Re: Antenna / VNA Question
Just measured dipole directly without twin-lead; right on expectations! Reactance was 0 at 9, 18.3, 27.8, 37.8, 47.3, 58.5, 67, 77.6, and 86 MHz. Alternate ones, when X moved from capacitive to inductive, had lowest R and SWR.
Begs a question: for practical use, isn¡¯t the gratifying direct measurement on the dipole less ¡°real¡± or useful than the measurement downstairs using the feedline?
toggle quoted message
Show quoted text
On Sep 22, 2021, at 1:33 PM, alan victor <avictor73@...> wrote:
?You have the antenna in the attic so I assume you can reach its center point. Maybe not. But if you can, just clip your vna to the feed point and take a measure. I had a 66 foot NVIS 40 meter antenna flat at 15 feet above the ground. Clipped the vna onto the wires, let hang, stepped away and observed its display with my spy glasses!!! Just kidding. Held, read it and it was spot on.
|
Re: Antenna / VNA Question
Andy,
What you have constructed is commonly known as a "doublet" antenna (a dipole fed with balanced feedline). The most famous version is the G5RV. If you search with Google on "doublet antenna" you will find numerous descriptions on how it works.
A couple of things to consider....
- As others have mentioned the resonant frequency of a dipole depends not only on the length/diameter of the wires but also on how far it is above ground and the proximity of other objects. In an attic you will have a lot of factors at work.
- The feedpoint impedance of a dipole in free space at resonance is around 73 + j0 ohms with horizontal arms. Changing the angle (like in an inverted V) will change the impedance. In a "real world" installation the feedpoint impedance will be much different due to the proximity of ground and other objects. The type of ground (salt water, marsh, sandy, soil etc) will also have an effect. As you change the height above ground the feedpoint impedance will change. (see attached graph)
- A transmission line acts as an "impedance transformer" . The impedance seen at the far end will not be the same as observed at the antenna feedpoint unless there is zero loss ("ideal" situation) AND it exactly matches the feedpoint impedance. For example if a low loss 300 ohm transmission line is a quarter wavelength long and you short one end you will see a very high impedance at the far end. Leave one end open and you will see close to zero at the far end. With 75 ohms a half wavelength transmission line will be close to 75 ohms at the far end.
- A transmission line may be unbalanced (like coax) or balanced (like twin lead or open wire feeder). The advantage of coax is that the transmission characteristics are not affected by external factors like rain. snow and nearby objects like a balanced transmission line. If you run twin lead from the attic to your radio room the presence of other objects (wood, gyproc etc.) will result in unbalanced operation and now the twin lead will effectively become part of the antenna.
- When you terminate a balanced transmission line it should be balanced or you upset the balance and the transmission line becomes part of the antenna.. The NanoVNA is an unbalanced device (gets worse with a USB cable attached) so you need to use a "balun" (BALanced - UNbalanced).
Roger
|
Re: Antenna / VNA Question
You have the antenna in the attic so I assume you can reach its center point. Maybe not. But if you can, just clip your vna to the feed point and take a measure. I had a 66 foot NVIS 40 meter antenna flat at 15 feet above the ground. Clipped the vna onto the wires, let hang, stepped away and observed its display with my spy glasses!!! Just kidding. Held, read it and it was spot on.
|
Re: Antenna / VNA Question
You are feeding a balanced antenna with a balanced line. No balun is required at the antenna. However, at your transmitter (or NanoVNA) you have an unbalanced input (unless you are using a tuner with a balanced output), so a banun would be appropriate here. Otherwise your twinlead will become part of the antenna.
Additionally, any time you have a transmission line with an impedance different from your antenna (in this case 75 ohm antenna and 300 ohm transmission line), your impedance at the other end will vary depending on the impedances involved and the length of the feedline, in wavelengths.
And, as others have said, any metal around your antenna will change the resonant frequency. Also, any metal near your twinlead will inbalance the twinlead and cause it to act as part of the antenna.
