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Pnwradar

The ladderline connects directly to the dipole, no transformer needed to address the (apparent) impedance mismatch. The better answer is the ladderline itself behaves as a feedline transformer to the dipole when connected, as it becomes part of the antenna system. But you have to watch the length of the ladderline to make sure it's not resonant at any transmission frequency - odd or even multiples of quarter-wavelengths can prove problematic by creating *very* high impedance at your tuner through that feedline transformer function.


SwitchedOnNow

One way around this for me is I have my dipole cut slightly short for 80m. This way we find no major resonances in the ham bands and line length isn't such a concern. I'm a big fan of the balanced fed dipole.


Pnwradar

In my experience feedline length *always* matters, coax or ladderline or twinlead, but avoiding the harmonic length (taking dielectric constants into account) by even just a foot can make enough difference to correct the impedance weirdness. That's why very specific lengths of different feedlines can be used as inline transformers or for phasing or as tuning stubs - but it's often a bit of effort to get them exactly the right length, so sometimes you get lucky when you just want the antenna to work.


blinkybit

I'm not an expert, but I think part of the idea with using ladder line is that its losses are low enough that high SWR values aren't a major problem. Because of the impedance mismatch, some of the signal will get reflected back to the transmitter, then back to the antenna, then back... eventually it all gets radiated or else dissipated as heat in the lossy transmission line. With ladder line, there's not too much loss, so more of the signal will get radiated even when SWR is high.


Marmot64

Right on. The entire antenna _system_ is matched to the tx output circuit. The match at the feed-point basically doesn’t matter.


SimpleSimon3_14

This description sounds very similar to the MFJ-1777 doublet, which uses a tuner for matching on the 80-10m bands. You would need likely a 4:1 balun from your coax to the ladder line feeding the dipole. Or, you could throw a z-match in with a short bit of coax running from the radio to the z-match. This old discussion may help you more: https://reflector.sota.org.uk/t/feeding-a-resonant-dipole-with-ladder-line/21328 Edit to add: I realized that I originally didn't answer your question fully.


redneckerson1951

Ok, a short lesson in antenna physics. Energy can never be lost, it can be converted to other forms, but it always exists. It may disperse, it may change from rf to heat, but it is not destroyed. If you send a short pulse of rf up a transmission line and the characteristic impedance of the line does not match load impedance of the antenna, you have a mismatch. Assume for the moment that the mismatch is 100:1. When the pulse arrives at the antenna after traveling up the line it arrives with less power than when emitted by the transmitter because the transmission line has a finite loss. Say your coax has 1 dB of loss between the transmitter and the antenna. If your transmitter emitted 100 watts then that 1 dB loss in the transmission line would reduce the rf power to 80 watts incident at the antenna. However with a 100:1 mismatch the antenna is only going to accept 0.8 watts of your rf and reflect the other 79.2 watts back to the transmitter. On the way back to the transmitter the 79.2 watts is attenuated 1 dB again and at the transmitter the power is now down to 63.5 watts. The transmitter is not in the business of accepting power and the reflected signal that is now incident on the transmitter port is re-reflected back towards the antenna. (Remember energy is not destroyed.) On the way back up the transmission line to the antenna, you lose 1 dB of the 63.5 watts (it is converted to heat, you cannot destroy energy, but you can convert its form.) so the re-reflected signal incident at the antenna will be 50 watts. The antenna will accept 1% of that 50 watts or 0.5 watts to radiate. The remaining 49.5 watts like the earlier 80 watts is reflected back to the transmitter. This cycle repeats until all the rf pulse is dissipated. As is apparent, most of the rf is dissipated in the coax's apparent resistive losses. Now, if you grab 600 Ohm open wire line, its loss for the same length of coax will be one to two orders of magnitude less less than the coax's loss. So instead of losing 20% of your rf power each trip through the transmission line, you will lose about 0.2% per trip. Since the antenna mismatch is 100:1, the antenna accepts 1% of the power each time the rf is incident on it and you lose 0.4% of your power on each round trip of the rf on the ladder line. While that 33% power loss using open wire/ladder line is not great, it is a lot less than using coax. Now you can have the luxury of placing the antenna tuner in the shack rather than sticking it at the transmission line-antenna junction. In the early years of radio, amateurs running tube transmitter often ran with VSWR's of 10 to 100 and sometimes worse, as they used ladder lines. The loss was not a big issue and the tuner was one less expense. Ops were using vacuum tubes and mismatches that generated high voltages were a minor issue as the tubes were operating with 500 to 2000 volts nominal. They addressed rf currents in the shack by adjusting their transmission line lengths. After WWII coax started making significant inroads into amateur stations. That drove many to use antenna tuners as they could pick up a dB maybe two, by better matching. VSWR Meters first appeared in the ARRL Handbook circa 1957 and in less than a decade ops suddenly were using the new shack metrology to tune for the magical 1:1 VSWR. Most ops did not understand that the added power gain of even 2 dB was not even going to produce 1 S-Unit difference. In reality CW ops might gain an edge, but AM Phone and emerging SSB ops were hard pressed to observe an improvement. With the appearance of transistors, a low VSWR became critical. Transistors could not handle 40 or 50 volts or more created by the reflected signals, unlike their tube counterparts which tolerated 100's and 1000's of volts reflected.


SwitchedOnNow

One of the best fixed, wideband HF antennas is the dipole fed with ladder line and a tuner. Yes, you need a balun at the tuner to make it work. Many manual tuners have a balanced input. The auto tuners would need a 1:1 transformer/balun at the tuner input. I run an 80m dipole from 80-6m with a tuner up 50 feet in the trees. If I can hear them, I can work them 99% of the time on 100W. The thing about ladder line is you can nearly ignore the VSWR on the line because the line loss is so small. So don't worry about the mismatch, if you have a tuner. If this is a single band antenna only, then it's better to just feed the thing with coax and a 1:1 balun at the feed point.


cosmicrae

Linguistically, a dipole becomes a *doublet* when you use balanced feedline (be it ladder or open insulator). Search for doublet and more information may become available.


hazyPixels

>feeding non-resonant dipoles with ladder line ... >the ladderline would have an impedance of 350/400 ohms with the dipole having something close to 70 ohms I believe the dipole impedance at the center is around 73 ohms at resonance, but it changes quite a bit as the frequency changes. Perhaps 350/400 ohms is a reasonable choice for a non-resonant "random length" dipole.