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#measurement Problems for reading SWR, Graph always mooves !
#measurement
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Hi all,
First time I post a question because i've an issue with my nanoVNA. Thanks to all for intersesting to it and many thanks for all your post ! I've learn so much things with all of you ! So when i try to measure the SWR with my NanoVNA, calibration is OK, but when I connect directly a small VHF / UHF Handheld in order to see how the swr is the lowest, the graphs always mooves. When i put my hands near the VNA i can read SWR is 1,5 at 140 MhZ, when i pull back my hands, i can read, 2,5 at 145 Mhz.... When a put the VNA verticaly i can read another measure, horizoly, another measure and it depend if i'm on a wooden table or a glass table.... So... it's really difficult for me to get a good and constant measure in order to see how the antenna work exactly ! Have you any idea ? I'm sure the nanovna is a good instrument but i do something wrong but i don't now what ! Thank's a lot for reading me and perhaps help ! 73'S QRO F4COH Pierre |
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Tom KD8AVF
Pierre,
I think you are trying to measure the antenna from a handheld transceiver? In such instances, the antenna uses the handheld transceiver as a ground plane to make up the other half of the antenna. Without the antenna attached to some tye of adequate ground plane, the SWR will be greatly affected by anything in close proximity. The antenna will use the nanoVNA test leads and nanoVNA itself as the other half of the antenna. Even your hand touching the nanoVNA or test leads become part of the antenna. I suggest using a magnet mount attached to a metal ground plane as a "testing jig". |
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Hello Pierre,
Great observations and questions. I am not an expert at this, but I do recall a response to a similar question a few weeks about using the nanoVNA for antenna measurements. I recall that the recommendation for measuring the characteristics (including SWR) of a vertical antenna is to have the antenna situated upon a good metal ground plane that is at least one-half (perhaps even more) of a wavelength in diameter around the base of the antenna, and that the coaxial cable length between the antenna and nanVNA should be at least two wavelengths. Thus, if you are measuring response of a 2-meter antenna, the antenna should ideally be placed on a metal ground plane that is two meters in diameter (1-meters in each direction around the base), and the coax between the antenna and VNA should be at least four meters long. Following these guidelines should help minimize the inconsistencies you described in your post. Another thing to consider is that when an antenna is placed on a portable/hand-held radio, you most likely will not have the same conditions as compared to those present when the antenna was tested as described above. In the case of hand-held radios, the human body makes up a portion of the ground ¡°plane¡± for the radio, and the SWR in actual practice can be highly dependent on orientation of the radio (horizontal vs. vertical vs. tilted), proximity to the radio to the face, use of an external microphone, etc. Therefore, measurements made with a nanoVNA under ideal conditions may or may not represent what the radio is actually seeing when in use. Good luck. 73 Ken - WB?OCV From: pjbellin@... Sent: Monday, March 1, 2021 07:59 AM To: [email protected] Subject: [nanovna-users] #measurement Problems for reading SWR, Graph always mooves ! Hi all, First time I post a question because i've an issue with my nanoVNA. Thanks to all for intersesting to it and many thanks for all your post ! I've learn so much things with all of you ! So when i try to measure the SWR with my NanoVNA, calibration is OK, but when I connect directly a small VHF / UHF Handheld in order to see how the swr is the lowest, the graphs always mooves. When i put my hands near the VNA i can read SWR is 1,5 at 140 MhZ, when i pull back my hands, i can read, 2,5 at 145 Mhz.... When a put the VNA verticaly i can read another measure, horizoly, another measure and it depend if i'm on a wooden table or a glass table.... So... it's really difficult for me to get a good and constant measure in order to see how the antenna work exactly ! Have you any idea ? I'm sure the nanovna is a good instrument but i do something wrong but i don't now what ! Thank's a lot for reading me and perhaps help ! 73'S QRO F4COH Pierre |
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Small handheld antennas are affected by close proximity to conductive and dielectric sources such as your body or hand.
