开云体育

You do not have to talk me out of soldering to the CPU.
I am really not trilled to do that anyway.
As far as model number - the battery seem to be "superguled" with sticky tape ( ?) so I did no attempt to
remove it. But the model # is irrelevant at this point anyway.

Being a coding geek I just got this "brilliant " idea.
The nanoVNA communicate with PC using USB - we have working nanoVNA-saver -
so why not
add another USB device - USB works in parallel - sort off - that device being ordinarily
Bluetooth adapter ( USB to Bluetooth )
and add this adapter as another means of communication

Or even better - replace the USB cable connection with such external ( no soldering to CPU ) USB - Bluetooth adapter .
All done in software...

PS anybody ported Python to C /C++ ?

There were MMIC amps that liked using carbon composite as the bias resistor,
since the large temperature coefficient made it well matched to keep the MMIC
biased properly.

Such is part of the art in electronics. But if a designer wants to get an advanced degree in electronic art, then he also needs to match the composition resistor's long-term drift to that of the MMIC! ;-)

Attached are some useful info for making VHF and UHF reference dipoies including a neat design for an impedance matching balun using semi-ridgid coax.

Thanks for the explanation. It's been quite a while since I studied the system design.

Have Fun!
Reg

Is it possible that your coax is not really RG-58A/U?

I created an EZNEC model of a free space 340 MHz inverted vee. The element lengths and angle between them were optimized for a 50+j0 ohm feedpoint impedance at 340 MHz. The complex feedpoint impedance between 200 and 500 KHz was then calculated over 101 steps and exported to SimSmith.

In SimSmith, 15 ft of various types of coax were added. The results are interesting. (This could have been done in EZNEC, but I find it easier to use SimSmith for this type of thing.)

With 15 ft of 75-ohm RG-59, the FREQ vs SWR plot looks almost identical to yours. The lowest SWR is at 352 MHz which is very close to the 353 MHz point you observed. My plot is attached.

Here's a quick and easy way to tell whether your coax is 50 ohm or 75 ohm. Calibrate the NanoVNA at the port as you originally did. Now connect the antenna with your 15 ft of coax. Sweep from 200 to 500 MHz. Now display the Smith chart instead of the SWR plot. If overall, the plot circles are roughly centered on 75 ohms, the coax is 75 ohms. (The 75-ohm point will be to the right of the chart center by about 10% of the chart width.) If the plot circles are roughly centered on 50 ohms (at the center of the chart), the coax is 50 ohms. Two example plots are attached.

When you calibrate at the end of the coax, you remove its effect, and you'll get the same results whether you calibrated with 50 ohm or 75 ohm coax. The SWR plot will then look like the one you got just as it did with my modeled version. Plot attached.

73, Dave
NU8A

Certainly, the feedline can act as an impedance transformer - but it's also a phase shift. In particular, it will change the phase of the reflected wave. Let's say I have an antenna that is resonant (X=0) but is a slight reflector (S11 mag = -20dB). If I had the VNA at the feedpoint, the SWR would be low, and the phase of S11 would be zero. Now, let's put in 50 degrees of feedline. Now, the S11 phase will be 100 degrees - 50 degrees on the way out, the reflection at zero phase, then 50 degrees more on the way back.

So that's why you want to calibrate at the end of the feedline - it also accounts for the loss in the feedline so you can measure the true SWR at the antenna. While everyone *says* "the SWR is the same every where on the feedline" that's not true for a lossy transmission line. The SWR gets better, the farther from the mismatch you are. Go out and get 100 feet of RG-174 and put any antenna at the far end for 2.4GHz. With 60 dB of loss, any antenna will have an awesome VSWR at the transmitter end. It's the MaxComm matching unit all over again.

Why you think it bug?
NanoVNA use 5k IF for measure (this firmware). But as it VNA and not SA - not need add filter for mirror supression.
So you see main IF peak and mirror 5+5kHz

Frank - Thanks for the comments and information. The calibration for the first attachment was the standard Open, Short, and 50 ohm Load calibration done at the Channel 0 SMA connector on the NanoVNA. The first attachment would show the antenna with transmission line as seen by a transmitter or receiver.

The calibration for the second attachment used the same Open, Short, and 50 ohm Load calibration but at the antenna end of the transmission line. That effectively removes the transmission line from all measurements to show the characteristics of the antenna itself. That is one of the more powerful features of a NanoVNA.

The two legs of the dipole were bent down slightly to improve the match. Technically it is an Inverted V Dipole. The feed line does come straight down from the feed point.

The coax is Berk-Tek RG-58A/U low loss foam coax. The characteristic impedance is 50 ohms.

I believe the first attachment shows the resonant frequency of the antenna just ahead of the low point in SWR. Compare that low with the second attachment.

73, Kent
AA6P

On 7/21/21 12:33 PM, Manfred Mornhinweg wrote:

We always knew clipleads are bad, the question is really "how bad is it"

- A 1/4W carbon composition resistor. It has an interesting but pretty bad behavior: Only at very low frequencies it has its rated resistance value, along with considerable parallel capacitance. By about 20MHz the resistance has dropped by 20%, then stays about constant! At 500MHz it's significantly inductive, and I don't know why, being the same size as the film ones. This is very much worse RF behavior than any of the film resistors! I have been preaching for decades that film resistors are a lot better than composition, but some people keep believing that carbon composition is good for RF, because very old books say so. Sure, back when the only choice was between carbon composition and wirewound, carbon composition was by far the better choice! But since the appearance of cheap film resistors about a half century ago, carbon composition is obsolete for RF.

