Will adding an amp to my SDR offer any benefit? - Page 1
Forum section: electronics / rf-microwave

author: Beamin
author_rank: Super Contributor
timestamp: October 10, 2017, 07:51:42 pm
content: http://www.ebay.com/itm/0-1-2000MHz-RF-Amplifier-Broadband-with-low-noise-Amplifier-LNA-gain-30dB/262929737452?_trkparms=aid%3D333008%26algo%3DRIC.MBE%26ao%3D2%26asc%3D20140109102600%26meid%3D665f9c2676bd4d038208f447543fd1fd%26pid%3D100041%26rk%3D9%26rkt%3D24%26sd%3D232103825103&_trksid=p5713.c100041.m2061QuoteDoesn't help bad English but will this just add noise to my SDRPlay? Or would this be better suited to a dongle where you are just worried about one frequency?I use my SDR for mainly HF stuff.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 10, 2017, 08:00:37 pm
content: If an extra amplifier helps with an SDR or other receiver depends a lot on the receiver and the antenna and not so much on the amplifier. Chances are high that 30 dB of extra broad band amplification is not a good idea. It would be a rather poor receiver that could profit from such an amplifier.The best "amplifier" is a good antenna.

author: Beamin
author_rank: Super Contributor
timestamp: October 10, 2017, 08:25:55 pm
content: Quote from: Kleinstein on October 10, 2017, 08:00:37 pmSo this is what I'm trying to figure out. Why doesn't it help? Is it that particular amplifier? Is there really no point in a receiver amplifier? I'm not trying to use it in place of a good antenna but rather to augment it.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 10, 2017, 08:59:44 pm
content: Any receiver is limited in how large the input signal is allowed to be. This limit applies not only to the signal of interest, but a usually much larger band. Especially modern receivers usually have enough amplification already inside, so that they would likely not profit from extra amplification, unless you are in a very quiet environment. Near to a strong radio station one might even need an attenuator (preferably tuned to that strong station) before the receiver.To decide if an amplifier helps, it depends on the environment (e.g. more likely in the Australian outback than in a large city). Also the receiver and the antenna are important factors. If at all it would be likely an preamplifier with lower gain and a more limited bandwidth - it only causes trouble amplifying signal outside the bands of the receiver.So if you want to use an extra amp it is more like low amplification that might help.

author: babysitter
author_rank: Frequent Contributor
timestamp: October 10, 2017, 10:43:43 pm
content: Prepare to get a adjustable attenuator (there hab been tv attenuators avalable for cheap in the past) and if you only look for a certain range to build filters either from cable (cable Pi configuration) or as helical filter (wire helix, capacitor, metal box.) 30 dB looks very big to me, I made some with ~10 dB recently.YMMV.Best regardsHendrik

author: radiogeek381
author_rank: Regular Contributor
timestamp: October 11, 2017, 12:53:23 am
content: Broadband amplifiers connected to an antenna are almost always a bad idea. Unless you live on Mars, the RF environment will provide that preamp with lots of signals to mix and generate spurious signals in the band that you actually want to listen to. AM broadcast stations will beat against each other and provide hours of listening enjoyment all across the shortwave bands. FM broadcast stations will appear where you thought only cellphones lived. Cell towers will beat against pagers, and so on and so on. (If this is new to you, google for "Intermod Products." The wikipedia article is comprehensive, if not very elegant.)If you want to see "clean" signals without lots of intermodulation products, you need front end filtering more than a pre-amp.Exception: if youreallyare to the point where the 5 or 6dB noise figure of your SDR is the barrier, add a <narrow band> preamp. Limit it to the chunk of spectrum you're really interested in. It is quite achievable to get a NF down to 1dB with not a lot of money, but a fair amount of twiddling and trial and error and error and error.Broadband amplifiers are handy around the lab, but are more trouble than they're worth when connected to an antenna.

