Don't Fear "The Snip"

Now that I have your attention...

 
(You too can turn your long dipole into these tiny wires!)

Back in the day when CB Radios were all the rage (We're talking late 1970's here), most of us had a pretty standard 1/4 Helical antenna attached to some kind of pole so we could have a base station.

We were kids then, so not a lot of money to spend on my radio hobby.  People told me I should tune my antenna for "optimal SWR", but I never did so because:

  • I really only ever contacted local folks, so range wasn't a problem.
  • I was afraid of cutting my precious antenna in case I cut too much off it.
  • It worked, so I just didn't mess with it.

These days however I want to contact people further afield than Sydney and with a limit of ten watts right now, I need all the help I can get.  It's definitely time to get past my phobia and do things properly.

As an aside, if you are taking your 4WD into remote regions and you're going to be relying on a radio for emergencies, you really want to make sure everything is tuned properly.  It might just save your life!

What's the deal with SWR?

If you put "SWR" into your favourite search engine you will probably get nothing on the first page about radio antennae, but lots about radio stations.  Try searching for "Standing Wave Ratio" instead, and you will still get a lot of conflicting information.  Here's what I believe to be the case, but first we need a little bit of theory:

If our antenna and it's cabling were perfect, every nanowatt of power our transmitter produced would be radiated out into free space.  As you can guess however, that is not the case here in the real world.

Any antenna has an impedance, and this (like anything AC-related) is going to vary with frequency.  There will also be one or more frequencies where our antenna is resonant.  It is at this point where our antenna system is the most efficient, and almost all the input power will be radiated.

You can demonstrate this effect with a length of rope or garden hose tied to a post and then shaking it to send a "wave" along the rope.  If the rope is not tied to anything, it's length is effectively infinite and the wave we create will expend all it's energy traveling away from our position. (Try this, it's fun!).

When we operate at a frequency where the antenna is not resonant, then a percentage of the energy we put into the transmission line and antenna is going to be reflected back towards the transmitter.  The physics here is actually pretty complicated so let's just assume this simplified explanation is correct for now.

This is the same case with our rope.  Tie one end to something like a pipe or stake in the ground, and by timing your flicking of the rope you will actually see your wave travel away from you, and some of the wave actually reflect back from the far end.  If you time the motion of the rope just right you will see waves which don't appear to move the rope.  Congratulations, you have created a standing wave!

In really simple terms, the Standing Wave Ratio (SWR) is a measure of how much energy is being reflected back to the transmitter, vs how much is being transmitted in the first place.  If this is the case, then we want to minimise the reflected energy.

(photo courtesy Diamond Antenna)

Getting a Good Figure

In an ideal situation, our ratio would be 1:1, but this is pretty hard to achieve in the real world, and this is where it starts to get complicated.  Depending on who you ask, you will get different results of what is an acceptable, good or bad value.  My belief is that we want to be less than 1.5:1, but a lot of American authors will tell you that 2.0:1 or below is acceptable.  I have found that with a little care it's possible to get 1.3:1 and below at our desired frequency.

Remember that our antenna is only resonant at a single, specific frequency.  Our goal therefore is to get the antenna to resonate at the frequency we use most of all, or failing that in the middle of the band in question.  Thinking about the 40m (7MHz) band in Australia, I have access to 7.000MHz - 7.300MHz.  If we tune for resonance at 7.150MHz then the reading for most conventional antennae can be expected to be acceptable across the entire band.

What do I need?

  • You need a good quality SWR meter.  I prefer the dual-needle models myself.
  • A short patch lead from your transceiver to your SWR meter.
  • Use a good quality pair of cutters.  You want to get a clean cut, especially with multi-strand wire.

Getting started

If you use a commercial antenna, it will almost certainly be a little too long for the band it is intended for.  Manufacturers do this on purpose  If you're making your own dipole, then allowing an extra 50cm or so is always a good idea.

I would also encourage you to make your adjustments with  the antenna in it's final location and using the balun (if appropriate), cable and connectors you will use in normal operation.  We want to tune the system as a whole.

How do I know if it's too long?

This is pretty easy to determine.  Measure the SWR at the bottom of the band, and again at the top (Min/Max Frequency).  Without worrying too much about the actual reading, the SWR will be higher at one frequency at one end.  If it is higher at the top of the band, then your antenna is too long (the most common situation). Conversely, a higher SWR at the bottom of the band indicates your antenna is short.

Making the cut

Let's assume that our antenna has a ratio of 3:1 at 7.000MHz and 4.5:1 at 7.300MHZ (Don't laugh, my Diamond W-721 trap-dipole gave these results at first).  We know that it's too long, but how much do I need to cut off? It is an exact science, but there are a lot of variables, so in essence, we guess.  The first time around, I cut 10mm (1cm) off each of the rat tails at the end.  This made almost no difference, so I cut 20mm (2cm) the next time.

On the second cut, the readings started to come down.  Emboldened, I kept removing 20mm from either end until I got to below 2:1, and then removed 10mm each time until I reached 1.1:1 at 7.000MHz, and just under 1.3:1 at 7.300MHz.  At this point, I figured "job done"

What if I cut off too much?

If you're making small cuts each time, it's unlikely you will do this, but if you did, you could solder wire onto the end.  Just make sure you make it strong, especially for wire dipoles.  Worst case would be to use an antenna tuner or replace the wires altogether and use these ones for a higher frequency band.

Other things to note

With a simple dipole, this is pretty much all you need to know to be successful, but there are other cases to watch out for:

Trap Dipoles such as the Diamond W series use LC Traps to form a multi-band antenna.  Always tune these from the highest band first (21MHz, the 7MHz, then 3.5MHz for example).

Depending on the design of the antenna, you might have a rat tail at the end for trimming, or you might  need to change the point at which you tie the antenna off onto each insulator.

If you are tuning a helical-wound whip type antenna, these usually have a plastic cap on the end.  Replace this after cutting and before you check the SWR.

Some mobile antenna designs do not need to be cut.  They have a moveable top section which is usually secured by means of a flush screw.  The same steps apply, but try to move the tip only a mm or two each time. (Thanks to the folks at GME for reminding me!)

Again on the helical whip devices they usually have a fibreglass or similar core.  You will need to cut this down so that the wire is located under the end cap.

But my transceiver has an automatic tuner...

In most cases these are pretty good but there is a limit to what the inbuilt tuner can work with, and it seems many of them will struggle if presented with a measurement exceeding 2:1.  You can use the method here to get your antenna to a point where your automatic tuner can take over.  That's up to you.

 
(OK, so I am just showing off here...)

Why not use an external tuner?

Of course you can do this if you want, but you are adding additional components into your signal path,  and that has to cost something.

What are you waiting for?

Your signals are counting on you!

 

Comments

Post a Comment

Popular posts from this blog

Recording weather with Arduino, Elasticsearch and Kibana 3

Auric: All about batteries

Build an obstacle-avoiding robot 1