Checking LW Ferrite Rod antenna with NanoVNA - Page 1
Forum section: electronics / beginners

author: vinlove
author_rank: Frequent Contributor
timestamp: January 28, 2023, 02:15:56 pm
content: I have a homebrew Ferrite Rod Rx antenna, which works good for LW band DX reception.The construction is a few ferrite rods wound with enamel wire connected to a variable capacitor, which tunes well for RX weak DX signals between 150 - 500 kHz.Now I would like to check this antenna with my new NanoVNA H4 for efficiency and resonance, so I could try extend the RX range to cover the MW band.When extending the RX range using the NanoVNA keep checking out the relevant parameters would help for saving time and accuracy.First of all, what would be the best process for checking the antenna in terms of setup and connections between the antenna and NanoVNA? The antenna works by magnetic fields tune up with inbuilt ferrite rod antenna of the radios, so it doesn't have any ports to connect to VNA.Secondly what parameters are we looking for? Because it is not a TX antenna, I presume SWR wouldn't be relevant for the measurements.

author: vinlove
author_rank: Frequent Contributor
timestamp: January 28, 2023, 02:54:04 pm
content: And is NanoVNA a right (the best) tool for this measurement?

author: wasedadoc
author_rank: Super Contributor
timestamp: January 28, 2023, 03:12:37 pm
content: Yes you can use a NanoVNA-H for that though all it can really do is help you verify that the new variable capacitor you'll need is suitable.Connect coil and capacitor in parallel and connect them to the centre and outer of the VNA's output connector (CH0). Sweep the frequency band of interest. Ideally at the tuned frequency of rod coil and capacitor setting the Smith Chart will show "due East", ie high resistance.Having said that it is better (and conventional) to have a coil with fewer turns for MW compared to LW.

author: vinlove
author_rank: Frequent Contributor
timestamp: January 28, 2023, 06:19:48 pm
content: Quote from: wasedadoc on January 28, 2023, 03:12:37 pmMy NanoVNA has port1 and port2. Is port1 = ch0? Is it for input or output? Sorry I am a totally newbie on VNA stuff.I see parameters such as reactance, inductance and resistance in some other antenna analysis sites. Not sure if NanoVNA can read these and direct to some path to resonance for the band.

author: radioheat
author_rank: Newbie
timestamp: January 28, 2023, 06:46:51 pm
content: The best process for checking the efficiency and resonance of your ferrite rod RX antenna using a NanoVNA would be to connect the NanoVNA's output port to the antenna's input (the variable capacitor) using a suitable coaxial cable, and then measuring the antenna's impedance (magnitude and phase) over the frequency range of interest (150-500 kHz and potentially extending to the MW band).When measuring with the NanoVNA, you will be looking for the frequency at which the antenna has the highest impedance, which is referred to as the resonant frequency. The antenna's impedance at this frequency will be at a minimum, and the antenna will be most efficient at this frequency. Additionally, you can also look at the shape of the impedance curve to see if the antenna is well matched over the frequency range of interest, which is important for good reception of weak DX signals.It's important to note that SWR (Standing Wave Ratio) would not be relevant for measuring the efficiency and resonance of a receive-only antenna, as SWR is a measure of the mismatch between the transmitter and the antenna system and is generally used in transmission.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 28, 2023, 06:50:06 pm
content: My NanoVNA-H has the top port on the left side labelled "CH0". It is the port used for S11 measurements.But, bluntly speaking, it seems that you don't know how to use a VNA. There are many Youtube videos and tutorials you should watch and understand before proceeding.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 28, 2023, 07:05:13 pm
content: Quote from: radioheat on January 28, 2023, 06:46:51 pmOne sentence says "highest impedance". Another sentence says "impedance ... at a minimum". One of those must be wrong!For DX working it is essential that the antenna in tuned to the frequency of interest. The shape of the impedance curve over 530 kHz to 1600 kHz when the tuning capacitor is at any particular setting does give particularly useful information.

author: iMo
author_rank: Super Contributor
timestamp: January 28, 2023, 07:51:16 pm
content: With nanovna (I did with nanovna-h4) you can measure the mu' and mu'' parameters of the ferrite material (see the ferrite materials datasheets for clarification). The measurement is pretty complicated - you do a s11 measurement (1-2 turns around the material), the data set is then fed into matlab and after some heavy math you get the graphs as in the DS.Otherwise with nanovna you may sweep the antenna to see where it tunes - make 2-3 turns at the end of the rod (a loose coupling as with griddip meter), the turns wired to ch0, and simply sweep from freq1 to freq2. While tuning the LC the max/min (it depends) will move around.The nanovna works the same as the grid-dip meter - you may do the same measurements:https://circuitdigest.com/tutorial/an-introduction-to-grid-dip-meterhttps://en.wikipedia.org/wiki/Grid_dip_oscillatorYou may also judge on the Q (quality) of your LC - the higher and narrower the by nanovna shown signal the better the quality. Q is important at LW, MW as it is related to the 1/Bandwidth (selectivity).Long time back the crystal set radios were tuned to the stations by the LC of the antenna only - thus it required pretty high Q of the antenna LC circuit to provide the necessary selectivity.https://www.allaboutcircuits.com/textbook/alternating-current/chpt-6/q-and-bandwidth-resonant-circuit/

author: vinlove
author_rank: Frequent Contributor
timestamp: January 28, 2023, 08:46:05 pm
content: Quote from: wasedadoc on January 28, 2023, 06:50:06 pmYeah, my NanoVNA H4 has arrived this morning. So I have had it for about 9 hours.I got it to learn Electronics and RF via practicals on the DIY antennas, BP filters and ferrite rods etc.Of course Youtube has many videos, but takes ages to go through them all. Forums are great for trying to get infos and advices quickly on what you are wanting to know or discuss.

author: vinlove
author_rank: Frequent Contributor
timestamp: January 28, 2023, 08:47:39 pm
content: Quote from: imo on January 28, 2023, 07:51:16 pmGreat detailed reply thanks.

author: vinlove
author_rank: Frequent Contributor
timestamp: January 28, 2023, 08:50:25 pm
content: Quote from: radioheat on January 28, 2023, 06:46:51 pmSure, great info. Thanks.

author: AFTORF
author_rank: Contributor
timestamp: January 28, 2023, 10:38:03 pm
content: For best accuracy, I think you have to make antenna measurement on day period because when night come, many LW stations are receiving with strong level. NanoVna or any VNA could be "jamming" and perhaps destroy with it.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 28, 2023, 11:13:40 pm
content: Quote from: AFTORF on January 28, 2023, 10:38:03 pmThe output signal from any version of NanoVNA is very much stronger than a normal broadcast signal picked up by the ferrite rod antenna so any impact on accuracy will not be significant. And only if using the VNA very near a powerful broadcast transmitter would the antenna produce a strong enough signal to damage the VNA.