The Myth of SWR #2

[@MrTommy001]

All I can say, as a relatively new HF ham is, I love this guy and his myth-busting descriptions.

[@Charlie-n7i]

Just newly subscribed; and, I have to share... Funny? 15-some years ago, when I was relatively new to Ham Radio, after battling the SWR and almost going insane trying to get a 1.5 or less SWR if possible, I researched the magical Antenna Tuner! I got one, put it inline and followed all the instructions from the owner's manual. A fellow Ham across town about 10 miles away on 10 meters without being solicited for a QRST (signal report) explained excitedly that my signal came up 2 db(!) and wanted to know what I had done. For 2 weeks I played the Hitchcock game with him before I confessed that I'd put an Antenna Tuner inline. It was one of those MFJ types for HF. And, my antenna that I was using from an apartment balcony was the Hustler 6BTV on the top floor of a 15-story apartment building where I lived at the time. KD8EFQ/73

[@martinpe9tig]

very well explained OM Sir even for me , the power isnt lost the ATU saves the output

[@Larry-AK0Z]

Absolutely true. Thank you for sharing!

[@paulm0hpd319]

You are still making waves Mark, ive seen several videos and comments on them trying to debunk what you have said in your videos, unfortunately its obvious they misunderstand you

[@BustRadioMyths]

They misunderstand science

[@paulm0hpd319]

@@BustRadioMyths you are not wrong, it's laughable really as they are making videos to try discredit you yet they have clearly misunderstood what you have said

[@LeopoldVDH]

Thanks Mark for clarifyi!g this much debated subject

[@stevegordon2869]

This video makes me just use my g5rv and smile.

[@jimwebb6564]

Sir, Thank you for your informative video. It has prompted me to listen to a number of other videos and writings on this and related subjects. I looked for Walt Maxwell's "Reflections III", but found quickly it was at least $100 for a new copy and $85 for a "fair condition" used copy. I intend to buy it but it may have to wait for a while. So I wonder if you could answer a question for me: You state that "reflected power eventually radiates". I've been unable to find an explanation of how this occurs. Does this eventual radiation occur under all conditions? Does the power radiate from the antenna, the feed line or both? What does "eventually" mean here, milliseconds, seconds, etc.? Any information you could share on this question would be much appreciated. Thank you! Jim Webb W4NTA

[@BustRadioMyths]

Hello Jim. Here is a link to Reflections. www.k3emd.com/downloads/Reflect.pdf Since reflected power is not lost it gets radiated. It's like asking, why does RF sent to an antenna get radiated? Because there's no where else for it to go! As for when reflected power gets radiated, Reflected energy flows back and forth between the mismatches at the source and load. After a few such journeys, the reflected wave diminishes to nothing, partly as a result of finite losses in the line, but mainly because of partial absorption at the load each time it reaches the load. In fact, if the load is an antenna, such absorption at the load is desirable, since the energy is actually radiated by the antenna. Reflected power does not get radiated from coax because it is shielded cable and shielded cable does not radiate, except in the case of common mode current. See my video on Does SWR Make Coax Radiate for an explanation if you are not familiar with common mode current. It's too bad the ARRL stopped publishing Reflections. There was a falling out between Maxwell and some at the ARRL.

[@jimwebb6564]

@@BustRadioMyths Thanks very much! What a great resource!

[@artbremer4076]

So, the way I'm comprehending it, as long as I have a proper quality ATU/transmatch in line, I shouldn't be worrying about neither VSWR nor my coax feedline length. However, if I would like to be as portable as possible but be the most efficient as possible and I don't want to bring a tuner or I don't have one: can I cheat by cutting my coax feedline to 1/2 wavelength and achieve a close match with my antenna's Z this way? I might not reduce the VSWR but with the absence of a tuning device , perhaps, I can fool the radio to "see" the coax Z and the load Z as the same due to 1/2 wavelength cycle properties... However, I feel like if it works, I will be restricted to one frequency and one frequency only ( the one my antenna happens to be resonant at on a selected band)

[@BustRadioMyths]

VSWR is determined only by the match at the antenna feed point. It will be about the same anywhere along the line unless you have common mode current or a long lossy line. So the half wave idea isn't really of any practical value. And without a tuner you are stuck with a narrow range of frequencies on one band. One location will probably differ greatly with another. Antenna tuners can be small and very light. Trying to go portable without one could be a problem. kzbin.info/www/bejne/r6S7gXqIoqumoLc

[@crazysloth9]

Does the reflection ever cause phase cancellation?