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This is also true when you attach th antenna to your handheld radio. The SWR will vary but you don't have to worry about transmission line loss since the antenna is directly connected to the radio. You can measure the swr in a lab environment by mounting the antenna on a ground plane and attaching the vna to the connector underneath the ground plane. Keep in mind this down not replicate the real world operating situation. Mike N2MS On 03/01/2021 3:13 AM pjbellin@... wrote: |
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Very good advice.?? Another possibility would be to cut a quarter wave wire, attach it to the shell of the antenna's connector, and let it hang down vertically while the antenna is oriented vertically upward.? How effective this is would depend on the electrical (not necessarily the same as physical) length of the wire and how it is affected by objects around it, but it would be easy to try.
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Note that an extensive ground plane ought to exhibit a very low impedance while an ideal quarter wave wire would be somewhere around 35 or 40 ohms, but we get the same sort of mismatch when we feed an ideal halfwave dipole with 50 ohm coax. 73, Maynard W6PAP On 3/1/21 5:27 AM, Ken Sejkora wrote:
Hello Pierre, |
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What a beautiful suprise to see all theses answers ! Thanks to all ! Many Many Thanks to all !
Now i understand, i begin to understand. Why i try to do this : in order to try to get the best antenna for the handheld, in order to have the minimum of reflected power, and test all my handys antennas I love handheld radio and i've done incredibles contact on top point (mountains) What do you advise to me in order to has the most precise result. I explain : i would like to try to have the result closest to reality. I think to try to mesure this evening the swr with the VNA vertical, the antenna directly mount on the VNA and the VNA in my hand, close to my head : like in the reality when i transmit. Many thanks to all, it's incredible : One question ask this morning : 5 answers coming from all around the world this evening.... Long life to radio Long life to hams Long life to all 73 QRO Pierre. |
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Pierre,
the SWR fluctuations you see on the NanoVNA simply show you how a handheld antenna performs in normal use! It doesn't offer fixed, good SRW to the radio. Instead it varies according to what's around, how you hold the radio, etc. The SWR you see on the NanoVNA is not the same the radio will see, because the NanoVNA is different from the radio. But there will be wild fluctuations in both cases. To find the best antenna for your radio, don't watch the SWR. Instead experiment which of them makes the radio transmit and receive the best. Go to an open place, and enlist the help of some fellow ham at a few km distance who can transmit at variable power, and who has a radio with a real S-meter. Test each of your antennas while communicating wth that ham. In each case, ask him to reduce the power until you hear him with significant noise, and then swap antennas and move the radio around and find out which antenna receives best, in which position. Also set your power so that his S-meter points somewhere in the middle of its range, and ask him to report the signal level for each of your antennas, while you move the radio in several ways. These tests should bring out which of the antennas provides the best performance, and whether any is more position-critical than any other. Be sure to make these tests on at least two or three frequencies spread over the band, because small antennas are often quite frequency-selective. Testing on just one frequency might give a wrong idea. SWR is very far from telling everything about an antenna. A 50? dummy load gives you perfect SWR, but no radiation! A very poor antenna might be so lossy that it behaves largely like a dummy load. And most handheld radios are rather uncritical regarding SWR. They need to be, since SWR varies so much depending on what's around the antenna. When I take my handheld radio into the field, I use a telescopic whip antenna, which I can extend just the right amount for the frequency in use. I marked these lengths for quick use. It allows me using the same antenna both on the ham band and on the marine band, with good performance. And the performance is significantly better than that of any rubber duckie. |
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There is a Japanese Web magazine, published in English, that had a 3 part series on testing "rubber duck" and other antennas on portable radios (HT's). They did SWR and field strength tests (Tx and Rx) using a modified Icom handheld radio as a test jig. I think members of this group will find the results interesting.....
