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#2
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On Sep 24, 4:35 pm, Walter Maxwell wrote:
On Mon, 24 Sep 2007 00:29:54 -0700, wrote: On Sep 23, 11:41 am, (Rick) wrote: I was reading in ON4UNs book, a section about Beverage antennas and decided to do some checking on mine, which is celebrating its ten year anniversary this month. snip This comment isn't re noise with the Beverage, but I believe it might be found interesting to those interested in Beverage antennas. Below is a quote from an article I published a few months ago in QST. The Beverage Antenna in WW2 by W2DU As a monitoring officer with the Radio Intelligence Division (RID) of the FCC in Hawaii during WW2 I was privy to some interesting situations. Our State Department was of course aware of the operations occurring in the Pacific Theater. The people there were also aware of the propaganda being spewed by the Japanese short-wave broadcasters. But State was curious concerning what the Japanese living on the homeland were being told-were they being told the truth, or the same propaganda as told on the short-wave broadcasts, or a totally different story. State asked the RID to determine whether we could obtain such information. We cruised the AM broadcast band and found several nighttime signals from Japanese mainland stations, but most were too weak to copy. However, JOAK, Tokyo, on 640 KHz was S9, but there was a problem in copying it. KFI, Los Angeles, was also on 640 KHz with an S9 signal-copying intelligence from JOAK was impossible. How can we eliminate, or reduce KFI's signal level. A Beverage Wave antenna, perhaps? We then proceeded to the northern portion of Oahu and constructed a Beverage one-half mile long, five feet above ground, aimed at Tokyo, and terminated with a 1000-ohm pot resistor to ground at the Tokyo end. We discovered that by varying the pot resistance we could null the KFI signal to almost zero. The resistance terminating the Beverage that produced the null was around 600 ohms. Because the matching resistive termination rendered the Beverage a traveling-wave antenna with no standing wave, the signal from JOAK was terminated by the input of our receiver, while the signal from KFI was dissipated in the matched resistance at the Tokyo end of the Beverage-no KFI signal reflected toward the receiver. Voila-JOAK was perfectly readable for recording! We sent the first recording to Washington, and State was delighted-requesting that we continue recording JOAK continuously. Consequently, our recordings were flown daily to Washington from Hickam Field in Honolulu. We were left in the dark concerning the information on the recordings, and how it affected the War effort, because State didn't share it with us. But it must have been pretty good, because State was on our case every day to make sure we sent them the recordings. Walt, W2DU Thanks for the add Walt. Gives me the idea to try a pot instead of a fixed resister, so I can tweak it for best results. MK |
#3
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On Mon, 24 Sep 2007 17:35:58 -0400, Walter Maxwell
wrote: The resistance terminating the Beverage that produced the null was around 600 ohms. Because the matching resistive termination rendered the Beverage a traveling-wave antenna with no standing wave, the signal from JOAK was terminated by the input of our receiver, while the signal from KFI was dissipated in the matched resistance at the Tokyo end of the Beverage-no KFI signal reflected toward the receiver. Hi Walt, A wonderful example of applied electronics revealing theoretical issues (e.g. the Z of the Transmission Line built as an antenna). 73's Richard Clark, KB7QHC |
#4
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NEC-2 and NEC-2 based programs like EZNEC do a good job of modeling
Beverage antennas. (The EZNEC demo isn't adequate for this.) The only trick is making the ground connection. In NEC-2 use Sommerfeld ground; in EZNEC it's called Real, High Accuracy ground. At each end of the antenna, create a semi-circle of radials, a few inches above the ground. For wire length, a free-space quarter wavelength is more than adequate -- the only requirement is that the effective resistance should be considerably less than the termination resistance. The semi-circles should be facing away from the antenna, so none of the wires extend under the antenna wire. Ground "connections" are made to the center of the semi-circle. This of course assumes that there are reasonably effective ground connections at both ends of the real antenna. Roy Lewallen, W7EL |
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