Antenna Choice / SWR: Do They Matter?

[marcspaz]

 

I had a very strange experience this weekend.  I wanted to share some information about antenna performance and SWR, in hopes of stimulating conversation and maybe helping debunk some common misconceptions about high SWR breaking radios.  Obviously, there is a limit to this, but with the typical gear and power levels we operate at, it seems to be a non-issue.

 

I have several radios in my Jeep, with a total of 4 antennas, depending on the band and radio being used.  I went offroading this past weekend and took all my antennas off but one small rubber ducky style to use with GMRS communications while on the trails.  I had this small, flexible antenna hooked up for the 5 hour trip to the park on Friday, all day wheeling Saturday and Sunday, and for the whole ride home.

 

While on the trails, I was getting anywhere from 100 yards to 1/4 mile of range out of my radio.  Most of the time, I was only using 5w, but when the terrain would cause issues, I would crank up the power to 50w.  This is pretty standard while offroad and using a flexible antenna.  Sunday, on the way home, I noticed I was getting less than a mile out of the radio.  That had me a little concerned, but not enough to pull over and check on anything.

 

We stopped for gas, so I decided to see if the antenna was loose and maybe check the SWR, to make sure everything was okay.  However, when I went to check on the antenna, I realized the rubber ducky antenna was on the wrong mount.  I had used my radio all weekend, running as much as 50w, with no antenna whatsoever on my GMRS radio.  100% of all of my communications for 3 days was all with no antenna, just radiant energy from the end of the coax... that's it.

 

I put the antenna on the right mount and used it for the rest of the ride home... only about an hour.  I got home and tested the radio.  Everything was 100% perfect.  The radio power output is still good, even after a 5 minute key-down on the dummy load.

 

So, while this is 100% non-scientific, I was still able to get about 50% of my typical range in the terrain I was in, even without an antenna.  With no antenna and obviously sky-high SWR, running an occasional 50w for 3 days, there was zero damage to my radio or coax.

 

Going back to the thread title, does antenna choice and less than perfect SWR matter?  I feel like the community may be overly sensitive to this issues.

 

Thoughts?  I am looking forward to some friendly discussion. 

 

[SvenMarbles]

That kind of puts things in perspective as people fuss over .2 in SWR lol. 
 

I suppose that radios nowadays are robust enough to just pretty much handle anything..

 

[SteveShannon]

6 hours ago, marcspaz said:

Going back to the thread title, does antenna choice and less than perfect SWR matter?  I feel like the community may be overly sensitive to this issues.

Less than we typically predict. I have said a few times that an SWR of 2.5 or so wasn’t the kiss of death commonly thought.  However, I’ve also attempted to transmit eight blocks down my street on 2 meters at 20 watts, using a Midland MXTA26 antenna, which is strictly a GMRS antenna and has a high SWR (I don’t remember for certain how high now but maybe up around 10:1) at 2 meters and found that I was unable to be heard. 

One article that I reread from time to time in on the ARRL site has influenced my understanding of high SWR. It purports that reflected power, no matter how high, eventually reflects again at the radio, this time back towards the antenna. Each time a portion is radiated by the antenna and a portion of reflected power bounces back and forth .At the antenna for the second (or third or whatever) a portion reflects again. Each time some of the signal is attenuated in the cable between the radio and antenna, converted to heat rather than RF radiation. With a no loss cable eventually all of the RF is emitted from the antenna, even if the antenna is high SWR. Unfortunately, there’s no such thing as a zero loss cable, but shortness and higher quality help.  The best available to a radio operator is to have no cable at all, such as an antenna directly connected to the radio, as in a handheld radio.  This (perhaps only in my opinion) is why handheld radios, including cell phones, can get by with SWRs that are up around 5:1 or 6:1.  It isn’t really that measuring the SWR of a handheld’s antenna is difficult. It’s that it matters little.

[WRYZ926]

As long as the radio has fold back protection that works properly then a high SWR should not hurt the radio. Now if the fold back protection is not working or the radio does not have that, then a high SWR can and will blow the finals. When a radio folds back, it cuts the power way down to protect itself from a high SWR. Most radios will cut the power back to 1 watt or less.

[Socalgmrs]

Ummmm.  Good to know that in your specific case with your specific radio, the high swr did not immediately fry your radio. However I would imagine the radio is damaged just not enough to notice at this time .  Do that every weekend and after a period of time and I’d bet money your radio will suffer.   As well, doing that or even less with a radio with out protections most likely will damage the radio very quickly.  

[WRXB215]

1 hour ago, WRYZ926 said:

fold back protection

I like to refer to this as a radio's "sense of self preservation." 

