100 Watts and a Wire: Episode 85

Discussion in 'Amateur Radio News' started by K0STH, Feb 14, 2017.

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  1. KO7V

    KO7V Ham Member QRZ Page

    My favorite is a center fed dipole cut 110 feet overall fed with 300 Ohm transmit balanced line with no cut-out windows. Fed with a balanced tuner that handles 1.5 kW with high voltages (home brew with switched input links and a balanced switch-tuned input inductor, and two large differential variable caps on the output to match the output Z, and RF current meters on each output connection to verify balance).. Yep, it is a big tuner. Tunes 1:1 on 3.5 to 30 MHz with low loss. It's up at 80 feet, and the DX comes back on the first few calls. K3 - SB-220 - bal. tuner - balanced line - CF dipole. I previously tried a homebrew OCF dipole with a 5KW Balun Designs balun with coax feedline, and (1) the balanced-line CF dipole works way better with way less feedline loss and (2) The coax and balun weight caused too much sag, that much LMR-400 was too heavy. Would work with a vertical support at the balun, but has less frequency flexibility. But Balun Designs makes a very good balun. The window line picks a up too much snow, ice and freezing rain compared to the continuous insulation. I have thought about DC shorting the antenna in the middle with a large RF choke and running DC 2 Volts at high current to melt the ice off the feedline...someday. Very hard to find the 300 Ohm transmit line without windows any more. You can get away with good quality VHF TV twinlead because the Z is mostly high and the RF currents are low. I tried that and it survived OK on 40 and 20M with full power. Cut dipole so it is not resonant on any band, and let the feedline and tuner fix the reactance, works beautifully. Oh, and be ready to re-tune when it rains, this is not coax, my friend.
     

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