SWR, Transmission Lines and Terminations

Well,

Following a discussion at a local radio club, I have been trying to put together a simple demonstration to illustrate standing waves on transmission lines. Now, this can be quite a complex topic, but here I will try and simplify the problem so we can see what's actually going on. There is a great explanation on Wiki here. W2AEW also has an excellent YouTube channel which covers some of this topic very well.

I am going to assume some prior knowledge about transmission lines, but here are the two basic questions that I often hear:

1. Why do I need to match my antenna to my coax and transmitter?
2. What is a good SWR and why does it matter?

Well, in the world of ham radio we match an antenna to a transmission line and a transmitter to minimise reflected waves, let's try and see why reflected waves are a bad thing.

My test setup here comprises a 50 ohm signal generator putting out a 10MHz square wave. We have a short length of 50 ohm coax to a BNC T-Piece and then another length of 50 ohm coax to a BNC connector.

The BNC connector on the end is connected to channel one of my scope (yellow waveforms) and the T-piece is connected to channel 2 of the scope (blue waveforms).

Now, with the 'scope set so that the end of the coax is seeing a 50 ohm load here are the signals that we see on the scope:

The signal at the end of the transmission line (yellow) is much the same as the signal part way along the transmission line (blue). We can even measure the delay from the t-piece part way along my transmission line to the end:

It is clear from the above scope screen that it has taken about 11ns for the signal to move from the T-Piece part way along my transmission line to the end. 

Everything looks exactly as expected because the line is terminated at the design impedence of 50 ohms.

Now, if I change the line termination impedance to be 1M ohm, this is the scope screen now:

Here, two interesting things have happened. Firstly the amplitude of the signal at the end of the line (yellow) has doubled, secondly the signal at the T-piece part way alone the line is very distorted.

Because there is a mismatch (in this example a very bad mismatch), the change in impedance will generate a voltage spike which in turn will create a current flowing in the opposite direction, and that is exactly what we can see.

In the case of the yellow waveform, the signal ariving at the end of the transmission line has started to reflect 100% back down the line, but because we are "looking" right at the end of the line, there is no delay so the two signals simply sum together to double the waveform amplitude.

I've annotated the blue waveform to explain whats happening here:

So here we have at point A the start of the signal traveling from the signal generator to the load, 11 ns later (we measured the time earlier and can see its the same here) we reach point B, at this point the reflected waveform is arriving in the opposite direction on the transmission line and the two sum to make the total amplitude of the waveform. Point C is the end of the outgoing waveform and point D the end of the reflected waveform. This is why we see the two signals adding but slightly out of phase with each other - each step in the waveform is the 11ns we know it takes for the signal to travel from the t-piece in my transmission line to the end (or the same distance but in the other direction).

I can set up the scope to illustrate this kind of thing in another way, and the resultant view looks like this video here:

So in this video you need to imagine that the yellow waveform is travelling from left to right from the transmitter to the antenna. Similarly the blue waveform is the reflected waveform travelling from the antenna back to the transmitter. The purple waveform in the middle is the resultant signal on the transmission line - hopefully it's clear that this wave is "standing" and not moving - hence the name. The standing wave peaks at twice the amplitude of each of the individual signals because it is the sum of the two.

So in this mock-up example all the power from the transmitter is reaching the end of the transmission line and reflecting backwards to go back from whence it came - that's very bad for the RF source.

If we think about the calculation of SWR:

Hopefully, we can see that if the reflected power is the same as the forward power then the SWR is infinite.

If we take a second example whereby the forward waveform amplitude is 1 Vpp and the reflected 0.5 Vpp (50% of the forward voltage), then the forward power is 2.5 mW and the reflected 0.625 mW, then the maths tells us:

that the SWR is 3 : 1.

So before the SWR reaches a value considered "acceptable" we need to have a reflected power equal to 4% or less of the transmitted power as that will deliver an SWR of 1.5:1 or less.

I also hope you can see for the SWR to be 1:1 the reflected power needs to be zero and chasing this ultimate aim is rather pointless.

Good, egh?

