Figure 1
This bit of the webpage is about making some simple antennas for 2m radios. The requirements I had were:
I got fed up with the rubbery black antennas that come with radios. Yes they work, but they are pretty crummy. If weeny rubber antennas worked they wouldn't use huge fiberglass coated antennas on masts! A small bit of ingenuity and bodging can easily come up with a twig that will work much better than a rubber duck antenna or dangly wire. The antenna is the single most important thing in the radio system, as it's what launches your signal into space and what collects other people's signals. Simply, if your twig doesn't cut it then you won't hear anyone and they won't hear you!
This page has a few ideas for simple antennas for 2m radios which are efficient and work well. They work just as well for receiving as well as transmitting (as is required by Mr Maxwell).
This is probably the simplest sort of antenna you can build. It is also very efficient and quite compact. A halfwave antenna is simply a piece of conductor that is electrically half a wavelength long. In the 2m band, a wavelength is 2m(!) so a halfwave antenna is approximately a metre long. It can be connected to the radio at either the end or in the middle, although different methods are needed.
The basic principle is to get two 1/4 wave conductors, and place them in a line. The connection to the radio is made in the middle via a short coaxial cable. The centre bit of the coax goes to one are, the shield to the other. The arms are each 50cm long.
Figure 1
Figure 1 shows a photo of a half wave antenna I built. The two arms are made of 2mm diameter enamelled copper wire. The centre of the antenna uses a small piece of circuit board I bodged using a knife
Figure 2
The circuit board has 3 small holes drilled in it to make a strain relief for the coax cable. I used thin cable (RG174 thickness, but silver plated wire with teflon dielectric so not RG174 crapness) with a SMB connector on the end because that's what my radio uses (now that I've put an antenna connector on it)
Figure 3
Figure 3 shows the end folded over to make it harder to poke your eye out with. Figure 4 shows that the thing is portable as it rolls up (but only a few times or the wire breaks!).
Figure 4
If you don't want to solder then you can replace the circuit board with a chocbloc connector which has the advantage that it can be unscrewed to pack away. Purists would argue that chocblocks are rated for 50Hz maximum not 144MHz, but one purist built such an antenna and discovered that it worked very well so ner.
A halfwave dipole has an impedance of 73 ohms, so the match to a 50 ohm cable (and radio) is not perfect but the loss is less than a dB. The performance gain from a halfwave dipole over a rubber duck antenna is very great, but has to be weighed against the fact that it is a metre long. Folding up the antenna will affect its performance to some degree. I've used my antenna inside my paragliding harness where it can be folded slightly to fit. Dangling it below would work even better but might fall off or snag on an obstacle on take off (bye bye radio).
The antenna produces linearly polarised waves, along the same axis as the wire so holding it vertical will produce vertical polarisation. For maximum effect, the receiving and transmitting antennas should face each other. A halfwave dipole antenna has a gain of 1.6 over an isotropic radiator, in the direction perpendicular to the axis of the antenna. Theoretically there is no radiation along the axis of the antenna.
I measured a VSWR of better than 1.5:1, at the centre frequency, and a VSWR of better than 2:1 over an 8MHz bandwidth. If you have the toys - VSWR meter + transmitter, network analyser etc. then tuning the antenna is achieved by snipping bits of wire of the ends of the antenna. Snip the same amount off each side! Shorter antennas work at higher frequencies. If you don't have the toys, 50cm long arms will do ok!
A Dipole has a low impedance at the midpoint (73 ohms) and is easy to drive using a balanced feed or coax, but has the inconvenience of needing access to the middle of the antenna. The impedance at the END of a halfwave dipole is high (theoretically infinite but practically 1-5k ohm) and needs some cunning to match it to a 50 ohm feeder. Simply connecting it to the coax (what do you connect the shield to?) will not work .
Figure 5
Figure 5 shows an end fed dipole made from 300 ohm TV twin lead feeder. This consists of
This antenna should behave electrically as a halfwave dipole. The advantage of this antenna is that it feeds from the bottom - performance wise it is no better than a halfwave dipole antenna.
Basic instructions
Figure 6,7
This antenna needs to be held out straight to work properly. Mine didn't work very well, (poor VSWR) which I might look into one day. It has got potential though.
Rubber duck antennas (and dangly coax antennas) are based on the fact that a halfwave dipole antenna can be simulated by making one half of the antenna reflect into the ground (Figure 8).
Figure 8
This is where the rubber antennas and dangly coax antennas fall down they don't have a decent ground. A decent ground needs to be a good conductor and at least 1/2 wavelength in size. This is how magnetic mounting mobile phone car antennas work, and they work well. The impedance of this type of antenna is 1/2 that of a 1/2 wave dipole ie 37 ohms. This still matches OK with 50 ohm coax.
This type of antenna is used for low frequency broadcast antennas where a very tall mast is used as a radiator working against the ground which has been bolstered up with wire to improve its conductivity.
A cunning method of simulating this type of ground is to use radials, which are conductors leading out radially from the centre. The impedance of the antenna can be tweaked to exactly 50 ohms by making the radials droopy. Figure 9 shows a groundplane antenna.
Figure 9
Each conductor is 1/4 wavelength long (50cm), and the radials droop at about 45 degrees. Geek toys allow you to adjust the angle of dangle and conductor lengths for maximum effect.
A clever way of making such an antenna is to build it out of a connector. Figure 9 is made from an N type connector (BNC, SO239, SMA etc will also work but are less robust so might fall to bits).
Figure 10
This type of antenna is ideal for mast use as it is relatively insensitive to metal below the radiator - it is shielded by the ground radials. A halfwave dipole works best if there is no metal within 1 wavelength or so which makes mounting it a bit tricky.
I use this antenna for listening to the aircraft band - It can pick up aeroplane telemetry from all around Europe if elevated on a mast.
I might put a more technical page up in future, (current status)
Have fun, and don't poke your eye out,
Hoppy. 7/8/03
A good explanation of how to build a 300 ohm feeder J pole can be found at http://www.packetradio.com/ant.htm although owing to the temporary nature of the internet, this link no longer works. Google and "J Pole antenna" will do it.
Toys at http://www.rohde-schwarz.com
Loans to buy toys at http://www.lloydstsb.co.uk