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When I am constructing an antenna, I have been using this device to plot the swr in my shop. I usually set a wide enough bandwidth to easily see the dip at resonance.
In the shop, connecting this device directly to the SO239 connector, the sweep on the screen is perfect. But in testing with the antenna installed with coax, and testing at the end of the feed line, the sweep seems strange. See attached
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That antenna is not the best with only 3 "stingers" as a skeleton image plane. It's pretty spartan! The "wiggles" you are seeing at the end of the coax reflect a small SWR on the coaxial line. The antenna is not strictly 50 ± j 0 ohms. As such, the wiggles are due to the standing wave within the coax due to a slight mismatch. But have a look at your cursor at "resonance". The trace is nearly flat surrounding the cursor. Nothing to be concerned about. Your NANOVNA is showing you the correct rendition of the response of the antenna connected to your 50-ohm coaxial line. Dave - W?LEV <> Virus-free.www.avg.com <> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> On Thu, Feb 6, 2025 at 11:46?PM jeffinwa via groups.io <k7jsm= [email protected]> wrote: When I am constructing an antenna, I have been using this device to plot
the swr in my shop. I usually set a wide enough bandwidth to easily see
the dip at resonance.
In the shop, connecting this device directly to the SO239 connector, the
sweep on the screen is perfect. But in testing with the antenna installed
with coax, and testing at the end of the feed line, the sweep seems
strange. See attached
-- *Dave - W?LEV* -- Dave - W?LEV
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I'm not sure why the ripple is in there. I have never seen that when I do an SWR measurement. The minimum of 1.2 isn't bad, but the frequency is 406 MHz. What band is the antenna for? Is it a commercially-made antenna? Zack W9SZ On Thu, Feb 6, 2025 at 5:47?PM jeffinwa via groups.io <k7jsm= [email protected]> wrote: When I am constructing an antenna, I have been using this device to plot
the swr in my shop. I usually set a wide enough bandwidth to easily see
the dip at resonance.
In the shop, connecting this device directly to the SO239 connector, the
sweep on the screen is perfect. But in testing with the antenna installed
with coax, and testing at the end of the feed line, the sweep seems
strange. See attached
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Hi,
What you are seeing is perfectly normal and simply represents the fact that the antenna feed impedance (at the antenna feed point) does not exactly match the coax impedance. It's certainly pretty close to matched though because you have a nice dip still - it's just not perfect.? But no need to change anything here.
The coax is acting as an impedance transformer with the impedance transformation cycling every half wavelength of coax.? If you switch on the smith chart trace on the vna you will see a number of circles corresponding to the cycles on your swr trace. Each trip around the smith chart represents one half wavelength of change in the coax's length.
When you did the tests in the shop, the feeder length (from the point on the vna where you did the calibration to the antenna feed point) was very short, much less than a half wavelength - so you didn't see any cycles in the swr. But now that you have a longer coax, you see them.
As an aside, of course the antenna impedance is going to be a bit different in its outside location than it was in the shop just because of the different environment. If you want to see the actual impedance of the antenna then you could do the VNA calibration at the far end of the coax, in situ. That would eliminate the effect of the coax from the readings and you should get a smoother and more accurate reading. But what you have there looks fine for normal operation.
73,
Andrew
VK2EZF
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On 7/02/2025 09:40, jeffinwa via groups.io wrote: When I am constructing an antenna, I have been using this device to plot the swr in my shop. I usually set a wide enough bandwidth to easily see the dip at resonance.
In the shop, connecting this device directly to the SO239 connector, the sweep on the screen is perfect. But in testing with the antenna installed with coax, and testing at the end of the feed line, the sweep seems strange. See attached
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On Thu, Feb 6, 2025 at 03:46 PM, jeffinwa wrote:
In the shop, connecting this device directly to the SO239 connector, the sweep
on the screen is perfect. But in testing with the antenna installed with coax,
and testing at the end of the feed line, the sweep seems strange.
The ripples you are seeing are not unusual. It is not due to the antenna - it is the transmission line and/or adapters being used. . When the impedance of the transmission line is slightly different than the 50 ohm impedance of the NanoVNA you get ripples which give a higher SWR at odd multiples of 1/4 wavelength (1/4, 3/4. 5/4 etc.) and lower at multiples of half wavelength (1/2, 1, 3/2 etc). Attached are two screenshots. 50 feet of RG213 with 50 ohms at the end was measured. SWR should be close to 1. In one plot a SMA to SO239 adapter was used and in the second a SMA to PL259 adapter with a barrel connector. Those female barrel connectors have about 35 ohms of characteristic impedance. Notice how much more ripple there is with the barrel inline.
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I suspect some inductance between your metal (?) mount and the feed line running parallel to it. Consider lengthening your feed line by a couple of feet and put a loop in it along the vertical somewhere.
