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SSB Antenna connection
"Meindert Sprang" wrote in message
... "Jack Painter" wrote in message newsxstc.52$9h.43@lakeread02... Hi Meindert, I don't understand your reasoning there, sorry. And Doug too, who referenced a steel ship, which is my reference as well. I have seen hardline (still 50ohm coax) in shipboard installations using the same Sunair ATU that I use, connected to the wire HF antennas. A backstay antenna is relatively short compared to the wavelength. It therefore has a high impedance. To match it to the 50 ohm of the transceiver, the impedance has to be transformed by an L-circuit with the capacitance at the low impedant side to ground and the inductance from the low impedance "hot" side to the antenna. If you would use coax at the high impedant antenna side, you get a terrible mismatch. The capacitance of this pice of coax adds to the L circuit at the wrong side, effectively giving you a PI circuit which is unable to match the high impedant backstay to the 50 ohms of the transceiver. It appears (to me) no different that the ungrounded dipole that I feed with coax from my land station tuners. Theoretically no. But your land dipole is probably much longer than a backstay and therefore has a lower impedance. By the way, does your coax connect directly to the dipole or do you have a balun (with a possible impedance transformation wich makes the coax have less influence)? Meindert, Thanks very much, that was a lightbulb going off (duh) that the backstay on less than a 70' yacht is going to have a seriously short antenna WRT wavelength! My wires and dipole are of course half wave devices and at desired frequencies do not even require a tuner at all. And yes I do use a 1:1 Balun (isolation only on the tunes dipole, 4:1 on random wires). And just because the specs of my Sunair Coupler _could_ deal with any wire 30' or longer, that would be a frivolous effort to try to tune, say 2182khz on so short a wire with 50ohm coax. It does work mediocre on an 80' wire but I am still somewhat surprised that any sailing vessel could get much performance (if any do) on MF from a (relatively short) backstay antenna. Closer to the 1/2 wavelength, I would think that coax would be more appropriate to the ATU-to-Antenna match than this GTO-15. Correct? And a 4:1 balun would in other cases make the match even more feasable, as well as the desirable electrical isolation from noise that a Balun can provide. 73 Jack Painter Virginia Beach, Va |
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