COFAW COax Frequency And Wavelength calculator.

This will help those learning the ropes and even those that already know.

COFAW will show you what lengths of your coax are happier for your radios.

---

The rest of the page below serves as the tutorial. DOS is at the bottom.

A screen shot of Cofaw at start up.

---

OK, let's run through the program quickly.

It's on and running and we click 'continue' and here is what you see next.

Nothing much here yet, let's get it to do something, anything.


Leave everything else alone for now, and let's just
click on the wide 'Calculate' button near the top.

---

Right, what you just did was,
calculate what two and a half wavelengths are for a particular type of co-axial
(coax) cable, (RG-6, Belden 1152A) at a particular frequency, (145.8 MHz).


We can see what two and a half wavelengths at this frequency in this coax are in Blue Text below the Transmission Line Type.
It's 3.8554 Metres.

---

Now we go across to the right hand side and click the top or up arrow on the 'Multiplier' window once.



The length of the coax has been increased a little bit, the default amount of wavelength.
(default is 0.125 or 1/8 of a wavelength, you can change it, click the button next to the arrows)
You will noticed that you now have a RED SWR looking at you instead of a nice green one.

Some data has changed, some hasn't,
in the middle of the page there are four yellow windows.
These windows show quick reference for you, for the length of a
radio wave, while travelling in this type of coax, for 1, 1/2, 1/4, and 1/8 wavelengths,
at the frequency you selected using the coax you have selected.

This is the length of the radio wave inside the coax cable.
It's not the same as when it is travelling through the air.


Above the Metric/Imperial and Print buttons is a block of data called,

Freespace Wavelength Data

This is the length of the radio wave in open space,
like between the house and the car on the road, or two hand held units.

---

Keep clicking the up arrow next to the 'Multiplier' window until you get to three.
The GREEN SWR comes back on. Eeeehaaa! it's good believe me.


What the little GREEN SWR is telling you is that this length of coax is a
good and happy length to use for this type of coax at the frequency you mentioned.

If you change the frequency, the result will be different.
If you change the coax, the result will be different.

---

Well, that's nice, what about a plain old normal common everyday radio setup? How do I use it?
Say for instance, the same frequency 145.800 and I have 10 metres of RG-58 coax.
OK, well select RG-58, (if you know the exact type it is better) from the Transmission Line Type.
drop down window, click the little button with a down triangle on it that is at the right of the window.

RG-58 coax is what you will commonly find at Jaycar, Dick Smith and other stores, for CB or mobile radio transmitter use.



Play with the multiplier until you can see the length of your coax or near about
to it, in the Wavelengths of Coax at this Frequency window.

We can also see the RED SWR.

---

Decrease the multiplier until the GREEN SWR appears.



You now have the length of coax to cut that is an SWR repeater for that frequency.
That means, when you make a power or SWR measurement at your radio end of the cable.
It will more than likely be very close.

If you use a coax cable that is 'any length will do', then your radio end of the cable measurements will more than likely be incorrect.
The waveform of the signal will be out of phase. Reflections will be cancelling and or adding to what you are trying to measure.
The output power and antenna tuning measurements will not be accurate.
If you do not have a length of cable cut for the job, in order to make accurate measurements, you should then do it at the antenna end of the coax cable.
Which if you think about it, is rather impractical in many or most circumstances.

---

Knowing that you will get more power up the spout if you tune your coax to your frequency,
so that you can measure the antenna properly. Would you do it?

Yes, you say, well, it makes sense to.
But, I live in the USA and don't like or use that metric stuff.



That's fine, click on the 'Metric/Imperial' button, and when it says 'IMPERIAL' click on the big 'calculate' button again.

---

Same frequency and coax as above but, in feet and inches.

---

This is the tutorial for the B1 DOS version of COFAW, written in, uh, hang on.....2000.

What the DOS version of COFAW looks like when you start it.

---

Start of by selecting the multiplier first, it will demand this if you try something else.

PRESS F6 for MHz, then select F1, to enter the 2nd multiplier, 27.5, then press enter.



Select F11, or F12 to change the type of Coax cable.

You can specify wavelength instead of frquency if you like.

To increase the length of the cable, press 6 on the numkey pad then enter.

To decrease the length of the cable, press 4 on the numkey pad then enter.

To specify a length of cable, press 2 on the numkey pad then enter.

Take note, just in case you get an error about parameteters being wrong if you run the DOS version on XP.
For some reason, on my XP PC, the DOS version works from the desktop folder OK, but not anywhere else???

---

Home
( UPDATED: August 24 2009 )