[@BustRadioMyths]

It causes phase reversal. When the wave hits the antenna phase is reversed 180 degrees. When it arrives back at the tuner it is reversed again by reflection. Now it’s in phase with transmitter power.

[@g0fvt]

Although I agree the pivotal word is "reasonable" if you take a case for example of a halfwave dipole cut for 40m and feed it with 100ft of RG58. All works well, but then you go looking around the 20m and 10m bands with the same antenna. An absolute disaster for feeder loss... I did exactly this. Taking the antenna down to change the feeder to open wire gained about 4 s-points on those higher bands. My latest antenna is a fan vertical with about 50ft of RG213, I never fitted elements for the 30m and 60m bands but it is still very effective on those bands. It is worth remembering too that SWR as measured at the shack will be lower than that at the antenna...

[@BustRadioMyths]

I would never use RG-58. RG-8x is about the same size and better coax.

[@g0fvt]

@@BustRadioMyths to be fair RG8X is not great either, plus I tend to use cable that I get for free!

[@BustRadioMyths]

Free? Absolutely!!!! I'm a cheap ham.

[@g0fvt]

@@spaceflight1019 that is a separate issue, though of course that technique can also increase common mode current.

[@DaDitDa]

Also, with VSWR on the feedline, the impedance (r+jx) at the input port of the feedline will not be the same as present at the output port (i.e., feedline-antenna interface) -- unless your feedline is electrically 1/2 wavelength or a multiple thereof.

[@ouijim]

Wish RF Choke was discussed more as essential equipment as RF can cause problems in Shack with Computer and other devices. I always use Antenna Tuner and RF Choke inline. Also using TrueLdderLine with Doublet Antenna at primary residence, amazing results. ; ) Thank You

[@BustRadioMyths]

Thanks for sharing. I have a video coming out in a couple weeks about using baluns and chokes to stop common mode current. Check it out!

[@paulm0hpd319]

@@BustRadioMyths I look forward to that one as a couple of recent videos I've watched have got it wrong

[@BustRadioMyths]

Right I watched one video where the guy said reflected power is also common mode current. Completely wrong.

[@paulm0hpd319]

@@BustRadioMyths that's a common misconception even some experienced radio amateur influencers are promoting this

[@timreisinger5377]

I'm confused about a few things: 1. If reflected power ping pongs back and forth, wouldn't the coax loss occur each time it travels from one end (transmatch) to the antenna feed point, and wouldn't this coax loss rapidly add up over many cycles (trips back and forth) to mostly dissipate as heat in the coax? 2. If a transmatch adjust the phase of the reflected wave to match the forward power, what effect does that have, if any, on how and when it eventually gets radiated? How can reflected power ever get radiated if the mismatch at the antenna feed point is always sending it back?

[@BustRadioMyths]

From antenna engineer Walt Maxwell: "Contrary to our prevalent, deeply ingrained belief, it is therefore not true that when a transmitter delivers power into a line with reflections, a returning reflected wave always sees the internal generator impedance as a dissipative load and is converted to heat and lost. It can happen under certain conditions of pulse-type transmission; for instance, if the generator is turned off after delivering a single pulse into the line while retaining its internal impedance across the line, the returning pulse wave will be absorbed. But if a conjugate-matched generator is actively supplying, power when the reflected wave returns, the reflected wave encounters total reflection at the conjugate match point and is entirely conserved, because it never sees the generator resistance as a dissipative terminating load. This is because the source and reflected voltages and currents superpose, or add at the match point, just as if the reflected power had been supplied by a separate generator in series with the source. And since the source voltage is generally greater than the reflected, the sum of their voltages yields a net current flow which is always in the forward direction. The reflected power adds to the source power, deriving reflection gain which compensates for the reflection loss suffered at the mismatched termination." Additional reading: www.iz2uuf.net/wp/index.php/2017/07/29/the-myth-of-reflected-power/

[@johnarcher9480]

Yes, but it can be an acceptable loss. If you have say 3dB loss (pretty high) and 25% of power is reflected back, 100w out turns to 50w at the antenna. 25% reflected back becomes 12.5, re-reflected to the antenna. At the antenna, the coax has again taken 1/2 so 6.25 to the antenna. 75% radiated 4.7w is added to the 37.25 that got radiated the first time. 1.5 goes back .75 gets to the radio and .38w back to the antenna. With a lower loss setup (say 10% loss) 90 gets to the antenna 22.5 comes back 20.25 gets to the radio/tuner, 18.23 gets back to the antenna 85.73 has now gone out the antenna. Another trip gets you another 2.77 out. So you get out over 88w Much better than the 42ish watts of the first system.

