- NOT done
Many hams never take on building an antenna. But for those who do the antenna can become a challenging project.
I'm going to cover building 5/8 wave j pole antennas for the popular 2 meter and 70 centimeter ham bands. With the building blocks I provide you should be able to build an antenna for other ham bands as well.
Here are a few general tips to follow when building a j pole antenna.
1. Take your time and measure. The more accurate you are at measuring the greater your success will be building an antenna. For example at 2 meters 1/2 an inch off could result in an antenna that doesn't work as expected.
2. If you are mounting your antenna in pvc pipe then secure it to the outside of the pipe while testing. You want to make sure your antenna works before you cut up that pipe.
3. Get an SWR meter before you build. You may be able to get by, but sooner or later you're going to have to get one.
The J pole antenna is an end fed dipole antenna. Instead of a 1/2 dipole I am using a 5/8 wave dipole. The dipole is end fed by a 1/4 wave shorted end transmission line. The coax from your transmitter is connected to the transmission line. See the figure below.
Lets take a look at the different elements of the antenna.
All J pole antennas are going to have this matching network. Instead of using twinlead we will make our own using small spacers. The reason is we want to use an air dielectric transmission line. If we use twin lead or ladder line with a solid dielectric then we will have to deal with the velocity factor of the materal. In terms of construction it means that your measurements need to be more accurate.
The basic J pole will have 1 of these elements. 5/8 wave dipole was chosen due to its slightly higher gain.
If you want to stack multiple 5/8 wave radiators you will need a 3/8 wave phasing element. The job of this element is to have the dipoles radiating in phase with each other.
The 1/4 wave matching line is balanced. The coax from your radio is unbalanced. The 1:1 balun will stop RF from traveling down the outer conductor of the coax. Some sources on the internet say you may not need it. From my experience you do and should use one.
The balun I'll describe will consist of a few turnes of coax. I wind mine on 1 inch pvc pipe.
I ended up having an unusual RF detector. My shack has an animated witch that makes noise when you make a loud noice. Turns out the hag is sensitive to RF as well. Without the balun 1 watt of transmit power will bring her to life.
Below are some antennas you can build. I used the same construction methods for each antenna. The difference between the antennas are the demensions.
Below is the diagram of the J pole antenna. The sections are labeled A through D. The instructions below will describe how to construct each section.
Building an antenna can be a rewarding project. I hope you take the time to build one.