Antennas for young DXers

When you look through the magazines and talk to other hams about DXing you always think about tall towers and big beam antennas. Erecting a tower and big mono band beam can be very expensive, labour intensive and takes much paperwork with the local council. However, most HF operators have simple wire antennas and still work the DX. Wire antennas are less expensive to make, are fun to erect and can give outstanding results.

There are many articles in plenty of ham radio publications including the ARRL Handbook and RSGB publications showing many proven designs for home brewing. I’ve built many wire antennas especially dipoles for various frequencies from 160m to 10m and more recently loops for 80m and 40m. The standard dipole antenna is by far the most versatile and easiest to build and set up and they work well. Remember the dipole is the standard by which all other antennas are compared. The term dBd refers to the gain compared to a dipole. A dipole will provide thousands of contacts and at a fraction of the cost of a tower and Yagi. Particularly for the lower bands of 40m and 80m for which Yagi antennas get a bit big for most NZ house and sections.

The dipole

The basic dipole antenna in the horizontal or ‘inverted V’ configuration is a ‘balanced’ antenna system. Single band dipoles are very simple to build and erect plus they require no ground system to work. Use the tried and true formula for calculating the overall length of the dipole and trim to suit. Formula is Total length = 468/freq in MHz. However, this gives results in “feet” which is an old Imperial measurement. I always use Total Length = 300/ freq. in MHz. This always gives a length 10% too long and that is good.

The term ‘high and clear’ is seldom the case for a real antenna. In the real world they are usually end up mounted low and close to trees, houses, power lines etc. that will detune them. Having the wire a little too long means you can trim it in length to suit the situation. Use an antenna analyser to set up the antenna. Remember if the dipole is moved or raised/lowered then the resonant frequency will change and the length will need to be adjusted.

Remember your antenna basic theory? You’ll recall that an antennas resonant frequency depends on its overall length. Usually corresponding to a low SWR. If you find that the antenna resonates at a frequency too high, you will need to add wire to each leg. Messy. If the antenna resonates at too low a frequency, you will need to shorten the legs. Much easier and less stressful.

Unless a dipole is located at least a half wavelength above the ground, the radiation will be a little bit omni directional (all directions) but with a strong preference to broadside.

Inverted V antenna

By mounting a dipole in an inverted V configuration a more pronounced omni directional pattern will result. There is an advantage with an inverted V in that only one high support if required but the ends should be kept well above touching height. The ends will have high voltages present when transmitting that can surprise a person or animal that touches them.

Balun

Both the dipole and inverted V antennas can be fed with coax satisfactorily but a balun will help with the radiation patterns. A simple balun (Balanced to un-balanced transformer) can be made with a coil of coax at the antenna connection. Use about 6 to 8 turns coiled at about 150mm diameter. There are references for coax baluns on the internet and many ham radio publications. 

Off Centre Fed Dipole (OCF)

The OCF dipole also known as the Windom antenna is a multi-band antenna that depending on its dimensions will operate well on 80m, 40m and 20m without an external matching unit. The original Windom antenna was fed with a single wire feeder but they will work fed with coax and the use of an ATU will allow multi-band operation. The balun at the feed point for these antennas should be a 1:4 or 1:6 current transformer not a coil of coax. While a Windom antenna is easy to build it is best to build one from a published design. This will ensure it will work first time and be less stressful. They are also available from various manufacturers but its not so much fun and in my view not in the spirit of ham radio.

The offset feed point of these antennas can be useful in some situations where the short leg can go towards the front of the house and the long leg to the rear. Remember, as with a wire dipole the legs can be bent and even drooped downwards if necessary.

Shack RF ground

As I mentioned in the last column, any antenna designed as ‘balanced’ will not be actually balanced in the real world. Therefore there will be common mode currents on the feed line whether twin feeder or coax. While the balun will help here, it is also important that a good RF earth is present in the shack.

A minimum earth system for the shack would be to use a single ‘bus bar’ and connect all and each radio item directly to the bus bar with heavy electrical cable or braid. Then connect the bus bar to at least one 2 metre ground rod via either a copper strap of 20mm x 3mm or very heavy electrical cable, as short as practicable. I consider this the minimum and it will help with stray RF in and around the shack, provide quieter receive and will help with TVI when the XYL is watching Shortland Street!

Summing up

Trees, while being a nuisance in tangling up the antenna wires when trying to erect a wire antenna, can also be your saviour. They make good supports. Not ideal when in full leaf and wet but better then nothing.

Try to erect the antenna as high up as practicable and keep it clear of roofs etc if possible.

If using coax use good quality new coax. RG-213 preferably but if small diameter coax must be used buy the best quality available with a good screen. Not the cheap CB type that will let you down.

Beg, steal or borrow a good quality antenna analyzer. There was a description of a home brew analyzer recently in Break-In. I have used one of these and they are good. Build your own like a real ham, you won’t regret it.

Like the speaker of an audio system, the antenna is the single most important item of the ham system. Time spent on planning and installing your antenna will pay dividends in the number of DX contacts.

Finally, think safety always! Keep clear of power lines and use personal safety equipment. You can’t work that illusive DX from a hospital bed or worse still, the grave.

Take a look at the following web site for useful information. www.chem.hawaii.edu/uham/antennas.html

See you in the pileups Mike ZL2CC