[@kingduckford]

Let us say that I want to use a 500 watt amplifier in my car for mobile use. Is it more practical to carry a small watt meter capable of reading my reflected power, or a cumbersome 600-800 watt tuner, that is also more fragile? What happens if I have a few ham sticks on my car, and I would prefer to not have to manually retune my MFJ 945E every time I switch antennas? Perhaps chasing a flat SWR has a value in these circumstances. The other example I can think of is, people with random wires and other extreme mismatches. I've read a a few threads on QRZ about people chasing endless bugs and RFI problems in their house or shack after trying to use an impedance matcher to make the antenna work, because it seems like some systems end up balancing themselves out in strange ways, or the extreme stand waves will seem to cause other problems. I think the bigger issue with SWR isn't loss of performance power, it is all the other problems and potentials that go with it. I still stick with the notion that a finely tuned antenna is best, because the tuner is always better at cleaning up minor SWR problems, than it is at trying to force a poor antenna to function. A finely tuned antenna can still get on the air, even if you have a tuner failure. Obsessing with flat SWR is one thing, but having a well tuned trap or monoband is still a better place to work from, then to rely on the tuner to do all your heavy lifting. One last complaint on tuners. If I use my tuner to get low SWR for 10 meters on my 5/8ths' vertical roof antenna, which is tuned for CB, my ALS 1306 refuses to operate on it. It reports that there is "too much 11 meter radiation" or something to that effect, another problem not solved by a tuner, but that a tuned antenna can cure. Just because you can get on the air, just because you can do fine work, just because you don't lose that much ERP, doesn't mean there aren't other issues SWR causes that a tuner does not cure. But, yes, I agree with you, and this is fine work. Thank you for making these.

[@paulm0hpd319]

I am wondering what other issues swr causes

[@BustRadioMyths]

Good points about mobil operation. I'd think a screwdriver antenna or a trunk mounted automatic tuner would be good for keeping the transmitter happy. As for feed line radiation (RF in the shack) that is caused by common mode current, not SWR. The treatment for that is at least one coax choke and a balun at the feed point. Coax is unbalanced so it's always good to use a balun at the feed point.

[@kingduckford]

@@BustRadioMyths I've been properly choking things very heavily for a few years now. It certainly kills noise and does a lot of good for the radio setup. Yet, even that seems to fail to fix some of the few mystery problems I can't solve. And some of them only pop up when I'm pushing the manual tuner to a heavy extent. Perhaps I need to look at my feedlines again sometime.

[@BustRadioMyths]

I've had to put RF chokes on a bunch of stuff in the shack. I don't have any problems now. palomar-engineers.com/rfiemi-solutions/ You can get clamp on chokes a lot cheaper on Amazon but I can't vouch for them. As for my antenna, an OCF dipole, I have a 9:1 Un Un at the feed point, a 50 ohm coax choke right after it and then another choke near where the coax goes into the tuner. I can work 160 through 6 meters with no more than 3:1 SWR on any band.

[@kingduckford]

@@BustRadioMyths I've been running Palomar "pipe bomb" inline chokes on each end of each HF feedline in my shack. A big PVC pipe filled with ferrite mix, they work very well. The cheap Amazon ferrites tend to be 43 Mix, and they do work well for CB/10 meter because that's where 43 is at its highest performance. They can be helpful, in great enough bead count, for lower HF, but then again a different mix suited for those frequencies will start to work better. Reading papers on that subject was quite interesting, as is the general subject of RFI. I've been running a Hustler BTV-6, with an immense radial field and good ground conductivity. With a trapped vertical, I've been able to run through my tuner on every band except 40 and 80 meters (no 160 meter on it) with it after taking a little time to tune it. I like it because I can use dual VFO to listen to two different pile ups at the same time, and switch between them without a tune up, especially barefoot. Only thing I have to change is the amp setting between bands if I want extra power. Makes band jumping a breeze, and allows great flexibility. So, like I say, my issue isn't with tuning for maximum performance, it is tuning for maximum laziness.

