ebay 0.1 - 2000MHz LNA - Page 1
Forum section: electronics / rf-microwave

author: metrologist
author_rank: Super Contributor
timestamp: February 22, 2019, 05:21:44 pm
content: I bought this one.Without LNA: Red trace is reference, 0dB attenuation and gain; Green trace is with internal preamp.With LNA: Yellow trace 0dB attenuation.

author: radiolistener
author_rank: Super Contributor
timestamp: February 22, 2019, 05:34:25 pm
content: can you perform some tests?If possible put generator on the input, how it works with different input level?

author: metrologist
author_rank: Super Contributor
timestamp: February 22, 2019, 06:55:13 pm
content: I do not have an RF generator - I would like a spa with tracking generator. Maybe I can borrow a VNA and use various input padding.

author: StuartA
author_rank: Regular Contributor
timestamp: February 22, 2019, 07:31:43 pm
content: I also bought a couple of these, but I have limited test gear and I'm a very amateur amateur.I did one test at 20MHz and measured their gain at ~30dB, which is what they claim. The gain is very dependent on the DC voltage used; the supplier of the ones I bought advised that at the higher supply voltage, the risk of self-oscillation increases.There's a website herehttps://pa0fri.home.xs4all.nl/Ant/Active%20antenna/Active%20receiving%20%20loop%20antenna%20eng.htmdescribing their use as the pre-amp for a magloop antenna, but it misses out some important details. Either the input or the output needs a balancing transformer, or the loop will see the RF coax line to the Rx as a "long leg".

author: metrologist
author_rank: Super Contributor
timestamp: February 22, 2019, 07:41:36 pm
content: A loop antenna will be one use case. Thanks!

author: radiolistener
author_rank: Super Contributor
timestamp: February 22, 2019, 11:41:27 pm
content: Quote from: StuartA on February 22, 2019, 07:31:43 pmthanks, very interesting articleQuote from: StuartA on February 22, 2019, 07:31:43 pmit can be tested by adding some ferrite rings on the coax line near the amplifier (make RF choke).If antenna performance will not be degraded, then coax line doesn't play significant role

author: metrologist
author_rank: Super Contributor
timestamp: February 23, 2019, 01:25:31 am
content: Also a good point. I used 10V input and the source is an OTA short antenna.

author: StuartA
author_rank: Regular Contributor
timestamp: February 23, 2019, 02:45:53 am
content: I got as far as connecting a loop (1m diameter) directly to the input, and the output of the LNA to a 20m coax feed to the receiver. For the initial test (listening on 5MHz), I was powering the LNA from a 12V battery ~300mm from it. This arrangement did not really work at all, I think because the coax braid is effectively connected to one side of the loop.I have not got around to testing a revised set up, but yes, putting some kind of RF choke on the ground of the LNA should be a step in the right direction.I have no other thoughts as to why it didn't work; any suggestions appreciated.

author: Bud
author_rank: Super Contributor
timestamp: February 23, 2019, 04:20:25 am
content: is a scientific ground that a 1m loop would work well at 5MHz?

author: radiolistener
author_rank: Super Contributor
timestamp: February 23, 2019, 10:01:37 am
content: Quote from: Bud on February 23, 2019, 04:20:25 am5 MHz is 60 meters wavelength. Any antenna with size 30 meters length or smaller will works.The key point here is that signal level on the antenna output will depends on antenna size.Smaller antenna size = smaller signal level on it's output.And here is a limit of the signal level that we can receive, this limit is noise floor level of your environment.This magnetic loop antenna works good with help of amplifier placed near antenna.And because this antenna with amplifier is placed on a long distance from home and other electromagnetic noise sources.Since home noises didn't affect it, the noise floor level is very low and antenna can pickup very weak signal.But the problem is that this signal is very weak (it's level is too small). If you put it directly to the cable, you will not be able to receive it on the other side of cable, because it will be flooded with noise from your home environment. This is why amplifier is added to this antenna and placed directly next to the antenna. It amplifies the weak signal and when it come to your home through cable, it will be strong enough to not be affected by noise from your home environment.This kind of antenna named "active antenna". And this is how it works. The key point here is that antenna together with amplifier should be placed on a long distance from any noise source, such as home or any kind of electronic equipment, power lines or just some piece of iron or some building.The Near Field region of antenna is very vulnerable to any noise source. Because any noise source placed in the Near Field region will be amplified by antenna much more than DX signal. Since this antenna is too small in size, it's Near Field region will be not so large, and it helps to avoid catch noise from environment. Also, since it's small it has small gain, but small gain of antenna is compensated by adding amplifier.The ideal distance  from antenna to any kind of conductor or noise source is:R = 2 * D^2 / lambda,where D - is the size of antenna , and lambda is wavelength .The wavelength can be calculated in the following way:lambda = c / frequency,where c is a speed of light:c = 299'792'458 .This equation works for almost all antennas. Butfor small antennas there is a lower limitfor Reactive Near Field region (the most sensitive part of Near Field region), for half wave dipole or similar antenna this lower limit isR = lambda / (2 * pi). So, if you reduce your antenna size more, the Near Field region will not be decresed, and you will get just a smaller gain level of antenna.For 5 MHz = 60 m, this limit is R = 60 / (2 * pi) = 9.55 .It means that you're need to place your antenna at least on 9.55 meters away from any conductor, power line, building or electronic equipment. Otherwise your antenna will be flooded with noise.

author: radiolistener
author_rank: Super Contributor
timestamp: February 23, 2019, 12:37:07 pm
content: added this LNA in my cart on aliexpress, will try how it works for short wave reception

author: metrologist
author_rank: Super Contributor
timestamp: February 23, 2019, 12:38:55 pm
content: This is a broadband amplifier so you might need some input filtering to your receiver to avoid front end overload. I was seeing about 5dB gain with 6V source.I am going to make some aluminum housings for these. What would be a good power connector?

author: Bud
author_rank: Super Contributor
timestamp: February 23, 2019, 02:23:22 pm
content: @StuartATry to measure the voltage on the device and current through the device and see if it matches the device datasheet. There may be a chance you got a fake transistor.

author: radiolistener
author_rank: Super Contributor
timestamp: February 23, 2019, 03:00:09 pm
content: Quote from: metrologist on February 23, 2019, 12:38:55 pmyes, but as I understand, it needs to take into account frequency response of mangetic loop itself.There is need to measure input of magnetic loop with vector analyzer.

author: StuartA
author_rank: Regular Contributor
timestamp: February 23, 2019, 09:01:36 pm
content: When I first received the two LNA which I bought, I checked them out at 20MHz, increasing the voltage from zero and measuring the current drawn and the approximate gain (signal generator and scope), and it seemed to me that they worked fine.However, the world of magloop antennas is 'challenging'. There is a huge amount of info on the net, but it is often inconsistent and contradictor.One of the most popularcommercialmagloops is one made by Wellbrook in the UK, which I think sells for something well over $400, and that has received widespread acclaim for its performance at up to 30MHz.Details of how to build a copy of the Wellbrook are given herehttps://www.george-smart.co.uk/projects/wellgood_loop/and that is what I'm currently doing.