[WSFG777]

I think anyone who has played with radio very long has done that.  Not good, but the radio designers know it’s going to happen and try to protect against it.  I have a Yaesu 2 meter mobile where I kept transmitting into an antenna which I didn’t know had bad connector, now that radio won’t receive… I’m not 100% sure it’s because I transmitted into an open load, but I expect it is.

thank you 

Joe K1VW/WSFG777

 

[WSAA254]

All interesting data points.  Just to add something, years ago (50) I had a hw-16 (all tube cw only).  Did not have a swr meter,

but was working on 80m at the time.  I probably used this for two years, then got a swr meter, (hw-16 fed a dipole) checked it, and

saw a swr of 6:1.   Very bad, however at the time I was in TN and contacted someone in SC at the time.  6:1 is not good of course, sold

the hw-16 later, the guy who bought it had the tubes checked out, and the final output tube was a little "weak" but all still worked.  So I agree,

bad swr numbers are not the kiss of death.   low is good, but if not its not the end of the world..  All the best  regards

[SteveShannon]

11 minutes ago, WSFG777 said:

I think anyone who has played with radio very long has done that.  Not good, but the radio designers know it’s going to happen and try to protect against it.  I have a Yaesu 2 meter mobile where I kept transmitting into an antenna which I didn’t know had bad connector, now that radio won’t receive… I’m not 100% sure it’s because I transmitted into an open load, but I expect it is.

thank you 

Joe K1VW/WSFG777

 

Google the service manual.  Most transceivers have a transmit receive relay that connects the transmitter to the antenna and disconnects the receiver from the antenna whenever transmitting.  That relay is there to prevent damage to the very sensitive receiver circuits while transmitting.  It might be a simple thing to fix.

[SteveShannon]

2 hours ago, WRYZ926 said:

As long as the radio has fold back protection that works properly then a high SWR should not hurt the radio. Now if the fold back protection is not working or the radio does not have that, then a high SWR can and will blow the finals. When a radio folds back, it cuts the power way down to protect itself from a high SWR. Most radios will cut the power back to 1 watt or less.

That’s the conventional wisdom, but I’m curious what the actual failure mechanism is. Is it an excess of voltage or an excess of current that destroys the finals?
We all oversimplify it by talking about “reflected power” but in reality both voltage and current are reflected by an imperfect impedance match.  Depending on the length of the transmission line, the reflected voltage or the reflected current will additively combine (constructive interference) at specific locations along the transmission line. If your transmission line is just the right length the voltage (or at a different length the current) will result in twice the value of the peak forward voltage (or current).  I anticipate that’s what actually causes the destruction of the finals and why it doesn’t happen to everyone in the absence of foldback protection.

[WRHS218]

For years I had no access to a swr meter and just trusted that what ever antenna, single band or multi band, would work for the bands for which they were advertised. Actually I didn't trust anything, I just didn't think about it. My amateur days were utilitarian at best. Wire up a radio, install an antenna that should be good enough and use the radio. Even so, I never had an issue with any radio I ran.

[WRYZ926]

13 minutes ago, SteveShannon said:

That’s the conventional wisdom, but I’m curious what the actual failure mechanism is. Is it an excess of voltage or an excess of current that destroys the finals?
We all oversimplify it by talking about “reflected power” but in reality both voltage and current are reflected by an imperfect impedance match.  Depending on the length of the transmission line, the reflected voltage or the reflected current will additively combine (constructive interference) at specific locations along the transmission line. If your transmission line is just the right length the voltage (or at a different length the current) will result in twice the value of the peak forward voltage (or current).  I anticipate that’s what actually causes the destruction of the finals and why it doesn’t happen to everyone in the absence of foldback protection.

 I would have to agree with what you said. I would have to do some research to give a better answer. And I am sure the type and length of coax does play some part in it.

I recently lost a radio due to a high SWR of 5  to 6. I had a 90 degree coax adaptor go bad and I didn't realize a branch had fallen down and was against my antenna. The combo caused my SWR to jump up and unfortunately I didn't catch it until it was too late. The finals were blown and now the radio only puts out 0.4 watts. Who knows how long that adaptor was bad. 

I trimmed the branches away from my antenna and removed the adaptor and my SWR went back down to 1.4 at 467.600 and 1.7 at 462.600.  I am running a Comet CA-712EFC with 35 feet of LMR400 to the wall and another 16 feet of LMR400 in the shack.

Needless to say I removed all of the other 90 degree adaptors from my other coax cables just to be on the safe side. The adaptors were either from DX Engineering or Max Gain. So while they weren't top of the line, they weren't Amazon junk either.

