out a request for the loan of an antenna. Within 10 minutes I had a Vtronix emergency antenna and an SWR meter on loan!

Loads of testing and combinations revealed the following:-

1. My below deck antenna cable connection was not perfect so I cut it out and made a soldered connection.
2. This reduced my SWR from 5.1:1 to 4.7:1. Not ideal, I clearly still have a problem, but at least my MFD alarm doesn't go off all the time now.
3. I tested with the emergency antenna and got a reading of 1.1:1 which proved to me that it's not the AIS or the splitter.
4. I had noticed that my VHF had been switching automatically from hi to lo power in the first couple of seconds of transmitting. I had thought it was a VHF fault but I now guess that its the VHF protecting itself from the SWR problem. Interestingly this problem did NOT occur when the SWR meter was connected. It was cured by replacing the cable between VHF and splitter even though they both tested the same with a multimeter and I could find no fault. (The "problem patch cable" was around 30" long and the replacement was maybe 2" shorter - other than that the appeared physically and electrically identical.)
5. Tests with the VHF and the SWR meter delivered a result of 1.7:1 significantly differently to the 4.7: 1 being shown by ProAIS 2 software connected to the AIS.
6. Bottom line is I can now transmit with the VHF on full power. My AIS can see targets 20 miles away (instead of just 3). but I am still to prove my AIS is effectively transmitting and an SWR of 4.7:1 is clearly not going to help, so I will replace the antenna and cable before I go sailing again. (If I am going to have to climb the mast, I want to do it just once and be confident it is fixed!)

Thanks once again for all the help.

[Cpt Pat]

Originally Posted by S/V Illusion

You can verify the precision of this cheap SWR meter online but I suspect it isn't a very reliable way of measuring power at the 25 W level or lower precisely enough to differentiate power by insertion at both ends of the coax to measure loss.

Parenthetically, simple attenuation along the coax run wouldn't cause the symptoms described.

Even a cheap meter is valuable for taking relative readings. If it incorrectly reads "20 watts" instead of an actual 25 watts at the transmitter, and "10 watts" instead of an actual 15 watts at the antenna end of the coax, you still know you are losing a significant 3 dB somewhere in the transmission path.

I am trying to give advice using inexpensive equipment that could be reasonably purchased and carried aboard. I doubt many skippers will have a quality time-domain reflectometer (TDR) available.

[S/V Illusion]

Originally Posted by Cpt Pat

Even a cheap meter is valuable for taking relative readings. If it incorrectly reads "20 watts" instead of an actual 25 watts at the transmitter, and "10 watts" instead of an actual 15 watts at the antenna end of the coax, you still know you are losing a significant 3 dB somewhere in the transmission path.

I am trying to give advice using inexpensive equipment that could be reasonably purchased and carried aboard. I doubt many skippers will have a quality time-domain reflectometer (TDR) available.

Last time - the lack of precision herein means the meter can read +/- x, I.e, + or - 15% of the scale or an error of 15 watts on a 100 watt scale - and vary each time it is read for a total error of 30 watts between high and low reading. Useless in terms of what you suggest.

The OP found his bad coax connector so hopefully this dead horse can rest now.

[Wotname]

Originally Posted by Silverfox

Guys, I wanted to update you with the outcome following your help and advice.

On the marina VHF net yesterday I put out a request for the loan of an antenna. Within 10 minutes I had a Vtronix emergency antenna and an SWR meter on loan!

Loads of testing and combinations revealed the following:-

............. It was cured by replacing the cable between VHF and splitter even though they both tested the same with a multimeter and I could find no fault. (The "problem patch cable" was around 30" long and the replacement was maybe 2" shorter - other than that the appeared physically and electrically identical.)
......

Thanks once again for all the help.

Thanks for the update; it is good news! Well done for tracking down the first problem. I'm sure you will find the rest of the problems next time - stay good friends with the SWR meter (and it's owner ).

The emphasised section shows how deceiving it can be to use a standard multimeter to diagnose RF faults. For the record, the "problem patch cable" was only electrically identical DC wise and no where near identical electrically at 150 MHz of AC. The multimeter is a good tool for diagnosing DC and low frequency AC but hopeless for RF (unless looking for an open circuit ).

EDIT: get yourself (or make) an 50 Ohm dummy load as explained upthread by other posters - it replicates a perfect antenna and is handy essential (IMO) for tracking down RF defects.

[stimpsonjcat]

Originally Posted by Silverfox

so I will replace the antenna and cable before I go sailing again. (If I am going to have to climb the mast, I want to do it just once and be confident it is fixed!)

Thanks once again for all the help.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Seal the antenna to coax on the mast with that and worry no more.

The odds are you have water in the coax.