How to measure SWR of ladder line?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K3RW, Feb 20, 2020.

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  1. W1VT

    W1VT Ham Member QRZ Page

    My 36ft high Inverted-L for 160 with elevated radials had a nice match with a 1:4 unun. The tree branch that supported it came down in Tropical Storm Isaias so I'm planning on replacing it with a self supporting version.

    Zak W1VT
    K3RW likes this.
  2. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    It is not the antenna feed point Z but the impedance that is presented along the transmission line. For example, if you are at 7 MHz using an 80 meter dipole, where the feed point Z is very high, 1/4 wave from the feed point along the transmission line going back the rig, the Z is very low. This low Z point on the line repeats again every half wave and by chance, given the transmission line's length you might wind up with this very low Z right at the transmatch terminals. Take a look at the Smith Chart. Mark a high impedance coordinate and then rotate that back 1/4 wave and you'll have a visual representation of how this works. At the extreme and with a lossless transmission line, if the end of the line is left open (infinite Z) 1/4 wave back the Z is a dead short or zero ohms.
    W2VW, W6MK and K3RW like this.
  3. K3RW

    K3RW Ham Member QRZ Page

    Is that 1/4 wave change true on ladder line as well as coax? Does the impedance of the transmission line (50, 75, 300, 450, etc.) change the chart at all for where the 1/4 wave changes occur?

    I'll admit that impedance matching isn't my strong suit.

    Its been oversimplified to me that an antenna that has a 50ohm feedline is just that, and tune the antenna for resonance and voila (!).

    Its also pestered me in tuning or building an antenna, especially a trap dipole, vertical, etc., where perhaps the length of coax to my antenna analyzer may have thrown off results. And instead of me seeing the feedline as part of the whole equation, I keep trimming line or shortening elements! I imagine I am not alone in this.

    In the low impedance situation presented on the shack end, would the 1:4 balun be best-suited at the antenna feedpoint on just about on the terminals of the transmatch itself?
  4. W6MK

    W6MK Ham Member QRZ Page

    I hope I can help clear this up conceptually.

    There are two impedance-matching points in an antenna system: The first is at the feed point where tuning the antenna (as by adjusting length) can adjust the antenna's impedance so that it matches (approximately) the nominal impedance of the feedline, usually coax at 50 to 70 ohms. This is called "SWR." The notion of antenna "resonance" can be useful here if the antenna's feed point impedance can be closely matched to that of the nominal impedance of a coaxial feedline. When there is a close impedance match there is little impedance transformation along the coax so that coax length is relatively unimportant in relation to feedline loss.

    For multiband antennas, parallel conductor feedline can be used because of the relatively low-loss characteristic of such feedline even when there is a significant impedance mismatch at the feed point.
    Coax feedline has higher losses when there is a significant impedance mismatch at the feed point.

    The second impedance-matching point is at the transmitter end of the feedline where the output of the transmitter needs to be fairly-well matched to the functional (not nominal) impedance of the feedline at that specific distance from the antenna.

    When there is a significant impedance mismatch at the transmitter end, an antenna tuner can transform the impedance at that feedline point to a value which is appropriate to the transmitter output circuit (usually about 50 ohms). The purpose of this impedance matching is NOT to tune the antenna to resonance--it is tuning the antenna system so that it can accept the maximum amount of power generated at the transmitter.

    Perhaps a most useful way of looking at a transmitter-and-external-antenna-tuner setup is that the
    tuner becomes the impedance-adjustable output stage of the transmitter. In the old days of vacuum
    tube transmitters, most of them had built in impedance-adjustable output circuits. Built-in tuners.
    Today's solid-state transmitters often have built-in narrow-range semi-automatic tuning circuits so
    to adjust to the slightly-varying impedance at the shack end of a coaxial feedline running to a well-matched antenna.
    K3RW, KP4SX, W1PEP and 3 others like this.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    In addition to @W6MK's excellent post #44, there is another consideration when using a random length of parallel conductor feedline to feed a "doublet" (a non-resonant center-fed dipole). Most hams believe that if you use a parallel line (owl, twin-lead, ladder-line) with a nominal 450 to 600 Ohm characteristic impedance, then the tuner always "sees" 450 to 600 Ohms.

    Not true. What the tuner/balun sees is almost always radically different!

    First, depending on the overall length of the doublet wire and the frequency, the starting feedpoint impedance (at the doublet center) could be anywhere from 10 Ohms to 5000 Ohms.