1 hour ago, Socalgmrs said:

Good to know that in your specific case with your specific radio, the high swr did not immediately fry your radio. However I would imagine the radio is damaged just not enough to notice at this time .  Do that every weekend and after a period of time and I’d bet money your radio will suffer.   As well, doing that or even less with a radio with out protections most likely will damage the radio very quickly.  

This is very true. As in my situation, I have no idea how long I was transmitting with that bad adaptor. It was definitely long enough to take the finals out in my radio.

[marcspaz]

3 hours ago, WRYZ926 said:

As long as the radio has fold back protection that works properly then a high SWR should not hurt the radio. Now if the fold back protection is not working or the radio does not have that, then a high SWR can and will blow the finals. When a radio folds back, it cuts the power way down to protect itself from a high SWR. Most radios will cut the power back to 1 watt or less.

 

I have seen the protective circuits in action on a meter... watching the power get rolled back as the mismatch increases or as heat increases.  Even some of my cheap radios seem to have some type of protection.

 

2 hours ago, Socalgmrs said:

Ummmm.  Good to know that in your specific case with your specific radio, the high swr did not immediately fry your radio. However I would imagine the radio is damaged just not enough to notice at this time .  Do that every weekend and after a period of time and I’d bet money your radio will suffer.   As well, doing that or even less with a radio with out protections most likely will damage the radio very quickly.  

 

I would agree, there is likely a limit to how long I can do that for before it does indeed break.  Thankfully, its not a 30% duty cycle for 3 days.  LOL

 

1 hour ago, SteveShannon said:

That’s the conventional wisdom, but I’m curious what the actual failure mechanism is. Is it an excess of voltage or an excess of current that destroys the finals?
We all oversimplify it by talking about “reflected power” but in reality both voltage and current are reflected by an imperfect impedance match.  Depending on the length of the transmission line, the reflected voltage or the reflected current will additively combine (constructive interference) at specific locations along the transmission line. If your transmission line is just the right length the voltage (or at a different length the current) will result in twice the value of the peak forward voltage (or current).  I anticipate that’s what actually causes the destruction of the finals and why it doesn’t happen to everyone in the absence of foldback protection.

I think there are two possible answers... one is excessive heat causing the materials to break down.  Sweating a chip is always bad.  LOL  The other would be a voltage peak exceeding the PIV / reverse breakdown voltage, causing a collector-emitter or base-emitter current leak.  In either case, we are breaking down the PN junction enough to allow current leakage and nuking the radio.  I would think as long as the current levels stay low, we can get lucky and just have to replace the PA.

[SteveShannon]

12 minutes ago, marcspaz said:

I think there are two possible answers... one is excessive heat causing the materials to break down.  Sweating a chip is always bad.  LOL  The other would be a voltage peak exceeding the PIV / reverse breakdown voltage, causing a collector-emitter or base-emitter current leak.  In either case, we are breaking down the PN junction enough to allow current leakage and nuking the radio.  I would think as long as the current levels stay low, we can get lucky and just have to replace the PA.

 

Agreed.  And actually both excess heat from current and breakdown from excess voltage could both be happening.  Even though peak voltage leads peak current by a quarter wave when they leave the transmitter, off peak current or voltage can still combine to exceed specs for the transistors.  This is one time where a very lossy transmission line is a benefit!

[LeoG]

There is no load on the radio.  Is there any chance for damage?  A high SWR produces power that backfeeds into the radio and that's what hurts the output stage.  If there is no antenna there is there the possibility of a backfeed even occurring?

[dosw]

8 minutes ago, LeoG said:

There is no load on the radio.  Is there any chance for damage?  A high SWR produces power that backfeeds into the radio and that's what hurts the output stage.  If there is no antenna there is there the possibility of a backfeed even occurring?

In the case of no antenna the pin at the end of the feedline connector, and the feedline itself becomes the antenna. In the case of no antenna, the pin in the center of the radio's antenna connector becomes the antenna. In either case, I would assume a very high SWR. You could test that first scenario by putting a farsometer between the radio and the feedline, with no antenna attached, and observe.

My Surecom isn't handy, but if I turn on my NanoVNA with nothing connected to the S11 connection, and set the display to SWR, it registers infinity, which may not be very good for a transmitter. When I put a section of 15 feet of RG58U on it, I get something like 70:1.

