Feedline Loss and Open wire Matching Questions

[KB5HAB]

1st as I understand it a loss of 3 db per 100 feet means a loss of half the applied power and add another 100 feet of the same cable and only 25% of applied power makes it to the antenna. Is this right? and 2nd when using a resonant dipole and feeding it with open wire line do I need to worry about matching the line to the antenna? I'm thinking about the open wire feeder because of the loss in cable and can use a 9:1 balun to match a 50 ohm cable to 450 ohm line at the station end but I'm wondering about the match at the antenna end. Do I need to attempt a match there or not and how to go about that if I do. I'll only be running 100 watts or less and I'm looking at a feedline of 250 feet or more from the peak of the house to the tree line. For now I wll be operating 3.5 to 14 mhz and don't want to buy a tuner for open wire line. Thanks.

[W5DXP]

1st as I understand it a loss of 3 db per 100 feet means a loss of half the applied power and add another 100 feet of the same cable and only 25% of applied power makes it to the antenna. Is this right?

Yes, that's right

2nd when using a resonant dipole and feeding it with open wire line do I need to worry about matching the line to the antenna? I'm thinking about the open wire feeder because of the loss in cable and can use a 9:1 balun to match a 50 ohm cable to 450 ohm line at the station end but I'm wondering about the match at the antenna end. Do I need to attempt a match there or not and how to go about that if I do. I'll only be running 100 watts or less and I'm looking at a feedline of 250 feet or more from the peak of the house to the tree line. For now I wll be operating 3.5 to 14 mhz and don't want to buy a tuner for open wire line.

No, you cannot "use a 9:1 balun to match a 50 ohm cable to 450 ohm line at the station end" of a single-wire multiband doublet. Such is an old wives' tale and the source of that idea should be corrected. The impedance "at the station" when feeding a 50 ohm antenna will vary from 50+ ohms to ~4000 ohms often with lots of reactance. Most gurus recommend a husky 1:1 current balun at the ladder-line/coax interface at the station and you may have to adjust the length of your feedline for optimum performance. I would just live with the losses in the open-wire line and enjoy your QSOs. When you get more experience, you may want to try to optimize your power transfer.

[N4EYZ]

This is helpful to figure a bit more precisely what losses may be:

http://www.arrg.us/pages/Loss-Calc.htm

[G3VZM]

1st as I understand it a loss of 3 db per 100 feet means a loss of half the applied power and add another 100 feet of the same cable and only 25% of applied power makes it to the antenna. Is this right? and 2nd when using a resonant dipole and feeding it with open wire line do I need to worry about matching the line to the antenna? I'm thinking about the open wire feeder because of the loss in cable and can use a 9:1 balun to match a 50 ohm cable to 450 ohm line at the station end but I'm wondering about the match at the antenna end. Do I need to attempt a match there or not and how to go about that if I do. I'll only be running 100 watts or less and I'm looking at a feedline of 250 feet or more from the peak of the house to the tree line. For now I wll be operating 3.5 to 14 mhz and don't want to buy a tuner for open wire line. Thanks.

I have used a 4:1 balun between 450 Ohm twin and 50 Ohm co-ax but would suggest running the twin all the way to the rig and using a balanced atu. There will be a mismatch at the antenna end of the feeder but it is unlikely to be a problem, especially as you will actually have a doublet capable of multi-band operation.

Having said that, feeder losses are not usually a big problem unless runing qrp (5W or less) or with a long than average feeder. If it radiates enjoy it.

73's , Fred G3VZM

[N0AZZ]

That is wrong and can not be used for all coax or hard line I use both and have no where near the 3 db per 100' I have 225' of LMR-400DB to a vertical and work QRP from it quite well am working on DXCC at this time. Would your 3 db loss applied to my 225' @ 5w output work out to be usable at all?

All I use with my IC-703+ is the internal tuner and it's good to go on 10-80m. Yours is a much different problem and antenna a external tuner will have to be used for you to tune all the bands you desire to work 20-80m is quite a spread. 100w on 80m may leave you lacking in the power department as you will hear much more than you can work. That band many who operate there will be running QRO I have difficulty running 100w there but may just be me.

[KB5HAB]

Yes, that's right



No, you cannot "use a 9:1 balun to match a 50 ohm cable to 450 ohm line at the station end" of a single-wire multiband doublet..

Now here's the rub. I have resonant dipoles on 75, 40, and 20 meters right now presently coax fed with swr of 1.5:1 or less where I operate. Not a single wire multiband doublet.
When I move the feedline will have to be lengthened. I may be misinformed but if the antenna is matched to 450 ohm line, and frequency doesn't change much, then why wouldn't there be a need to match the feedline at the station end? I don't use a tuner now and don't want to have to buy a 250 to 500 dollar one if it's not needed anyhow. Running open wire feeder to the station itself is not feasible as the feeder will have to cross the roof and go through a couple of walls to get from outside to the rig.

Now according to that loss calculator the loss in db is less for 250 feet than some charts show for 100 feet of the same cable. Let the confusion begin.

[W5DXP]

I have resonant dipoles on 75, 40, and 20 meters right now presently coax fed with swr of 1.5:1 or less where I operate. Not a single wire multiband doublet.

That would have been good information to include in your original posting. So it sounds like a fan dipole with close to a 50 ohm resonant feedpoint impedance on those three harmonically related bands.

You could make the open-wire transmission line a multiple of half-wavelengths long on 80m which would make it also somewhere around a multiple of half-wavelengths long on 40m and 20m. That would tend to reproduce the antenna feedpoint impedances.

Or you could, as you originally suggested, use a 9:1 balun at the antenna feedpoint to raise the impedance to 450 ohms and a 1:9 balun at the ladder-line/coax junction to reduce the impedance to 50 ohms. Ideally, you would not need a tuner and since the SWR on the 450 ohm ladder-line will be close to 1:1, the losses from SWR will be minimized.

[G3TXQ]

Like Cecil said, it will work provided you use a 9:1 transformation at both ends of the ladderline run; but I wonder whether it's really worthwhile:

250ft of a good quality coax like RG213 will have about 1.8dB of loss at your highest frequency; with something a bit more "exotic" like LMR-400 it would be 1.1dB.

By comparison, 250ft of commercial Wireman 553 ladderline has 0.6dB of loss; 0.4dB for the slightly lower loss Wireman 554. To those figures you would need to add the loss of the two baluns which are needed!

Seems to me the ladderline solution is a lot of trouble for little (if any) gain.

73,
Steve G3TXQ

[AA7EJ]

As presented I would suggest checking out Maxwell's book on
reflections and transmission lines, to learn about antenna matching.
Secondary compare the overall performance /expenses of a dipole with over
250 feet of feed line versus another antenna with shorter feed line.
As always , your antenna selection depends on many factors - your operating interests
Being one of them. Maybe a random ( long wire) antenna strung towards your tree line
from your house would work better for you.
I do not know your exact situation, but if you are planning to experiment with antennas
I would suggest to invest in small manual tuner ,good for 100 watts or so , before spending
Money on expensive good grade coax or ladder line.
Also consider the comments on operating 80 meters with "only" 100 watts , otherwise you may
Be disappointed.

[M3KXZ]

Having said that, feeder losses are not usually a big problem unless runing qrp (5W or less) or with a long than average feeder. If it radiates enjoy it.

73's , Fred G3VZM

Even running QRP, feeder losses aren't usually a problem. The power lost in the feeder is a percentage of the power applied, and at low power levels the power lost is also low. It's not like the feeder will sap 5W of power regardless of power applied.