Jock Elliott wrote: ↑Thu May 04, 2023 11:39 am
Recently, I became interested in chasing down some low-power community FM broadcast stations.
Yes, I know it is possible to build yagis to cover the commercial FM broadcast range, but I was hoping for something more compact and more readily portable...
Cheers, Jock
Hi there, Jock, you may try the "jungle job" from Gordon "Dick" Bird (G4ZU - SK), the antenna is really simple and can be seen at the bottom of
this page (and in the image below)
- junglejob.png (218.12 KiB) Viewed 20757 times
basically it's a two elements Yagi antenna with the radiator element bent to form a "V" shape, while the image shows the antenna built using aluminium pipes for boom and reflector and wire for the dipole, it can be built using wood (or telescopic fishing rods) for the boom and reflector support and wire for the dipole and reflector; the gain is around 10dBi and the antenna shows a nice directional pattern with a front/back ratio about 19dB, the feedpoint impedance is in the range of 50 Ohm so it matches the standard coax, although a 1:1 balun could be used to avoid distorting the pattern due to imbalances, the
image below shows what to expect from such an antenna (from a quick NEC simulation)
size is quite manageable, the calculations are shown in the first image above, the antenna can also be used for TX and the size remains manageable for antennas calculated up to the 20 meters Ham band oh and... by the way it will work fine for the 2m band
Forgot, as for all horizontal antennas, installation height has an effect on the pattern and the gain, a reasonable and manageable height will be 0.60 Lambda (reason explained
here) which, for the FM band would be around 1.83m (6ft)
NOTE: the formulas given for lenghts will give them in meters
just to be totally clear
Frequency 98.000 MHz
Lambda (300/MHz) = 3.061 meters
Reflector (150/MHz) = 1.530 meters
Dipole (147/MHz) = 1.500 meters (each arm=0.750 meters)
Boom (0.17 lambda) = 0.520 meters
Height (0.60 lambda) = 1.836 meters (recommended, not mandatory)
HTH