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Hi,

Just entered this interesting hobby of Radio SDR for RMO, DIY Antennas and all that stuff.

Got my Nano VNA H today and I have a question. Do I need tot ake off my Low Noise Amplifier for measuring my DIY antenna ?

Thanks in advance for all tips.

regards Rainer

Yes, if you wish to measure the characteristics of the antenna.
No, if you wish to measure the characteristic of the amplifier.

Carol

Hi Carol,

Thank you very much. I will have to dismantle everything then. :-)

regards Rainer

Hi,

One more question in regard to my Set up.

It is a DIY 4 element YAGI antenna for the frequency of 174 310 000 Hz and the LNA is Bias Tee powered from the connected RTL-SDR receiver.

Is there a way to measure something putting the Nano VAN in through operation or will I damage my Nano VNA when I power my LNA via Bias Tee from my RTL-SDR device ?

Maybe a dumb question and perhaps nonsense but as I said I am total Newbie and want to learn.

Thanks for your patience.

Rainer

A schematic diagram posted a similar thread on Reddit shows there is a DC current path through resistors on the NanoVNA ports.
You can possibly damage the NanoVNA ports if you apply a DC voltage from the bias T to them. You need to attach a DC blocking capacitor in series with any connection to the NanoVNA on which a DC voltage is present.

Carol

Hi Carol,

Thanks. I guess any capacitor can be used in order not to block the frequency or better said a capacitor which lets through my frequency of 174 MHz. Right ?

Just guessing :-)

Rainer

Interesting.
Thanks for bring this up!

Looking at the schematic on page 10 of the pdf pointed to by the reddit post,
I'd guess the first resistor to burn due to DC injected into the TX SMA port would be R14.
Current there is roughly Vtx/(49+56), and power at R14 is I**2 * R = (Vtx/105)**2 * 56 Watts
Assuming R14 is good for 1/10 Watts before it burns out:
0.10 = (Vtx/105)**2 * 56
sqrt(0.10/56) = Vtx/105
Vtx = 105 * sqrt(0.10/56) = 4.43 Volts DC max into the TX SMA.
A thorough analysis of that resistor network might bring the max DC voltage even lower.

On the RX SMA port, first to go is the 100 ohms at R24, placed directly across the port.
0.10 = Vrx**2/100
0.10*100 = Vrx**2
Vrx = sqrt(10) = 3.16

So if DC exceeds 3 volts into either port, a blocking cap should be used
to prevent burning out a resistor in the nanovna.

Given all the blocking caps further back at the SA612A mixers and Si5351 clock generator,
I believe the nanovna should still give accurate results with a DC bias on the TX and/or RX port.

My nanovna has RFI covers over those resistors,
the figure of 0.1 Watts max per resistor assumes 0805 resistors.
If it's 0603 resistors, then it's 0.063 Watts max per resistor,
and you'd best keep DC voltage below sqrt(0.063watts*100ohms) = 2.5 Volts.

Jerry, KE7ER

I agree with Jerry. Use an appropriate capacitor for RF such as ceramic or mica, not electrolytic or paper.

Carol

Jerry, Simple solution if worried about application of DC voltages to
the ports, is to use an inline SMA-M to SMA-F DC Block available from
Amazon for anywhere from $7-10 US for a pack of two on up to $45 or so
from higher end manufacturers. Rated loss is a few 10ths of a dB to
6GHz, or in the case of Nooelectric's piece, 8GHz, but it's $20 ea. If
you review these, read the customer feedback, generally positive from
hams and other engineers.
This serves a couple purposes : blocks DC voltage as well as provides
an extension to save the instrument connector SMA threads. I assume if
calibrated once installed, any impedance offsets will be zero'd out. I
thought I had one for instrumentation but can't find it to check.
Anybody else have any experience with these?

Regards, Ted (KD7AQO)

-----------------------------------------From: "Jerry Gaffke via
groups.io"
To: [email protected]
Cc:
Sent: Monday October 5 2020 10:42:54AM
Subject: Re: [nanovna-users] Newbee with Nano VNA device

Interesting.
Thanks for bring this up!

