The LLA antenna - Page 1
Forum section: electronics / rf-microwave

author: A.Z.
author_rank: Super Contributor
timestamp: August 17, 2021, 03:22:18 pm
content: I just stumbled upon this antenna design from Matt Roberts (KK5JY - the same who modeled the Loop on Ground)http://www.kk5jy.net/lla/the antenna is basically a run of ladder line suspended horizontally at about 1m (3'6") from ground using plastic poles, terminated at one end with a resistor and fed using a 9:1 transformerThe design is still "experimental", that is Matt didn't yet have the time to fully test the antenna and assess its performances, yet the design and the resulting radiation lobe is interestingNow, being the kind of curious folk I am, I decided to run my own simulations using the 4NEC2 program, first I created a model for the antenna as described by MattCode:' symbolsSY hght=1 ' height from groundSY freq=3.650 ' test frequencySY wire=0.000512 ' wire radiusSY spac=0.0889 ' wire spacingSY leng=10 ' wire lengthSY segm=51 ' segments in wireSY segs=1 ' segments in ends' horizontal wiresGW 1 segm 0 0 hght leng 0 hght wireGW 2 segm 0 0 hght-spac leng 0 hght-spac wire' end junctionsGW 3 segs 0 0 hght 0 0 hght-spac wireGW 4 segs leng 0 hght leng 0 hght-spac wire' groundGE -1GN2000130.005' loadingLD14114700LD500058000000' feedingEKEX01101.00' frequencyFR0000freq0' endENand the simulation results confirmed what described by Matt, I also tried using a short instead of the terminating resistor and then leaving the far end open, and again, the results match what Matt wrote, later on I had an idea and decided to see what kind of pattern such an antenna would present if installed vertically, so, I modeled the same antenna turned vertical, with the base (feedpoint) at 1m from ground and the top at 11m, here is the model for such a vertical version of the LLACode:' symbolsSY hght=1 ' height from groundSY freq=3.650 ' test frequencySY wire=0.000512 ' wire radiusSY spac=0.0889 ' wire spacingSY leng=10 ' wire lengthSY segm=51 ' segments in wireSY segs=1 ' segments in ends' vertical wiresGW 1 segm 0 0 hght 0 0 leng wireGW 2 segm 0 -spac hght 0 -spac leng wire' end junctionsGW 3 segs 0 0 hght 0 -spac hght wireGW 4 segs 0 0 leng 0 -spac leng wire' groundGE -1GN2000130.005' loadingLD14116500LD500058000000' feedingEKEX01101.00' frequencyFR0000freq0' endENyou may notice that I replaced the 470 Ohm terminating resistor used in the original design with a 650 Ohm one, since in my simulations that gave a much better and flat match, the results of the simulation were quite interesting and can be seen in this imageas you can see, the antenna, even if vertical, shows a directional pattern and a pretty low radiation angle, the "gain" is negative, but that in my book means that the antenna should have a pretty good SNR in reception and then, thanks to the flat SWR it could be used from 160 to 10 meters, then ok it remains to see how well it will work in TX; also an idea for the vertical may be putting a relay at the feedpoint, that could allow to reverse the feeder connection so in practice reversing the directional patternBottom line, if someone here has some length of unused ladder line, it may be interesting to experiment with the LLA idea, either in horizontal configuration or in vertical one

author: coppercone2
author_rank: Super Contributor
timestamp: August 18, 2021, 05:20:39 pm
content: Very interesting, I wonder what impedance the ladder line is.

author: A.Z.
author_rank: Super Contributor
timestamp: August 18, 2021, 05:38:36 pm
content: Quote from: coppercone2 on August 18, 2021, 05:20:39 pmJust a matter of taking the time to read the KK5JY page which I linked

author: coppercone2
author_rank: Super Contributor
timestamp: August 20, 2021, 07:34:33 pm
content: I have a transformer designed to test weird impedance (HP patch panel), but it has a fixed switch with the highest setting being 900 ohms (also 600 and 200?), but its ladder line territory.Doubt it would fall nicely into one of those taps.. but I wonder if the antenna can be standardized so the same ladder line impedance is used for different things, I still need to get through that articlemy interest is more along the lines of is there something specialized you can do with 900 ohms so I can use the transformer set I have with it for some purpose.

author: E Kafeman
author_rank: Regular Contributor
timestamp: August 23, 2021, 06:53:06 pm
content: As it is a transmission-line, more or less terminated with its characteristic impedance is antenna efficiency by design low.If transmission line is a carefully designed ladder with two parallel lines, a coaxial cable or single wire above a well conducting ground, they are all variations of relative lossless transmission lines as long as they are terminated with its characteristic impedance. Short or open end or just less good selected end termination will improve antenna efficiency as transmission line then will act as a single wire monopole antenna. For a coaxial cable will RF signals travels along outside of braid if making it mistuned terminated.As a kind of transmissionline do it reminds me about a similar kind of antenna/transmisionline known as a leaky wave antenna which I once did evaluate.A long time ago did I live near a railroad. It was since long unused. Along railroad was a telephone landline, once used by railroad company for its internal communication but not any longer used. It consisted of about 10 separate single wires and in total about 30 miles or longer in length.As telephone lines typically are screwed a notch, selecting an isolator at slightly different location for each pole, to avoid high capacitive coupling by avoiding parallel lines, could two wires be hijacked as aleaky transmission lineantenna due to constant minor variation in wire distance, if well tuned at rigg location.However, every few miles is this kind of landlines pupinized with a coil to reduce wire losses and compensate for capacitive effect:See this link for explanation:https://www.wikiwand.com/en/Loading_coilEach location where these coils was placed created a more lossy spot from RF view, field strength increased as transmission line not was as ideal.As shortwave antenna was it no real advantage with this kind of antenna but it did work well for receiving low frequency submarine traffic and listening for ELF noise. It was also my best antenna for receiving a particular low power FM broadband radio, ~100MHz, located in a nearby city where these lines ended. I assumed it also was a good antenna to pick up tunder-storm discharges at long distance which could lead to local high current and even make phone-lines create local firework due to my tuning circuits.At one occasion did I use two of these phone lines as a 27 MHz antenna at a foxhunting competition. The transmitter was located in middle of a railroad bend, to not be too visible from distance. Transmitter was well tuned for minimum local losses, almost no local field strength measured. To further reduce field strength was my transmitter built in a well isolated box with power and LF signals carefully filtered and with a full transformer balanced output and whole radio was hooked up between the lines.Very funny as every foxhunting competitor tried to find the transmitter near some of the phone poles with pupinizing coils along railroad, long away from actual transmitter location.I designed a few other amusing foxhunting antennas, designed to make it a bit hard to localize transmitter location but this antenna was actually not my design, it was just a reuse of old phone lines.