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Tune Around! Topics For
Technicians! |
BUILDING and
Tuning A DIPOLE THE EASIER WAY! Here are a couple of methods that should work well for you! You may find the second method to be easier to use, but try both! This short bit of information hopefully will save you much time when building a dipole. It concerns hams that want to make a dipole, (any frequency), so they usually begin by using the standard formulas below : 234 / design frequency = each dipole side length in feet. or 468 / design frequency = total length in feet The only problem with this, when you put it in the air, it isn't even close to 1.0 SWR. In fact the antenna is resonant way low in frequency or just the opposite, too high. So......up and down, up and down, up and down.... you start chopping off or adding pieces of wire until the antenna comes up to your design frequency. This can take many, many trips to the rig, back to the antenna, back to the rig, etc, etc and can be very time consuming to get the dipole tuned where you want it. The solution to fewer trips up and down guessing at tuning the antenna: Frequency scaling using a bit of simple math! Example: Lets say you want a dipole for 18.130mhz. You just used the standard formula 234/freq = length of each half in feet or 468/ freq = both lengths total in feet...you then cut each wire to 12.9 feet from the formula.....(234 / 18.130) You put the antenna in the air and to your horror the antenna is resonant at 17.80! (You rotten no good sacafragamatza punka mocha fritzalitz !!!!!!!!!) (Choice words edited out due to family rated website) What to do? Here comes that bit of math with a formula to save the day! The formula = Old Frequency (17.80) / New Frequency (18.130) X Original length from the formula, (12.9 feet) = New dipole length (12.7 feet). Now since it's much easier to use inches rather than 10th's of a foot, doing the math to find out how many inches 12.9 feet is: 12 feet + .9 feet = how many feet and inches? 12" X .9 = 10.9 inches. Add this to 12 feet.....12 feet + 10.9 inches rounded off = 12 feet 11 inches. Doing the same with the new dipole length.....gives us, 12 feet + 8.4 inches = 12 feet 9 inches (rounded off) for the new length! So there is a difference of about 2 inches between the old length and the new length. Now since the new length is shorter than the old length...you cut of 2 inches from the original length that you arrived at using the formula the first time. This should get you very very close to the exact frequency using this formula! If by some quirk of Murphey's law it still is off a bit, just repeat the procedure again with no guesswork involved!!!!! Here is another example using the same frequency, 18.130mhz and you find the best swr is 18.500mhz. Now the antenna is toooo short....but how much? 18.500 / 18.130 X 12.9 feet = 13.16 feet so........, the difference between the new length and the old length is 13.16 feet - 12.9 feet = .26 feet which is what would have to add to the original length to make it longer and much closer to the correct length. So we get .26 feet added to 12.9 feet = 13.16 feet for 1/2 of the dipole! Just in case you need to know, a foot divided into 10th's of a foot = 1.2 inches per 10th of a foot. Don't let this formula confuse you. It is not a formula for designing a 1/2 wave dipole......not even close...it only helps you tune the antenna much quicker than the cut or add and try method! Math comes to the rescue from lessons in school, years ago, that you thought you would never use! Thanks to the teachers, math made your antenna building experience much easier! "Don't forget to add length when the dipole is too short and to take away length when it is too long! Practice the formula in your spare time using various design frequencies and results. This formula works very well and has been around for a long time. Just thought I would send it your way. I use this technique all the time, I'm just too old (smart)..... to make all those trips anymore. I am interested in getting it on the air, not on the ground! Have fun!" 73 This submitted by John / N0KHQ / St. Louis Editor's note: Here is another way of stating the formula in real life words: Unwanted lowest swr frequency divided by Wanted frequency multiplied by results of standard formula equals new length for half of the dipole. Here is a second method that works just as well!
~Dipole Tuning for Lowest swr~ So in the planning stage of putting up your dipole you have decided to
put it up designed for one of your favorite hf bands, but your
not looking forward to all of that frustration of designing it by using
the "formula", and finding out that the antenna is either too
long or too short! We all have "been there, done that". This old standby dipole formula has been around since way before time started, and has been arrived at by much experimentation by thousands of hams. But in most cases, it will only get you somewhere near the proper dipole length. No, it is not exact science due to all of the variables within the antenna environment. And when you do use it, the tedious antenna tuning process comes into play which we all hate and get frustrated with sometimes. In our fast paced lifestyle, we would love to cut out as many of the tuning steps as possible. Here is an easy method of tuning your dipole for lowest swr without all that up, down, trim, back up, check swr again, back down, trim, check swr again and repeat this process over and over, until you get that lowest swr you want on your dipole. Save yourself lots of time and trouble by reading further: Of course you must first decide on what frequency you want your dipole centered on...this is the first decision you have to make. So for the first try at the design, you use the old standby formula in hopes of getting it close to the proper length. You decide, as an example, that you want to operate on the 20 meter band with your dipole, so you plug in the formula into your calculator and get started. 468 / freq in Mhz = total length in feet for a half wave dipole. You want to center your dipole on, as an example, of 14.300Mhz. So, using the formula 468/14.3...we get 32.72 feet...or about 32 feet 8 inches for the total antenna length. Now putting standard "practice" into place, we add a couple of feet to
that length for tuning purposes, giving us about 34 feet or so as a
guess. (It is easier to take away from the length as it is to add
length later.) We now get all of our wire, feed line, insulators, supports, etc, together and put this new dipole up in the air at it's final operating height so we can check the swr in hopes that we will be close to 14.300Mhz as our lowest swr frequency. To our dismay, we find that the swr is lowest on 14.050Mhz....RATS! That's not even close! This makes the total length way tooooo long for operation at our design frequency of 14.3Mhz. Now.....here is where the "Easy Way" of tuning the dipole comes into play.
