To: |
"Ford Peterson" <ford@cmgate.com>, <topband@contesting.com> |
---|---|

Subject: |
Re: Topband: Two Wire Beverage Query... |

From: |
"Tom Rauch" <w8ji@contesting.com> |

Reply-to: |
Tom Rauch <w8ji@contesting.com> |

Date: |
Sun, 22 Aug 2004 10:56:40 -0400 |

List-post: |
<mailto:topband@contesting.com> |

analyzer into account. So I'm using a good model over perfect ground. In the case of both 160M and 80M, the model shows a "R" value of just under 5 ohms (4.9 on 160M and 4.7 on 80M). The no load version of the model measures 0.6 -j979 on 160M and 2.4 -j514 on 80M. Looking at the vertical without a load is difficult to read R and j. Both instruments could read the j term but only the Autek could report an impedance of 32 -j920 at 1800 and 10 -j445 at 3500.>> A word of caution. You cannot depend on either bridge to be accurate when far from 50 ohms. Even a one bit data error in the A/D conversion can create huge errors. nearest comparable is the first item of Two 5' rods coming in at 85 ohms. After considerable heroic efforts your 7 radials and 3 - 5' rods comes in at 29 ohms -- within spittin' distance of the two 8' rods in my soil. Quite a difference huh?>> ....and it is useful data. Taking this information one step further, my 680 ohm characteristic impedance bidirectional beverage wire should be terminated at 680 less 30 or 650 ohms? This is a huge departure from the table in ON4UN's 3rd Ed book, which suggests 680 ohms to 340 ohms or so.>>>> When in Beverage mode, the two wires are PARALLEL. You have to tie the conductors in parallel at each end and measure surge impedance. You can do this two ways. You can measure the highest R and lowest R with a wide frequency sweep on an accurate impedance measuring device and find the square root of high R times low R at zero J crossing. Say you measure 800j0 and 200j0 the result would be 400 ohms. The other way is to hunt and peck for minimum SWR VARIATION as the termination is adjusted. Do this with your final ground systems in place. Again, both wire MUST be connected in parallel at each end. The resulting resistance will be the "antenna mode" termination resistance. DO NOT deduct ground resistance or make any corrections, since you want to know the sum of ground resistance and antenna surge impedance anyway. You already know the transmission line impedance. 73 Tom _______________________________________________ Topband mailing list Topband@contesting.com http://lists.contesting.com/mailman/listinfo/topband |

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