my HAM radio page
Radiation of radiowave happens via Antenna.
We know many different antennas but we’ll try to explain different types :
Random wire means to use undefined wire length (not related with wave length) mostly used with Antenna Tuner for better balancing. Consist of two wire undefined characteristic. Two wires we call radiation (or hot) and counterpoise (cold, grounding). Radiation wire can be in different position – vertical, horizontal or in between/under angle – sloper. Counterpoise should be laying on the ground in direction of radiation.
Here are some hints of length to USE (so no quite random):
RW length to USE (m) | 128,9 | 124,1 | 105,8 | 61,9 | 45,1 | 36,3 | 32,6 | 25,6 | 21,6 | 17,7 | 12,5 | 10,8 | 8,8 |
RW length to USE (ft) | 423 | 407 | 347 | 203 | 148 | 119 | 107 | 84 | 71 | 58 | 41 | 35,5 | 29 | ft |
Here are some hints of length to AVOID (so no quite random) :
in meters
Frequency MHz | 1/2 Wave | 2nd Multiple | 3rd Multiple | 4th Multiple |
1,9 | 75,0 | 150,0 | 224,9 | 299,9 |
3,8 | 37,5 | 75,0 | 112,5 | 150,0 |
7,2 | 19,8 | 39,6 | 59,4 | 79,2 |
10,1 | 14,0 | 28,0 | 42,1 | 56,1 |
14,2 | 10,1 | 20,1 | 30,2 | 40,2 |
18,1 | 7,9 | 15,8 | 23,8 | 31,7 |
21,3 | 6,7 | 13,4 | 20,1 | 26,8 |
24,9 | 5,8 | 11,6 | 17,4 | 23,2 |
28,5 | 4,9 | 9,8 | 14,6 | 19,5 |
in feets
Frequency MHz | 1/2 Wave | 2nd Multiple | 3rd Multiple | 4th Multiple |
1,9 | 246 | 492 | 738 | 984 |
3,8 | 123 | 246 | 369 | 492 |
7,2 | 65 | 130 | 195 | 260 |
10,1 | 46 | 92 | 138 | 184 |
14,2 | 33 | 66 | 99 | 132 |
18,1 | 26 | 52 | 78 | 104 |
21,3 | 22 | 44 | 66 | 88 |
24,9 | 19 | 38 | 57 | 76 |
28,5 | 16 | 32 | 48 | 64 |
source : http://www.hamuniverse.com/randomwireantennalengths.html
dipole to have symmetrical antenna (hot and cold ‘wires’ are the same length). We can divide them :
Popular type of antenna with asymmetrical length of poles. result of asymmetry is high-impedance (2,000-4,000 ohm) wire antenna that resonates on its fundamental frequency and all harmonics above. This antenna needs to be balanced (to bring the wire’s high impedance feedpoint closer) to the desired 50 ohms with baluns (unun 49:1 or 64:1), which eliminates the need for an antenna tuner in most cases.
A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor usually fed by a balanced source or feeding a balanced load.
‘Magnetic’ because they pick up the magnetic component of an electromagnetic field, unlike the usual antenna forms (dipoles, yagi antennas, vertical antennas), which only respond to the electrical component.
The small loop antenna is known as a magnetic loop since it behaves electrically as a coil (inductor)
Magnetic antennas can be built very compact, good for limited spaces or ‘mobile’ use, e.g. on boats with limited space. Radials or other counterweights are not necessary, the radiation is largely independent of the distance of the antenna to the ground.
At the resonance frequency, magnetic antennas are quite narrow-banded. This has the advantage that on the one hand only few harmonics are radiated, on the other hand the antenna acts as a very narrow-band additional preselector and thus prevents overdriving of the usually quite broadband receivers by signals far away from the receiving frequency.
Nice practical MLA is construction of G4FON. Antenna uses Coax RG-58 or better it’s shield (3,5mm). I changed to RG-59 with shield diameter 4,4mm. Feeding comes on bottom via toroid FT114-43 (40m -5 turns, 30m-3 turns, 20m-2 turns). On top is variable capacitor (or trimmer via switch to precalibrate)
Some more info:
There are many coax cables for different operations (and different characteristics):
Different types of Connectors or better RF connectors :
Balun (BALanced-UNbalanced) and Unun (UNbalanced- UNbalanced) are used to improve transmitter (transceiver) to antenna radio transmission.
Traps are in short frequency switches.
Choke are meant to prevent signals passing along the outside of the braid. It’s simple device can be used to cure television interference by acting as a braid-breaker.