RX only linear loaded antenna - Page 1
Forum section: electronics / rf-microwave

author: A.Z.
author_rank: Super Contributor
timestamp: September 29, 2020, 08:55:55 am
content: Hello all, time ago I decided to experiment a bit to find out which antenna would better fit a limited space (and budget) radio listener setup, I started by picking an SDR (the Msi.SDR "blue dongle") and collecting various bits and parts for the experiment, the self imposed limitation was that the antenna should fit my (10m x 2m) 2nd floor balcony; the very first antenna I set-up for the experiment was a vanillla "random wire" placed in a "zig-zag" configuration with a counterpoise and fed using a NooElec 9:1 BalUn, the antenna worked pretty well but as expected was pretty noisy and for sure I could do better, so I moved on trying a dipole, a T2FD and a passive loop too, then I stumbled upon a page describing the "cobra" antenna (https://www.hamuniverse.com/cobraantenna.html) which is basically a linear loaded dipole (LLD) built using two three conductors wires for the arms, the wires are connected to form an "S" (hence the "cobra" name); at first I thought it was just another "miracle antenna" (snake oil and all that), but then, since some run of 3-conductors wire wouldn't cost me so much, I decided to go on and give it a try (again RX only !)The first setup resulted into a two 4m arms (3-conductors, totalling 24m wire) which initially I directly fed using the NooElec 9:1 BalUn I already had (not optimal, but ok for a quick check), the antenna worked pretty well, so I decided to go on and set it up "by the book", picked a run of window line and a pair of toroids, connected the window line to the dipole center, wound a dual core "guanella style" 4:1 BalUn and connected it to the coax feedline, performances were much better, so I kept using it for quite a while, but then... being the kind of folk I am, I decided to fiddle with it a bit more, I started by adding two runs of about 3.5m insulated wire (single conductor) to the end of each LLD arm, once installed the wires formed an horizontal rectangle (open) with the single wire sections at about 1m from the 3-wire arms; with such a simple modification signals were boosted quite a lot and the noise floor remained nicely down, at that point, I decided to experiment with the feedpoint, so removed the window line and the 4:1 BalUn and replaced it with the NooElec 9:1, while it probably is a bit too much, it allowed to gain even more signal, so at least for me, that config was a keeper; after using the "tweaked" LLD for some months, I consider it a pretty good antenna for limited spaces, running WSJT-X (lately JTDX) on FT8 I was able to pick up spots from all around the world, including Antarctica, so the performances seem to be pretty good (I also compared the "tweaked" LLD to a miniwhip, and while the whip works decently, the LLD beats it hands down), plus the antenna seems to work pretty well from LW (yesterday picked up the Tipaza transmitter on 252KHz) up to VHF (was able to clearly receive communication between ATC and aircrafts around 120MHz)Notice that I'm far from being an expert in antenna modeling (and I don't own a VNA), I played a bit with 4NEC2, but I'm pretty much at loss when it comes to putting together a NEC model file, so I didn't run any simulation and sincerely I don't know what kind of pattern such an antenna hasNow for the question; I'm considering the idea of turning the antenna into an horizontal "delta loop", the idea is to connect the end of the two 3-conductor wires (arms) together using a run of insulated wire, but I wonder if it would be better reversing the setup and using two runs of single wire from the feedpoint and a run of 3-wires cable between them, so here's the question:which config do you think could be better for the loop setup described above ?

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 29, 2020, 03:22:38 pm
content: first: it's not fully clear to me, why you take 3-conductor cable for an antenna; that serves absolutely no purpose; just take a single conductor, stranded wire or similar which is sufficently weather/UV resistanta delta loop is a good antenna; but forget the 1:9 unun; since it's a symmetrical antenna, you're supposed to use a 1:1 current balun.but I'm asking me if it's worth it to run full size antennas for receiving purpose only. there are very good active receiving antennas like the mini/midi whip or active loops.personally I'm almost entirely using active antennas like the above ones in the RX-path of my ham station only, at least for medium and short wave while using the full size loops and dipols only for TX.receiving is mostly a matter of the S/N-ratio, and the MidiWhip according to DL4ZAO beats my full size antennas on short wave easily.

author: A.Z.
author_rank: Super Contributor
timestamp: September 29, 2020, 03:45:37 pm
content: Quote from: HB9EVI on September 29, 2020, 03:22:38 pmFirst of all, thank you for the reply, then...The reason is in the subject, that is, linear loading, just like using loading coils and for the same reason, it's just another way to load itQuoteThe original design uses a run of "open wire" (ladder or window line) and a 4:1 current balun to feed it, and as I wrote I went for that approach, but then, I decided to experiment a bit, and since the resulting antenna offered better performance (in RX !!) after testing both for some months, I decided to go for the 9:1 w/o the "open wire" feeder, and now my doubt is about the delta loop, if you think that feeding it with a 1:1 is the way to go, no problem, I may just pick one of the two cores I used for the 4:1 which are already wound "guanella way" and use it; but remember that I'm planning to use the antenna (again RX only) on different bands/frequencies and I'll feed the antenna using 50 Ohm coax, not "open wire" lineQuoteQuoteExactly so (S/N ratio) and in my case, the LLD has far better S/N than the miniwhip, on the other hand the small loop (http://www.kk5jy.net/rx-loop/) has better S/N ratio but then, when compared to the LLD pulls in less/lower signalsJust in case, the attached pic shows the station spotted on FT8 at the moment (16:00 UTC, RX started at 11:00 UTC)