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Why Does Everyone Use 450 Ohm Line, Not 300?
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by K0OD on November 1, 2009
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It would seem that good 300 ohm window line, not the old TV stuff, has many advantages over 450 ohm or even 600 ohm open wire line for feeding multiband HF doublets: Lighter, cheaper, far less visible, adequate for legal max, less wind resistance, easier to thread into shack etc.
So why don't hams use the 300 ohm window line?
I picked up 140 feet of the stuff yesterday (at about 39 centers per foot). Have I overlooked shortcomings of this product?
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RE: Why Does Everyone Use 450 Ohm Line, Not 300?
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by W0BTU on November 1, 2009
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I built a couple of folded dipoles (and 4:1 coax baluns) out of 300 ohm TV twin lead for once for myself and once for a friend.
They were cut for 40 meters, but also worked on 15 meters.
They worked great!
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RE: Why Does Everyone Use 450 Ohm Line, Not 300?
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by N4JTE on November 1, 2009
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I might be speaking from my butt here, but I think the inherent wider seperation and insulation properties of well manufactured 450 ohm ladderline tends to make it a more consistant and forgiving feedline when running into a antenna looking for 50 ohms, or any variation thereof. The loss incurred in a mismatched feedline depends on the feedline. 300 ohm tv line or equivelent will probaly be more prone to tvi, signal loss and resultant problems more than a 450 ladderline.
Regard's
Bob
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RE: Why Does Everyone Use 450 Ohm Line, Not 300?
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by WB2WIK on November 1, 2009
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Most of the 450 Ohm "window line" is less than 450 Ohms, anyway.
It's not very important.
However, the higher the Z, the lower the current and the less the loss.
The only reason we stick with about 600 Ohms is it's a good compromise between conductor loss and radiation loss. Going to wider spaced line would have even less conduction loss, but radiation loss starts to increase.
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RE: Why Does Everyone Use 450 Ohm Line, Not 300?
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by WX7G on November 1, 2009
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People do use transmitting (and receive) 300 ohm ladder line. The Wire Man, Inc. stocks transmitting 300 ohm ladder line as part number 562.
You point out several advantages of 300 ohm line based on smaller physical dimensions. What you did not point out is that HF loss is higher for a smaller line.
How about for equal conductor size? Given equal conductor size the HF loss of matched, dry 300 ohm line will be 1.5X that of 450 ohm line.
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RE: Why Does Everyone Use 450 Ohm Line, Not 300?
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by K0OD on November 1, 2009
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"300 ohm tv line or equivelent will probaly be more prone to tvi,"
Any comments on the TVI aspect? That's an important consideration to me.
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RE: Why Does Everyone Use 450 Ohm Line, Not 300?
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by N3OX on November 1, 2009
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"Any comments on the TVI aspect?"
Yep. Doesn't make sense.
A lot of people conflate high mismatch and high common mode current because it's generally harder to decouple *high voltage* feedpoints from the common mode circuit.
But the differential mode impedance of the line has nothing to do with that. Only the impedance of the feedpoint (and even then it's common mode impedance, which has a lot to do with balance and less to do with its differential mode impedance, though they do interact) and the length of the line.
Feedpoints with high voltage with respect to the rest of the common mode circuit path make it easy to drive troublesome currents even when connected to a point in the common mode circuit that has high common mode impedance. Even then, if balance is good, the line doesn't radiate (because both conductors are getting driven with equal and opposite high voltage.
There's no reason at all that 300 ohm line would give you more TVI than higher impedance line. High SWR doesn't cause the feedline to radiate.
73
Dan
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RE: Why Does Everyone Use 450 Ohm Line, Not 300?
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by W8JI on November 2, 2009
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There is a big long thread about line loss in Elmer's related to ARRL measurements of ladder line.
The thing to remember is conductor resistance greatly dominates the loss in a clean dry line at HF. It is NOT the dielectric losses or type of dielectric or spacing to a large extent(even if you get conductors a foot apart on 20 meters).
While what WX7G says is true, it also assumes the line is matched when loss is calculated or measured. You really have to look at the entire system, and the increased losses often won't be apparent (or might not appear) in a real system.
We assume current is less (making I^2 R conductor losses less) when the feedline is spaced wider because we assume the antenna is re-adjusted to the higher surge impedance. If the antenna is not changed to decrease current in the higher-Z line, then losses don't go up significantly as line impedance is decreased through closer spacing because current essentially stays the same on shorter lines.
If we have a long enough line then there are so many current peaks a lower Z line of the same conductor diameter would increase loss from increased current IF the antenna was greatly mismatched.
A second factor is impedance at the tuner. A lower Z line operates with a lower average SWR over HF in a multi-band dipole, and presents a more modest impedance to tuners. I think 350 ohms or so is the optimum value. We might want to give up a little loss in the line to keep the tuner from seeing 15 ohm load impedances.
There really is a whole lot that weighs into this, much of which is interesting but meaningless in the real world.
I just installed an extended double zepp on 80 meters, a perfect looking antenna 150 feet high and in the clear made from number 10 copperweld wire. It has less gain than a dipole but a narrower pattern because I used 450 ohm window ladder line to feed it. But it sure looks good. It also won't stay up long with that wind load at the center.
I'll soon be feeding it with one foot spaced #14 wire, and it will have a couple dB gain over the dipole. No one else will notice the 2 dB, but I will know it changed. I will also know because the antenna won't blow down from wind catching on the feedline.
I would probably use 300 ohm transmitting line if I had a short feeder and regular antenna tuner in a multiband dipole. Mostly because of the tuner, and because the antenna would be much better in the wind.
BTW, don't use ARRL figures for ladder line loss.
Tom
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RE: Why Does Everyone Use 450 Ohm Line, Not 300?
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by K9IUQ on November 2, 2009
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When I was 14 yrs old and newly licensed and poor I had the local TV repairman give me some old used 300 ohm tv line. The cheep stuff.
I made a 40 mtr dipole fed with this junk. Did not have a tuner. Did not even know what an antenna tuner was. I took this 300 ohm tv line and soldered a PL 259 onto it so it would screw into my Eico 720 xmtr.
I worked DX and WAS with this antenna for a couple of years.
It was only many years later that I learned this would never work....
Stan K9IUQ
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RE: Why Does Everyone Use 450 Ohm Line, Not 300?
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by WX7G on November 2, 2009
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Correction. I should say the loss of 300 ohm line having the same conductor size is higher by the square root of 1.5, which is 1.2X.
Having said all I have, I use receiving 300 ohm line for the reasons the original cites; small size, low wind loading, and it can be closed in a window. All I can find these days is the stiff line at Radio Shack. I'd like to find some of the thinner line they used to stock.
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