Part1 Part2 Part3 Roger |
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Hi Pierre,
What do you advise to me in order to has the most precise result.Operating a handheld transceiver has many imperfections, not the least of which is the operator himself who is a pretty good conductor (70% salty water in a human body) and will absorb or reflect some of the RF. For an end-fed vertical antenna (which is the typical antenna design on a handheld radio) the optimal ground plane would have a 1/4" wavelength diameter. That sounds at first glance easy to achieve but there is a serious problem hidden in that statement. The wavelength is inversely proportional to the frequency and you probably want to operate that handheld device on one (or more) amateur radio band and not just on a single fixed frequency. To optimize for single frequency operation you take 1/4 of the wavelength and use a circular metal disk with that length as radius. That will result in a high-Q antenna + groundplane system that has a low SWR only in a very narrow frequency range. A compromise is to use a square groundplane for testing which means that the radius of the groundplane varies in every direction and the resulting system of antenna + groundplane has a low-Q (wide bandwidth). This should result in low SWR values over an entire band assuming the square groundplane is sized such that length of each side is less than 1/2 wavelength of the highest frequency in the band and the length of the diagonal is more than 1/2 wavelength of the lowest frequency in the band. The 1/2 wavelength corresponds to the desired 1/4 wavelength radius when the antenna is mounted in the center of the square groundplane. I explain : i would like to try to have the result closest to reality.Because of the many variables that you will encounter when operating in real life, it sadly isn't realistic to simulate it. The best you can do is ensure a consistent environment in which you are testing the antennas to make sure you test each antenna under the exact same conditions. This does mean to keep the human being as far away as reasonably possible from the device under test. Maynard's suggestion of using a 1/4 wavelength wire to improve the antenna performance of a handheld radio works well and does result in a measurable gain (about 3dB). Those wires are often referred to as Tiger Tails and you can find more information by searching for Tiger Tail Antenna. It is a very inexpensive way to improve the performance of a handheld radio. 73. Thomas |
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On 3/1/21 10:33 AM, Roger Need via groups.io wrote:
There is a Japanese Web magazine, published in English, that had a 3 part series on testing "rubber duck" and other antennas on portable radios (HT's). They did SWR and field strength tests (Tx and Rx) using a modified Icom handheld radio as a test jig. I think members of this group will find the results interesting..... interesting, but I'm not sure I buy his antenna test fixture: 1) in practice people hold the ht (or it's next to your body with a mic/headset) 2) he has no choke on the coax, so the coax is "part of the circuit" It could well be that the "match" is better (or worse) than he has shown. This whole "measure performance of antennas on mobile devices" is a really, really tricky thing. (As Apple found with the iPhone 4, with the? feedpoint shorted by your hand). I'd measure them on a feedthrough connector in the middle of a *big* conductive plate(cardboard + aluminum foil. Something that is at least a wavelength in *radius* - so for a 2m HT antenna, you're looking at a 12 foot diameter circle. or on a plate that's at least radius > height of antenna |
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As previously mentioned, the problem is because of the lack of a 'ground plane' or return circuit to the nanovna (or HT).
Search the web for "tiger tail" or "rat tail" and ht or handheld radio. There are many results about adding a 1/4 wave counterpoise wire to the HT. Some have measured and posted the results of tests, which were quite good. QRP "walkabout" or pedestrian mobile HF operators often use what they call a "drag wire" counterpoise, which trails along behind them on the ground. I made one for my UV5R HT, using some #28 silicone covered, very flexible wire that is also quite limp. It seems to have helped a lot. I intend to operate battery portable, and bought this BNTECHGO brand wire from Amazon for making small, lightweight counterpoises and maybe (shorter) antenna wires. At first, after reading about the 'tiger tail' counterpoise, I tried a leftover piece of very thin magnet wire, but it was too stiff and inconvenient. The limp silicone wire is much better, although a little bulkier (but still tiny). To connect it to the HT, I stripped the end for about 3/4 inch and bent it into a circle. Then adjusted the circle diameter to fit easily over the threaded antenna connector on the top of my HT. After soldering the little loop, i place it over the threaded SMA post on the radio, then screw on the antenna. That clamps the counterpoise wire in place. That last bit, with the antenna base being wider than an SMA cable connector, would be the difficulty with this method with the nanovna. A simple way would be to connect a 1/4 wave wire to an alligator clip, and attach the clip to the SMA connector shell. I've done this, both for a multiband HT antenna, and for a mobile antenna. It works. (Moving the counterpoise will still affect the nanovna readings, but much less.) Hand position will not have such a large effect, either. And it's cheap and easy to try! -- Doug, K8RFT |
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Tom N8ZI
I also added a tiger tail radial to my ICOM HT.
