I'm searching for the truth. Please read.
W8JI
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Join Date: Oct 2003
Posts: 2,853
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Any one additional comment you can pass back to "tube shortwave" about this:
"If they had a shred of knowledge in this department, they would know that a balancing resistor on the OPPOSITE side of a folded dipole does not absorb "half the transmitter power" at all. "
That wasn't the statement at all. He isn't even honest enough to quote someone accurately.
The statement I made (it is in black and white here on the Zed, so there is no debating he misquoted) is the resistor sometimes dissipates less than half of the power applied to the T2FD antenna, and on other bands it dissipates much more than half the applied power. Nothing worse than a critic who fabricates the very statements he is openly critical of.
He goes on to say:
"If that were true, that resistor would blow up every time I run 1500 watts on CW. In fact, at most, the resistor will absorb 1/4 of the transmitter power if the antenna is that far out of resonance on my chosen operating frequency. The other 3/4 is radiated with a very small amount coming back (<10%). Most of the time, the balancing resistor isn't doing very much."
That's clearly wrong, as any analysis or measurement has shown. What "tube shortwave" is unaware of is the owner of Amp Supply found that antenna design in some publication and decided to build it commercially. I believe he eventually cooked up a deal with B&W to buy the "design" of that, and a terminated V, antenna. The original matching transformers were the filament chokes from amplifiers, wound at Prime Instruments.
I actually directly measured the dissipation in the resistor and it was pretty high overall.
There is considerable mutual coupling between the dipole half with the resistor, and the dipole half with the feedline. Anyone who knows anything about antennas understands mutual coupling of two very long wires (90 feet or so long) spaced a few inches apart is extremely high, plus the ends of the wires are directly connected. The resistor is, for nearly all practical purposes, directly connected across the feedline.
The power division depends largely upon the unterminated impedance at the feedpoint. On bands where the antenna is a conventional half-wave or 3/4 wave dipole length, that impedance is low and the resistor has limited effect on the system. On bands where the feedpoint would have a high unterminated impedance, the resistor soaks up most of the energy.
This is why the SWR stabilizes. What would be a terrible SWR on some bands is reduced by diverting energy into the resistor on those bands.
When testing the antenna as a consultant doing work at Amp Supply, I built an antenna at home and ran the "resistor" side through ladder line to a dummy load. I had an RF current meter in series with the load, and measured load current. I still have the huge resistor I used here, I use it as a dummy load for testing high impedance balanced systems. The measured result was basically as LB Cebik and many others have analyzed and published.
If you search the web you will find many articles telling exactly what the T2FD antenna is, and what it is not.
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check this out....good reading.
