Is 450 ohm twinlead better

Discussion in 'General Technical Questions and Answers' started by K0CMH, Mar 12, 2008.

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  1. K0CMH

    K0CMH Ham Member QRZ Page

    I understand why balanced line has much less loss that coax.

    However, I have wondered why 450 ohm balanced line seems to be preferred some much over simple (cheaper and easily available everywhere) 300 ohm TV "twin lead".

    Since most tuners have a 4:1 balun for balanced feed line, I would think the 300ohm would be a closer impedance. My login would say that the tuner output is looking at a situation where the 4:1 balun is operating at 50 X 4 = 200 ohms. Attaching to a 300 ohm feed line would be 100 ohms difference. Attaching to a 450 ohnm feedline would be 250 ohms difference.

    I could understand a possible advantage if at the antenna input there is a very high impedance, then the 450 would be closer than the 300.

    Is there an error in my logic, or something I am not taking into consideration, besides the fact that usually TV Twin lead is pretty small wire gauge and may not handle higher wattage well?
     
  2. K9ZMD

    K9ZMD Subscriber QRZ Page

    Because balanced transmission line has relatively low loss even at high SWR, the match at the feed point is not critical for HF operation. Since the feed point impedance of a multi-band antenna is not going to be 50 ohms from band to band - it is going to vary widely - neither 300, 450, or 600 ohm balanced line will be a good match on all bands. Bottom line, if the SWR is happens to be higher with 450 (or 600) ohm balanced line than with 300 ohm line, the power loss will still be very low and the difference in loss between them is just not worth worrying about.

    Many hams do use 300 ohm transmission line quite successfully; I've used it many times to feed multi-band doublets at the 100 watt level for CW and SSB. The power handling capability is limited (as you guessed), but some hams even risk 200 watts if the duty cycle is low. I don't know of anyone who has tried QRO with 300 ohm TV line, but if someone is willing to admit doing it, I'd like to see the "after" pictures. :p

    As an afterthought, I should add that the small gauge wire used in TV twin lead isn't the only limiting factor. The solid dielectric, even the foam dielectric used in some varieties, can't handle the voltage present in high power operation. In addition, the surface of the solid dielectric collects moisture and dust that adds to power loss. The wider spacing and air dielectric of ladder line & window line improves power handling capability. At the same time, there is considerably less surface where moisture and dust could collect and effect the transmission line.

    Gary, K9ZMD
     
    Last edited: Mar 12, 2008
  3. N6YG

    N6YG Ham Member QRZ Page

    Other then more power handling and slightly less loss. I see no reason for using 450 ohm window line. Or are you asking about 450 ohm twinlead and not windows line?? There is Twinlead, Window line and ladderline. It seems like a lot of hams get these mixed up. while they are all balanced line there are subtle differences

    I don't like the common 450 ohm window line as I have found it very unstable in wind and rain. Interestingly enough the 300 ohm twinlead seems more stable?? The 450 ohm window line seems to be an eyesore, it really stands out. The stuff is visible blocks away.

    For low power, I use the 300 ohm stuff form Rat shack. For higher power applications I completely bypass the 450 ohm window line and I build true 600 ohm ladderline. Even though the home brew stuff is bigger it doesn't stand out as much. I paint the insulators with different colors of military camouflage paint so the insulators disappear into the background.. It's amazing how well that camouflage paint works. It works so well I've had people walk right into the ladder line as I was building it.

    As far as impedance is concerned, its not that critical. The loss is so low that even with a high SWR the difference between 300, 450 or 650 ohm balanced line is negligible (Assuming low power =>100 watts). The only real difference is stability and power handling. What's really interesting is that on paper the 450 ohm window line should be more stable then the 300 ohm twinlead. But in my experience the 300 ohm twinlead is more stable not only that but its easier to bring into the shack.
     
  4. N6YG

    N6YG Ham Member QRZ Page

    Okay Gary you beat me.. I think you did a better job explaining it to. Kinda why I just lurk in the Q&A area..

    As far as the power handling capability of 300 ohm twinlead (foam exterior type)
    I will admit to testing it. The highest power level I have been able to use with the 300 ohm stuff is about 400 watts. At that point I started seeing imbalances. Realistically I would never use it above 100 watts

    The big problem with the 300 ohm stuff is the conductors are really small. This makes the connection's really critical. The conductor is so small that you can't even think about nicking a conductor, they must be perfect. I know this is probably common knowledge but I'm going to repeat it anyways. I have a simple trick for not damaging the conductors. Its kind of like cutting glass, you basically just make score marks in the insulators.

    Here's how I do it. The trick to not damaging the conductors is to use a sharp razor blade to carefully score the insulator, don't cut all the way through to the conductor. All you want to do is score the insulator enough so it will tear. After I make the score mark I flex the insulator enough to make the insulation tear on the score mark. After carefully striping the insulators I crimp and then solder ring terminals on both ends. I also use the same method for stripping coax. It always leaves a perfect conductor..
     
  5. WS2L

    WS2L Guest

    Personally and everyone you ask will have a different opinion on this but I prefer the 450 ohm feedline over anything else for HF. I have built numerous antennas for friends and myself and we all have been happy with it.

    Try both and see which one works best for you. I never tried 300 ohm line so I might be missing out on something but I will stick with the 450 ohm stuff for now.
     
  6. K0CMH

    K0CMH Ham Member QRZ Page

    OK, so what I seem to get from the answers is that the low loss in balanced line more than makes up for any mis-match at the connection points. Is that correct?

    In other words, the dB loss at the tuner connection and the antenna feedpoint would be less (when added together) than what I would get with the run of coax. Yes/No?
     
  7. AB1GA

    AB1GA Ham Member QRZ Page


    Almost, but not quite there.

    Where you have an impedance mismatch, you get reflections, i.e. part of the power going out comes back. In your scenario, there's a reflection at the tuner/feedline interface and the antenna feedpoint. Reflection is a lossless process.

    The loss occurs in the feedline due to wire resistance and dielectric loss. In some cases, such as antennas with low radiation resistance, wire loss in the antenna and ground system are major players as well.

    When a line is perfectly matched, there is a certain amount of loss which depends on the design of the line. When the SWR is above 1, there is an excess loss which depends on the SWR and the loss with a perfect match.

    Coax cable has a much higher loss per unit length than open wire line or window line, so not only do you lose more to begin with, even the excess loss at high SWR can be felt. The loss of window or ladder line is so low that even at high SWR, the excess loss is rarely noticable. So balanced line, even with a high SWR, can outperform coax which is perfectly matched.
     
  8. KL7AJ

    KL7AJ Premium Subscriber QRZ Page


    Actually, since the gradual demise of over-the-air TV, 300 ohm twin lead has been getting harder to find. In fact, long before cable was around lots of people were replacing twin lead with coax. The supply of 450 window line has been fairly constant, however. I've always had tons of the stuff, while my 300 ohm stash has just been evaporating.

    eric
     
  9. K4AX

    K4AX Ham Member QRZ Page

  10. W5DXP

    W5DXP Ham Member QRZ Page

    When the SWR > 1:1, you will *never* see 300 ohms using a 300 ohm line. The impedance seen by the balun can be anything except 300 ohms. That seems to be what you are missing. Attaching to a 300 ohm feed line would be an unknown number of ohms different.
     
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