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The simple use of math will help save you lots of time tuning a dipole for lowest SWR!

Here are a 3 different but similar methods that should work well for you in tuning your dipole for lowest swr without many steps and repetitive trial and error! You may find one of the explainations of the different methods shown below to be easier to use, but try all of them! Use the one that you understand the best and that is more simple for you to use. Practice with all of them using your own practice "lengths and frequencies".

This short bit of information hopefully will save you much time when "tuning" a dipole whether you have built it yourself or purchased commercially.  Yes, most commercially purchased dipoles need tuning.

These methods can be used by 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. Most builders cut long and trim the total length for lowest swr on their design frequency.

The only problem with this, when you put it in the air, it isn't even close to that magic 1:1 SWR that most of us seek.

In fact, after your first swr check, the antenna may be lowest in swr 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 with the lowest swr.
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:

By using a bit of simple math should help do the trick! 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 (each leg) to 12.9 feet from the formula.....(234 / 18.130) or 2 times that length for the total length in feet, then cut in half using the 468 constant formula.
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 due to family rated website)

What to do?
Here comes that bit of math with a formula to save the day! Get your calculator out.

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) per half.
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 per half.
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 for each half of the dipole!

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 making the antenna resonate higher in frequency.

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!" 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 total length for half of the dipole.

Method # 2

Tuning a Dipole Antenna for Resonance!

Find a Ham Radio Dipole Resonant Length

Most dipoles will require a little "trimming or adding" to resonate at the desired frequency. Here is a good recommendation in 7 steps.

1. Cut your dipole wire some 2-3 % longer than the length given by the standard formula.

2. Make a note of the length obtained in step 1.

3. Raise the dipole to its operating height.

4. Measure the SWR at several frequencies within the intended frequency band. (Use only a few watts and pick a quiet time on the band to make your tests).

5. Note the frequency (F = min) at which minimum SWR is obtained.

6. Multiply (F min from step 5) by the antenna length recorded in step 2.

7. Divide the result of the above multiplication by the desired frequency of operation, to obtain the final length.

Trim both ends of the dipole down to the final length obtained in step 7. ( So by using these 7 steps, you have only raised the dipole twice!

Here is a 3rd method that works just as well!

~Dipole Tuning for Lowest swr~
The Easy Way!
Find the correct "Constant"

Due to the way modern ham radio transmitters are designed,
we all try to tune our dipoles for the lowest swr so our transmitter will provide maximum output to the antenna without a tuner involved.

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".

That "formula" for a half wave dipole, as we all should have learned here in the U.S., is based on a "constant" number of 468, which is divided by the center frequency we wish to use for our dipole, giving us our total length in feet for the dipole.

468 / freqmhz = total feet length

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 construction and it's 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:

What Frequency?

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 standard dipole 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 in feet.

Now putting standard "practice" into place,  we guess to add some extra length 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 note that this length includes the length of each wire at the extreme end loops that are attached to the end insulators and at the center insulator at the very end of the coax feed line in the center of the dipole. This is part of the antenna that many forget about. It radiates too! So your total radiating length for each half of the dipole starts where the shield does not cover the center conductor of the feed line.

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 to where we wanted it! 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.  
(Call it "lowest swr freq" if you like, but call it something so you wll know what it is!)

You have now recorded the length (remember, you wrote it down earlier) AND the frequency of lowest swr. These are 2 important parts of our new calculations. Without them, you are back to square one.
So we can now do a bit of simple math to get our needed length for lowest swr at our design frequency.

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 in our example.

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 existing antenna system.

You need to find that magic "constant" number that was arrived at by your final tuning result of your 75 meter antenna when you first finished it. 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.
112 times / 3.9Mhz = 436.8 (This number was actually the end result constant  of your final tuning attempt which gave you the lowest swr and changed the length of the antenna using the original dipole formula)!!!!!!

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.

This should get you very close!


New Dipoles first attempt in the air!

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!

2. Find the frequency (F) of the lowest swr of your first attempt dipole.
(Check several frequency points between top and bottom end of the band.)
Record the lowest swr frequency!

3. Multiply length from step 1, (L) times frequency (F) in step 2 of lowest swr to get new constant.

Step 1 L x Step 2 F = new constant

4. Now use the new "constant" obtained from step 3 above in the formula rather than the 468 to get new length in total feet.

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 OR have the correct length of your existing antenna 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. 

Hopefully if you did the math correctly, you will only have to make no more than a couple of attempts at tuning your antenna for lowest swr rather than taking up your valuable on the air time by the cut, and try over and over method! Have fun!

Math does not lie, but sometimes your antenna, it's composition and the environment plugs in a variable or 2 into to the "standard" formula. 
Using the methods in the articles above should help with getting you on the air much faster!
Please send us feedback on how either one worked for you. If you have an easier method, let us know! No antenna analyzers allowed...please. Just an swr meter and your skill.

n4ujw AT hamuniverse.com

73 ~ N4UJW


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