The small SWL beam (1...30 MHz)

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Andrew (grayhat)
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Re: The small SWL beam (1...30 MHz)

Post by Andrew (grayhat) »

13dka wrote: Fri Jul 15, 2022 8:23 am
Andrew (grayhat) wrote: Fri Jul 15, 2022 7:08 am Ok, I think we may stick with "SDL" (Small Directional Loop) or "SDRL" (R for Receiving)
I suggest SLOB then, you know it really helps if the acronym is a word (took me 2 years until I could memorize "NCPL" and 10 for "G5RV/HB9CV/W3DZZ"), or the antenna has a completely unrelated name like "Double Bazooka" for a "coaxial folded dipole antenna". :) Hmmm...Bazooka...
LOL !! Well, "Bazinga" would be cool, although maybe probably too "nerdy" :lol:

Ok, serious now
Andrew (grayhat) wrote: Fri Jul 15, 2022 7:08 am those shouldn't be a major issue as long as the horizontal run of the coax is choked with snap-on ferrites
Not sure what you mean by "horizontal run"? In my mind the coax will likely not make that much of a horizontal run, depending on its bending capability) (which is admittedly not so great for the H-155 I'm using), the more snap-on ferrites it gets, the less it will be able bend.
Let me try a "masterpiece of art" :D
sdl_coax.jpg
sdl_coax.jpg (75.3 KiB) Viewed 2032 times
the above shows the coax routing, horizontally along the "cross support" arm and then down vertically, judging from currents simulation in NEC, the most effective place for snap-on ferrites should be along the horizontal run of coax
BTW that is the part that causes me the most headaches, having this NooElec Balun sitting horizontally in its junction box at the end of the boom, with the coax putting some pull on the boom and the prospect of chaining another box with the preamp to the balun box. I should have thought of that, there are 90° SMA plumbing pieces available on Amazon. :/
there won't be any "pull" with the config shown above, the coax will just be supported by the same structure supporting the loop; as for the RF preamp, my idea is that placing it at the center of the cross (the point where the horizontal coax turns down) will avoid loading the arm too much

As for currents, here's what the NEC model shows
sdl_curr.jpg
sdl_curr.jpg (31.11 KiB) Viewed 2029 times
which shows why I'm thinking that the horizontal segment of coax should be the best suited for the ferrites
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Re: The small SWL beam (1...30 MHz)

Post by 13dka »

Andrew (grayhat) wrote: Fri Jul 15, 2022 7:47 am
As a note, the difference between a 530 Ohm or a 540 Ohm load resistor is negligible, so 2x270 Ohm resistors in series will just fit
I figured. I'll have to use 2x 1kOhm parallel -> 2x 47Ohms parallel for 523 Ohms (the 525pc resistor box doesn't come with 270 Ohms*). With some good layered shrink tubing that should work without an extra box.

* Because I bought 525 resistors and 600 ceramic capacitors, Amazon offered me a business account today :mrgreen:
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Andrew (grayhat)
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Re: The small SWL beam (1...30 MHz)

Post by Andrew (grayhat) »

13dka wrote: Fri Jul 15, 2022 8:43 am
Andrew (grayhat) wrote: Fri Jul 15, 2022 7:47 am
As a note, the difference between a 530 Ohm or a 540 Ohm load resistor is negligible, so 2x270 Ohm resistors in series will just fit
I figured. I'll have to use 2x 1kOhm parallel -> 2x 47Ohms parallel for 523 Ohms (the 525pc resistor box doesn't come with 270 Ohms). With some good layered shrink tubing that should work without an extra box.

Well, you may use 2 x 1K in parallel and get 500 Ohm, add a 47 Ohm in series and you'll have 547 Ohm, pretty close :) as for coax routing and chokes, see my previous post
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Re: The small SWL beam (1...30 MHz)

Post by 13dka »

Andrew (grayhat) wrote: Fri Jul 15, 2022 8:37 am Let me try a "masterpiece of art" :D
Ah yes, of course - the inside of the loop is probably less prone to be disturbing the pattern and that would indeed solve the problem with the load and the preamp, that could sit just inline with the balun box, at the feed line end equipped with this:

sma_90.jpg
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Andrew (grayhat)
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Re: The small SWL beam (1...30 MHz)

Post by Andrew (grayhat) »

13dka wrote: Fri Jul 15, 2022 8:56 am Ah yes, of course - the inside of the loop is probably less prone to be disturbing the pattern and that would indeed solve the problem with the load and the preamp, that could sit just inline with the balun box, at the feed line end equipped with this:
no need for the angular adapter, see this
sdl_struc.jpg
sdl_struc.jpg (31.78 KiB) Viewed 2017 times
the 9:1 will be placed sideways in the box, with the connector facing the inside of the loop, from there a run of coax, hosting snap on chokes, goes from the 9:1 to the center box hosting the preamp and, from there the coax exits vertically running along the support pole, down to the receiver, I believe that such an arrangment should give the best balance and won't load the support structure too much (btw, sizes aren't "scaled" in the above drawing)
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Re: The small SWL beam (1...30 MHz)

Post by 13dka »

Yeah I understood this, it's just that I'm usually reluctant to bend coax sharply, even the double foil-shielded stuff, particularly that stiff Belden H-155. Probably unnecessary in this case since that's being bent only once and at the antenna where the integrity of the shield isn't that important. However, the knee adapter would avoid this altogether..

