Antenna Strength

Hi,

I would like to know what is the strength of the default antennas that we get with limesdr.

If I want to buy new set aof antennas, what should I consider to buy?

I was looking for this, but am not sure whether this suits my SDR. Does the mentioned antenna work for 90-100MHz, 433MHz, 800-1.2GHz, 2.4GHz Band?

Can Someone help me to pick antenna for these mentioned bands?

-Prady

Will the antenna be used for receive only or receive+transmit?

Do you want an omnidirectional antenna or one with directivity?

Physics prevent creating an antenna with great characteristics in all the bands you mention.

If you describe your use case(s), it will be easier to offer advice.

I don’t have a particular advice for an antenna yet (and as @mfalkvidd said, it really depends on the use case which antenna design is best), but I can provide some general background information on the antenna issue.

Connector type

First of all the LimeSDR and the LimeSDR-Mini have two different connectors. The LimeSDR comes with a U.FL connector while the LimeSDR-Mini has an SMA connector, more precisely an SMA female connector without reverse polarity. Some WLAN equipment comes with those reverse polarity connectors, which would not fit on the LimeSDR-Mini. So make sure you have an antenna with the right connector type or fitting adapters. (Any adapter can cause attenuation of your signal though.)

Impedance (50 ohms vs 75 ohms)

Antenna cables as well as antennas itself have an impedance which varies depending on application. Antennas with coaxial connectors usually have an impedance of 50 ohms or 75 ohms (75 ohms is used for radio broadcast and TV, 50 ohms for most other purposes). The LimeSDR and LimeSDR-Mini have an impedance of 50 ohms, i.e. in the optimal case, the antenna should show an impedance of 50 ohms for those frequencies where you want to operate on. However, the mismatching loss (technically: unwanted reflection due to mismatching) when using 75 ohms equipment is neglectable for powers below 1W. Nonetheless, a 50 ohms or 75 ohms antenna will show a drastically different impedance (e.g. 2000 ohms) for frequencies it hasn’t been built for. In that case, the losses are more relevant. In addition to those losses through reflection (because of wrong impedance), antennas can and will have an efficiency factor lower than 100%, which means that some of the energy is dissipated into heat.

For receiving

For receiving, matching the impedance and having a good efficiency factor isn’t as important as for transmitting because in the receiving case you can increase the LNA low-noise amplifier gain and thus compensate the weakness of the signal at the price of also amplifying other signals (on other frequencies) that may disturb your reception. A bigger problem in the RX case is when the antenna has a good impedance match and low attenuation on frequencies where you do NOT want to receive (but where there are strong signals that can disturb your reception). Particularly in the range of 90-100 MHz, you may find strong signals that you’d normally want to block out, when receiving on other frequency ranges. Thus an antenna that is matched for those frequencies might be a bad idea if you want to receive frequencies in other ranges. (Notwithstanding, the LimeSDR-Mini comes with good filters as I noticed, so it might not be a practical issue in your case.) It’s best if the antenna is most sensitive on frequencies you want to receive and least sensitive on frequencies which you want to keep out (particularly it may be wise to keep the 88-108 MHz range out unless you specifically want to receive weak signals in that frequency range).

Transmitting

For transmitting, the antenna should at least be roughly matching 50 ohms for the frequencies you want to transmit (which should be the case when the antenna is specified for a particular frequency range) and have a reasonable efficiency factor. Otherwise, the energy isn’t converted into radiation but reflected back into the transmitter (SDR or amplifier) or dissipated into heat. I assume that transmitting with 5 mW, any reflection will not destroy the SDR (not sure on this though?), but it would mean you don’t get your power out. If you use an amplifier, however, a mismatched or non-connected antenna can destroy the amplifier, depending on whether the amplifier protects itself against a mismatched load.

The antenna you mentioned doesn’t seem to be specified for any other range than 2.4 GHz, so I would not expect it to be a good TX antenna for 90-100 MHz, 433 MHz, or 800-1.2 GHz. Whether it’s sufficient for your use case might depend on how much power you need out in the air.

3 Likes

I will be using it for receive+transmit and Its omnidirectional use.

I am working in Security field, I have to demonstrate workings on 433MHz, transmit and receive(sniff). FM, Telecomm Network Band, WiFi bands.

@DL9JBE OMG. WOW. That was very helpful. Thank You so much for that.

And, about transmission, I want to transmit it for short distance. Like atleast expect it to cross a room. Right now its only near my desk. I can not transmit beyond this area. So, in that case, I have to use different antennas for other frequencies?

And sometimes if I want to catch FM signals, am not able to do it on lime. I do not know why. Could you tell me why is that happening?

-Prady7

If you fail to get across the room, then I’m not sure it’s an antenna issue. Other reasons for a low range may be:

  • Improper gain settings (you can set the PAD gain to 52 dB, but as far as I understand you should keep IAMP at 0 dB unless you want to modify the I/Q amplitude in the digital domain, which can lead to clipping).
  • Improper bandwidth settings (I had wrong bandwidth settings, and it caused massive attenuation which I didn’t notice until I actually measured the output one day).

