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I bought this dipole wire antenna brand new. It came with the 1-1 Balum and 41’ of ladder line. I can only hang the wire 30’ off the ground. 11 feet of the ladder line hangs close to the ground with the Balum. I am getting high SWR with an auto tuner on all bands except a small portion of 40 meters. 7.130-7.200. I have been told not to let the ladder line on the ground. This is a problem. Now, can I cut the ladder line to keep it vertical with the balum? Or will cutting the ladder line de-tune the entire ZS6BKW invention?

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  • $\begingroup$ Thank you for the technical reply. You lost me a bit since it’s been many years in the electronic theory stuff to deal with. Zo and zl impedance stuff that I don’t remember anymore. I bought this antenna from ham radio supplies. I will contact them on who the manufactuer is. And go from there. $\endgroup$ Mar 19, 2022 at 17:31
  • $\begingroup$ As a point of interest, who is the manufacturer? $\endgroup$
    – user21417
    Mar 20, 2022 at 14:15

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The original design did not specify a balun; the ladder or window line is intended as a $\lambda/4$ transformer (at 40m for the original design). Thus, the length of this line is actually chosen to manage the load impedance $Z_L$ for the length of the dipole. The theory of operation and experimentation by the designers determined that with a 300$\Omega$ "transformer" and 50$\Omega$ coax feed-line a balun (or unun) is not necessary. This is the case even with modern finals, or at least this is what the authors claim.

They also claim that an ATU is not necessary for the majority of bands.

The line itself should not run along the ground, and it should drop from the dipole/flat-top at 90$^\circ$.

This antenna is tuned by selecting the frequency and length for the flat-top and tuning with the "transformer", aiming for an ideal feed-point impedance for the coax feed-line. According to the references I'm reading, they did so with a lookup-up table (i.e., it was not measured) for $Z_L$ and some assumptions about the Velocity Factor and characteristic impedance of the transformer line.

To determine the length of the matching section we use the standard transmission line equation which gives us ZIN if we know ZL , ZO, the frequency and the length of that line. By re-arranging the equation we can find the length at any given frequency and ZO once we've used the "look-up" table to find ZL . Of course, ZIN is fixed by the required standing wave ratio on the 50 ohm cable to the rig. Usually this VSWR may not be more than 2:1, and is always specified by the transceiver manufacturer.

ZR1DQ - Antennas - Theory - G5RV Multiband Dipole Antenna

This suggests that to tune this for the band(s) you are interested in, and to get the 1:1 SWR values declared and published for the 20m and 40m bands, some experimentation and calculations might be necessary. I don't think this is a "hang it and tune it" antenna, and it was probably shipped longer than necessary -- including the window/ladder line -- to allow for tuning.

  • Try without the balun/unun. The original design not only did not specify a balun, it was determined to be unnecessary. It may be throwing off your measurements. It may be critical for operation. But it is an unknown variable at this point.
  • The "transformer" line is, ideally, run straight down to above the earth and then the feed-line connected to that. Since we do not live in an ideal world, a gentle curve without getting too close to the earth is probably fine. What too close happens to be was not (as far as I know) part of original modelling.
  • You might have to go back to the original documentation on this antenna. There are references online and in the ARRL antenna book that should be easy to find. This antenna requires sweating the details about the various impedances to reach the compromise it promises.
  • One thing is for sure: you cannot just lop off the ladder/window line at some convenient length, as the length of this line is crucial to the theory of operation for this design.

Of course, since this is some commercial variant of the design, perhaps the manufacturers know better and the balun is there so the transformer line length is less critical and to limit RF getting into the actual coax feedline. We do not know because we do not know what else was changed or assumed by the manufacturers. This makes the antenna a hybrid design. You might be able to ask the manufacturer how they determined the various impedances and lengths and why they think a balun/unun is necessary.

What kind of balun might give us some hints about what the manufacturers are trying to do, so if you get no joy from them and you still want to use the balun you should probably find out what sort of balun it is.

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  • $\begingroup$ A 1:1 balun or choke is required when you go from balanced line to unbalanced coax. I've experienced this myself with a G5RV, it was better behaved and easier to tune with a coil of coax at the base of the ladder line. But as you say there's no impedance transformation required, the antenna tries to transform to near 50 ohms. $\endgroup$
    – tomnexus
    Mar 19, 2022 at 15:14
  • $\begingroup$ @tomnexus yes, of course. But the theory of operation for this design specifically leaves any matching to the window/ladder line used as a matching transformer. The ladder line is the unun, at least in the original theory of operation. If this hybrid design is so out of spec with respect to the original design I would still recommend keeping it as simple as possible and adding more matching only if necessary. As of now the entire antenna system is an unknown. I would still advise working from the known to the unknown. Deal with CM later if necessary! $\endgroup$
    – user21417
    Mar 19, 2022 at 15:31
  • $\begingroup$ But OP said a 1:1 balun, which is just what you need (assuming it's not broken), no additional matching, just allows you to connect the balanced antenna to the unbalanced coax without getting too much RF in the shack (which also upsets the overall matching by changing the shape of the antenna). So I think it's wrong to say "a balun is unnecessary" - it is essential. $\endgroup$
    – tomnexus
    Mar 20, 2022 at 4:11
  • $\begingroup$ I've added mealy-mouth lawyer words to make it clear what I'm saying: this design is what it is and that is well documented, this commercial version is also whatever it is, and some experimentation, measurements, and possible calculation might need to be done to get the results the original authors (and commercial vendor) suggest is possible. Whether or not an unun interferes with this will require experimentation and measurement as well. $\endgroup$
    – user21417
    Mar 20, 2022 at 14:12

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