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#1
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Feedline impedance query
If I join 2 lengths of 93 ohm coax to give 46.5 ohm impedance will the db
loss on the line be greater,less or the same as a single lengthof the coax? ZL2DG |
#2
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Feedline impedance query
On Sun, 18 Jun 2006 16:34:19 +1200, "David & Margaret McBeth"
wrote: If I join 2 lengths of 93 ohm coax to give 46.5 ohm impedance will the db loss on the line be greater,less or the same as a single lengthof the coax? ZL2DG Presumably by "join 2 lengths" you mean to connect two lengths in parallel. If the loss factor L is the loss in one length of the cable, won't the loss in two parallel lengths be Plost=Pin/2*L+Pin/2*L, so Plost/Pin=L? Owen -- |
#3
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Feedline impedance query
On Sun, 18 Jun 2006 05:10:58 GMT, Owen Duffy wrote:
On Sun, 18 Jun 2006 16:34:19 +1200, "David & Margaret McBeth" wrote: If I join 2 lengths of 93 ohm coax to give 46.5 ohm impedance will the db loss on the line be greater,less or the same as a single lengthof the coax? ZL2DG Presumably by "join 2 lengths" you mean to connect two lengths in parallel. If the loss factor L is the loss in one length of the cable, won't the loss in two parallel lengths be Plost=Pin/2*L+Pin/2*L, so Plost/Pin=L? Hi Owen, Hmm, each time I look at these equations, something is missing, but I don't know what (probably because it is missing). Simply put, two lines in parallel lose as much power as one line. Nothing to be gained in that regard. 73's Richard Clark, KB7QHC |
#4
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Feedline impedance query
On Sat, 17 Jun 2006 22:16:58 -0700, Richard Clark
wrote: If the loss factor L is the loss in one length of the cable, won't the loss in two parallel lengths be Plost=Pin/2*L+Pin/2*L, so Plost/Pin=L? Hi Owen, Hmm, each time I look at these equations, something is missing, but I don't know what (probably because it is missing). Devilishly clever technique that. I can't really help, I didn't see it even after I pressed the send button! Simply put, two lines in parallel lose as much power as one line. Of course, we both assumed the lines operates with VSWR=1. Nothing to be gained in that regard. Pardon the pun! Owen -- |
#5
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Feedline impedance query
On Sun, 18 Jun 2006 05:33:00 GMT, Owen Duffy wrote:
Simply put, two lines in parallel lose as much power as one line. Of course, we both assumed the lines operates with VSWR=1. Hi Owen, You assumed twice. SWR will only mean more loss - the same loss for one or two lines either way. 73's Richard Clark, KB7QHC |
#6
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Feedline impedance query
David & Margaret McBeth wrote:
If I join 2 lengths of 93 ohm coax to give 46.5 ohm impedance will the db loss on the line be greater,less or the same as a single lengthof the coax? You have not supplied enough information. What is the load impedance in each case? -- 73, Cecil http://www.qsl.net/w5dxp |
#7
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Feedline impedance query
Richard Clark wrote:
Simply put, two lines in parallel lose as much power as one line. Nothing to be gained in that regard. Doesn't it depend on the value of the load impedance, i.e. upon the SWR? -- 73, Cecil http://www.qsl.net/w5dxp |
#8
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Feedline impedance query
It is perfectly obvious, at DC and low frequencies, the resistances of the two inner conductors are in parallel with each other. So with a fixed voltage at one end there will be a greater power dissipated in a load resistance at the receiving end. Outer conductor resistance has a secondary effect. By connecting two cables in parallel the resulting attenuation will be smaller. But as Cecil implies, for exact calculations, cable impedance, cable length, generator and load resistances, and SWR enter the argument. In general, it is a complete waste of time, trouble and cost to connect coaxial cables in parallel merely to obtain a different Zo impedance. If a reduction in line loss is the objective, then the most economic and effective procedure is to use thicker wires in the transmission line and let line impedance look after itself. ---- Reg. G4FGQ |
#9
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Feedline impedance query
Reg Edwards wrote:
In general, it is a complete waste of time, trouble and cost to connect coaxial cables in parallel merely to obtain a different Zo impedance. Sounds like someone might have some free 93 ohm coax and be wanting to use it on a dipole. -- 73, Cecil http://www.qsl.net/w5dxp |
#10
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Feedline impedance query
David & Margaret McBeth wrote: If I join 2 lengths of 93 ohm coax to give 46.5 ohm impedance will the db loss on the line be greater,less or the same as a single lengthof the coax? ZL2DG Assuming the cables are matched in each case, loss is the same. Think about it this way. You have half the power through each cable. This means whatever loss you have, each cable now has half the power loss. When they are summed back together, losses are now the same. Say you had one cable with 100 watts, and power loss was 1 watt. Now you use two cables and 50 is 50 watts in each and loss is 1/2 watt. You sum them together and you get 1 watt loss, and now have 99 watts. That's the matched case. In the mismatched case in limited conditions, what Reg says will work. For example at VHF and lower frequencies most cables are dominated by conductor losses. At cable less than 1/4 wl long will have decreased loss when it has some SWR in a direction that decreases current. By paralleling two cables that are LESS than 1/4 wl long, as long as conductor resistance losses dominate, you can find situations where losses decrease if the mismatch is in a direction that decreases line current. If the lines are long increased SWR always causes increased loss. You'd have to be careful to avoid making a change that causes each line to have increased loss from standing waves, because if that happened line loss would increase. Since 93 ohm line has terrible loss, I'd avoid it. A good line would be a short length of parallel 75 ohm line, but then who needs a 37.5 ohm line? I did that once on a 160 vertical, but the .05dB power I saved wasn't the reason. The real reason was so the 35 ohm antenna feedpoint stayed where it was instead of being transformed by the line. 73 Tom |
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