[@timbookedtwo2375]

Good explanation. I am confused about antenna efficiency. I have seen lots of mathematical stuff about it but no really good straightforward explanation. I am a retired "bean counter" and cannot follow the math. Also, I expect efficiency depends on the material the antenna is made of and its construction, as well as "capacitance and inductance." Could you make a video - or several - about this topic? SV0SGS PS: I mostly use resonant antennas. I do have a couple of non-resonant antennas and put an automatic "antenna tuner" at their feed points.

[@BustRadioMyths]

Antenna efficiency is basically how much power is radiated verses how much goes into it. The rest is rocket science. Above my pay grade. :)

[@DaDitDa]

@@spaceflight1019 Antenna gain is not the same as antenna efficiency. Example: A high gain (i.e., highly directional compared to an isotropic radiator) Yagi antenna can be constructed with poorly conducting materials that make it inefficient; i.e., Power_radiated / Power_applied < 1. But whatever power is radiated by this inefficient Yagi will exhibit gain compared to an isotropic radiator.

[@AndrejaKostic]

@timbookedtwo2375 It's a bit hard to explain without math, but... When you put energy into antenna, some part of it will be radiated out as RF. We use something called "radiation resistance" to model that "loss" of energy. So I'll call that value Rr. Then, and this might be a bit hard to digest at first, all of the real capacitances and inductances are modeled as complex numbers, where (handwaving a little bit, don't look too closely), the real part models the energy lost as heat, and the imaginary part models the energy bouncing around. We add the actual material resistances of antenna and everything in the path, and we get resistance of losses. Let's call that RL. So our antenna efficiency is Rr/(Rr+RL). Therefore, if the radiation resistance is much higher than the resistance of losses, antenna will have high efficiency. If the resistance of losses gets close to the antenna's radiation resistance, then we get large losses. If they're the same, losses are 50%, and if the losses are more than radiation resistance, then they're more than 50%. Now the issue with all if this is the following: We can't really measure the antenna radiation resistance directly, it needs to be calculated by programs or RF wizards, and loss resistance might be low, and could be therefore difficult to measure, and you'd need to measure it when your tuner is in matched state, and it will also be affected by the things such as skin effect. Then, if your tuner is connected to antenna by coax, there's coax loss too. Now all of this is not related at all to the radiation pattern antenna produces.

[@timbookedtwo2375]

@@AndrejaKostic thank you for the explanation. on a practical level how can I determine/estimate the efficiency of my antennas?

[@AndrejaKostic]

@@timbookedtwo2375 So lot's of handwaving here, but the radiation resistance of "simple" antennas depends on the relationship between the length of the antenna and the wavelength we're using it on. So the shorter the antenna, the lower the radiation resistance. There's a table with some values on say English Wikipedia article for radiation resistance. What I did, was I experimented with modelling programs such as 4NEC2, to get an idea of what I can expect for simple antennas. Then, if an antenna is shorter, I expect it to have lower radiation resistance, and if it's longer, it will generally have higher. This leaves us with estimation of losses. Some initial ways of estimating them are to just measure the DC resistance of the antenna (say for dipoles/doublets/monopoles), and then this will give an order of magnitude. If the antenna is made of strong material, one way would be to say put 1 A of DC current through it, and then measure the voltage on it. If you happen to have a thermal camera, this might also show unexpected hot spots on say connections. But for RF, we also have the issue of skin effect and skin depth and so on. There are skin depth calculators on the Internet for various materials, so they might give a sense by how much the DC values need to be derated. All of this going in-depth is important mostly if you're using very short antennas, or magnetic loops and similar stuff. If you have a good (as in low-loss) tuner, and can pump the antenna with lots of power, even when it's short, then the currents through it will increase and all of the usually unimportant loss effects will start mattering. On the other hand, for long antennas, usually you'll have large voltages, and not so large currents. And as always, keep in mind that the actual direction where the antenna is radiating is a separate topic: A long antenna might have gain in directions which might not be very useful and so on.

[@k9eihamradio]

I'm happy that you're taking on the mythos of SWR in ham radio. My concern is that you're only focusing on one issue and not explaining other factors like Earth losses, antenna efficiencies, antenna feed-point impedance, and and surrounding objects in the antenna near field that can cause issues to an antenna system. I think, and, you might agree that 6 minutes just isn't enough time to fully discuss SWR. With that in mind it has encouraged me to read and study this more. Thanks for the references Mark!