FM J-pole made from 300 ohm twin lead

[ebacon]

Here is an FM antenna that I made this morning. It works well and thought it worth sharing since many people have questions about FM antennas.

http://www.flickr.com/photos/93893819@N05/sets/72157639233712853/

 

[gridleakrick]

What does the 24 1/2" dimension refer to?
How does this antenna compare to a standard 300 ohm dipole?
Rick

 

[JBL GUY]

What does the 24 1/2" dimension refer to?
How does this antenna compare to a standard 300 ohm dipole?
Rick

That antenna appears to be close to the variation of the J-Pole antenna, called the "Slim Jim" antenna.

See picture below.

If the 300 Ohm twin lead were to be connected together at the top of the antenna then it would be the Slim Jim variation. This variation, when mounted vertically is claimed to have more gain than the standard J-Pole antenna (the claim is lower angle of radiation and slightly more broad banded).

The position of the coaxial cable connection is based on feed line theory.

That part of the J-Pole antenna (the U shaped part) is functioning as a shorted quarter wave feed line.

At the base of the antenna, bottom of the U part of the antenna, the impedance is zero, and at the other end, the open end, the impedance is infinitely high.

In the center the impedance equals the intrinsic impedance of the material used to construct the antenna. In this case it would appear to be 300 Ohms.

The feed point is chosen at at point where the impedance along the shorted quarter wavelength feed line is equal to the impedance of the cable that is connected to the antenna.

I have not run the numbers on ebacon's design, but his dimension (the 24 1/2" dimension) is what is left over when the distance up from the shorted end (the feed point of the antenna) is subtracted from an electrical quarter wavelength in the material used to construct the antenna.

The actual dimensions will vary depending on the velocity of propagation rating of the material used to construct the antenna. It is always a value less than 1. This means that the physical length of the antenna elements will be less than their basic electrical length.

If one looks at the various plans and antenna calculators on line, one will find a fair amount of variation in the suggested dimensions.

These antennae benefit from proper decoupling of the feed line from the antenna. For the coil choke balun, the length of the wire in the coil should be electrically a quarter wavelength long at the frequency of interest or the center of the FM band. That will result in flat wound coil of 4 turns.

Proper decoupling of the antenna and feed line serves two purposes. That is to reduce common mode RF signals on the shield of the coax that may affect the reception pattern of the antenna and to reduce common mode noise that may be picked up by the shield of the coax and enter the antenna at the feed point and travel down the feed line to the receiver.

With this antenna, as all antennae, it is location, location, location and application. Without specific information, it is difficult to predict which antenna, dipole or J-Pole will function the best.

 

[Noodle]

Wonder how it would do for transmitting? Any thoughts JBL GUY?
I got an Analyzer.

 

[JBL GUY]

Wonder how it would do for transmitting? Any thoughts JBL GUY?
I got an Analyzer.

J-Pole antennae work OK as a transmitting antenna.

The same rules apply, good RF decoupling, and correct dimensions and construction for the best performance.

I used a double set of decoupling radials similar to the picture below. This helps to insure a consistent radiation pattern. Also it is possible to tilt the radiation pattern down a small amount (lower the radiation angle).

In general, if a J-Pole antenna is used for a VHF or UHF transmitting antenna, the closer to horizontal the major lobe of radiation is the better.

This is why some prefer the "Slim Jim" version of the J-Pole antenna.

Then there is the collinear or "Super J-Pole" antenna, that when designed and constructed correctly will exhibit some gain (not quite 2 dB) relative to the standard J-Pole antenna, as shown in the second picture.

There are improvements that can be applied to this antenna, such as replacing the horizontal phasing element with a correctly designed phasing coil. This will increase the gain by almost another dB, approaching a total gain relative to the standard J-Pole antenna of just a little less than 3 dB.

Again, I will recommend 4nec2 antenna modeling software for those that have an interest in antenna design.

Some screen shots of the output of this software.

The last 2 pictures are rough examples of the output of antenna modeling software when applied to a J-Pole antenna. The third picture is the current distribution on a J-Pole antenna and the next picture is the radiation pattern for a J-pole antenna.

The feed point impedance can be improved, this is just a rough example.


And the last picture is a reminder of the "Slim Jim" version of the J-pole antenna.

 

[Noodle]

J-Pole antennae work OK as a transmitting antenna.

The same rules apply, good RF decoupling, and correct dimensions and construction for the best performance.

I used a double set of decoupling radials similar to the picture below. This helps to insure a consistent radiation pattern. Also it is possible to tilt the radiation pattern down a small amount (lower the radiation angle).

In general, if a J-Pole antenna is used for a VHF or UHF transmitting antenna, the closer to horizontal the major lobe of radiation is the better.

This is why some prefer the "Slim Jim" version of the J-Pole antenna.

Then there is the collinear or "Super J-Pole" antenna, that when designed and constructed correctly will exhibit some gain (not quite 2 dB) relative to the standard J-Pole antenna, as shown in the second picture.

There are improvements that can be applied to this antenna, such as replacing the horizontal phasing element with a correctly designed phasing coil. This will increase the gain by almost another dB, approaching a total gain relative to the standard J-Pole antenna of just a little less than 3 dB.

Again, I will recommend 4nec2 antenna modeling software for those that have an interest in antenna design.

Some screen shots of the output of this software.

The last 2 pictures are rough examples of the output of antenna modeling software when applied to a J-Pole antenna. The third picture is the current distribution on a J-Pole antenna and the next picture is the radiation pattern for a J-pole antenna.

The feed point impedance can be improved, this is just a rough example.


And the last picture is a reminder of the "Slim Jim" version of the J-pole antenna.

Good info provided here. Thank you!

 

[ebacon]

What does the 24 1/2" dimension refer to?
How does this antenna compare to a standard 300 ohm dipole?
Rick

24 1/2" is the dimension from the coax shield connection to a cut in one of the twin leads. If you look at the photos you can see the cut. There is a Sharpie pen mark near it.

As far as gain goes, my recollection is that a J-Pole has twice the gain of a Dipole (I might be wrong on the magnitude, but a J-Pole does have better gain).

As with all antennas, orientation is a big factor in how well it receives. In my case the bulk of radio stations are in Detroit which is roughly south of my house. I mounted the J-Pole pointing east-west as its reception lobes are 90 degrees to that, or north-south in my case.