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I want to sweep my 2 meter antenna from the end of a 15 to 18 foot coax. This is because I need to get the antenna out in the clear away from nearby objects to get a good indication of the actual VSWR. The antenna has an SMA connector. I have two pieces of coax I can use. One is 15' of LMR-100 with SMA connectors on both ends. The other is an 18' piece of RG-8X with PL-259 connectors at both ends. In either case I can move the plane of calibration to the end of the coax where the antenna connects.
So I have two questions for those who have a lot of VNA experience in this group:
1) Since LMR-100 is lossy at 2m frequencies, will I lose much precision in the sweep by calibrating at the end of the coax?
2) Am I better off using RG-8X knowing that I will need to adapt the PL-259 to SMA at both ends? I understand that PL-259 connectors are not so good at VHF and adding adapters will only make them worse.
Which is the better alternative?
Albert KK7XO
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Hi Albert
You can cal at the end of the coax, but you don't have to.
Lets say the coax has about 2 dB of loss.2 dB of the signal is lost going up, and 2 dBis lost coming back down.Thus the antenna is 4 dB worse than you though it was.
In dB a measured Return Loss of 20 dB RL was really 16 dB RL.
In SWR ....... the math takes a while! hihi.? ? ??
In either case, you are making sure the antenna is on frequency, the coax shouldn't change that.?(Yes, I know a few exceptions)? and if you adjust the antenna for best RL/SWR, then you have adjusted the antenna for best RL/SWR.? It's as good as you are going to get!
Good luck with your project.? ?Kent WA5VJB? ?Antenna Editor CQ Magazine
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On Sunday, September 19, 2021, 10:33:16 AM CDT, kk7xo via groups.io <kk7xo@...> wrote:
I want to sweep my 2 meter antenna from the end of a 15 to 18 foot coax.? This is because I need to get the antenna out in the clear away from nearby objects to get a good indication of the actual VSWR.? The antenna has an SMA connector.? I have two pieces of coax I can use.? One is 15' of LMR-100 with SMA connectors on both ends.? The other is an 18' piece of RG-8X with PL-259 connectors at both ends.? In either case I can move the plane of calibration to the end of the coax where the antenna connects.
So I have two questions for those who have a lot of VNA experience in this group:
1) Since LMR-100 is lossy at 2m frequencies, will I lose much precision in the sweep by calibrating at the end of the coax?
2) Am I better off using RG-8X knowing that I will need to adapt the PL-259 to SMA at both ends?? I understand that PL-259 connectors are not so good at VHF and adding adapters will only make them worse.
Which is the better alternative?
Albert KK7XO
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That's interesting. I didn't think of using return loss instead of VSWR. I was thinking my VSWR would be much improved at the end of a lossy cable and I couldn't think of a way to correct for it. But return loss is the way. Simple math actually.
But I wonder how calibrating at the end of the lossy coax will affect my sweep. Do I lose dynamic range?
Albert
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You loose a few dB of dynamic range, but since a 20 dB RL is the same as a 1.1 to 1 SWR,I'm sure you have plenty of dynamic range! hihi? ?And now you know why so many of us like to work in Return Loss vs then R's.? Kent WA5VJB
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On Sunday, September 19, 2021, 11:09:16 AM CDT, kk7xo via groups.io <kk7xo@...> wrote:
That's interesting.? I didn't think of using return loss instead of VSWR.? I was thinking my VSWR would be much improved at the end of a lossy cable and I couldn't think of a way to correct for it.? But return loss is the way.? Simple math actually.
But I wonder how calibrating at the end of the lossy coax will affect my sweep.? Do I lose dynamic range??
Albert
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On 9/19/21 9:09 AM, kk7xo via groups.io wrote: That's interesting. I didn't think of using return loss instead of VSWR. I was thinking my VSWR would be much improved at the end of a lossy cable and I couldn't think of a way to correct for it. But return loss is the way. Simple math actually.
But I wonder how calibrating at the end of the lossy coax will affect my sweep. Do I lose dynamic range?
Albert You'll lose a little bit of dynamic range, but, really, how much do you need.? If your antenna has a |S11| of -20dB or -30dB, then the fact that the dynamic range is now 50dB instead of 60dB isn't going to be a big thing, is it? TBH, if it's easy, I'd calibrate at the end of the coax - if nothing else, the impedances are not transformed by the transmission line.? Subtracting loss from RL doesn't account for the phase shift.? Just be careful if the coax changes loss when you move it, etc. The other thing that can trouble you unexpectedly is that for a lot of antennas, the coax is part of the antenna system (either by design, or happenstance).
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Yes, you will lose a bit of dynamic range. However, consider the "raw" dynamic range of the NANOs. At 2-meters, you have more than enough dynamic range in the instrument to throw away a bit. Actually, neither coax is appropriate for VHF. The LMR-100 is lossy at 2-meters as is RG-8X. Neither is too good at 2-meters. Then throw in the nasty SO-239 / PL-259 on the RG-8X. Use the LMR-100 and don't sweat the small loss of dynamic range. . Dave - W?LEV On Sun, Sep 19, 2021 at 4:09 PM kk7xo via groups.io <kk7xo= [email protected]> wrote: That's interesting. I didn't think of using return loss instead of VSWR.
I was thinking my VSWR would be much improved at the end of a lossy cable
and I couldn't think of a way to correct for it. But return loss is the
way. Simple math actually.
But I wonder how calibrating at the end of the lossy coax will affect my
sweep. Do I lose dynamic range?