So there are many things which can cause your problem.
|
Re: Antenna / VNA Question
Without a proper balun (not a transformer !!) or common mode choke, the
NANO and anything it is sitting on or your hand or arm will become part of
the measurement. When you make impedance measurements directly connected
to a balanced source/load, the NANO needs to be sitting on something which
is a good insulator with no other conductors or biological material
attached or closeby - nothing, not even the USB cable. This applies to
HF frequencies. When the physical size of the NANO becomes greater than
0.10 (10%) of a wavelength at the frequency of interest, all bets are off.
You're OK at HF.
Since you know the impedances at the shack end of the feedline, the length
of the feedline, and the impedance of the feedline, you can use SimSmith to
eliminate the influence the feedline. SimSmith can be found (it's free)
at:
and an excellent tutorial at:
Then you know what you are really dealing with.
BTW: The NANOs compare extremely favoriably with far more expensive
(HP/Agilent/Keysite) instruments. And there is no doubt closeby objects,
especially near the wire ends, are lowering the calculated resonant
frequency.
Also remember, resonance is defined as ¡ÀjX = 0 (crossings of the central
horizontal line on the Smith Chart), not necessarily SWR =0.
Dave - W?LEV
toggle quoted message
Show quoted text
On Wed, Sep 22, 2021 at 2:24 PM KENT BRITAIN <WA5VJB@...> wrote: D. The 468 rule of thumb is not that accurate.
That is just a good starting point. It does not allow for the diameter
of the conductor or any nearby objects. The difference between bare Copper
Wire and Insulated Copper wire can be as much as 10% difference. Any
nearby insulating material 'dielectrics' will lower the resonate frequency
of an antenna. Good luck with your project, Kent WA5VJB
On Wednesday, September 22, 2021, 09:16:33 AM CDT, Andrew Kurtz via
groups.io <adkurtz@...> wrote:
I have a dipole antenna in my attic, made of bare 14 gauge copper wire.
It is 49.916 feet long, and the feed point is within 1.5 inches of the
center. Thus, the ¡°rule¡± that f=468/L suggests the optimal, tuned
frequency should be 9.38 MHz (right?). And at that frequency X should be
about 0 and R should be radiative R at about 75 ohms (right?). 55 feet of
¡°300 ohm¡± twin-lead connects this balanced antenna to my ¡°shack¡± in the
basement.
In the basement, I analyzed the "twin-lead plus dipole" using my
nanoVNA-H4, calibrated with 100 points each between 0.5 and 5 MHz, then 5
and 10 MHz, then 10 and 15 MHz, then 15 and 25 MHz. The outputs looked
very reasonable (I think): Smith charts making nice circles, reactance X
rising periodically from capacitive to inductive smoothly, then crashing
very fast from inductive to capacitive, and real R and SWR lowest where X
is rising and quite high where X is falling.
But here comes the question: minimum R and SWR, and X crossing 0 from
capacitive to inductive, occurred at 3.1, 8.1, 11.5, 17.3, and 23.6 MHz ¡ª
nowheres near the expected 9.38 MHz. Also, I can¡¯t see a regular multiple
of wavelength between those frequencies. Here are possible answers:
A. My understanding is incorrect. (I am very new to RF stuff.)
B. The feed line really changes things. And do you advise some sort
of balun between antenna and twin-lead or twin-lead and receiver?
C. The nanoVNA is not that accurate.
D. The 468 rule of thumb is not that accurate.
By the way, the minimum resistance measured at each apparent tuned
frequency was around 30 ohms, versus the expected 76 or so. Is this
significant?
Andy
--
*Dave - W?LEV*
*Just Let Darwin Work*
|
Re: Antenna / VNA Question
On 9/22/21 7:16 AM, Andrew Kurtz via groups.io wrote: I have a dipole antenna in my attic, made of bare 14 gauge copper wire. It is 49.916 feet long, and the feed point is within 1.5 inches of the center. Thus, the ¡°rule¡± that f=468/L suggests the optimal, tuned frequency should be 9.38 MHz (right?). And at that frequency X should be about 0 and R should be radiative R at about 75 ohms (right?). 55 feet of ¡°300 ohm¡± twin-lead connects this balanced antenna to my ¡°shack¡± in the basement.
