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Ladder Line to Coax Baluns

Discussion in 'General Technical Questions and Answers' started by W2BLC, Aug 21, 2008.

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  1. W2BLC

    W2BLC Ham Member QRZ Page

    There are several ladder line to coax baluns available. Reading the advertising does little to explain why one make/model might be better than another.

    What I want to do is to convert my 160 meter dipole to ladder line (about 80 feet required from the house to the feed point) and run 15 feet of coax out through the wall to the balun. Currently I have many coax fed dipoles and wish to reduce the herd to two or three all band types. Coax fed OCF is also being considered.

    I could bring the feedline directly into the house through the wall, but unless there is a very strong reason for this, I would prefer to use coax (easier to handle and ground).

    I use the PalStar at1500DT (differential tuner) and typically run about 800 watts output.

    Some user experience would be great to hear.
  2. W5DXP

    W5DXP Ham Member QRZ Page

    When the impedances being dealt with are unknown, as they usually are in multi-band, ladder-line-fed dipole applications, a hefty 1:1 current-choke is usually the balun of choice. 1000 ohms (or more) of choking impedance may even be too low for some of those sky-high impedances. I requested a particular manufacturer run some choking impedance measurements on their expensive 4:1 voltage balun wound on a T-300A-2 powdered-iron toroid (a Sevick design). The results were disappointingly low.

    An effective 1:1 choke can be implemented by winding ten or so turns of RG-400 (teflon) coax on an FT-240-x toroid of material appropriate for the required frequency range. Toroids of different materials can be stacked in an effective manner for covering wide frequency ranges.
  3. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    What you propose should work fine. You may have a small amount of loss in the coax but probably not enough to worry about.

    My only criteria would be to get one with the highest power rating. Something on the order of 5 KW would work well. Even if your not running a lot of power a high power Balun will be less likely to have problems when the impedance (incident voltage) is very high.
  4. N4KZ

    N4KZ XML Subscriber QRZ Page

    You will find balun information on DX Engineering's web site to be very helpful in determining the different kinds of open-wire-to-coax baluns and the type you might need.

    73, N4KZ
  5. VE3PTC

    VE3PTC Ham Member QRZ Page

    what w5dxp said !!
    anyhoo, you may consider another route. i have for the longest time, when using long dipole's for multi-band work, always used bal-line from the feed point all the way to the tuner. its a great system, if you can use it, for lower losses etc.
    if you are worried about the bal line in the shack/house, can i suggest that you could use 2 similiar lenghs of coax. i find 93 0hms coax works well. join the braids together at the tuner end, but not to, and feed point, and use both the inners, as a balanced, yet shielded line. its lossy compared to bal line/open wire feeders, but will give you what you need to exit the shack etc.
    only my .02 cents worth.
    regards, bob. ve3ptc/gm0ley
  6. G0GQK

    G0GQK Ham Member QRZ Page

    There was a time when every antenna design which used 300, 450, or 600 ohm open wire feeders recommended using a 4:1 balun. It seems these days that people are now recommending a 1:1 balun, so what is a 4:1 balun to be used for if not with open wire feeder ?. I once read that a 1:1 balun will only function properly if it is used for resistances between 50-75 ohms.

  7. W8ATA

    W8ATA Ham Member QRZ Page

    Take a look at Bob's ,KZ5R, Model 4115t at Bob builds by hand several of Dr. Jerry Sevick's designs. High quality products that are affordable.

  8. VK2TIL

    VK2TIL Ham Member QRZ Page

    "There was a time when every antenna design which used 300, 450, or 600 ohm open wire feeders recommended using a 4:1 balun".

    The principle seems to be that 300, say, divided by 4 is 75 ohms and therefore a good match to 50 ohms.

    Well; no.

    As DXP said, the impedances are unknown (unless measured or calculated). It is a common misunderstanding that a line, whether 50 ohms, 300 ohms, whatever, presents that impedance; it only does so in some special cases.

    In most "random" cases the line transforms the (unknown) feedpoint impedance to some equally-unknown impedance which may be high, low or in-between.

    "It seems these days that people are now recommending a 1:1 balun, so what is a 4:1 balun to be used for if not with open wire feeder?".

    A 4:1 (1:4) balun is used to transform an impedance to four times/one-quarter of its value.

