there is always an amount of personal satisfaction to be had by undertaking a bit of successful home brewing. if we can build something that works as well as a bought one, looks as good and costs much less we can consider this a big win.
when it comes to amateur radio, one area where we can achieve a lot with investing little is in the construction of antennas. this article describes the basic theory of operation and construction details for just such a winner!
a bit of background first....i was in need of a dual band antenna to use with a vhf/uhf fm rig in the shack. like many of the radios in current production, it has a single antenna output to cover both bands. commercially made dual band antennas are available at varying prices and it was the matter of price that presented the obstacle. after considering a few alternatives i decided that a simple j pole could be built quite cheaply, would be a solid performer, and although not apparent at first, could be easily made into a dual-band antenna with a single feed line and no duplexer. i set about building two of these antennas, one for home base use and the other for out portable in field days and other events. construction of both antennas took part of an afternoon and as they were made with materials from the ever-increasing 'junk box', they cost basically nothing to make.
how it works. the idea of two bands from a single band j pole is quite simple. a dipole will provide a good match and radiate at the third harmonic of its fundamental frequency. if we multiply 146 mhz (the centre of the 2 metre band) by 3 we get 438 mhz - near enough to the fm portion of the 70 cm band. as the j pole is an end fed half wave - the same principle applies. however, providing a match and radiating efficiently are two different things. the method of getting this j pole to work more effectively on the third harmonic is by means of a quarter wave stub, which decouples everything above the required half wave radiator on the uhf band. on vhf, this decoupling stub is just seen as a bit of inductance, which shortens the vhf radiator length slightly, having no major effect on 2m performance. the decoupling stub is a piece of rg-58 or rg-213 coaxial cable cut to a quarter wave in electrical length and shorted at the top. on the uhf band, this stub presents high impedance and cuts off everything from the stub upwards. the included photographs will provide a much clearer explanation of how the unit is put together.
another thing you will notice from looking at the pictures of the example antennas is that they are in fact not made from aluminium or copper, as the common image of a j pole seems to be, but are in fact made out of ordinary old 300 ohm tv ribbon. there are several good reasons to use this material: it is cheap, much easier to work with and can be easily remade if a mistake occurs such as in the final tuning process. it starts to get expensive replacing aluminium!
ribbon j poles work as well as their aluminium counterparts and are much lighter in weight, less expensive to build and easier to weather proof. keep in mind at this point, for those of you that may not have looked at j poles before, the 300-ohm section (or indeed, the bottom quarter wave section of any j pole) does not radiate. this portion of the antenna uses a parallel stub a quarter wave long at 2m. any radiation is cancelled - similar to the way that feed line radiation is cancelled in parallel lines of hf antennas. as the end of a half wave presents high impedance relative to the coax, the quarter wave stub provides a means to match the two. the point of connection for the coaxial feeder is in fact the 50-ohm point of the matching stub.
both antennas are housed in fibreglass radomes making weatherproofing simple. one of the antennas has been specifically made with portable operation in mind. the smaller of the two units that are pictured is light enough to be used on top of a telescopic fishing pole extended to 25 feet high. this happened to meet another one of my own criteria when first looking for an antenna to build - portability. i wanted something that would work well and could be made light enough to go up on the fishing pole for portable work - not wanting to cart lengths of metal tubing around with me. this whole set-up can easily be carried in any vehicle without the need for roof racks or other means of carrying metal tubes and it can be erected and on air in minutes by just one person – without the use of any tools!
construction.to begin, prepare the 300-ohm ribbon as shown. add the uhf half wave wire and trim this wire to tune the antenna to uhf. next, prepare the decoupling stub as shown, either rg-58 or rg-213 cable is fine. (i did find that with 20 watts of tx power the rg-58 starts to warm up after a minute of so of transmission, not the case with the rg-213.)
start with the stub a little longer than the length given and slowly trim the stub from the bottom to tune it for lowest swr once the top of the stub is shorted, leave it as is. soldering of the coax will heat up the dielectric of the cable causing it to change value, if you tune the stub without first soldering it, and then solder later on, you will find the j pole out of tune significantly! it is far easier and more accurate to leave the shorted top of the stub alone and slowly trim the bottom, each time reattaching it to the radiating wire. trim very carefully - 1mm here can make a difference of 1 mhz! alternatively, you could simply short the coax with a pin but this method may cause problems with the antenna going out of tune after some time in service & variations in temperature.
once these steps have been completed, attach the top vhf radiator and trim to tune on the 2-metre band. it will have no effect on what you have just done on uhf.
the antenna should cover 146 - 148 mhz with an swr of 1:1 and 433 - 439 mhz with an swr of no more that 1.5:1 at the two extremes. it will have a nice flat dip in the centre of this section and in fact, both my antennas came in at better than 1.5:1 on uhf. not bad for 6 mhz bandwidth!
although not essential, i opted to make a choke of about 10 turns of the main feed line just under the feed point of the antenna to decouple the feeder from the antenna and minimise any feed line radiation.
points to note: the measurements given should be used as a starting point, as the final application of the antenna will change the measurements and tuning slightly. for example, this antenna made up without a radome is slightly longer that with the radome.
if pvc conduit or water pipe is used to house the antenna it could well tune differently to the measurements supplied here. both the radomes used in the construction of these antennas are fibreglass. the 300-ohm ribbon is quite capable of handling the output power levels of current transceivers in the 50-watt range on vhf and 35 watts on uhf.
on the air..... both antennas are excellent performers when it comes to the business end of the project. all of the repeaters in my area, both on 2m and 70cm (some of which are quite a distance from my qth) are worked almost as well as with a commercially made dual band vertical about 20 feet higher on the tower. my enjoyment of this project was enhanced when i could see how well these simple wire j poles worked. also, i had all the materials on hand already so they cost me nothing to make!
so what have we done?there we are, a need for a couple of antennas was satisfied at the best possible price, nothing, and the two antennas provided me with an enjoyable afternoon of construction that resulted in an article that performs, in my experience, as well as anything i could have bought. the bought one wouldn't have been as much fun!!
i would be pleased to hear from anyone who has a bash at making one of these antennas to hear how it worked out. replies can be directed to
vk3dmp@wideband.net.au references: adapted from an article by edison fong, wb6ion (online) from february 2003 qst - arrl.
1. completed portable antenna – very lightweight but solid performance. pictured here with coax and telescopic mast.
2. here is the antenna being assembled on a fibreglass support.
3. quarter wave 300-ohm matching section – note the wire attaches to one side only!
4. quarter-wave uhf decoupling stub made from rg-213