Lesson well learned. Make sure you cover the exposed end of the wire before use. I was too eager to check for signal improvement, keyed down the PTT and that Tiger tail burnt a chunk of my hand. The exposed end of wire was laying across my hand and gave me my one and ONLY encounter with a RF burn. It took 3 days to heal. |
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KV5R
Hello Pierre,
Yes, what the others have said, your hand changes the loading on half the antenna, both with HT and Nano. If your goal is max range mountain-topping, you can easily make a much better antenna than a short "rubber duck." Here is a portable, adjustable VHF/UHF dipole I made years ago with TV rabbit-ears: You can also easily make 3-5 element yagi wiith PVC pipe and aluminum tape. Model it in MMANA-GAL first. 73, --KV5R |
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On 3/2/21 9:18 PM, Solder Soldier via groups.io wrote:
My approach is to attach the antenna to the NanoVNA to mimic the handie-talkie as best as possible. Hold the VNA as you would a radio and make your measurements. A rubber ducky on a ground plane is not a realistic test and my guess that the engineering is not even optimized for a ground plane.It kind of depends on what you're trying to measure. If you're confirming S11 for the design, then a ground plane works great - you model it over a ground plane, build it, and see if it matches the model. If you're making pattern measurements, then you need some sort of "arm and head phantom" to replicate the RF properties of your hand, body, and head. That is difficult. Holding the VNA in your hand is close, but you have a hard time pushing the right button if you're holding the VNA up to your head. What most people do is model the "hand/head/body" with the antenna to get the design optimized. Then, they model that antenna over a ground plane, which is what they use for manufacturing verification.? If you're real fancy, you have a series of test jigs in various configurations that are easier to replicate and which you can also model. Maybe you have a lossy dielectric clamp that holds the radio/phone next to a dielectric head (or a bag of a mixture of salt water and shredded plastic)? There's a whole literature on making RF phantoms with all sorts of recipes for the material to replicate human body tissue RF properties. It just depends on what you're trying to do. If you're just curious about the match for various rubber duckies that's sort of different than "optimizing the radiated RF field".? For the latter you'd need good information about the output Z of the radio, which is almost certainly not 50ohms regardless of load. But maybe someone will figure out a way to use the NanoVNA to make a load pull test set. |
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On Wed, Mar 3, 2021 at 03:21 AM, Jim Lux wrote:
I do not think the importance of this can be understated. 50 Ohms has always been a nominal, not exact, goal for system design. The efforts to modify the manufactured tool to get the port impedance to exactly 50 Ohms subverts the best case for such precision - repeatability among instruments. Output impedance is only one of the tradeoffs in the design of an RF output stage and is less important than others such as stability and efficiency. And it is a dynamic characteristic, typically changing with temperature, load, frequency, and drive - not stuck at some magic matching number. In the end, after nominal matching, optimizing the radiated RF field requires measuring the radiated RF field in situ, not measuring some magic impedance on a bench. To a lesser degree, discrete components are similar - their characteristics are dynamic and measurements need to taken with consideration for the conditions of end use. |
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On Mon, Mar 1, 2021 at 12:58 PM, Manfred Mornhinweg wrote:
When I take my handheld radio into the field, I use a telescopic whip antenna,That's a great idea to mark it... I have a few telescopic antenna's I use on scanners, and always just guesstimate. -- Regards, Chris K2STP |
Roger Need
Chris K2STP
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