Looking at the schematic on page 10 of the pdf pointed to by the
reddit post,
I'd guess the first resistor to burn due to DC injected into the TX
SMA port would be R14.
Current there is roughly Vtx/(49+56), and power at R14 is I**2 * R =
(Vtx/105)**2 * 56 Watts
Assuming R14 is good for 1/10 Watts before it burns out:
0.10 = (Vtx/105)**2 * 56
sqrt(0.10/56) = Vtx/105
Vtx = 105 * sqrt(0.10/56) = 4.43 Volts DC max into the TX SMA.
A thorough analysis of that resistor network might bring the max DC
voltage even lower.

On the RX SMA port, first to go is the 100 ohms at R24, placed
directly across the port.
0.10 = Vrx**2/100
0.10*100 = Vrx**2
Vrx = sqrt(10) = 3.16

So if DC exceeds 3 volts into either port, a blocking cap should be
used
to prevent burning out a resistor in the nanovna.

Given all the blocking caps further back at the SA612A mixers and
Si5351 clock generator,
I believe the nanovna should still give accurate results with a DC
bias on the TX and/or RX port.

My nanovna has RFI covers over those resistors,
the figure of 0.1 Watts max per resistor assumes 0805 resistors.
If it's 0603 resistors, then it's 0.063 Watts max per resistor,
and you'd best keep DC voltage below sqrt(0.063watts*100ohms) = 2.5
Volts.

Jerry, KE7ER

On Mon, Oct 5, 2020 at 09:07 AM, Carol KP4MD wrote:
> A schematic diagram posted a similar thread on Reddit
> Mute This Topic:

Links:
------
[1]


/> > shows there is a DC current path through resistors on the
NanoVNA ports.
> You can possibly damage the NanoVNA ports if you apply a DC voltage
from the
> bias T to them. You need to attach a DC blocking capacitor in
series with any
> connection to the NanoVNA on which a DC voltage is present.
>
> Carol
>


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Thanks everybody.

I guess this is what I need

Right ?

Rainer

I have yet to need a DC block.
If I did, would likely just put down something like
a $0.01 0805 N0G 0.01uF cap on the device I was inspecting.
Or cut an SMA cable in half, add an SMA jack at the end of the coax with a series cap.

Size the cap to be reasonably low reactance at the lowest frequency of interest,
where reactance_in_ohms = 1/(2*pi*Hertz*Farads)
I believe that any reactance in the cap (within reason, keep it under 50 ohms),
be it capacitive or inductive, would get corrected for by the calibration step.
So maybe use your VNA to see if the cap has some serious inductive reactance
at the highest frequency of interest.

Then check out something that does not have a DC issue using the VNA,
comparing results with the DC block in place and without, re-calibrating for each.
Verify that you get the same readings.

For saving the VNA's SMA connectors from wear,
I just leave two short SMA-to-SMA cables in place on my VNA.
The only connector that will wear out is the far end SMA connector of each cable.
The short cables also prevent mechanical stress on the VNA circuit board.


The DC blocks from Minicircuits or Pasternack are in the $50 to $150 price range.
I'm sure there are cases where these are a good idea,
but I'm just puttering around here with a $50 vna.
I can afford a slight risk of blowing out the front end.

However, if you do want to look like a professional,
here's two DC blocks at $20 total on Amazon,

You probably want the SMA-MF(50khz-8ghz).
They claim to have an SMA-MF(DC-6ghz), I have no idea how
you would build a DC block that allows signals through down to DC.
Didn't look hard, but haven't yet seen anything cheaper than that $20 for two.
Except the $0.01 for an 0805 cap.

Jerry, KE7ER

Make sure you select the correct device, that web link could be for any of 6 different devices.
I'd probably get the SMA-MF(50khz-8ghz)
Perhaps somebody else has a better suggestion.

Jerry, KE7ER

Hey Rainer, it's what I would use.. Says package of two in add, I
noticed one guy reported he only received one. Nobody else reported
that however. Probably an error on the marketers part.
Interestingly one reply indicated he measured up to 9GHz with little
to no insertion loss, although specs say 6GHz.
I've used higher quality ones (much higher cost Pasternack, and
others, rated to 18GHz) up as high as 24GHz, with no adverse effects.

Just be careful to not place too much lateral pressure on the longer
connection to preclude damage to the circuit board SMA soldered
connections. By the way, if you have a SAA-2N (N-connectors) there are
also DC Blocks for N.

Regards ..... Ted (KD7AQO)

----------------------------------------From: "Rainer"
To: [email protected]
Cc:
Sent: Monday October 5 2020 12:23:18PM
Subject: Re: [nanovna-users] Newbee with Nano VNA device

Thanks everybody.