Well? You must have played a little by now?

Well,

Last time I mentioned that I had bought myself a Rigol DSA815 Spectrum Analyser with the built in Tracking Generator. I'm just about to head back to MAN for a flight to A71 land but I have had a bit of a fiddle this morning. First off, I made myself a very simple SWR bridge using some bits and bobs lying around, it's similar to the one I made back here for the power meter:

http://g0mgx.blogspot.co.uk/2012/10/well-since-progress-i-discussed-last.html

but smaller and not designed for the power, it looks like this:

What we have here at the top of the photo is a LH connector for the RF source, the top RH connector for the load (antenna). On the bottom the 2 parallel 100R resistors form a 50R termination for the forward power sample port and the reflected power sample port is connected to the bottom connector.

So, if I now connect the output of the Spectrum Analyser tracking generator to the top left (RF source), an antenna under test to the top right (RF Load) and the input to the Spectrum Analyser to the Reflected power sample port, we have a very expensive antenna analyser thingamabob!

With my recently constructed 40/80M dipole from here:

http://g0mgx.blogspot.co.uk/2013/04/well-further-to-my-musings-last-time.html

coupled to the RF Load port and the instrument appropriately configured, here's what we see:

Now, I've used the built in marker capability within the Spectrum Analyser to do a few things here:

What does this all tell me?

• The 80M part of the antenna is nicely tuned within the band at 3.533MHz
• The 40M part of the antenna is a mile out at about 7.765MHz
• The 5dB width of the 40M resonant point is about 165KHz

I haven't done the maths yet so I don't know if my selection of a +5dB measurement is appropriate to see the bandwidth of decent SWR - I will do that later. But a summary of the summary?

It's too damn short!

Interesting though, egh?

My Power Struggle is Over

Well,

Following all the palaver associated with my Power Meter project; I've finally completed it.

Here's the finished article:

The front panel (which is covered in Fablon) isn't anything like as bumpy as it looks in the photo to the naked eye; it's perfectly acceptable.

I've finally decided that the switches need to switch between short and long sample times (as before) but the second switch that I had used to peak hold the meter, now switches the units primary display units from dbM to dbW - each of these has the power converted to watts in brackets.

The unit had quite a bit of use this weekend in the WAE RTTY contest - and yes I was sending and receiving QTC traffic! - but the meter certainly seems to work at full UK power from my linear, it's a damn site more accurate than anything else I have here, but most impressive was the ability of the unit to read really low reflected powers so I can tune the antenna far better with this unit than I can with any of the analogue meters I have here. This meter can tell be that I have -20dbW reflected power when an analogue meter wont even be moving.... I'm actually quite impressed.

Cat's not overly enthusiastic about it:

Impressive though, egh?

 

Feel the Power!

Well,

Since the progress I discussed last time:

http://g0mgx.blogspot.co.uk/2012/10/its-becoming-real-power-struggle.html

with this power meter project; I've had (yet another) build of the directional coupler:

This time its been constructed from double sided PCB material and uses FT-82-67 toriods.

Connecting the forward and reflected ports to my 'scope (terminated with 50R) with a RF generator (this one is made my Kenwood) connected to the transmitter port and a dummy load connected to the antenna port, here's what I found on the 'scope (I've got the setup wrong and the blue reflected port reading on the 'scope needs to be divided by 10):

I then connected the coupler to the ports on the power meter and also included a 30dB attenuator. The attenuator was constructed using this site here as a Pi attenuator:

http://www.microwaves101.com/encyclopedia/calcattenuator.cfm

The setup for the next phase looks very much like this:

 

So, once this was put together, I then constructed an excel table with frequency across the top and the AtoD down the side, this table looks like this for the forward port of the coupler:

So, once I had these values I can the look into the equation that will take me from the A-to-D reading back to power - this is basically what the power meter needs to do - here's the equation:

So solving this equation as a pair of simultaneous equations - in fact a pair or more of simultaneous equations - I see this:

and this:

so basically I get different constants depending on what lines of the table I use as the input to my simultaneous equations.