author: cdev
author_rank: Super Contributor
timestamp: October 11, 2017, 02:38:47 am
content: @Radiogeek381 really nails it here - I can't really add to it.Assuming your antenna is adequate (which often means getting it high up and into the clear) maybe what you might want to look into first is a preselector..https://en.wikipedia.org/wiki/PreselectorAlmost all SDRs benefit greatly from something called a preselector (or band pass filter).SDRs often have issues with selectivity.. As others said.. if you add a preamp with most antennas, make it one with some selectivity in front of it.Also, for HF, you could consider just building your own high pass filter to reduce AM broadcast band signals. 560 to 1650 KHz. You might want to listen to 160 meters, which is 1800 KHz, so ideally, you might want a sharp roll off at around 1.7 Mhz.Unless you want to listen to VLF, or Am broadcast band, thats kind of a no brainer.. Almost guaranteed to help improve the signals by removing a very strong potential cause of interference..You could build a bypass switch into your filter so you can remove it from the signal chain - or not- with a switch.Quote from: radiogeek381 on October 11, 2017, 12:53:23 amOn HF with few exceptions, an amplifier is rarely needed, however, sometimes you are in a really noisy environment in your QTH, but its quiet outside..and then an LNA can be useful in amplifying the signals outsiderelative toyour noise, inside...Thats kind of my situation, to a point. I had some success doing that, allowing me to hear 10 meter beacons that I literally had never heard before.I had almost never heard anything weak on 10 meters before, because of the power line noise.So I tried something called a PA0RDT Mini Whip..its technical name is "e-field probe"..the key concept is relative strength of signal to noise..The inside of some modern homes have very noisy RF environments and then an e-field antenna may be helpful by moving the front end outside to a quieter location, -probing the e-field with a very high impedance probe. The e-field probe antenna also uses the coax as part of its antenna.The web-sdr in the Netherlands (http://websdr.ewi.utwente.nl:8901/) uses an e-field probe antenna.. with great success.However then you have to take extra care that the noise from your house doesn't follow the coax feed line to the input of the preamp.. outside.. You might do that by wrapping multiple turns of your coax around a ferrite core to act as an RF choke.. before the coax enters your house. Also grounding it out there.. and letting the portion inside your house float. Also, power it with a battery, not an AC-connected power supply.This is easier to see explained by demonstration than anything else.. This kind of antenna may be as good as it gets for HF for people in some noisy locations, IF the coax into the house can be isolated with a good RF choke and the antenna preamp powered by a battery, not AC power.You can find many more resources by searching on "pa0rdt mini whip".So, here is a general rule about amplifiers.. the front end noise performance of the receiving system is determined by the noise figure of its input, wherever that input is.Coax might be lossy and might pick up noise..so it might be best to put any LNA - if you use one, outside where its quiet. At the antenna. And prevent noise from inside from migrating out to your sensitive preamp's input. Use a common mode choke and perhaps look up "galvanic isolation" at the receiver and try that if you still have noise issues.A 1:1 RF transformer.Here is another situation where they may help a lot. On UHF - Lband - with GPS signals - 1575 MHz, an LNA *at or near the antenna* is quite useful because you often have a long, lossy coax run between the antenna and the receiver- Again the LNA has the effect of moving the point where the noise figure gets determined to the antenna side of that long transmission line. So, almost all commonly used GPS antennas include them now.Older GPSs literally wont work without that help. So there are two good examples where an LNA helps.

author: Beamin
author_rank: Super Contributor
timestamp: October 11, 2017, 10:58:06 am
content: So what would happen if you transmitted through that pa0rdt antenna?

