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Part 6 - Matching and SWR

SWR or VSWR - These have to be the most widely known yet most misunderstood terms in Amateur Radio!

To prove it I researched and wrote all this and then had to change it on the advice of the great Kurt N. Sterba, the antenna guru of World Radio Online. Kurt kindly set myself, like so many others, on the right track after I asked him to review these pages. Please see the December 2011 edition for the article. Here we go........

Nearly everyone knows the Nirvana of SWR is 1:1 but what does it mean?

Firstly let me guide you to some good reading that will greatly help, as with all my scriblings, I'm just touching the surface and not including all the details, so the more you read the more you will understand.

A great article on what SWR is all about is in the 'Even more out of thin air" book by PW Publishing Ltd. The article, by the late Fred Judd - G2BCX, on whose books a lot of us grew-up with; is titled 'VSWR Problems at VHF' (page 78) but don't let the title put you off. It is a great explanation of SWR not just at VHF. The book is a good investment and at £6.75 a great little present!

The ARRL Handbook also has a lot of information about SWR but these myth busters in Section 21.14 are a must to remember: 

  • A high SWR does not cause interference
  • A high SWR does not cause transmission from the feeder
  • You can get out a decent signal with a high SWR!

I would add two more to that list:

  • An aerial with a low SWR just means you don’t need an ATU to use it
  • The lowest SWR point is not the point of resonance

WARNING: We must be careful here. Your Transmitter hates a high SWR and will tell you so by going POP! Don't connect your Transmitter up to an open or short circuit or try to replicate any diagrams here, they are just to help you understand.

It is all about Matching

Forget about SWR and waves for the moment - they can confuse matters.

We talked about matching before and how if everything was 50 Ohms in the aerial system then everyone was happy. I think I mentioned pipe diameters, well let's talk a bit more about RF.

In the following pictures the feeder has an impedance of 50 Ohm, the same as the Transmitter. Look at the diagram below. There is no antenna!

 

The Transmitter is sending the RF up one wire in the feeder, it can't go anywhere, so it gets reflected back to the Transmitter.

Look at the diagram below. There is a short where the aerial should be...

 

The Transmitter is sending the RF up one wire in the feeder, it hits the short and again gets reflected back to the Transmitter. This may not make sense, why not just go through the short back to the Transmitter? Well I'm afraid it just doesn't, and Fred will tell you why when you buy the book!

OK so they are the two extremes, an open and a short. Let's look at the ideal:

  

Wow - our Transmitter is sending the RF down the feeder and it's seeing the 50 ohm Aerial. Now the two are matched - all the power fed into the aerial will be used by the aerial (hopefully transmitted!)

Let's go a little way from ideal now and have an aerial with an impedance of 100 Ohm.

 

Now this is somewhere between ideal and an Open circuit. Some of the RF will be reflected - in other words, if the aerial impedance does not match the feeder's impedance there will be reflected RF back to the Transmitter.

Reflections at the transmitter

The feeder is an important part of the system and its length can be very important, especially if there is a mismatch at the aerial. The feeder’s impedance at the transmitter gets altered and, depending on the feeder’s length (compared to the RF wavelength), gives a mismatch between the Transmitter and the feeder. As it is not matched anymore, the reflected wave at the transmitter can get reflecting back again to the antenna, this can be good as it will try to enter the aerial again. The reflected wave does not hit your Transmitters PA and warm them up! Kurt explains that......

"When the reflected wave reaches the transmitter the two waves, transmitter power and reflected power, combine to produce a forward wave that is in phase with the transmitter wave. All of the power in the reflected wave is now added to the transmitter power and sent back to the antenna. Note that when the aerial is not an exact match to the transmission line, the power going down the line to the aerial is always greater then the transmitter power. 

But one might ask, if the reflected power does not heat the PA, why do transistors transmitters have protective circuits that shut the rig down when the SWR gets too high? Well, for one thing transistors zapped by too high a voltage. The peak voltage of the transmission line goes up as the square root of the SWR. If the transmitter is at a high voltage point of the line it may be good bye transistors

Also, a low impedance load can cause excessive current to flow. The transistor current can easily double in high SWR situations. So to protect against these problems, the output is automatically reduced when the SWR goers too high. This is voltage and current protection not reflected power protection."

Now what have I added to the diagram, the loss of the feeder. All feeders have a loss (see the spec's for RG58 coax at  X dB/m at 100MHz). Basically it will loose a certain amount of RF at a certain frequency.

When the system was matched, the RF travelled down the feeder and suffered the loss of the feeder once - OK, we wish it didn't but it is the best we can get away with.

When the system is not matched; the RF travelled down the feeder and suffered it's loss once, then some of it got reflected so less RF got to the aerial. Wait, it has to go through the feeder again and loose more RF, and when it gets to the Transmitter it gets reflected again and has to go up the feeder again, more loss.

So if you have an unmatched system with a poor feeder that has a high loss. The RF that can’t enter the Aerial gets bounced up and down the feeder trying to enter the aerial or the Transmitter.

The energy that enters the transmitter can destroy it. The energy that enters the aerial is good. But each time it looses energy in the feeder. So a poor match with a poor feeder will make a very inefficient system.

On the other hand, if you have an unmatched system with a fantastic feeder that has a very low loss. A high SWR will make little difference to the system as the RF will be reflected with little loss and keep hitting the aerial hoping to be accepted! That’s assuming you have a way of stopping it getting into the transmitter! (an ATU perhaps?).