In the basement, I analyzed the "twin-lead plus dipole" using my nanoVNA-H4, calibrated with 100 points each between 0.5 and 5 MHz, then 5 and 10 MHz, then 10 and 15 MHz, then 15 and 25 MHz. The outputs looked very reasonable (I think): Smith charts making nice circles, reactance X rising periodically from capacitive to inductive smoothly, then crashing very fast from inductive to capacitive, and real R and SWR lowest where X is rising and quite high where X is falling.
But here comes the question: minimum R and SWR, and X crossing 0 from capacitive to inductive, occurred at 3.1, 8.1, 11.5, 17.3, and 23.6 MHz ¡ª nowheres near the expected 9.38 MHz. Also, I can¡¯t see a regular multiple of wavelength between those frequencies. Here are possible answers:
A. My understanding is incorrect. (I am very new to RF stuff.) The presence of the attic materials (your roof, etc.) reduce the resonant frequency (significantly).?? A simple experiment, at a higher frequency so it's more manageable - make a dipole that's a meter or two long and suspend it in air. Then put it on the ground. The other resonances aren't at "exact" multiples in a real dipole. The other thing is that your feedline may be acting as part of the antenna.? And there may be other wires in your attic that are interacting. B. The feed line really changes things. And do you advise some sort of balun between antenna and twin-lead or twin-lead and receiver? A program like SimSmith can show you what happens with the feedline.?? Short answer is that it wont change the frequencies at which X=0, but it does change the impedances a lot. 55 feet of twinlead will change the Z quite a bit - For a quarter wavelength, for instance, Zin * Zout? = Zline^2.? Your 55 feet isn't a quarter wavelength, but let's say it was, and your antenna was actually 70 ohms.? At the end of the feedline you'd see Zout = 300^2/70 = 1.3k. If your feedline were half a wavelength, then Zin=Zout , so you'd see 70 ohms.? A pretty big difference. C. The nanoVNA is not that accurate. Assuming you've done the cal right, the NanoVNA is probably telling you the right numbers. D. The 468 rule of thumb is not that accurate. 468 is a rule of thumb for a dipole "in free-ish space" - Put anything that is has an epsilon that's not 1 near it, and the resonance drops.
By the way, the minimum resistance measured at each apparent tuned frequency was around 30 ohms, versus the expected 76 or so. Is this significant?ions,
Without going through a bunch of calculations for your feedline, it's hard to tell.
|
Re: Antenna / VNA Question
D.? The 468 rule of thumb is not that accurate.
That is just a good starting point.? ?It does not allow for the diameter of the conductor or any nearby objects.? The difference between bare Copper Wire and Insulated Copper wire can be as much as 10% difference.? ? ? Any nearby insulating material? 'dielectrics' will lower the resonate frequency of an antenna.? ? Good luck with your project, Kent WA5VJB
toggle quoted message
Show quoted text
On Wednesday, September 22, 2021, 09:16:33 AM CDT, Andrew Kurtz via groups.io <adkurtz@...> wrote:
I have a dipole antenna in my attic, made of bare 14 gauge copper wire.? It is 49.916 feet long, and the feed point is within 1.5 inches of the center.? Thus, the ¡°rule¡± that f=468/L suggests the optimal, tuned frequency should be 9.38 MHz (right?).? And at that frequency X should be about 0 and R should be radiative R at about 75 ohms (right?).? 55 feet of ¡°300 ohm¡± twin-lead connects this balanced antenna to my ¡°shack¡± in the basement.
In the basement, I analyzed the "twin-lead plus dipole" using my nanoVNA-H4, calibrated with 100 points each between 0.5 and 5 MHz, then 5 and 10 MHz, then 10 and 15 MHz, then 15 and 25 MHz.? The outputs looked very reasonable (I think):? Smith charts making nice circles, reactance X rising periodically from capacitive to inductive smoothly, then crashing very fast from inductive to capacitive, and real R and SWR lowest where X is rising and quite high where X is falling.
But here comes the question:? minimum R and SWR, and X crossing 0 from capacitive to inductive, occurred at 3.1, 8.1, 11.5, 17.3, and 23.6 MHz ¡ª nowheres near the expected 9.38 MHz.? Also, I can¡¯t see a regular multiple of wavelength between those frequencies.? Here are possible answers:
? ? A.? My understanding is incorrect.? (I am very new to RF stuff.)