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2017, 01:33:46 pm
content: Quote from: radiogeek381 on October 11, 2017, 12:53:23 amEven worse will be the mixing products produced in the first mixer since its dynamic range will be reduced by all of the gain before it. Assuming no preselection filtering, the last active stage before IF filtering will be the one which produces the most intermodulation and that is the first mixer which explains why high IP mixers are popular.Radio front ends often include an RF amplifier *only* to lower leakage of the first local oscillator by providing reverse isolation.For applications where the ambient noise level is low and maximum sensitivity is desired, the RF amplifier only has to provide enough gain to overcome the receiver's input noise figure. If ambient noise is high, then the RF amplifier only degrades performance and should be left out. For instance the bands at 10 meters and below rarely require an RF amplifier to lower noise figure.Feedline loss directly adds to the receiver's input noise figure so adding an RF amplifier at the antenna often makes sense. This is commonly done with remote GPS antennas for example.

author: cdev
author_rank: Super Contributor
timestamp: October 11, 2017, 01:48:28 pm
content: You can try using a variable attenuators (the "potentiometer type" - which are cheap to buy or easly to make..) made for cable TV - on you HF receiver. Its helpful to prevent front end overload with an LNA, often in combination with a preselector and use of RF gain to adjust the other front end way downward.. You may have one LNA that has very low IM and it wants a fairly low level input that is pre filtered of the frequencies you don't want. Turn the last RF gain (like on an RLSDR) down almost all the way too. Also use a good ground.A magnetic loop is a great preselector.. If only that level of selectivity was easier to put in a box for use with other antennas. It is possible to do pretty well, but very hard to get that razor sharp selectivity with tuned circuits in a preselector without a lot of loss.. (if anybody knows a good circuit please post)A big loop with a decent variable cap has a very high "Q" factor.. thats why they are so selective.Quote from: babysitter on October 10, 2017, 10:43:43 pm

author: babysitter
author_rank: Frequent Contributor
timestamp: October 11, 2017, 04:19:07 pm
content: As an example, my LNA design.https://wiki.schaffenburg.org/Projekt:Gainblock_Schaffen1105(Actually need to post Layout/Schematic)BRHendrik

author: Beamin
author_rank: Super Contributor
timestamp: October 12, 2017, 03:41:07 am
content: Quote from: babysitter on October 11, 2017, 04:19:07 pmIs there a link that translates that page? My german is not very gut.

author: Lord of nothing
author_rank: Super Contributor
timestamp: October 12, 2017, 06:21:11 am
content: I need to use an amp to get some VHF/ UHF Signal with my ordinary 20€ USB Dongle.Maybe some SDR who are build for that purpose are better.

author: cdev
author_rank: Super Contributor
timestamp: October 14, 2017, 11:39:07 pm
content: Beamer, you should consider changing the title of this thread to use the word "LNA" instead of "amp"..

author: radiogeek381
author_rank: Regular Contributor
timestamp: October 14, 2017, 11:49:36 pm
content: Quote from: cdev on October 14, 2017, 11:39:07 pmHmmm. I think most professionals in the field consider SDR to mean "Software Define Radio" not receivers.In any case, the three SDRs in front of me right now are all capable of receiving and transmitting. One is an Ettus USRP N200 with an RF deck that tunes from 10MHz to 6 GHz (actually will tune down to DC). Another is an Ettus USRP B210 that tunes from 50MHz to 6 GHz. The LimeSDR is the third, though the transmit side may not be ready for prime time.And I've never interpreted amplifier as transmitting only. Most of the amplifiers in my shop are LNAs, but some are power amplifiers.The title is just fine. (IMO).

author: cdev
author_rank: Super Contributor
timestamp: October 15, 2017, 01:30:50 am
content: I edited my post to remove the second part..you're right.. my bad!

author: Beamin
author_rank: Super Contributor
timestamp: October 15, 2017, 06:04:15 am
content: Quote from: radiogeek381 on October 14, 2017, 11:49:36 pmWeird stuff happens when you tune down to DC. I like how you have an alternate reality universe of mirror signals down past the DC 0 mark. I also like measuring the 60hz buzz and weird reflections and sped up recording down really low.