How to Build a Dipole FM antenna
The antenna rod is made of 6 mm copper tube I found in a shop for cars. It is actually tubes for the breaks, but the tube works great as antenna rods. You can use all kinds of tubes or wire. The benefit of using a tube, is that it is strong and the wider tube diameter you use, the wider frequency range (bandwidth) you will also get. I have noticed that the transmitter gives highest output power around 104-108 MHz so I set my transmitter to 106 MHz.
The calculation gave the rod length of 67 cm. So I cut off two rods at 67cm each. I also found plastic tube to hold the rods and to give it a more stable construction. I use one plastic tube as boom and a second to contain the two rods. You can see how I used black duct tape to hold the two tubes together. Inside the vertical tube are the two rods and I have connected a coax to the two rods. The coax is twisted 10 turns around the horizontal tube to form a balun (rf choke) to prevent reflections. This is a poor mans balun and lot of improvement can be done here.
Copper Material
I placed the antenna on my balcony and connected it to the transmitter and turned on power supply. I live in a medium city so I took my car and drove away to test the performance. The signal was perfect with crystal clear stereo audio. There are many concrete building around my transmitter which affects the transmitting range. The transmitter worked up to 5 km distance when the sight was clear (could not obtain line-in-sight). In city environment it reached 1-2km, due to heavy concrete.
Dipole FM Antenna Construction
I find this performance very good for a 1 W amplifier with an antenna which took me 45 min to build. One should also take in account that the FM signal is Wide FM, which consume much more energy than a narrow FM signal does. All together, I was very pleased with the result.
Antenna Testing and Measuring
Thanks to a complex antenna analyser, I have been able to get a plot of the antenna performance.
The red curve show the SWR and the grey show Z (impedance). What we want is a SWR of 1 and Z to be close match to 50 ohm.
As you can see, the best match for this antenna is at 102 MHz where we have SWR = 1.13 and Z = 53 ohm. I did run my antenna at 106 MHz, where the match is worse SWR = 1.56 and Z = 32 ohm.
Conclusion: My antenna was not perfect for 106 MHz, I should re-run my filed test at 102 MHz. I will probably get better results and longer transmitting distance. Or I should make the antenna a bit shorter to match the frequency 106 MHz.
See more: Wireless FM Transmitter
