Hi guys,
I am a complete newbie at radio and SDR, so please bear with me…
I recently received my LimeSDR and am loving this whole new world.
I recently saw an article that shows how you can use the LimeSDR as a VNA. The function that interests me most is to see what the optimum frequency is of a particular antenna. (S11 return loss??)
So I have bought a few bits and pieces which I hope are sufficient:
1 x Lime SDR
1 x ebay RF Bridge by 60dBm.com
1 x 50ohm dummy load (SMA)
and a couple of sma pigtails.
What I am missing it seems is a 10db attenuator. Do I really need this? Could I blow up my LimeSDR?
So I have connected the LimeSDR TX2_H to the Input of the RF bridge, the 50ohm load on the REF connector, the antenna I want to test on DUT, and the OUT back to the RX1_H on the LimeSDR.
Is that correct? Will this setup work?
Thanks very much in advance. I have been a networking engineer (CCIE) for 20 years now, but have never messed around in RF land.
Appreciate any input 
loiphin.
No one bothered? Surely someone should know about this…
@loiphin,
I saw your post from a couple of days ago and figured someone would jump in. I’ve been on the LimeSDR trail since the beginning (going on 2 1/2 years now) and from what I see in your post you’re doing it right for the correct transmit and receive ports. Bear in mind that as-is the range your transmit and receive will work will be restricted to the UHF and beyond bands - those ‘_H’ ports have definite range limitations that are documented on the MyriadRF site. There are modifications that can be made that will allow the receive ‘_H’ ports open up to 2 MHz to about 2 GHz decently, but as-is (and don’t quote me - it’s been awhile) the receive range is restricted to (about) 200 MHz to 3.8 GHz as-is. Again there are curves on the performance of each port on the MyriadRF site for the LimeSDR - so that’s where your VNA will work best unless you pull the shunt capacitors off the LimeSDR to open it up for lower band antennas. But if you’re just checking or designing Wifi antennas and the like using the Lime as-is should be fine for that.
I would think that adding a 10dB pad in the receive path will help with overload from the transmit path and will buffer (somewhat) any transients due to ESD - so I think that’s a good plan. There are examples of the LimeSDR being used as a VNA on the MyriadRF site and the program to steer it is open source - so you should be able to get it off Github.
That’s my 22 cents on the whole issue - let us know how you get along with your Lime being used as a VNA.
73 de Marty, KN0CK
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Thank you Marty for taking the time to come back to me I have already watched a number of your videos, playing with the LimeSDR
I also did the MN18 mod on my LimeSDR, just waiting for some better antennas in the post.
I will wait for the 10db attentuator from china and then give it a go. I want to hopefully emulate what one of these ‘cheap’ VNA antenna testers can do (Andreas does a review on one - https://www.youtube.com/watch?v=ZpKoLvqOWyc).
loiphin.
I think it’s mainly to present a better match. E.g. small attenuators are often placed between amplifier stages to improve matching.
It’s not the same setup and it’s probably difficult to say without testing. Give it a go and see what you get with e.g. a 50R load, short and an antenna whose performance at a given frequency you know.
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