Wow - we got this far with no mention of waves!

So Reflected power is a sign of a mismatch, how do we measure it?

Well there are two ways, you can measure the impedance of the antenna with a fancy gizmo like an MFJ Antenna analyser or a resistive bridge or you could put a meter in the feed line that measures the reflected power. If it’s in the feed line I can sit in my shack and not be up a ladder.

Do we all agree the meter in the feeder sounds a little like a good idea and sounds amazingly similar to an SWR meter? OK let's go for that option.

So we can measure the reflected RF and the forward RF on the feeder with a meter (and a little circuitry). great, let's just leave it there and always try to get the reflected power to be as low as possible. Why is this not the case? There is a simple reason.....

If you can not get rid of that 2 Watts (I making this number up) reflected RF what does it mean. Well in a 5 Watt QRP system it's 40% of the available power but in a 400 Watt system it's only 0.5% of the available power -  we need to compare the reflected power with the forward (or incident) power we are trying to get out.

A good way of comparing things is a ratio. 10 of these for 1 of those; the road slopes up 15 metres in every 100 metres, it is always 15 times taller than me. These are all comparisons against something - a ratio.

Ratios are written as (for the above examples): 10:1;  15:100; 15:1

Hang on - My SWR meter says 2:1 and 5:1 hey, it's showing a ratio!

Below is a nice Forward and Reverse RF meter - notice that there is a scale in the middle where you can measure the SWR where the two needles cross - job done!

So now we know that we want to measure the mismatch, that measuring the reflected power and comparing it to the forward power will give us a ratio we can use to see how well matched our system is - you are measuring SWR!

Well almost!!! The SWR is really the ratio of the Feeder impedance (Zo) to the Terminating (aerial) Impedance (Zt) and is Zo:Zt if the Zo is greater than Zt or Zt:Zo if Zt is greater than Zo.

No mention of RF power there so are our results really correct? Well not necessarily, because we want to measure the SWR in our warm shacks at the end of the feeder we are measuring the reflected power after it has gone through the feeder twice and lost a lot of its energy (if it is a poor feeder) so we could get a low reflected power and a therefore a good SWR from a very poor mismatch and very poor feeder!

But what about the waves?

OK - we got this far without actually saying what the Voltage Standing Wave Ratio is. So do I really need to? Well go on then....

RF is made of Waves - when the RF gets to the end of the feeder and is reflected it travels back along the feeder. Every wave has a peak and a trough (max and min) and the forward wave and reflected wave interact to 'create' another wave on the feeder. If you could see this wave it would appear to be stationary - it is standing and it's a voltage wave. The Ratio is the Ratio of its maximum and minimum values.

So there we have it. The Voltage Standing Wave Ratio (VSWR) is nothing more than a method of measuring the mismatch of the aerial to the feeder.

Now look at how we usually connect up our SWR meter and where our ATU goes. 

 

The SWR meter is there to allow you to adjust the ATU for the minimum SWR/reflected power at which point it should be a match for the feeder i.e. 50 Ohm impedance. This then protects the Transmitter from the reflections coming back from the aerial. The ATU is providing the Transmitter with a match.

The Transmitter is sending its RF into a matched load so it very happy and you don’t have to replace your PA!

The other side of the ATU is a different story – that can have a big mismatch and RF is bouncing back and forwards from the ATU to the Aerial. If the feeder losses are low, the reflection will go back to the aerial and try to be be transmitted.

If the feeder losses are high then all that reflected RF energy is wasted in the feeder.
Is ATU the proper name for an ATU?
Antenna Tuning Unit - well from what we've seen here the ATU does not tune the antenna, it matches the antenna and feeder SYSTEM to the Transmitter impedence.
So is this all it does? No....
Kurt explains that the the Tuner DOES tune the aerial (not quite if there are line losses though). When we use the ATU to match the system we effectively cancel out the the reactances at the antenna feed-point, no reactances leaves just the resistive element and that is the definition of a resonant aerial!
But remember it's primary funtion is to protect your output circuit by matching the antenna system to the transmitter's output impedance.  PA Protector - PAP, do you think that could catch on? ".....I've adjusted my PAP for lowest SWR!"
Summary

Let’s return to what we said in the beginning:

  • A high SWR does not cause interference – quite right, why would it? The RF travelling back in the feeder is no different to the RF going forward. 
  • A high SWR does not cause transmission from the feeder – again correct.  The  reflected RF is contained in the feeder. ONLY when there is an imbalance in the feeder will it transmit and SWR has nothing to with that.
  • You can get out a decent signal with a high SWR! – I hope all this has shown that although some signal will be reflected, if your feeder losses are low it will rebound and try again!
  • A high SWR on coax is a bad thing. Coax has a high loss (compared to twin feeder) and the SWR indicated reflected waves which are attenuated more and more as they travel back and forth down the coax.
  • A high SWR on twin feeder is not a bad thing. Twin feeder has very low loss and so that power travelling back and forth is not attenuated anywhere near as much as it would be on coax. That is why HF broadcast stations use open feeders and not coax.

And the one I added:

  • An aerial with a low SWR just means you don’t need an ATU to use it. It certainly doesn’t mean it’s a good aerial. A large enough 50 ohm resistor as a dummy load will give a 1:1 SWR but nothing radiated so be wary of all these great claims!

    For proof positive have a look at my doublet and tuner post here