    But we have to know the original impedance to make an assessment; if, say, the original impedance is 1 000 ohms (we won't get into complex impedances here) a 4:1 transformer will give 250 ohms.

    But, if the original impedance is 8 ohms, we will transform that to two ohms.

    We have to know what we are doing!!

    "I once read that a 1:1 balun will only function properly if it is used for resistances between 50-75 ohms".

    There are two kinds of "balun" or "balun/transformer".

    The "choke" balun puts impedance on the outside of a co-ax shield; this is done by coiling the line or by adding ferrite loading.

    This opposes current trying to flow on the outside of the shield where it doesn't (usually) belong. The "choke" action is a result of the characteristics of the coil (its inductance) or of the particular ferrite material used.

    A "transformer" balun usually uses a transmission line wound on a core (air, iron, ferrite).

    The surge impedance (Zo) of this line is important. For a 4:1 device, the optimum Zo is the geometric mean (twice the smaller, half the larger) of the impedances being matched.

    So a device matching, say, 200 ohms to 50 ohms will use a line with Zo of 100 ohms; matching, say, 20 ohms to 5 ohms will/should use a line with Zo of 10 ohms. Lines of very high or very low Zo are difficult/impractical to construct so a compromise is usual.

    Most commercial balun/transformers seem to be optimised at about the 200:50 level; a pair of wires wound parallel usually has a Zo somewhere around 100 ohms. They may not work well if used to match, say, a high impedance (end of a half-wave) or a low impedance (short vertical).
  9. K2PJW

    K2PJW Subscriber QRZ Page

    I have a similar setup except the dipole is cut for 80M. I tried both 1:1 and 4:1 baluns. The difference I could see was where the settings on the tuner needed to be for things to be resonant. After some time I ended up bringing the 450 windowline into the shack and connecting it directly to the antenna tuner. I like this setup much better for me. I run 700 watts on 80-20M and no RFI problems, that DID surprise me. Either way, I think you will really like the antenna after you run windowline to it. I love mine.
  10. W5DXP

    W5DXP Ham Member QRZ Page

    An ordinary folded-dipole has an SWR close to 1:1 on the 300 ohm twinlead. Such a system is said to be "flat". That is a good application for a 4:1 or 6:1 balun to bring the impedance down close to 75-50 ohms. But if the SWR is unknown, the impedance is also unknown. If the SWR is not 1:1, then the Z0 of the line will never be encountered by the balun. Not everyone understands that fact of physics.

    That may be true for some TLT designs. Not necessarily true for W2DU baluns and other current-chokes.

    Please see VK2TIL's excellent posting. Let's see if those concepts can be illustrated with a simple example.

    Let's assume we have an antenna system using Z0=450 ohm ladder-line with SWR=10:1. Should we use a 4:1 balun? a 6:1? a 9:1? Maybe a 1:1?

    Baluns are known to function better with purely resistive impedances rather than highly reactive impedances. Here are a couple of rules of thumb.

    1. The minimum purely resistive impedance encountered will be Z0/SWR. In the above example, Z0/SWR = 450/10 = 45 ohms. Obviously, a 1:1 balun would be best.

    2. The maximum purely resistive impedance encountered will be Z0*SWR. In the above example, Z0*SWR = 450*10 = 4500 ohms. A 9:1 balun will try to bring that impedance down to 500 ohms but don't expect any TLT balun to eliminate common-mode current under that high resistance condition.

    3. What about the case where the resistive part of the impedance is equal to 450 ohms? The reactive part will be +/- j1170 ohms. Baluns are known not to function very well in highly reactive situations.

    Many TLT balun systems could be improved by simply adding a hefty 1:1 choke on the TLT balun input.

    Seems the best one can do in the above example (if the impedance is unknown) is a hefty 1:1 common-mode current-choke with a very high choking impedance. With conventional transmission line transformers, one is usually shooting in the dark when dealing with non-resonant systems. In particular, TLT baluns using powdered-iron toroids usually provide a pitifully low common-mode choking impedance.

    My all-HF-band no-tuner dipole resolves the problem by using the length of the ladder-line to resonate the entire system thus relieving a lot of the stress on the balun. The 1:1 balun (choke) sees only resistive loads between 35 ohms and 85 ohms, i.e. low-voltage/high-current situations.

    It is often possible to relieve the stress on the balun simply by changing the length of the ladder-line.
    Last edited: Aug 22, 2008
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