According to your sketch we basically create a common mode choke out of a coax pigtail and clip-on ferrites. I just checked my drawers and found some cable pieces I obviously ordered for something in the past before I forgot I did, or for what I ordered them but of course they are either 2m long or have the wrong connectors (don't we all know that?). The need of a choke behind the balun is understandable but I was initially planning on / hoping for this:

Balun_LNA_naked.jpg
Balun_LNA_naked.jpg (53.98 KiB) Viewed 2012 times

Wouldn't the choke behind the LNA still be sufficient? That way I could simply clip ferrites to the end of the feedline and let it drop gently down the pole behind that part. Otherwise I do have fitting lengtht SMA window pass-thru pigtails with some super crappy,thin and lossy coax inside (thinner than RG-174), that would do with a gender changer adapter, on the plus side that stuff bends around everything but I'd rather avoid this and have 40cm straight 5mm coax to the LNA (which needs to go on the shopping list first).

BTW, I just checked - 2.5mm is the absolute maximum still fitting in the the terminals of the balun. Phew! :)
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Andrew (grayhat)
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Re: The small SWL beam (1...30 MHz)

Post by Andrew (grayhat) »

quick note, will reply more extensively later; if you're using the NooElec v1, ensure to cut the "R1" trace to achieve full galvanic isolation !

Image

in short, the V1 has a tapped transformer, antenna side, and the tap is grounded, but for loop antennas it's better to have full galvanic isolation, so cutting that "R1" (in reality it's just a track on the backside of the PCB)

Image

we'll achieve it and willing to go back it will be easy to solder a jumper at that point
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Andrew (grayhat)
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Re: The small SWL beam (1...30 MHz)

Post by Andrew (grayhat) »

13dka wrote: Fri Jul 15, 2022 10:46 am Yeah I understood this, it's just that I'm usually reluctant to bend coax sharply, even the double foil-shielded stuff, particularly that stiff Belden H-155. Probably unnecessary in this case since that's being bent only once and at the antenna where the integrity of the shield isn't that important. However, the knee adapter would avoid this altogether..
well, the alternative to bending would be using a short "pigtail" between the balun and the preamp (boxes) and then going straight down with the coax to the receiver, then ok, the alternative would be the knee adapter
According to your sketch we basically create a common mode choke out of a coax pigtail and clip-on ferrites. I just checked my drawers and found some cable pieces I obviously ordered for something in the past before I forgot I did, or for what I ordered them but of course they are either 2m long or have the wrong connectors (don't we all know that?). The need of a choke behind the balun is understandable but I was initially planning on / hoping for this:
exactly so, the idea is to avoid adding too much weight to the structure arm supporting the feedpoint by moving the preamp to the center of the "cross" (structure), but if you feel like moving everything (balun, preamp and chokes) along the arm will be ok, then go on that way, all in all you are building it, and in any case, modifications to the structure may be done at a second time, if needed
Wouldn't the choke behind the LNA still be sufficient? That way I could simply clip ferrites to the end of the feedline and let it drop gently down the pole behind that part. Otherwise I do have fitting lengtht SMA window pass-thru pigtails with some super crappy,thin and lossy coax inside (thinner than RG-174), that would do with a gender changer adapter, on the plus side that stuff bends around everything but I'd rather avoid this and have 40cm straight 5mm coax to the LNA (which needs to go on the shopping list first).
Well, to tell the truth, at this point I don't know; probably it will be ok; as for the LNA, if you want for a "maiden fly" you may omit it and then add it later if cranking the RX preamp won't suffice, but if you prefer going for belt and suspenders, then having the LNA won't hurt; as a note, going for my suggested approach, that is balun, pigtail w/chokes, preamp would also allow to quickly bypass the preamp just by disconnecting it from the pigtail and coax and connecting the latter together
BTW, I just checked - 2.5mm is the absolute maximum still fitting in the the terminals of the balun. Phew! :)
a way to fit larger wires is to use an electric terminal block (also called "mammut" or "mammoth") in that case the block will sit inside the same box hosting the balun, the antenna wires will come to the block terminals and from there two short runs of thinner wire will go to the balun
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Re: The small SWL beam (1...30 MHz)

Post by 13dka »

Andrew (grayhat) wrote: Fri Jul 15, 2022 12:56 pma way to fit larger wires is to use an electric terminal block (also called "mammut" or "mammoth") in that case the block will sit inside the same box hosting the balun, the antenna wires will come to the block terminals and from there two short runs of thinner wire will go to the balun
Yeah, another thing I forgot to buy is the alternative to that - a nice pair of M4 bolts and nuts/wing nuts/washers to use cable lugs, making that all as "modular" as it gets. BTW, the balun and the preamp weigh like 20 grams together and I'm planning to omit the preamp first, as long as I don't screw it to the boom I can easily insert the preamp on demand. I'll keep things flexible and zip-tied until I know what works best.

Re balun, the one on the picture is the one I have with the R1 connection cut.
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Andrew (grayhat)
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Re: The small SWL beam (1...30 MHz)

Post by Andrew (grayhat) »

13dka wrote: Fri Jul 15, 2022 1:24 pm Yeah, another thing I forgot to buy is the alternative to that - a nice pair of M4 bolts and nuts/wing nuts/washers to use cable lugs, making that all as "modular" as it gets. BTW, the balun and the preamp weigh like 20 grams together and I'm planning to omit the preamp first, as long as I don't screw it to the boom I can easily insert the preamp on demand. I'll keep things flexible and zip-tied until I know what works best.
if you'll go for nuts/bolts... go for all brass; as for the modular approach, you are right, until we won't have a definitive design/setup it's the best approach
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