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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
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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
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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
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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.
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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
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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*
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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.
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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.
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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
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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?
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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.
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Hooray! No... now you have divided the problem. Divide and conquer.
NOW read all of those prior emails and begin to get your head wrapped around the issue of HOW DO I GET MY ANTENNA CONNECTED TO MY TRANSMITTER?
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Hi Andrew,
? ? The 468/f formula is based on (a) a perfect conductor and (b) a dipole antenna in free space.? In the real world, antenna systems are affected by the environment, such as a height above ground, metal objects near the antenna, the type of wire, whether it's insulated, and a host of other factors. Also, did you calibrate your NanoVNA for the frequency range of interest?? That too may play a part in what you're seeing.
Generally, 468/f will get you in the ballpark; it's best to cut a little bit long, test your antenna in its operational configuration (i.e. installed in your attic) and then adjust from there.? Also, keep in mind that it's not just about SWR and reactance...A dummy load will have a 1:1 SWR.? If it tunes up well and works well with your radio, that's all the proof you need; it's called 'antenna theory' for a reason.? A balun won't hurt either, but if it works without one then you don't need one.? Feel free to keep us updated!
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On Wednesday, September 22, 2021, 02:00:51 PM EDT, Andrew Kurtz via groups.io <adkurtz@...> wrote: 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? 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.
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On 9/22/21 11:00 AM, Andrew Kurtz via groups.io wrote: 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? Well, sometimes you can't get to the antenna (or your being close to it perturbs its behavior) There are two approaches to this: 1) calibrate the VNA with the feedline inside the calibration plane (i.e. put the open, short, load at the end of the feedline) 2) Measure the feedline by itself, and mathematically remove it from the measurement. Both have their strong/weak points. For instance, if you measure the antenna and today it measures differently than it did last time, you still have the "is it the feedline or the antenna that changed". One interesting approach is that with antennas with distinct resonances, off resonance, it usually presents a pretty big mismatch.? So you can run your TDR sweep and get a decent measurement (since, usually, TDR is done with a short or open at the far end), and if your feedline has changed, you'll see it in the TDR
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On Wed, Sep 22, 2021 at 11:00 AM, Andrew Kurtz wrote:
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?
I mentioned in another post that the transmission line acts as an impedance transformer. What you see at the end of your 300 ohm twin line will depend on the following factors: antenna feedpoint impedance, characteristics of transmission line and transmission line length. I have attached some example that illustrate this point. Lets say your antenna measured 75 ohms resistance and 0 ohms reactance at 9 MHz. You installed 43.5 feet of generic 300 ohm tubular cable (away from nearby objects). You measured with your NanoVNA and got an SWR of 1.58 with R=0 and X near 0. Looks good because this corresponds to 1/2 wavelength of cable at 9 MHz. It was then shorted to 32.6 feet or 3/8 wavelengths. Now the SWR is 12.78 with R = 144 and X = -263. Next it is shortened to 21.8 feet or 1/4 wavelength. SWR increases to 23.6 with R=1179 an X = -9.857. It is clear that length has a dramatic effect on what you measure. These values were all calculated with the downloadable Transmission Line Details program. Annotated screenshots attached However if we used RG-59 or RG-6 which has a characteristic impedance of 75 ohms the SWR measured at the NanoVNA would be close to 1.5 for all cable lengths above. Roger
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Wow, I didn¡¯t realize that! Thanks...
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On Sep 22, 2021, at 3:49 PM, Roger Need via groups.io <sailtamarack@...> wrote:
I have attached some example that illustrate this point. Lets say your antenna measured 75 ohms resistance and 0 ohms reactance at 9 MHz. You installed 43.5 feet of generic 300 ohm tubular cable (away from nearby objects). You measured with your NanoVNA and got an SWR of 1.58 with R=0 and X near 0. Looks good because this corresponds to 1/2 wavelength of cable at 9 MHz. It was then shorted to 32.6 feet or 3/8 wavelengths. Now the SWR is 12.78 with R = 144 and X = -263. Next it is shortened to 21.8 feet or 1/4 wavelength. SWR increases to 23.6 with R=1179 an X = -9.857. It is clear that length has a dramatic effect on what you measure. These values were all calculated with the downloadable Transmission Line Details program. Annotated screenshots attached
However if we used RG-59 or RG-6 which has a characteristic impedance of 75 ohms the SWR measured at the NanoVNA would be close to 1.5 for all cable lengths above.
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On Wed, Sep 22, 2021 at 05:49 PM, Andrew Kurtz wrote:
Wow, I didn¡¯t realize that! Thanks...
Getting - and using - a NanoVNA really starts a learning curve! And - there's SO much to learn . . . . . -- Doug, K8RFT
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Very good explanation Roger.
I might add to get ride of USB interference I put a choke on the USB cable and it kept it isolated and the Nano VNA calculated the same values on HF frequencies with no ground issues with it plugged in or not.
Of course if you have your hand on the VNA it does affect it.
I also have my VNA on plexiglass
Kinda reminder not to be holding on to it when making a measuremnt. Also keeps stuff away from the VNA.
De K8HTB Joe
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Roger,
What software are you using with those screen shots showing ¡°Transmission Line Details¡±? I have never seen that display before¡.? Thanks in advance¡.
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Regards,
Chris K2STP
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Looks like TLDetails:
Vince N2JRS
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-----Original Message----- From: [email protected] < [email protected]> On Behalf Of Chris K2STP Sent: Thursday, September 23, 2021 12:06 PM To: [email protected]Subject: Re: [nanovna-users] Antenna / VNA Question Roger, What software are you using with those screen shots showing "Transmission Line Details"? I have never seen that display before....? Thanks in advance.... -- Regards, Chris K2STP
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Roger Need
Chris K2STP
vh2