How the LNA can help with your antenna woes
Posted: 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 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 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
(Part 2 will follow when this post has been approved)
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 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 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
(Part 2 will follow when this post has been approved)