However, despite the different constants, if I then use the equation and the constants to re-generate my powers from the ADC values the curve seems to be a very good fit. Here is the original readings plus the two equation outputs plotted:

So, all in all, I'm rather confused about the whole thing! But whatever the confusion, the device certainly seems to be very frequency stable i.e. very little variation in AtoD values as frequency increases. It's all good.

The software I have for this meter contains a huge set of lookup tables for the AtoD reading to power conversion; clearly these all need to change!

Cat's not helped much:

 All good though egh? I'd love to know what you think.

It's Becoming a Real Power Struggle

Well, blimey!

How hard can it be to get a piece of PIC assembler software to display some characters on an LCD display? The answer seems to be very.

I've started putting my power meter together, it's the one I started here:

http://g0mgx.blogspot.co.uk/2012/10/a-bit-of-power-struggle.html

Well, eventually I managed to assemble all the parts and put the boards into an enclosure I have here:

I've actually made quite a good job of the metalwork so far, I've intentionally not cut the hole for the display as I don't yet know which one of the many I have here I would end up using. Here's the view of the front panel as-is now, the meter looks OK but isn't the large meter I found, as that's just too big for the case!

So, PIC programmed, LCD display wired in, power on - nothing. Just some very faint black squares on the display that I can make more evident or go away using the on-board contrast control.

So, still not knowing if the software is intended to interface to an HD4470 compatible or not, the question was what to try next?

I ended up cutting the software back to just the display routines and simply programming the PIC with the code to display just a few characters on the LCD. Result - nothing.

So, next I added a simple loop at the end of the LCD routine calls to turn on and off one of the digital output ports every second. I attached a suitable LED to the port and tried again - as expected the LED started to flash. This told me a few things:

• My PIC programming was working and my software was making it into the chip;
• The software was reaching my loop for LED flashing so it wasn't getting stuck anywhere;
• The routines for the LCD weren't working.

Now, you may (or may not) recall I did quite a bit of fiddling with LCD displays and Pinguino and more recently Arduino back here:

http://g0mgx.blogspot.co.uk/2012/02/four-isnt-quite-same-as-2.html

So from that messing about I had a reasonably good idea how the initialisation of these LCDs should look, and the code I was staring at didn't quite look the same. So I changed the sequence of numbers sent to the LCD to initialise it, re-programmed the PIC and tried again. Nothing.

Hmm, next I looked at the delays in the software between writing the Enable line on the LCD high and then low again - you kind of have to do an "open sesame" on the LCD every time you send it either data or a command and the delays in the assembly software I was looking at used the processor "nop" instruction. This is really a null operand and hence takes one (I think) clock tick to execute. I changed this to be a 1ms delay and.... Bingo! We have characters on the display:

So, my next puzzle was the fact that I could display the characters 0-9 OK, but any attempt at alpha characters resulted in, what looked very much like, Japanese characters appearing on the display. After much cursing, general muttering and many re-programming of the PIC to try different things, I found a short between two of the address lines on the board. Doh! It took me over 6 hours to find that as I was only looking in software....

So it does seem that I am moving in the correct direction. I have also found that the Assembly code I downloaded from the QST in-depth website for this project, doesn't assemble on my version of MPLAB. I've had to change a few things:

• declared names are clearly case sensitive in my assembler and not used in that way in the code - many capitalisation changed;
• literal values used with the "loadw" command - the decimal declarations used generated value too big warnings, all I have done is declare the values as the hex equivalent and prefixed with "0x" - I don't understand why this is different, but it assembles;
• The declared names in the .inc file for the processor are also case sensitive so again, a number of changes needed there.

I then incorporated my changes to the display routines into the downloaded software and now I am seeing what I expected to see on the display.....

It's time to test the unit, I'll let you know how I get on. I suspect I need to re-make the directional coupler (again) as I posted a photo on the eHam.net homebrew forum and asked for some advice, here's a very polite post telling me what a pile of junk I have made:

eHam comments

Hard work, frustrating but all good though, egh?

