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

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.)
WRITE THIS LENGTH NUMBER (34 feet) DOWN. YOU WILL NEED IT 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.

Example:

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!

Recap:

New Dipoles first attempt in the air!