? ? B.? The feed line really changes things.? And do you advise some sort of balun between antenna and twin-lead or twin-lead and receiver?
? ? C.? The nanoVNA is not that accurate.
? ? D.? The 468 rule of thumb is not that accurate.
By the way, the minimum resistance measured at each apparent tuned frequency was around 30 ohms, versus the expected 76 or so.? Is this significant?
? Andy
|
I have a dipole antenna in my attic, made of bare 14 gauge copper wire. It is 49.916 feet long, and the feed point is within 1.5 inches of the center. Thus, the ¡°rule¡± that f=468/L suggests the optimal, tuned frequency should be 9.38 MHz (right?). And at that frequency X should be about 0 and R should be radiative R at about 75 ohms (right?). 55 feet of ¡°300 ohm¡± twin-lead connects this balanced antenna to my ¡°shack¡± in the basement.
In the basement, I analyzed the "twin-lead plus dipole" using my nanoVNA-H4, calibrated with 100 points each between 0.5 and 5 MHz, then 5 and 10 MHz, then 10 and 15 MHz, then 15 and 25 MHz. The outputs looked very reasonable (I think): Smith charts making nice circles, reactance X rising periodically from capacitive to inductive smoothly, then crashing very fast from inductive to capacitive, and real R and SWR lowest where X is rising and quite high where X is falling.
But here comes the question: minimum R and SWR, and X crossing 0 from capacitive to inductive, occurred at 3.1, 8.1, 11.5, 17.3, and 23.6 MHz ¡ª nowheres near the expected 9.38 MHz. Also, I can¡¯t see a regular multiple of wavelength between those frequencies. Here are possible answers:
A. My understanding is incorrect. (I am very new to RF stuff.)
B. The feed line really changes things. And do you advise some sort of balun between antenna and twin-lead or twin-lead and receiver?
C. The nanoVNA is not that accurate.
D. The 468 rule of thumb is not that accurate.
By the way, the minimum resistance measured at each apparent tuned frequency was around 30 ohms, versus the expected 76 or so. Is this significant?
Andy
|
$23.99 ???? for a tool that usually costs a few bucks
in any network equipment store, not to mention the usual Chinese
vendors on Ali and Ebay.
toggle quoted message
Show quoted text
On Wed, 22 Sept 2021 at 14:58, RandMental <randmental@...> wrote: I bought these for RG213 and RG58 - Don¡¯t waste your money
Still using a carpet knife and side cutter!
-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of
DougVL
Sent: Wednesday, September 22, 2021 12:29 PM
To: [email protected]
Subject: Re: [nanovna-users] Stripping Coax
Check
<>
--
Doug, K8RFT
|
|
|
I'd imagine 'not the shield', aka the ('hot'?) center conductor.
73, Willie N1JBJ
toggle quoted message
Show quoted text
On Sep 19, 2021, at 1:44 PM, Mike C. <mg@...> wrote:
What is a "warm" wire in the CH0 connector??
On 9/19/2021 11:32 AM, eugen.trifu@... wrote:
I had the same problem, which appears when making the calibration and SHORT probe does no electric contact is made with the CH0 connector.
Solving was by tightening the warm wire in the CH0 connector.
|
The typical life of flash in many SOC micros is around 10K erase/write cycles.?
I found this link on an STM32 forum that has some good tips on extending flash life.?
I had asked several months ago how difficult it would be to add an 8 pin SPI flash chip (eg:25f128)? to the NanoVNA for extra storage space.? ?Is support already in the ChibiOS routines? The Nano-F has this.
?I have 25f256 chips I can attach to my H,H4 to test any routines with.
Thanks,
Larry
On Wed., 22 Sep. 2021 at 12:21 a.m., DiSlord<dislordlive@...> wrote: In my last firmware i add option DONE IN RAM (in calibration menu)
This allow do calibration and not save it in flash (prevent flash damage from frequent rewriting, and not need use save slot)
This calibration reset after device power off.
|
|
|
What is a "warm" wire in the CH0 connector??
toggle quoted message
Show quoted text
On 9/19/2021 11:32 AM, eugen.trifu@... wrote: I had the same problem, which appears when making the calibration and SHORT probe does no electric contact is made with the CH0 connector.
