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## Re: [TowerTalk] coax stubs /bandpass filter

 To: towertalk@contesting.com Re: [TowerTalk] coax stubs /bandpass filter TexasRF@aol.com "Tower and HF antenna construction topics." Tue, 30 Mar 2010 18:56:34 EDT mailto:towertalk@contesting.com>
 ```Jim, to further put the numbers in perspective, using the 10 to 1 example, the impedance could be between 5 ohms and 500 ohms (50/10 - 50 X 10). The nominal 30 dB notch with .05 ohm short would have attenuation from 20 dB to 40 dB. One would be smart to make sure the notch is placed at one of the 500 ohm points along the cable. Wonder how you go about determining where that is? Could you assume the plate load capacitor is a point of minimum impedance and go from there? If so, the high impedance points would occur every odd quarter wavelength (at the suppression frequency) along the cable, right? 73, Gerald K5GW In a message dated 3/30/2010 5:43:29 P.M. Central Daylight Time, jim@audiosystemsgroup.com writes: On Tue, 30 Mar 2010 18:10:41 EDT, TexasRF@aol.com wrote: >This is a very simplistic analysis as no adjustments were made for the >mismatch losses incurred in the example. Your analysis and description is excellent as far as it goes. But let me take it a bit further (and closer to reality). The other part of the problem is, "what is the impedance on the transmission line at the point where the stub is connected, AT THE FREQUENCY THE STUB IS SUPPRESSING?" That is NOT an easy question to answer, but it has a LOT to do with how effective that short circuit is. Let's take the example of a line that has a mismatch at the antenna of perhaps 10:1 at the harmonic, and a mismatch at the transmitter of perhaps 20:1. These are VERY realistic assumptions -- both the transmitter and the antenna provide a fairly low SWR at the operating frequency, but almost never do so at the harmonic (unless the antenna happens to be something like a log periodic). At various points along that line, the impedance varies from a few ohms to several hundred ohms, and at all points other than the peak and null, has both resistive and reactive components. Without knowing the impedance at both ends and the electrical length of the line, there's no way to know where the peaks and dips in the impedance fall. If you happen to pick a point on the line for your stub where the impedance is low, it does almost nothing. If you happen to pick a point on the line where the impedance is high, it does a LOT. And anywhere in between, the results vary from not very much to a lot. :) 73, Jim Brown K9YC _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread [TowerTalk] RotorEZ Documentation, (continued) [TowerTalk] RotorEZ Documentation, Pat Cahill Re: [TowerTalk] RotorEZ Documentation, Jack Re: [TowerTalk] coax stubs /bandpass filter, Richard (Rick) Karlquist Re: [TowerTalk] coax stubs /bandpass filter, Chris Wendling Re: [TowerTalk] coax stubs /bandpass filter, Jim Brown Re: [TowerTalk] coax stubs /bandpass filter, k2qmf Re: [TowerTalk] coax stubs /bandpass filter, TexasRF Re: [TowerTalk] coax stubs /bandpass filter, Jim Brown Re: [TowerTalk] coax stubs /bandpass filter, TexasRF <= Re: [TowerTalk] coax stubs /bandpass filter, Jim Brown Re: [TowerTalk] coax stubs /bandpass filter, Gary Schafer Re: [TowerTalk] coax stubs /bandpass filter, Jim Brown Re: [TowerTalk] coax stubs /bandpass filter, Gary Schafer Re: [TowerTalk] coax stubs /bandpass filter, rk