| May, 2005 |  |
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| May, 2005 | |
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A particular 80 meter inverted Vee antenna for HF(80-10 meters) is fed with coaxial cable. Results using a tuner at the transmitter have not been as expected so Ron K3PF was asked for some advice. He responded that first the antenna must be resonant on80 meters. One could see that would help skip using the tuner on 80 meters but how was resonance on 80meters important when a tuner would be used on all of the other bands? He provided this very informative response.
Tuning the antenna to resonance is important if you are using COAX because high SWR can cause large losses with long runs of coax. If the antenna and the coax are both 50 ohms, you have a perfect 1 to 1 match with minimum loss. However, if the resistance of the antenna, AT THE FEED POINT, is something other than 50 ohms, you will have a mismatch. The higher the feed point resistance, the higher the SWR and more loss in the COAX. A perfect resonant dipole antenna is about 75 ohms at the feed point. This is still a good match of 1.5 to 1 (75 ohms divided by 50 ohms = 1.5). Therefore, a resonant dipole fed with 50 ohm coax is an efficient system. Assuming you have TUNED the dipole to 80 meters, the harmonically related bands will not be as good a match as the fundamental, but, the match should not be too bad. NOTE: Antennas with an SWR of 2 to 1 are generally accepted as a good match.
If, on the other hand, you do not tune the 80 meter dipole, the SWR will be high unless you are lucky enough to have cut the antenna to resonance in the first place. I NEVER was lucky. I always had to tune my antennas to resonance. In this case, the harmonically related bands will have high SWR, too. Reason - An antenna that is too long or too short for 80 meters will be too long or too short on the harmonically related bands.
Tuning an antenna is very important if you use COAX and you DO NOT use a tuner AT THE FEED POINT. If the antenna is not tuned, there will be a mismatch that causes standing waves in the transmission line. Standing waves translates to loss. The higher the mismatch, the greater the loss in the COAX. And, the longer the coax, the greater the loss.
You can get away with an UNTUNED NON RESONANT antenna by using a tuner AT THE FEED POINT. Why? The tuner is a matching transformer. It matches the 50 ohm COAX to whatever resistance is at the antenna's FEED POINT.
Therefore, multi band operation works well Because the tuner retunes and matches the feed point resistance to the coax's 50 ohms. When the antenna LOOKS like 50 ohms to the coax, the system is matched. The TUNER matches the antenna FEED POINT to the 50 ohm coax.
When using a balun on a COAX FED balanced antenna such as a dipole or inverted vee, the balun should be attached to the center of the antenna. Why? The antenna is balanced. The coax is unbalanced. The balun transforms the balanced antenna to the unbalanced coax. In a perfectly matched system, a 1to 1 balun is required. Why? The balun is a transformer. If you use a 4 to 1 balun, you will transform the antenna's feed point to 4 times its resistance. For example, a 50 ohm antenna will look like 200 ohms to the coax. 200 ohms (antenna) divided by 50 ohms (coax) = 4 to 1 SWR. If you use a 1 to 1 balun, the 50 ohm antenna and 50 ohm coax are a 1 to 1 match.
Ron K3PF
| May, 2005 | |
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