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Hi folks,
I somewhat agree with what others have said and Rogers demonstration shows one issue but let me make some additional observations/comments. My comments below ignore some minor effects due to the nature on how the NanoVNA circuit works when measuring impedances that are not close to 50 ohms (example: not close to the center of the Smith Chart).
First off Jeffs antenna only has a feedpoint impedance close to 50 ohms at resonance somewhere in the 400 to 500 MHz range, and you can get close to 50 ohms at resonance by adjusting the angle on the radials. Most importantly feedline loss (attenuation) makes SWR look much better than it really is especially at high frequencies where feedline loss becomes significant, so minor dips in SWR start looking like significant dips due to large feedline attenuation.
Also Jeff used a scale of 1 unit per division on his SWR plot which gives him a full scale reading of 9 on his SWR plot which is zoomed out too much especially considering that feedline loss makes SWR look much better than it really is. I recommend that Jeff set his SWR plot scale to 0.25 per division which would make his full scale plot reading approximately 3.0. If he does this I think he will easily see that his minimum SWR is actually higher than the 406 MHz frequency his marker is set on and his other dips (valleys) that are not at the minimum SWR point will move farther away from his minimum valley making his true resonate point more obvious.
Note: I built an antenna similar to Jeffs this evening as a sanity check and tested it mounted almost directly to the NanoVNA-F using a few adapters. Also connected the antenna to the NanoVNA-F using an 18 inch piece of RG58, and also using 100 feet of RG58, and the differences in the SWR plot between all 3 configurations is very noticeable regarding the generation of multiple dips (valleys) especially when using the very long run of RG58 which is coiled up on a spool that has a lot of loss at 430 MHz.
Also note common mode signal on the feedline did not appear to come into play much during my tests as touching the feedline or NanoVNA-F body had very little impact on the SWR plot (just minor difference).
Just FYI,
Don (wd8dsb)
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Here is a picture showing my test vertical mentioned in my above post when I had it connected directly to the NanoVNA-F using just a couple of adapters. Note in this picture the NanoVNA-F was calibrated for and displaying 500 KHz to 1 GHz and displaying 301 points. SWR full scale = 3.5
Don (wd8dsb)
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Two more pictures using 10 feet of RG58 Coax connected to the vertical. First Picture is with SWR Plot scale set to 0.25 per division and second picture with SWR plot scale set to 1.0 per division.
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Hello Roger,
I may have missed something here, but it seems to me your trace labels are reversed.
The "top - SMA barrel" trace has much larger SWR ripple and a predominant capacitive reactance loop on the Smith Chart. I suggest this is actually the SO239 barrel.
Also, the markers are at different frequencies which make the upper trace SWR at the marker "look" better.
The lower trace has much lower SWR ripple and a much tighter reactance loop at ~50 Ohms on the Smith trace. More like an SMA barrel.
Cheers...Bob VK2ZRE
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On 7/02/2025 12:10 pm, Roger Need via groups.io wrote: On Thu, Feb 6, 2025 at 03:46 PM, jeffinwa wrote:
In the shop, connecting this device directly to the SO239 connector, the sweep
on the screen is perfect. But in testing with the antenna installed with coax,
and testing at the end of the feed line, the sweep seems strange.
The ripples you are seeing are not unusual. It is not due to the antenna - it is the transmission line and/or adapters being used. . When the impedance of the transmission line is slightly different than the 50 ohm impedance of the NanoVNA you get ripples which give a higher SWR at odd multiples of 1/4 wavelength (1/4, 3/4. 5/4 etc.) and lower at multiples of half wavelength (1/2, 1, 3/2 etc).
Attached are two screenshots. 50 feet of RG213 with 50 ohms at the end was measured. SWR should be close to 1. In one plot a SMA to SO239 adapter was used and in the second a SMA to PL259 adapter with a barrel connector. Those female barrel connectors have about 35 ohms of characteristic impedance. Notice how much more ripple there is with the barrel inline.
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I like look at the swr in real life conditions. If one is using this as an antenna on a handle talkie shouldn't you simulate this by removing the radials.?
Mike N2MS
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On 02/07/2025 1:03 AM EST Donald Kirk via groups.io < wd8dsb@...
wrote:
Here is a picture showing my test vertical mentioned in my above post when
I had it connected directly to the NanoVNA-F using just a couple of
adapters. Note in this picture the NanoVNA-F was calibrated for and
displaying 500 KHz to 1 GHz and displaying 301 points. SWR full scale =
3.5
Don (wd8dsb)
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Hi Mike,
The application was not for use on an HT but rather for an outdoor installation using 3 radials and I wanted to see what the actual feedpoint impedance and SWR looked like as close to the feedpoint as I could get using a quick setup. The only reason I’m holding the NanoVNA in my hand is so folks could see the NanoVNA and antenna in the same picture while I held my camera in my free hand.
Don wd8dsb
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On Fri, Feb 7, 2025 at 04:08 AM, Bob Ecclestone VK2ZRE wrote:
I may have missed something here, but it seems to me your trace labels are
reversed.
