How the LNA can help with your antenna woes

Useful information regarding antennas for SDR products.
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How the LNA can help with your antenna woes

Postby 13dka » Sat Mar 19, 2016 6:20 pm

I'm certainly not the only one who noticed that the LNA can work wonders with a variety of not so optimal or impossible antenna contraptions, random wires, poles, rods, or loops. I don't understand why this works though. First off, LNAs that are not connected directly to the antenna are often said to be useless or at least much less effective. But if I got that right it should be even worse with the RSP1, the (switchable) LNA sits even behind the filters in the tuner chip, and not directly behind the antenna input (where another, non-switchable discrete LNA lives). Yet it still seems to have an impedance matching effect or something, whatever really happens there sure helps a lot making makeshift antennas work that wouldn't work if attached to other receivers.

To my surprise, the LNA even does wonders on shortwave and below! One of the eye-opening encounters with the LNA magic was a simple 70cm diameter wire loop, which I made to check if the "small loop" concept would work for me as an indoor antenna (before I shell out 400 Eurobucks for a Wellbrook loop). Without the LNA, next to nothing came out of that loop between 0 and 5 MHz, with the LNA it came to life and brought acceptable medium wave reception and 80m hams with much reduced local QRM. So the loop did what it was supposed to do. Here's a screenshot of the loop reception on long wave with the LNA off and on:

Image: 70cm loop on LW with LNA off and on
1-2_LF_Loop_LNA_OffOn.jpg (51.08 KiB) Viewed 4957 times

So I made a bigger 1m diameter loop (similar to the Wellbrooks) out of some more rigid 3mm wire (not even copper) and compared that to an indoor dipole on 80m:

Image: 1m indoor loop vs. dipole
1-3_80m_Indoor_Loop_vs_Dipole.jpg (75.72 KiB) Viewed 4957 times

As you can see, it won't pick up weak DX stations but it gives you a better SNR than an indoor wire/dipole and allows for comfy listening despite 2 laptops and a flatscreen TV running in the same room. Of course I used 5m of coax to get the loop as far away as possible from these QRM sources. If you have absolutely no chance to get some antenna outsides, a loop may be your best friend. If you're also on a tight budget, the best initial investment is in coax, SMA connectors and some wire, the RSP's built-in LNA will help you make due with that simple stuff and no external LNA, at least for a while.

But of course an outdoor antenna is hard to replace. This is the indoor dipole when used outsides:

Image: Same dipole outdoors
1-4_80atHome_Dipole_outside.jpg (42.38 KiB) Viewed 4957 times

(Part 2 will follow when this post has been approved)
Last edited by 13dka on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Posts: 136
Joined: Sun Feb 14, 2016 8:40 am

Re: How the LNA can help with your antenna woes

Postby 13dka » Sat Mar 19, 2016 6:50 pm

(Part 2)

Indoor antennas may give you considerably less mileage. There will be more noise indoors, reducing your SNR and modern buildings make that even worse by attenuating the signal - wall and roof insulation with aluminum foils, heat and UV protection glass, roofing tiles with metal particles, you name it... will shield your antenna from HF. New or modernized buildings and roofs can be surprisingly anti-HF. A balcony can already help much with that, but you're still in the QRM cloud around the house, which is said to extend at least 5 meters beyond the walls etc.. Your goal is to get as much antenna as possible as far away from walls and roof as possible. Anyway, there's a lot to read about that on the internet.

If you don't want to spend much money or like experiments, you might end up with some wire contraption outside. I bought some cheap 0.75mm braided speaker wire and made an off-center fed dipole (due to the coax being only 5m), which works surprisingly well on 80 and 20m. Well, actually it doesn't work well at all but again, the LNA is fixing the many shortcomings of the thin wire, the asymmetric feeding and the improper impedance matching.

Image: 20m reception with LNA off and on
2-1_20m_outdoor_dipole_LNA_off-on.jpg (51.18 KiB) Viewed 4954 times

A few days ago I discovered the tiny NooElec "one-niner" balun and impulse-bought one, thinking it might help with the not so great 40 and 15m bands on that dipole, and with some ranges where the spectrum shows quite a low output. Then, after googling a bit about the device I was thoroughly confused: It's a 9:1 balun, which doesn't seem to match what's needed for a symmetric antenna (such as a dipole), and since it's a balun and not a 9:1 "unun", it doesn't really serve end-fed long wires either - whatever the concept behind this balun might be, it's completely undocumented. However, since my outdoor dipole is fed off-center (5m + 9m) I assumed high-ish impedances at the feeding point and thought that perhaps it would help.

The result was completely unexpected - while the noise rose to a much more homogenous level across the shortwave bands, the signals did not rise with it and even disappeared on some frequency ranges.

Image: Spectrum 10-15 MHz without/with balun
2-2_NOBALUN_BALUN_10000-15000.jpg (64.32 KiB) Viewed 4954 times

20m was almost useless all of a sudden, without the balun (and with the LNA on instead) I got more signals than the Twente University web SDR on 20m, with the balun it looked much like the "LNA off"-image on top of this post. 40 and 15m didn't improve either and while VLF was pretty awesome without the balun, it was pretty much gone now.

Image: Spectrum 20-25 MHz, balun increases local QRM only
2-3_NOBALUN_BALUN_20000-25000.jpg (27.33 KiB) Viewed 4954 times

Were the reviews of that thing just yet another stupid internet product hype?

I got the answer when I switched the LNA on and off across the shortwave - its positive effect was gone in most ranges, or in other words, the antenna that needed the LNA to work really good did not need it anymore and defeated its impact on reception. The balun would have actually improved the antenna for me (well a little bit, and only at some frequency ranges) if I wouldn't have an RSP1 and the LNA in it and I would have written a positive review as well. But all it did for me was raising the noise level and decreasing the SNR compared to the LNA-only operation before, and the LNA could not fix that anymore.

Somehow the LNA has a much more positive effect on that dipole than a balun, and I guess even a properly matched transformation ratio from a less ridiculously tiny balun wouldn't give me much better results. If I were to use a balun, I'd also have to design and tune the antenna properly. I removed the balun next day and the distant signals on 20m were back. Even though the dipole is obviously quite unapt for that band when used with some random radio, the LNA makes it work, and how! I'm hearing stations from North and South America and the Carribean every afternoon (despite the currently not so great conditions, Canadians are actually often "booming in" and listening to ABC from Shepparton on 12065 kHz was quite enjoyable every afternoon so far.

Bottom line: whatever oddball makeshift contraption you will try to use as an antenna or whatever space limitations may force you to do that in first place, the built-in LNA may help to make it work in many cases, or it will give you a preview on how some design might work when you add an e.g. LNA4ALL, it generally makes you more independent from properly constructed and tuned antennas, even though it might not replace them. :)
Last edited by 13dka on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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