any reasonably-sized directional antennas for FM broadcast radio reception?

[Jock Elliott]

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

[Andrew (grayhat)]

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 23387 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

[Andrew (grayhat)]

as for the Jungle-Job, in case someone is interested, here's the 4NEC2 model for the antenna, feel free to play with it as desired

Code: Select all

CM G4ZU "Jungle job" antenna (2 element compact beam)
CM https://www.essexham.co.uk/gordon-dick-bird-g4zu-information
CM feedpoint impedance around 50 Ohm, use 1:1 balun
CE

' frequency
SY freq=29.000              ' design frequency
SY wave=(300/freq)          ' wave length

' materials
SY copp=58000000            ' copper
SY alum=37700000            ' aluminium
SY stel=1390000             ' steel

' radius
SY wrda=0.0125              ' alum.pipe 25mm diam.
SY wrdb=0.0100              ' alum.pipe 20mm diam.
SY wrdc=0.0080              ' alum.pipe 16mm diam.
SY wire=0.00125             ' copper wire 2.5mm diam.
SY pole=0.025               ' support pole

' calc factors
SY dfac=144                 ' dipole factor (147)
SY rfac=145                 ' reflector factor (150)
SY bfac=0.236               ' boom factor (0.15...0.17)
SY hfac=0.60                ' height factor

' height, arms, boom
SY hght=(wave*hfac)         ' height from ground
SY darm=(dfac/freq)/2       ' single dipole arm
SY rarm=(rfac/freq)/2       ' single reflector arm
SY boom=(wave*bfac)         ' boom
SY bomb=(boom/3)            ' backside boom
SY bomf=0.05                ' forw boom

' reflector
SY rsec=(rarm/3)            ' reflector sections
SY rena=rsec                ' end of first section
SY renb=rena+rsec           ' end of second section
SY renc=renb+rsec           ' end of third section

' dipole vertex angle
SY angl=97                  ' angle between arms
SY anga=90-(angl/2)         ' vertex angle of arm 1
SY angb=180-anga            ' vertex angle of arm 2

' segmentation
SY segl=81                  ' segments
SY segm=11                  ' segments
SY segs=5                   ' segments

' feed
SY fwir=1                   ' feed element
SY fseg=1                   ' feed segment

' start of geometry

' dipole (copper wire)
GW  1 segl     0    0 hght -(darm*sin(anga)) -(darm*cos(anga))  hght wire
GW  2 segl     0    0 hght -(darm*sin(angb)) -(darm*cos(angb))  hght wire

' reflector sections (aluminium pipes)
GW  3 segm -boom     0 hght           -boom             rena   hght wrda
GW  4 segm -boom     0 hght           -boom            -rena   hght wrda
GW  5 segm -boom  rena hght           -boom             renb   hght wrdb
GW  6 segm -boom -rena hght           -boom            -renb   hght wrdb
GW  7 segm -boom  renb hght           -boom             renc   hght wrdc
GW  8 segm -boom -renb hght           -boom            -renc   hght wrdc

' boom (aluminium pipe)
GW 10 segs -boom  0 hght  -bomb  0 hght wrdb
GW 11 segs -bomb  0 hght  -bomf  0 hght wrdb

' mast (steel pipe)
GW 12 segm -bomb 0 hght -bomb 0 0 pole

' end of geometry
GE 1

' ground parameters
GN  2  0  0  0  13  0.005

' elements loading

' dipole
LD  5  1  0  0  copp
LD  5  2  0  0  copp

' reflector
LD  5  3  0  0  alum
LD  5  4  0  0  alum
LD  5  5  0  0  alum
LD  5  6  0  0  alum
LD  5  7  0  0  alum
LD  5  8  0  0  alum

' boom
LD  5 10  0  0  alum
LD  5 11  0  0  alum

' mast
LD  5 12  0  0  stel

' enable ext kernel
EK

' feeding
EX 0 fwir fseg  0  1.0 0.0

' test frequency
FR 0 1 0 0 freq 1

' end of model
EN

note; the most acute readers will notice that, in the model, I sized the boom at 0.236 lambda, that value popped out while trying to improve the antenna and it serms to be a good pick, just try to change the frequency (freq in the NEC model) and check it by yourself

[Andrew (grayhat)]

Another easy to build and quite efficient antenna is the H-DoubleBay by W4GRW, the antenna can be built for the HF or for the V/UHF bands, the original website disappeared in the internet limbo, but luckily was archived so a good copy is still available here

http://web.archive.org/web/202102160504 ... hdoublebay

the image at the front page shows the antenna built using just wires, spacers and ropes, and that's ok for use on HF bands where the antenna can have
a quite big size, but it can be easily built for VHF using just some plywood and wire and will result in a pretty manageable antenna

http://web.archive.org/web/202102230354 ... conference

http://web.archive.org/web/202102230354 ... ebayphotos

a standard "doublebay" (not a quad or octa one) for VHF could probably be built using a broomstick, some pieces of PVC pipe (horizontal spacers) and some wire, the performance will be pretty good, the NEC model shows a gain around 10.6dBi with low takeoff angle and a nice bidirectional "8" pattern, and then that antenna won a lot of VHF contests, so it may fit well for hunting faint transmitters

Here's an example, this one is for the 2 meters band

hdb_144.jpg (163.89 KiB) Viewed 23324 times

[Jock Elliott]

I wonder if the jungle job could be assembled using replacement telescoping antennas that are sometimes available for shortwave radios?

Three telescoping antennas, a bit of wood and some wire?

[Andrew (grayhat)]

I wonder if the jungle job could be assembled using replacement telescoping antennas that are sometimes available for shortwave radios?

Three telescoping antennas, a bit of wood and some wire?

well, yes, if the antennas are long enough they will fit as a reflector, add two pieces of wire for the dipole and some thin rope to tie (and insulate) the ends of the V shaped dipole to the ends of the reflector, a broomstick for the boom which will hold the telescopic antennas reflector and the dipole center and there you go