There were MMIC amps that liked using carbon composite as the bias resistor, since the large temperature coefficient made it well matched to keep the MMIC biased properly.

A 10 MHz input to CH1 produces logmagnitude plots with peaks at 9.990 and 10 MHz.

Source is a tinySA w/ a 40 MHz BP filter. Clean to within what my 8560A can detect :-)

Have Fun!
Reg

That W1DX article appeared in QST three times, November 1991, April 1977, and March 1956. There is a lot of misinformation about transmission lines/SWR in the amateur community and I suspect ARRL feels it is worthwhile to republish the article once in a while. There are several excellent books about the subject available that can clear things up. I highly recommend the ARRL publication "Another Look At Reflections" by the late Walter Maxwell, W2DU. Although he puts on the afterburners once in a while, his information appears to be correct - and a real eye opener for someone who has misinterpreted transmission line theory.

K0AM

I'm curious if you first calibrated the transmission line with a 50 ohm load at the far end. Does the transmission line come away from the antenna perpendicular to the elements? A half-wave dipole impedance will start at low values near the ground (unless you have a LOT of money, a 160 meter dipole would not show 73 ohms), then tend to wiggle around a 73 ohm value as ultimately achieved in true free space. Are you attempting to adjust the impedance that the antenna is showing to the RG-58 characteristic impedance? Check to make sure it is 58 and not 59!

Just a few thoughts...
K0AM

There was specific mention of a 1(?)-second timeout on AT commands in one of the recent links in this thread.

73, Willie N1JBJ

We always knew clipleads are bad, the question is really "how bad is it"

Well, pretty bad! :-)

Since I had the test jig set up and calibrated, I put some other 100? resistors in it. Test results of each are attached. The guinea pigs were:

- A run-of-the-mill 1/4W carbon film resistor. It's pretty decent.

- A 1/4W metal film resistor of the same size, an ultra-cheap Chinese product with very thin and flimsy wires, but otherwise OK. Slightly worse than the carbon film, as expected, but still pretty decent and usable.

- A 1/4W carbon composition resistor. It has an interesting but pretty bad behavior: Only at very low frequencies it has its rated resistance value, along with considerable parallel capacitance. By about 20MHz the resistance has dropped by 20%, then stays about constant! At 500MHz it's significantly inductive, and I don't know why, being the same size as the film ones. This is very much worse RF behavior than any of the film resistors! I have been preaching for decades that film resistors are a lot better than composition, but some people keep believing that carbon composition is good for RF, because very old books say so. Sure, back when the only choice was between carbon composition and wirewound, carbon composition was by far the better choice! But since the appearance of cheap film resistors about a half century ago, carbon composition is obsolete for RF.

- A 2 watt metal oxide film resistor. Due to its larger size, it has much higher inductance than the quarter watters. Probably it had many turns cut during trimming at the factory.

- Just for fun, I'm also including a high precision 0.1% resistor. It's about the same size as the 2W MOX. It's a Danotherm type PDW103, with gold-plated terminals. Beautiful and very useful to calibrate ohmmeters and similar applications. But definititely NOT for RF, as the measurement shows! This thing must be wirewound, with a zillion turns! I don't have more information about it. It's usable at DC and line frequency, but not even at audio, having 73? inductive reactance at 10kHz!!!

The artifact of my test jig or calibration, around 600MHz, remains present in all the measurements, so don't pay attention to that. I don't know whether the results above 700MHz or so have any validity. Better look just at the range from zero to 500MHz, in all my graphs published in this thread.

Read also this
/g/nanovna-users/message/22407?p=,,,20,0,0,0::Created,,posterid%3A4247573,20,2,20,82990168

Jim, no attachments.

Dave - W?LEV

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

Larry, Thanks for the heads-up to the mod.

One more comment; After replacing the bezel and bottom panel. The marker could be moved with the lever to the left, but not to the right. The marker kept moving left as long as I kept the lever pushed to the left. When I released the lever, the marker reversed direction and kept moving to the right, with no further action on my part, Pushing the lever to the right had no effect. But, it did not hang the vna, even when I pushed the switch downward.

This tells me that something is erratic with the lever/switch. But, why doesn't the problem appear with the original FW? Maybe it has something to do with how the algorithm handles sensitivity to contact debounce.

Jack K1VT

Thanks all for the comments.

I did try three ferrites at the antenna end of the transmission line and did not see any change. Two were the clamp on type. I believe they were designed for frequencies above HF but I don't know the ferrite mix. The third ferrite was a mix 31 FT-240 core with several turns of the coax wound around the core.

I was far enough away from the antenna in both cases to not see any effect on the readings. Calibrating at the end of the coax is also done because you don't want the NanoVNA in close proximity to the antenna.

QST published a very informative article in November 1991 called "My Feed Line Tunes My Antenna!". The author was Byron Goodman W1DX.

I believe the SWR and Phase plots in the first attachment may be entirely normal and explained by transmission line effects since the antenna impedance is not 50 ohms. Note that the frequency scan was relatively wide. These kinds of effects might not be noticed to the same degree with HF antennas.

73, Kent
AA6P

Jack,
A few of us have removed the jog SW and added 3 push buttons instead along the front cover. Have a look in the MODs section in files for info.