As for the antenna, not sure if you are afraid of connecting some wires and/or soldering or crimping: If you just want to send across the room, maybe a random wire, or better a long-wire antenna will suffice. Simply connect an (unshielded) wire to the inner wire of the coax end. You may also use a bit of coaxial cable as feed line (i.e. between the SDR and the one-wire antenna), which can improve issues to due lack of bad HF grounding as the coaxial cable can serve as an improvided counterpoise due to sheath currents (which are normally unwanted but might not be an issue when transmitting 5mW).

The setup would be as follows:

SDR -> shielded coaxial cable -> unshielded single wire

The shield of the coaxial cable is connected to the SDR but left unconnected at the side where the single wire is connected.

Note that this is a very quick & dirty approach, but it might suffice for your scenario (and cost almost nothing).

If you try this, I’m happy about your feedback.

Amendment:

If you want to transmit in the GHz range, a too long coaxial cable as feed line is bad, as it can cause massive attenuation.

@DL9JBE That is very helpful… Thak you much. I am also attaching the error regarding the board.

prady@ubuntu:~$ sudo LimeQuickTest
[ TESTING STARTED ]
->Start time: Thu Aug 6 05:10:43 2020

Gateware version mismatch!
Expected gateware version 2, revision 23
But found version 2, revision 22
Follow the FW and FPGA upgrade instructions:
http://wiki.myriadrf.org/Lime_Suite#Flashing_images
Or run update on the command line: LimeUtil --update

->Device: LimeSDR-USB, media=USB 3.0, module=FX3, addr=1d50:6108, serial=00090726074F1F16, HW=4, GW=2.22
Serial Number: 00090726074F1F16
Chip temperature: 52 C

[ Clock Network Test ]
->FX3 GPIF clock test
Test results: 45220; 48976; 52732 - PASSED
->Si5351C test
CLK0: 17554 / 17554 - PASSED
CLK1: 17554 / 17554 - PASSED
CLK2: 17554 / 17554 - PASSED
CLK3: 17554 / 17554 - PASSED
CLK4: 17554 / 17554 - PASSED
CLK5: 17554 / 17554 - PASSED
CLK6: 17554 / 17554 - PASSED
->ADF4002 Test
Result: 10 - PASSED
->VCTCXO test
Results : 5112906 (min); 5113041 (max) - PASSED
->Clock Network Test PASSED

[ FPGA EEPROM Test ]
->Read EEPROM
->Read data: 12 08 06 12 08 06 02
->FPGA EEPROM Test PASSED

[ LMS7002M Test ]
->Perform Registers Test
->External Reset line test
Reg 0x20: Write value 0xFFFD, Read value 0xFFFD
Reg 0x20: value after reset 0x0FFFF
->LMS7002M Test PASSED

[ RF Loopback Test ]
Note: The test should be run without anything connected to RF ports
->Configure LMS
->Run Tests (TX_2-> LNA_L):
CH0 (SXR=800.0MHz, SXT=805.0MHz): Result:(-15.7 dBFS, 5.00 MHz) - PASSED
CH1 (SXR=800.0MHz, SXT=805.0MHz): Result:(-17.2 dBFS, 5.00 MHz) - PASSED
->Run Tests (TX_1 -> LNA_W):
CH0 (SXR=1800.0MHz, SXT=1805.0MHz): Result:(-18.7 dBFS, 5.00 MHz) - PASSED
CH1 (SXR=1800.0MHz, SXT=1805.0MHz): Result:(-25.3 dBFS, 5.00 MHz) - FAILED
->Run Tests (TX_2-> LNA_H):
CH0 (SXR=2500.0MHz, SXT=2505.0MHz): Result:(-29.9 dBFS, -11.48 MHz) - FAILED
CH1 (SXR=2500.0MHz, SXT=2505.0MHz): Result:(-31.8 dBFS, -11.49 MHz) - FAILED
->RF Loopback Test FAILED

=> Board tests FAILED <=

Elapsed time: 2.48 seconds

This is prompting FAILED. Can you please help me what is happening? Also, I am not able see limesdr in sdrangel or gqrx. I have installed few times. in different OS also. But, There is some kind of problem.

Please update the gateware as instructed. E.g. run:

LimeUtil --update

When running LimeQuickTest please ensure that the board is cold (52C suggests it’s been running a little while) and that nothing is connected to any RF ports.

If you could do the above and post the results again.

Sure, will perform that. When you say nothing is connected to RF ports, do you mean I have to remove antennas or disconnect the pins on board?

Also, what if I want to transmit something in GHz. Like a router does. Is there a way to do it with an antenna.

Would you recommend some source or topics for me to read and get strong in the above mentioned concepts. I am from Computer Science background so your suggestion would help a lot. I would like to read and know more about SDRs and Antennas for the doubts I have.

-Prady

Not sure what you mean by pins on board. The RF ports on LimeSDR Mini are two SMA connectors and LimeSDR USB has U.FL connectors. So in either case there should be nothing (no cables or antennas) connected to these when running LimeQuickTest.

Sure. Got it.

Thanks alot. I will perform that