You have already found the frequency of lowest swr to be 14.050Mhz. 1. RECORD that frequency in your notes where you put the
length (34 feet) of the antenna. You have now recorded the length AND the
frequency of lowest swr. These are 2 important
parts of our new calculations. Without them, you are back to square one.
2. Now take the actual length of the antenna that you wrote down (which was 34 feet in the example) and multiply it by the frequency (in MHz) of the lowest SWR. The resulting number will be your new constant, to replace the old standard formula constant of 468 in the standard dipole formula. Do it like this: 34 X 14.050 = 477.7....this is the new constant to be used in the formula. Now using the new constant in the modified dipole formula, divide the new constant, 477.7, by the frequency you want to have in the middle of your preferred range. Remember that was 14.3Mhz. 477.7 / 14.3 = 33.40 feet. This is 33 point 4 feet, not 33 feet 4 inches!Now convert .4 feet to inches.....this is almost 1/2 of a foot...so
about 5 inches. (You could do the decimal conversion to be exact, but at hf, this is not needed.) Your new length should now be 33 feet 5 inches.
This is the length the antenna should be to have lowest swr on 14.300Mhz. Now you need to adjust the one you have in the air to this length. After doing the above calculations and changing the length of the dipole, put the antenna back up into it's final position at the exact same height as your first try without changing any other part of the antenna, just like you did the first time, when you used the old 468 number constant. Your "new" tuned antenna should now give you, at or very near, the lowest SWR at the desired frequency of your original design, when you used the new constant number in the formula. If for some reason you later want to trim an HF wire antenna, (say, you
decide to move to a different portion of a wide band), don't waste
your time by trimming or adding a little at a time until
you get the lowest swr. You can make a very close estimate of
how much to cut or add based on the band and how far you have to change it
by repeating the process
above. When you built that 75 meter dipole and tuned it for lowest swr, your final tuned length was, let's say....112 feet for lowest swr on 3.9Mhz. Remember, this is just an example....forget that old formula for now. You want to change your present center frequency, 3.9Mhz, to the lower part of the 80 meter band, say around 3.6Mhz. From experience, you know you will have to lengthen the antenna....but how much? All you have to do is take an swr reading for the lowest swr on your present antenna. This should be at or very near to 3.9Mhz unless something has changed with your antenna system. You need to find that magic "constant" number that was arrived at by your final tuning result of your 75 meter antenna. Remember, you did have to tune it after using the original standard formula so after the length was changed, this changed the original constant used in the formula. Now just multiply 112 feet (your present antenna
length) by your old frequency. 436.8 is your new constant for use in the formula so, 436.8 / 3.6Mhz (your wanted center
frequency) = 121.33 feet for total length. Recap: 1. First write down the total length of your new antenna
after using the old formula. (468/freqMhz=total length in feet) Hopefully
you kept notes! Existing dipole (modify it's center design frequency) 1. Find lowest swr frequency (Old center Freq) of the existing antenna. 2. Find length (L) of existing antenna. You did write it down I hope! 3. Multiply length (L) times OLD dipole FREQUENCY (where the swr was lowest) (Old F) to get it's resulting constant number. Existing L x Old F = new constant Use the new constant, (not 468), in the formula again using your new design frequency. New constant / new frequency = total length in feet It is assumed that your
dipole is used without an antenna tuner and you want the lowest swr
possible on your design frequency. It must be noted that you can only use
this "find the formula constant" method AFTER you have used the old
formula first for getting close to the antenna center frequency. This
is due to any number of variables in the original antenna installation due
to surroundings, height above ground, bare or insulated wire, angle of
antenna relative to it's center, (horizontal, inverted V, etc), diameter
of conductor, insulation thickness, etc. Math does not lie, but sometimes
your antenna, it's composition and the environment plugs in a variable to
the formula. Using the methods in the articles above should help with
getting you on the air much faster! 73 ~ N4UJW
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