[wayoverthere]

I'll bring up the theory that with modern gear more able to self-protect, high SWR is less of a danger than it used to be, especially with good quality gear.

not long after i switched trucks, i moved my comet SBB5 ham antenna from a fender bracket on the ranger, to a fender bracket on the new Ram.  it was great on the ranger, 1.1-1.3 across all of 2m and 70cm.  few nights later, chiming in on the local linked repeater, and get a report that running mid (usual) or high power, i'm VERY scratchy, almost unreadable, even trying the other two other repeaters in the area (2 uhf & 1 vhf, which I can usually get into on a handheld with a rubber duck without issue).  check SWR, and find 2.8 on UHF, and over 3 on VHF, which explains the poor signal. Hooking it up to the analyzer showed SWR is all out of whack on 2m and 70cm, but it was weirdly resonant on 220 (like 1.2 or so). Testing some other whips I had on the same mount, the 2x4SR is stellar, so I'm rolling with that.

the only explanation i can see there is where the sbb5 sat fully above the hood on the Ranger, the shape of the hood on the Ram has the base sitting about an inch below the main "plane" of the hood; the radio (FTM500) has shown no ill effects after the fact. In the end, the 2x4 is the only one that reads good on that fender bracket on the Ram (though the 1/2 wave signal stalk didn't read good on the Ranger, either...tuning that is one of those ongoing to-do list items).

Edited October 25 by wayoverthere
minor clarifying

[nokones]

That is exactly what the "Queen" was trying to convey in one of his videos several months ago.

[GreggInFL]

As an aside, this is the kind of thread that makes this forum so valuable.

You guys rock.  I love this bar.

[piggin]

15 hours ago, marcspaz said:

 

I had a very strange experience this weekend.  I wanted to share some information about antenna performance and SWR, in hopes of stimulating conversation and maybe helping debunk some common misconceptions about high SWR breaking radios.  Obviously, there is a limit to this, but with the typical gear and power levels we operate at, it seems to be a non-issue.

 

I have several radios in my Jeep, with a total of 4 antennas, depending on the band and radio being used.  I went offroading this past weekend and took all my antennas off but one small rubber ducky style to use with GMRS communications while on the trails.  I had this small, flexible antenna hooked up for the 5 hour trip to the park on Friday, all day wheeling Saturday and Sunday, and for the whole ride home.

 

While on the trails, I was getting anywhere from 100 yards to 1/4 mile of range out of my radio.  Most of the time, I was only using 5w, but when the terrain would cause issues, I would crank up the power to 50w.  This is pretty standard while offroad and using a flexible antenna.  Sunday, on the way home, I noticed I was getting less than a mile out of the radio.  That had me a little concerned, but not enough to pull over and check on anything.

 

We stopped for gas, so I decided to see if the antenna was loose and maybe check the SWR, to make sure everything was okay.  However, when I went to check on the antenna, I realized the rubber ducky antenna was on the wrong mount.  I had used my radio all weekend, running as much as 50w, with no antenna whatsoever on my GMRS radio.  100% of all of my communications for 3 days was all with no antenna, just radiant energy from the end of the coax... that's it.

 

I put the antenna on the right mount and used it for the rest of the ride home... only about an hour.  I got home and tested the radio.  Everything was 100% perfect.  The radio power output is still good, even after a 5 minute key-down on the dummy load.

 

So, while this is 100% non-scientific, I was still able to get about 50% of my typical range in the terrain I was in, even without an antenna.  With no antenna and obviously sky-high SWR, running an occasional 50w for 3 days, there was zero damage to my radio or coax.

 

Going back to the thread title, does antenna choice and less than perfect SWR matter?  I feel like the community may be overly sensitive to this issues.

 

Thoughts?  I am looking forward to some friendly discussion. 

 

I have a magnet mount that came with the midland 500. 6.6 to 1 with no antenna. 12 feet of lossy rg178 with nothing attached was 3.3 to 1.

 Some mobile magnet mounts have fairly high loss in the long provided coax. Measure yours with a active swr device and you might be surprised!

I don't have any idea what the power amp is in my midland but if it is like other radios it probably has an rf power module in it. The ones in my Icom 2730A are rated for 60 or 65 watts max but run at 50 watts. I mention this because the modules are rated for 20 to 1 swr no damage at over 60 watts!!!!

Very rugged. Good practice also will reduce drive if high SWR is detected. I know nothing about how the midland is built or any other GMRS radio is made but...I would be surprised if the makers reinvented the wheel to make mobile radios. The modules are economical and reduce parts count. They are probably pretty robust! And lossy cable will radiate some...which is why you could use it at all. Great story!

[AdmiralCochrane]

Voltage vs amperage doesn't matter, it's the wattage/heat.  That is the primary reason you can (almost always) get away with it for short transmit cycles.  I don't recall a damaged final that appeared to have pure breakdown due to overvoltage.  Also pretty sure if it was voltage that mattered, there would be more instantaneous damage vs damage over time (heat buildup).