 

A Bit More of a Power Struggle

Well,

What a palavar! You remember the Power Meter project from here:

http://g0mgx.blogspot.co.uk/2012/10/i-wonder-how-much-power-is-there.html

and here:

http://g0mgx.blogspot.co.uk/2012/10/a-bit-of-power-struggle.html

well, I ordered the strange surface mount resistors I needed that were 52.3 ohms from Farnel. They have a £20 minimum order value so I bought all sorts of other stuff that I didn't really need; they arrived today and I have completely goofed over the type I have ordered. The ones that arrived today are so small, you need a magnifying glass to even see the component on a white piece of paper! They are less than 0.5mm long - no use to me and my soldering iron at all!

I've nearly finished the power supply for the project today and the main board is also now fully populated - the PSU just needs a mains input on the back:

The power supply is in a case I found in a really neat shop in the centre of Derby called RF Potts; they sell all sorts of great electronic bits and bobs in there; so this Verobox was £1.90 (so I bought 5!):

Here's the main board:

So, progress is slow and steady! I've emailed the project designer today to ask what type the LCD display needs to be - I'm sure it'll be a HD4470 compatible, but it would be nice to know for sure - the less unknowns in these scenarios the better!

Looking good though, egh?

A Bit of a Power Struggle

Well,

You may remember that I started a Power/SWR meter project back here:

http://g0mgx.blogspot.co.uk/2012/10/i-wonder-how-much-power-is-there.html

as I said then, I had ordered the circuit boards for the project from Far Circuits, and they have duly arrived.

Unfortunately, I am a few bits short of a full kit within my vast collection of components, and have had to order a couple of SMT components (52.3R resistors!) and a 2.5V reference plus a couple of other small parts.

I should be able to complete the project when those bits arrive; I'm also struggling to figure out if the display used is a HD4470 compatible device, or something else completely. I cant actually find a way to figure that out.

The boards look like this so far:

The two smaller boards underneath the meter are the A to D converters, these take the voltage out of the directional coupler I made in the last post and convert it to a digital data stream; the devices used are Analogue Devices 8307 and they are logarithmic. The main board is at the bottom of the photo on the left hand side and the PSU is at the bottom right.

The meter you can see is really neat; this was an eBay purchase, although it's bigger than I anticipated (it's actually exactly the size the seller said it was but I didn't bother reading it). So I may need to rummage for something suitable to enclose this project when its complete as I haven't got anything here bog enough to house the meter neatly - I do have a smaller meter of the correct value so I may end up using that.

Cats been helping again:

I'll post more when the missing pieces of the jigsaw arrive.

Good though, egh?

Tuning - with less noise!

Well,

For some time I have been using an LDG Electronics AutoTuner here in the shack. It's been an OK experience and the ability to change bands and then have the tuner sense the RF frequency and auto adjust itself back to the previously stored settings for antenna and band appealed greatly. Set it up once and never touch it again - perfect!

The theory was fine, but it didn't quite work in practise. The device never quite matched the antennas correctly - I always found it necessary to tweak the automatically chosen settings and then store them manually plus on most bands I couldn't quite get a 1:1 match. My biggest concern with the unit was that the SWR reading on the tuner itself didn't ever match the readings at the radio or with the separate meter I have here. The net result was that I was tuning for the best match read by the radio, which was normally quite a miss-match according to the tuner.

Anyhow, to cut a long story short, I have purchased a MFJ roller coaster based tuner:

Looking at this picture worried me a little. There very much looks like there is a dent in the top of the unit! However it's completely invisible unless taken by a camera with a flash but I should probably consider reporting this back to the supplier.

Anyhow, using the device as a tuner seems extremely easy. MFJ tend to have a bad reputation and are often called "Mighty Fine Junk"; however, my experience is all good. This tuner does exactly what is says on the tin. I've completed some tuning charts which I will try and print out and place somewhere near the unit itself in the shack.

The cat has taken on a whole new degree of madness; he has now taken to sleeping in the cover of my 'scope on a high up shelf in the shack:

I'll keep you posted on the tuner, looks OK though, egh?