Solving was by tightening the warm wire in the CH0 connector.
|
Re: PC Boards for QEX Step Attenuator Available
The author discusses this some and there are numerous response graphs in the QEX supp files. Can you read the article or do you need a copy
Dick K9IVB
|
In my last firmware i add option DONE IN RAM (in calibration menu)
This allow do calibration and not save it in flash (prevent flash damage from frequent rewriting, and not need use save slot)
This calibration reset after device power off.
|
On 9/21/21 2:05 PM, William Smith wrote: OK, that's adjustable for different jacket/braid/dielectric/center diameters, do you have to swap the cartridge for different strip lengths?
73, Willie N1JBJ
yes and no. There's a bunch of "slots" into which the blade drops to adjust the "length". Then the hex screw sets the depth. If you're using two kinds of strips consistently, then getting extra cassettes is useful. I just used to keep notes on where to put the blade, and how many turns of the hex for each one. And eventually, the blade does get dull (esp the one cutting the shield in the middle). On Sep 21, 2021, at 4:58 PM, Jim Lux <jim@...> wrote:
On 9/21/21 1:20 PM, William Smith wrote:
Hey Jim,
Do you have a pointer to the fancy adjustable one? I've been having a hard time finding anything other than 2 and 3 blade non-adjustable ones
73, Willie N1JBJ Paladin CST Pro
looks like it
On Sep 21, 2021, at 2:56 PM, Jim Lux <jim@...> wrote:
the fancy adjustable one
|
Depending upon the version of the NanoVNA and the firmware version the
NanoVNA uses 101, 201, 301, or 401 points of measurement. That says that if
you have a calibration range of 3 to 30 MHz and the device is using 101
points the NanoVNA measures approximately every 267 KHz.Now if you reduce
the range to the 7050 to 7060 MHz that is 10 KHz. That is very much less
than the 267 KHz used by the device. Yes the device is still calibrated for
the 3 to 30 MHz but may not accurately measure the narrower span. EXAMPLE
1st point is 3000 KHz
2nd point is 3267 KHz
3rd point is 3534 KHz
and so on.......
I would recommend that you calibrate the 3 to 30 MHz range for
board measurements but also due an additional calibrate in the specific
frequency range of interest. If you calibrated in the 7000 to 7100 range
then there will be a measurement point every 990 Hz.
The basic rule is to recalibrate with changes in frequency or changes in
jumper cable length.
The numbers quoted are approximate.
Regards,
*Clyde K. Spencer*
toggle quoted message
Show quoted text
On Tue, Sep 21, 2021 at 7:03 PM Joe WB9SBD <nss@...> wrote: Ok,
say I set up Stimulus for 3 to 30 Mhz.
And then do a calibrate
Open
Short
load.
Now it is supposedly calibrated for anywhere between 3 and 30 Mhz correct?
Now I look at the who range and decide to zoom in on say 7 to 7.3
Start 7 Mhz
stop 7.3 Mhz
is it still calibrated?
How about really narrow like 7050 to 7060 Khz
still calibrated?
Joe WB9SBD
|
Joe,. The Nano has discreet frequency steps you can set. Since you have an H4, you can have up to 401 steps between the lowest and highest swept frequencies.?
You need to keep this in mind as everything between the steps is interpolated.?
On Tue., 21 Sep. 2021 at 7:28 p.m., Joe WB9SBD<nss@...> wrote: Thanks, I was thinking of something like that.
Joe WB9SBD
toggle quoted message
Show quoted text
On 9/21/2021 6:12 PM, alan victor wrote: Always calibrated however, unit does interpolation. Therefore, if you desire best accuracy over a given range, then cal over THAT range.
|
Joe, Another good way to learn about the NanoVNA is to read old posts in this group. Many typical questions have already been answered. For example this post which deals with your question. /g/nanovna-users/topic/73210908#12819Roger
|
Roger Need
RandMental
Mike C.