The "top - SMA barrel" trace has much larger SWR ripple and a predominant
capacitive reactance loop on the Smith Chart. I suggest this is actually the
SO239 barrel.
Th trace labels are correct. The "SMA barrel" plot was done by attaching a SMA to PL259 adapter to the NanoVNA. Then a SO239 barrel was added and the RG 213 attached to that. At the end was a 50 ohm non-inductive load. There will be more ripple in the SWR than using just a SMA to SO239 adapter. The reason is that SO239 barrels are not 50 ohms characteristic impedance. I have seen others report them as around 35 ohms. Also, the markers are at different frequencies which make the upper trace SWR
at the marker "look" better. The point was to show the larger ripple when there is more impedance mismatch. All plots used the same scale.
The lower trace has much lower SWR ripple and a much tighter reactance loop at
~50 Ohms on the Smith trace. More like an SMA barrel.
The lower SWR ripple trace was done using a SMA to SO239 adapter and then attaching the RG213. There is still an impedance discontinuity but it is not as bad as when an SO239 barrel is used.
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Here is another test result. 10 feet (3 meters) of RG316 with SMA connectors on one end an 100 ohm SMA resistor on the other end. SWR was 2 at 1 MHz and then went through peaks and valleys with decreasing SWR trend due to cable attenuation.
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How good are those SMA to PL259 and SMA to SO239 connectors that are popular with some NanoVNA users? A back to back test was done using a NanoVNA-H4. Results attached....
Roger
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I have to say that my experience with this group is the best I have ever had.
omg! Years of forums with arrogant responses, and mostly no help. Thank you
all for letting me learn without the slams. Means a ton to me. As a Ham for over
forty years, I have read about standing waves, reflected waves, etc with feed lines,
and antennas. But to be able to see this on a screen with these vna's, amazing.
thanks to all here
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I agree, I’m very new to all of this and it is a bit overwhelming but it is a great group, especially the pros!
--
Sent from Canary ()
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On Friday, Feb 07, 2025 at 2:19 PM, pastor.healer via groups.io <pastor.healer@... (mailto:pastor.healer@...)> wrote:
I have to say that my experience with this group is the best I have ever had.
omg! Years of forums with arrogant responses, and mostly no help. Thank you
all for letting me learn without the slams. Means a ton to me. As a Ham for over
forty years, I have read about standing waves, reflected waves, etc with feed lines,
and antennas. But to be able to see this on a screen with these vna's, amazing.
thanks to all here
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Hi Folks,
A few last comments and then I'm moving on. Today I modeled (approximately) Jeffs antenna using 4NEC2 and I made a few assumptions based on Jeffs installation. I built my antenna last night and wound up with my radials bent downward at an approximate 30 degrees angle which provided a very good match at resonance and therefore I used 30 degrees bent downward radials in my model. I also guesstimated Jeffs antenna is about 2 feet from the nearby tall mast that I estimated to be approximately 8 feet tall and I left all other variables out of the picture.
Bottom line using the results of the 4NEC2 model is that the presence of the nearby mast hardly influences the feedpoint impedance at resonance (just a couple of ohms) and therefore the SWR is hardly impacted by the nearby mast but the presence of the nearby mast does cause some scalloping of the antennas radiation pattern when looking at the horizontal plane (azimuth) with peaks and valleys in the azimuth being around +/- 2.5 dB of where the azimuth pattern would be if mounted in the clear (in the clear the azimuth pattern is a pure circle even when just using 3 radials). Also note the presence of the nearby mast did not introduce minor dips in the SWR plot, the SWR plot was very smooth. Therefore the 4NEC2 model supports the theory and actual observations that the minor SWR dips (valleys) that Jeff sees when using coax and the NanoVNA to measure his antenna are not a result of the antenna or nearby mast but rather the feedline and associated connectors as we (especially Roger and I) have discussed above.
Just FYI and 73,
Don (wd8dsb)
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One other thing that might be taken into consideration is calibration of the NanoVNA. Are you testing the antenna, or the antenna system?
When I want to test a new home made antenna, I calibrate the NanoVNA with an open, short, and 50 ohm load (assuming 50 ohm coax) to the antenna end of the coax. This allows measuring the antenna and not the coax.
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On Fri, Feb 7, 2025 at 02:07 PM, Roger Need wrote:
How good are those SMA to PL259 and SMA to SO239 connectors that are popular
with some NanoVNA users?
Above HF/VHF anything with a "UHF" connector involved is going to exhibit increasingly poor performance. In addition nobody that I know of makes a good-quality UHF calibration kit so you will have to use adapters which degrade the calibration. I have purged all of them from my station in favor of type N, which is good beyond 12GHz, high quality ones beyond 18GHz. I use N, BNC, SMA, and reserve type F for RX-only use. 73, Don N2VGU
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Roger Need
Donald S Brant Jr