    Second, depending on the electrical length (in wavelengths, considering the velocity factor of the parallel line) and the frequency, the transformed input impedance at the shack end of the parallel line can be anywhere from 5 Ohms to 5000 Ohms. Particularly, trouble happens at frequencies where the parallel line is either an odd multiple of 1/4wl or any multiple of 1/2wl.

    If using a unbalanced T-network tuner followed by a balun, you are asking the balun to operate over a huge impedance range, which it does poorly, with a lot of loss in the balun itself at both impedance extremes. It the tuner is trying to drive 10 Ohms or less, it has high internal I^2R losses. If the tuner is trying to drive 2500 Ohms or more, it is likely to arc internally, or the tuner simply is not capable of tuning those extremes...
    K3RW and K0UO like this.
  6. W6MK

    W6MK Ham Member QRZ Page

    I can usually defer to WA7ARK's perspectives, but I am wondering about the
    stated "trouble" with parallel line lengths of 1/2 wavelength or any multiple. I do accept that odd
    multiples of 1/4 wl are likely to be problematic. Even multiples of 1/4 wl are also 1/2 wl multiples! Probably a mistyping!

    I use heavy-duty 1:1 baluns ahead of my tuner(s) because such baluns, if well-designed, are better
    able to handle the stresses of extreme impedance ranges than baluns which also serve to transform
    K3RW likes this.
  7. 2E0ILY

    2E0ILY Ham Member QRZ Page

    Thank you for that clear and concise explanation without undue referral to maths! My question is, please, what sort of impedance mismatch can tube type amplifiers usually handle? For example, an elderly Collins 30L-1 or an Alpha 86? Does whether they have a Pi or a Pi-L output usually make a difference to their range? Thanks!
    K3RW likes this.
  8. W6MK

    W6MK Ham Member QRZ Page

    My tube type rigs (E.F.Johnson Adventurer; Heathkit DX100B) could tune anything that I could come up with. In those days, as a young teenager (13 to 16 y.o.) my usual approach to an antenna was a piece of wire of random length (that would run from the house to the far end of the backyard). The wire was plugged into the center terminal of the output receptacle of the transmitter (Johnson: RCA phono receptacle; Heathkit: UHF receptacle).

    So almost any antenna reasonable, or unreasonable, I would assume would work with a linear amplifier, depending specifically on the values of the inductor and capacitors in the output circuit.
    I would assume that an impedance range is provided with an amplifier instruction manual.

    Keep in mind that the output circuit requirements in solid state linear amplifiers and vacuum tube linears are different. There are many output circuit adjustments available in a vacuum tube device which can limit possible damage from high voltages or currents in the tank circuit.
    Last edited: Aug 10, 2020
    K3RW likes this.
  9. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Regardless of the characteristic impedance of the transmission line, the same action takes place. The impedance repeats every half wave along the line and the high impedance and low impedances happen 1/4 wave apart. The best way to visualize this action is to get familiar with the Smith Chart or computer programs that deal with them. The Smith Chart is applicable to all characteristic impedances by normalizing the impedances involved by dividing them by the characteristic impedance of the transmission line involved. Take a look at the standard Smith Chart and you'll notice that the center is 1.0. This where the impedance being plotted is equal to the characteristic impedance of the line (50/50 or 300/300 or 600/600 all equal 1.0)

    In regard to the placement of a balun in the antenna system, it depends on the impedances involved and the type of transmission line. I find it best to use a 1:1 balun at the transmatch output. However, as mentioned in this discussion by others, a balun can not handle just any impedance so I avoid the cases where the impedance is very high. To do this (avoid Hi Z's) I do some measurements to see what the impedance is that the balun will need to handle. If you cannot measure the Z consider the what the antenna's feed point is on the bands of operation and consider what the transmission line is going to do to this Z at the balun's terminals. If the f.p. Z is high you have the 1/4 wave issue where the Z at balun can be very low or with the same high f.p. Z a half wave issue will make this high Z show up at the balun. The best compromise might be to avoid 1/4 wave and 1/2 wave long lines or multiples thereof.

    Baluns can surely handle the lower impedances OK. However, the high Z case leads to high voltages across the balun windings (from either output terminal to the input's shield). This causes high flux density in the core which is what leads to overheating (keep the flux below 100 gauss is the recommendation). I leave it here. Hopefully you remember Steve, G3TXQ (SK). He has discussed this at length over the years and maybe you can search for these discussions. Also, Steve's info and more is on his personal website found just by searching his call sign.
    K3RW likes this.

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