Friday, June 29, 2012
SMALL BACK YARDS - THE BANE OF HAMS
It seems that there is an assumption that if you have a small backyard you are doomed to some sort of compromise antenna. My response to that dire prediction is, “It isn’t necessarily so.”
If you have 70 feet in a straight line that you can put up a wire even at 15 feet above the ground, you have what it takes to make a simple (no traps, no ground radials, no compromise) antenna that will work well from 40 meters through 10 meters and even 6 meters. (Please remember to be cautious of power line when installing any antenna system. They’re real killers.)
It turns out that the simplest antenna to build is one of the best radiators of RF. Sixty seven feet of #14 copper wire with a convenient length of either 300 Ohm TV ribbon or better 450 Ohm ladder line for feed line will do the job very nicely. Bring the feed line down as perpendicular to the antenna as possible. It does not have to be perpendicular to the ground as long as it stays 8 feet or so above the ground. When you get to where you want to enter your house install a balun. Here a 4:1 balun will work with most tuners and 450 Ohm ladder line, but I prefer a 9:1. In my experience more tuners, both built in and stand alone, perform better with 450 Ohm ladder line and a 9:1 balun. If you use 300 Ohm TV ribbon then a 4:1 is a better choice unless you can find a 6:1 balun which would be ideal. You should be aware that TV twin lead has more loss than ladder line, but both beat an all coax feed line in this application by a country mile.
This antenna performs as a common ½ wave dipole on 40 meters. On the higher bands it behaves as a collinear. That is why you want to feed it with twin lead or ladder line. Ladder line is preferable because it has significantly less loss than TV lead. You may be tempted to put the balun right at the feed point of the antenna and then run coax all the way back to the transceiver. That will work but the loss in the coax will be very high especially 17 meters and up.
Here’s why. There is a common misconception that by putting a balun in the feed line even when there is a high SWR that somehow the balun will correct the mismatch. This is just not true. The balun only transforms the mismatch to another value that may or may not match the feed line impedance. The feed point on this antenna and most collinear runs from 20 Ohms to 800 Ohms depending on the frequency you are running. The following table compares the loss between a balun at the antenna feed point and 70 feet of RG58 and 60 feet of 450 ladder line between the feed point and a 9:1 balun then 10 feet of RG58 to run through the wall to the tuner and transceiver in the ham shack.
BAND 60 ft ladder line and 70 ft RG58 with a balun 70 ft RG58
10 ft coax w/balun at the ant feed point no balun
--------------- ------------------------ ----------------------------- --------------
6 meters 8.52 dBi ERP 5.84 dBi ERP 3.93 dBi ERP
10 meters 7.84 dBi ERP 4.89 dBi ERP -2.66 dBi ERP
12 meters 8.24 dBi ERP 6.10 dBi ERP 0.21 dBi ERP
15 meters 6.96 dBi ERP 4.65 dBi ERP 4.50 dBi ERP
17 meters 8.34 dBi ERP 5.79 dBi ERP -1.58 dBi ERP
20 meters 6.83 dBi ERP 4.02 dBi ERP -3.14 dBi ERP
40 meters 5.71 dBi ERP 4.37 dBi ERP 5.67 dBi ERP
ERP – Effective Radiated Power for the antenna system
Note – I did not have the necessary loss specs for TV twin lead for this comparison, but it will fall somewhere between 60 ft ladder, a balun with 10 ft of coax arrangement and the all coax with a balun arrangement.
Another misconception is that an antenna must be resonant to radiate. This is also not true. The loss of signal strength in a non-resonant antenna system is almost always due to high loss in the feed line caused by mismatch between the feed line and the antenna’s feed point. A low loss feed line such as ladder line, TV ribbon, or open wire feed line carries the transmitter’s power to the antenna where it is radiated efficiently despite high SWR cause by non-resonance. The performance of a non-resonant antenna will be comparable to a resonant antenna when feed line losses are kept to a minimum. However, you must still do something to prevent the high SWR from damaging the transmitter. This is the job of a tuner. For multi band antennas that do not have provisions for achieving 50 Ohm feed point impedance at all the expected operating frequencies, a low loss feed line and tuner arrangement is a reasonably inexpensive solution. Another solution is a remote auto-tuner at the feed point of the antenna. This is effective, but considerably more expensive. And it is often difficult to manage the weight of the auto-tuner at the antenna’s feed point unless it also happens to be at one of the antenna’s support points. If an auto-tuner is available, it will work just as well at the end of the ladder line where it enters the house. If you use an auto-tuner be sure it has provisions for feeding a balanced line. If not, then a balun will be necessary between the balanced line and the auto-tuner. It would be best to check with auto-tuner manufacture about how to implement that.
Some will note that this antenna looks suspiciously like a G5RV and that is a valid observation. The major difference is that the balanced line can be any convenient length and the coax can thus be kept to a minimum length. In this type of antenna one should always try to achieve the shortest run of coax possible and thus reduce the feed line losses to a minimum. It is true that the balanced line that is cut to a specific length on the G5RV does act as an impedance matching device. But again it will provide a good match at some frequencies and not so good at others just like a balun. The loss that occurs in the coax will still be present. So always keep the coax as short as possible and the balance line as long as is sensible so as to reduce the loss in the antenna feed system to a minimum.
Happy HF’n and 73
PS - By the way, this antenna can be bent or sloped but its efficiency will drop some and often the bandwidth will get narrower making it difficult or impossible to tune some bands.