A Cheap Homebrew 2m Antenna
A simple 2m antenna can be made out of antenna twin feeder. The antenna has good gain and is easy to make. It does need to be erected away from metal things and needs a simple choke at its base – easily made of 4 turns of coax 6 inches in diameter a few inches (4”-5” max) from the attachment point of the coax to the antenna . The details vary a bit depending what type of twin feeder you have, there are two common types – slotted 450 Ohm Black feeder and 300 Ohm solid clear plastic feeder sometimes used for old TV antennas. I will use this 300 Ohm TV feeder as I have stacks of it (RS used to sell it). If you want some of it just ask.
450 Ohm Slotted feeder to a homemade HF dipole Clear 300 Ohm TV feedline being prepared.
The aerial below is known as a Slim Jim. Wikipedia says G2BCX. Fred Judd designed it in 1978. The ‘J’ in the name is because it is related to a J-Pole, it’s a folded dipole version of it. Otherwise I suppose it would be called a Slim Fred. The layout has three main characteristics. It is a folded dipole half-wave vertical (an end fed half wave) fed by a quarter wave of feedline (the bottom third of what you build) which acts as a transmission line transformer, finally there is a tap on the transmission line to match the 300 Ohms of the feedline to the 50 Ohms of the coax. Quite a lot going on but we can ignore the theory and just build it.
Also as usual, the theory is wrong in real life. You must experiment a bit with every antenna, my cable was sold by RS, other cables may behave differently. I needed 61 inches despite the theory saying much less (53” taking my measured VF into account) It has the gain of a dipole, despite what some manufacturers say.
In Imperial units you need only remember that it is 58 to 61 Inches long, with top and bottom shorted. There is a one inch gap that starts 19 inches up one side and the coax is attached roughly 4 inches up from the bottom with the shield of the coax going to the side with the gap in it.
You might need to move the coax feed point plus or minus half an inch to get a good SWR. At 58 inches long my best SWR was 2.2 to 1 at 145.125MHz with the antenna hanging in free space (3 or 4 feet away from the walls in my shack. At 61 inches long I get 1.12 to 1 with the feedpoint at exactly 4 inches.
Keep your SWR ideally below 1.5, although 2 to 1 will work ok. The SWR will change when you hang it up, or stick it in a plastic pipe. Some report a 5% drop in frequency inside a PVC pipe. I think the overall length is more important than the feedpoint – the lack of a pronounced resonance – and more importantly the lack of symmetry about resonance are indications of common mode current – flowing down the outside of the coax, the turns of coax are not doing a very good job, they are better than nothing and they will get you on the air
Some turns of Coax are vital – otherwise you get RF in the shack and your SWR meter will “go strange”, your lips will tingle when you kiss your microphone, don’t kiss your microphone. NB, my feeder needs 61 inches, despite my measuring its velocity factor as 0.86. I need to do further research to check my measurements.
There are several Slim Jim Calculators online – they give lengths to two decimal places, you can type in a desired 2m Frequency, and get super accurate calculated values… that will be wrong.
Inevitably you need to experiment and adjust if you want a really good SWR.
Here are some graphs of performance, note this is still work in progress, Note that taping the antenna to a small (15x10mm) piece of wood affected the SWR, note that inserting this wood and antenna up a wide PVC pipe affected the SWR and pushing the antenna alone up a narrow PVC pipe affected the SWR.
I will experiment further and put final construction details on my blog at https://MI5AFL.blogspot.com. Finally I am happy to give anyone some ribbon cable and a length of coax if you want to make your own. Email me at MI5AFL@arrl.net if interested.
