open end half wave dipole antenna

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As of November 1st, 2003, a dear friend named Sir Harold made his own Open Half-Wave Dipole Antenna. He said that as soon as he was finished with it, he would write a documentary/with pictures, to let all the visitors read about his own venture.

So my friend, just CLICK HERE to read Sir Harolds' Maiden Journey with the Dipole Antenna.

I have used this special type dipole antenna with the 200mW FM Transmitter and the 7 Watt. It has performed very well indeed. Construction is fairly easy and the whole assembly is light-weight. If you any questions concerning it or should you decide to make it, give me a call and I will help in any way that I can...

At one time it was thought that communcations in the VHF (Very High Frequency - which includes the 87 through 108 MHz FM Broadcasting Band) range and higher, would be restricted to 'line-of-sight' paths. Although this has not proven to be the case. But the term line-of-sight is still useful in understanding propagation. If there were no elements giving interference to RF waves, they would theoredically travel in a straight line (thus the term coined...line-of-sight). But in fact, radio waves are normally refracted or bent, slightly earthward. the reason is not hard to understand. The normal drop in temperature, pressure, and water vapor content of air with increasing altitude affects the refractive properties of the atmosphere. So, under average conditions, radio waves are refracted toward Earth enough to make the horizon appear 1.34 times farther away (Radio Horizon) than the normal view of the horizon (line-of-sight).

A simple formula can be used to estimate the distance to the 'Radio Horizon' under average conditions:

Example: The distance to the normal view of the horizon (line-of-sight) for an antenna 20 feet above average terrain would be 4.72 miles. But the distance to the 'Radio Horizon' for an antenna 20 feet above average terrain would be 6.32 miles! So it looks like radio frequency waves travel a little further than the eyes can see...thank you, mother nature!

This antenna was used in a vertical fashion as to produce an omni-directional radiation. In theory with its' vertical positioning, the signal should give an all-around 'even' signal perpendicular to the vertical axis of the antenna. Look at the picture below. If you were to position yourself directly above the antenna when it is finally positioned in a vertical manner, the electro-magnetic waves would radiate out in all directions. When your antenna is finally made, and you set out to put it at a particular height, use the above formula to find out where 'your' Radio Horizon is!

100 Watt Soldering Gun

Rosin Core Solder

Two Coat-Hangers (3/32 in diameter)

PVC Piping (discussed in text)

Lag Bolts/Washers (discussed in text)

Electrical Tape

RG-59 Coax Cable (discussed in text)

RF Connector (discussed in text)

Make sure you have all the materials before you start construction of the project. Once that has been done, you may proceed with the...

RG-59 Coaxial CAble comes in either an aluminum or copper sheild (braiding). To use solder in all your connections in this project, one must purchase the copper braid. If you can only get aluminum braid, you can also use screw and nuts to also achieve a good and strong 'solderless' connection.

Find two coat-hanger wires that have the diameter of 3/32 of an inch. Take the two coat hanger wires and straighten' them out... as straight as you can get them. From now on, the coat-hangers that are the conductors for the dipole, will be called 'rods'. (All the FM Transmitters in this website were designed to transmit on either 87.5, 87.7 or 87.9 MHz. This is deemed necessary, since the coils are preset and need no final adjustment. It is just the variable capacitors that you have to deal with...in order to capture the said frequencies.)

In order to find out the right length needed for the two rods, I have made a a special webpage just for that. It covers two formulas needed to find the exact resonant length of your particular frequency. I have given an example...and once you understand how the length is acheived, you can then plug in your own figures. Just click HERE for calculating your own lengths.

Once you have found the length needed for each rod, place them aside. Now take two pieces of the 1 inch PVC piping and cut them to the length of one of the rods...less six inches. In other words...if a rod length is 30 inches, cut both pieces of the 1 inch PVC piping at 24 inches. This will mean that each rod (when the antenna is completed) will extend out from each piping approximately six inches. This is needed as to keep the whole assembly (PVC piping) as light as possible. There is no reason for the 2 pieces of piping to be the exact length of each rod. These rods (coat hangers) are strong enough to hang out 6 inches from the PVC piping, without bending in strong wind.

Next, take a PVC "T" coupling and glue the two cut pieces of the 1 inch PVC piping into the "T" coupling...the picture below shows how it should look when glued.

Next, take the two coat hanger wires that were cut to proper lengths and scrap the enamel off of one side (about 1/4 of an inch) of each coat hanger; this will be in preparation for soldering later. Position them on top of the 1" PVC Piping. Secure them with electrical tape. The picture below is what it should look like.