I guess this is what I need Mute This Topic:

Links:
------
[1]

/>
Right ?

Rainer


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Thanks everybody for the tips.

Rainer

Hi,

I was thinking about this DC Blocks.

If I understand correctly those DC Block block the DC current and without DC current my Low Noise Amplifier does not work, correct ?

So if that is the case, why not just turn Bias Tee OFF and I can measure through ... right ?

BTW I ordered the pack of two. For what do I need them if above is OK ?

Rainer

On 10/6/20 8:11 AM, Rainer wrote:

Hi,
I was thinking about this DC Blocks.
If I understand correctly those DC Block block the DC current and without DC current my Low Noise Amplifier does not work, correct ?
So if that is the case, why not just turn Bias Tee OFF and I can measure through ... right ?
BTW I ordered the pack of two. For what do I need them if above is OK ?
Rainer

if your LNA is one that has a relay to bypass it when it's off, then yes, turning off the power would be what you need.

Rainer,

If you remove power from the bias-tee, the LNA no longer works.
But it almost certainly does not short input to output,
and if it did it would not do so without adding extra loads to ground.

Unless it happens to be extremely fancy and have Jim's relay included in the package.

You need to remove the LNA somehow to measure the antenna with the nanovna.

Jerry, KE7ER

To measure the antenna and feedline 'system', disconnect the antenna
feedline from the receiver and connect the end of the feedline to the VNA.
You can not make a credible measurement of the antenna/feedline 'system'
with the preamplifier connected to the feedline.

Dave - W?LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*

Hi,

I have here to same set ups.

RTL-SDR >> 4.5 meter coax 75 Ohm >> LNA >> Antenna

I disconnected the RTL-SDR from the coax an antenna set up and connected the Nano VNA to both set ups and yes I can measure SWR, Logmag and get a Smith circle.

Compared the values of both and they are different which is absolutely clear for me as Yesterday we optimized one antenna to get a better signal at 174 310 000 and mine was not optimized.

So I guess I spent the bucks for the DC Blocks for nothing but does not matter. It is leanring material and perhaps one day I can use them somewhere else :-)

Both set ups are running pointing at the same transmitter 960 km away from here and later in the afternoon we will compare data of Meteor scatter.

Thanks everybody for the comments which made me analyze everything more.

One interesting thing is that now having measured with the LNA in the line the good SWR value did move to a higher frequency compared to optimizing without the LNA.

I am talking about a DIY 4 element YAGI antenna for 174 310 kHz. you can see my meteor scatter data in the rmob.org under Rainer Ehlert in Mexico San Luis Potosi. The second set up is still under testing and will be in the same city on the roof of a friend about 4.5 km away from my site to the Northeast.

Lots of things still to learn. Radio waves are magic :-)

Rainer

On Tue, Oct 6, 2020 at 12:47 PM, Rainer wrote:
Hi,

Forget everything I said about measuring the antenna with the LNA included. Sorry for not listening to the experts here. This was nonsense what I wrote. I joined the Ham Antenna group in groups.io and got some more info.

Rainer

Hi,

I have here to same set ups.

RTL-SDR >> 4.5 meter coax 75 Ohm >> LNA >> Antenna

I disconnected the RTL-SDR from the coax an antenna set up and connected the
Nano VNA to both set ups and yes I can measure SWR, Logmag and get a Smith
circle.

Compared the values of both and they are different which is absolutely clear
for me as Yesterday we optimized one antenna to get a better signal at 174 310
000 and mine was not optimized.

So I guess I spent the bucks for the DC Blocks for nothing but does not
matter. It is leanring material and perhaps one day I can use them somewhere
else :-)

Both set ups are running pointing at the same transmitter 960 km away from
here and later in the afternoon we will compare data of Meteor scatter.

Thanks everybody for the comments which made me analyze everything more.

One interesting thing is that now having measured with the LNA in the line the
good SWR value did move to a higher frequency compared to optimizing without
the LNA.

I am talking about a DIY 4 element YAGI antenna for 174 310 kHz. you can see
my meteor scatter data in the rmob.org under Rainer Ehlert in Mexico San Luis
Potosi. The second set up is still under testing and will be in the same city
on the roof of a friend about 4.5 km away from my site to the Northeast.

Lots of things still to learn. Radio waves are magic :-)

Rainer