The next thing to take care of, since this design is to be in a vertical fashion, is to continue making extensions on the PVC Piping. There are two more pieces of 1 inch PVC piping to be used in this project. One will be inserted into the only hole left in the 'T' ' connection. The length of this 1 inch PVC piping must be at least 1/2 the total length of your dipole antenna. In other words, if one of the rods measured 32 inches, then the length of the PVC piping must be at least that said length. It can be more than 32 inches...but no less. The reason for this is that the feed line (RG-59 coax cable) should be routed away from the antenna at a right angle (perpendicular) for as great a distance as possible. This will help current imbalance in the line caused by RF (radio frequency) pickup from the dipole rods. The ARRL Handbook states...a distance of at least 1/4 wavelength or greater. By keeping a distance of one measured rod, you will then satisfy the ARRL's suggested distance.

After you have measured and cut the 1 inch PVC piping and inserted it into the 'only hole left' in the 'T' coupling...insert the other end into a 90 degree elbow.

Then cut another piece of PVC piping three feet long and insert it into the remaining hole of the ninety-degree elbow. The picture below is how is should look like when the glueing is completed.

NOTE: Make sure you know how all the pieces go before applying the glue. That PVC glue bonds in seconds, and you surely don't want to have to go back to the store for another supply of PVC!

Next, measure out the total distance you will be running your coax cable...that is...from the antenna to the transmitter. Add at least 3 more feet, for the addition of a home-made 1:1 balun that you will make.

Now, we will make a1:1 balun. This device is needed to change the 'unbalanced' RG-59 coaxial cable, to a 'balanced' one. Although the dipole antenna is a 'balanced' 75 ohm impedance antenna, the RG-59 is a 75 ohm 'unbalanced' transmission line. In order to expedite the full of amount of FM transmitted signal flowing in the coaxial cable, to the antenna....a balun MUST be used between the two.

Just CLICK HERE for the complete instructions on how to make a home-made 1:1 Balun. Once it is made, just come back to this webpage, to continue the project.

Once the balun is made, look at the picture below to see exactly where it is to be place on the antenna. It is put within 2 inches of the feed point on the dipole. In other words, the 'feed point' is located where the terminals of the coaxial cable are soldered to the two rods of the dipole.

Measure out the distance you will be running your coax cable...from the antenna to the transmitter. Add at least 3 more feet, for the addition of a balun.

Now, we will make the 1:1 balun, that is needed to change the 'unbalanced' RG-59 coaxial cable, to a 'balance' one. Although the dipole antenna is a balanced 75 ohm impedance antenna, the RG-59 is a 75 ohm 'unbalanced' transmission line. In order to expedite the full of amount of FM transmitted signal flowing in the coaxial cable, to the antenna....a balun MUST be used with the dipole antenna. inserted in between the end of the coaxial cable and the feed point of the dipole antenna. The picture below shows where the balun will be inserted. Please CLICK HERE for the full instructions on how to make a home-made 1:1 balun for the open half-wave dipole antenna. When you have finished making it, you may come back to this webpage, for further finalization on the erection of the dipole antenna.

With this type of antenna, being erected in a vertical fashion (that is, perpendicular to the earth's ground), your coaxial cable should be routed away from the antenna at a right angle. A right angle departure of 1/4 wavelength 'must' be taken into account, as the angle of the signal radiation from this type of antenna is perpendicular to the rods of the dipole antenna. You surely do not want your coax interfering with the full radiation signal your transmitter is giving off!

You can continue tapeing the coax all the way down the PVC. Take a look at the picture below...it will show everything I have just explained.

Once you have secured the coax all the way to the transmitter, you can then either purchase an RG-59 Male Connector that will adapt to your 'female connector' on the transmitter. Radio Shack carries an assortment of these connections. Bring your transmitter to the 'Shack' and see if you can find 'one' RG-59 male connector that will fit in your transmitter's antenna output. Different transmitters can have different size male connectors.

If there is no 'male connector' on the output of the transmitter, you can just solder like you did on the dipole rods...remember, the center wire will go to the antenna connection of the transmitter and the outside braided will go directly to ground on the transmitter.

There you have it, my friend. How well it is built and mounted, will determine how 'long' it will give you great service!

P.S. ...and to a special friend, who needed to make an antenna 'real bad' for his transmitter!