Albert
-- *Dave - W?LEV* *Just Let Darwin Work*
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Thanks. The reason I'm doing this is because I made a blog post about how to design a 2-meter twinlead J-Pole using SimSmith and the NanoVNA. One of my readers asked me to post a final sweep of the SWR. I have a sweep, but it is at the end of 15 feet of LMR-100 and I know it will look better than it really is, and I didn't want to cheat.
I will post the result in RL, and hope the reader can deal with that over SWR. I think probably he can.
Just for fun I think I will sweep SWR with the calibration plane at the end of the coax and compare.
Albert KK7XO
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On 9/19/21 9:12 AM, KENT BRITAIN wrote: You loose a few dB of dynamic range, but since a 20 dB RL is the same as a 1.1 to 1 SWR,I'm sure you have plenty of dynamic range! hihi? ?And now you know why so many of us like to work in Return Loss vs then R's.? Kent WA5VJB
I think |S11| = -20dB is more like 1:1.22 VSWR 1:1.1 is 26 dB 1:1.5 is 14 dB 1:2 is 10 dB
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SO-239 are not so bad at VHF, and in theory the affect generated by the connectors, in general, is equal across a broad frequency range. If you are within that range, then the connector type is not an issue. In general, as with everything, there are examples where this fails, but not many...
Same for Coax... As long as you are within the design frequency range the cable, the issues caused, are generally the same across various frequencies.
You will see different loss figures for cables, but across most ham bands, the losses are close to equal, across a narrow frequency range. That is to say, if you loose 1 db at 144 MHz., you will probably loose close to 1 db at 145 MHz. Again, as with everything, there are exceptions. As a rule though, if you are within the design specs of the cable and connector, you can just use them as if they are interchangeable, RG-8, LMR400, RG-8x, etc.
Even if you were on an a design edge, and you had slight to medium differences between the lower end of 2 meters, and the upper end of 2 meters, (loss wise), the calibration process would fix this, assuming you calibrated using the antenna end of the coax as your reference plane....
The nanoVNA is a wonderful device!!
73, and thanks,
Dave (NK7Z)
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources
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On 9/19/21 8:33 AM, kk7xo via groups.io wrote: I want to sweep my 2 meter antenna from the end of a 15 to 18 foot coax. This is because I need to get the antenna out in the clear away from nearby objects to get a good indication of the actual VSWR. The antenna has an SMA connector. I have two pieces of coax I can use. One is 15' of LMR-100 with SMA connectors on both ends. The other is an 18' piece of RG-8X with PL-259 connectors at both ends. In either case I can move the plane of calibration to the end of the coax where the antenna connects.
So I have two questions for those who have a lot of VNA experience in this group:
1) Since LMR-100 is lossy at 2m frequencies, will I lose much precision in the sweep by calibrating at the end of the coax?
2) Am I better off using RG-8X knowing that I will need to adapt the PL-259 to SMA at both ends? I understand that PL-259 connectors are not so good at VHF and adding adapters will only make them worse.
Which is the better alternative?
Albert KK7XO
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If you sweep with your calibration plane at the antenna, and at the VNA, I would like to see those results...
73, and thanks,
Dave (NK7Z)
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources
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On 9/19/21 9:19 AM, kk7xo via groups.io wrote: Thanks. The reason I'm doing this is because I made a blog post about how to design a 2-meter twinlead J-Pole using SimSmith and the NanoVNA. One of my readers asked me to post a final sweep of the SWR. I have a sweep, but it is at the end of 15 feet of LMR-100 and I know it will look better than it really is, and I didn't want to cheat.
I will post the result in RL, and hope the reader can deal with that over SWR. I think probably he can.
Just for fun I think I will sweep SWR with the calibration plane at the end of the coax and compare.
Albert KK7XO
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On 9/19/21 9:23 AM, Dave (NK7Z) wrote: If you sweep with your calibration plane at the antenna, and at the VNA, I would like to see those results...
73, and thanks,
Dave (NK7Z)
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources Here's an example of a 6BTV with 100 ft of RG-8X. Blue trace is cal at the NanoVNA Orange trace is cal at the end of the Coax. You can pretty clearly see the loss of the coax increasing with frequency. Also attached is a plot of the measurement of two 100 ft cables, (new and old) with the cal at the NanoVNA end, and the far end of the coax just open.
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Terry Turner, W5ETG, now a silent Key, was the first to develop the twinlead J-Pole back in the 1960's.? ?That puppy has certainly traveled far!? ?The hard part today is finding Twinlead! hihi
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On Sunday, September 19, 2021, 11:19:29 AM CDT, kk7xo via groups.io <kk7xo@...> wrote:
Thanks.? The reason I'm doing this is because I made a blog post about how to design a 2-meter twinlead J-Pole using SimSmith and the NanoVNA.? One of my readers asked me to post a final sweep of the SWR.? I have a sweep, but it is at the end of 15 feet of LMR-100 and I know it will look better than it really is, and I didn't want to cheat.
I will post the result in RL, and hope the reader can deal with that over SWR.? I think probably he can.
Just for fun I think I will sweep SWR with the calibration plane at the end of the coax and compare.
Albert KK7XO
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Hi Jim,
THANK YOU! I also have a Rig Expert AA-600. If I sweep at the shack, I get a 2:1 bandwidth that is far wider, than if I sweep at the antenna feedpoint.
Why does the nano not show this same effect? What am I missing?
Is it because the calibration the nano provides, removes the effects of the feedline, while the AA-600 does not, because it does not have a calibration option?
73, and thanks,
Dave (NK7Z)
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources
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On 9/19/21 9:38 AM, Jim Lux wrote: On 9/19/21 9:23 AM, Dave (NK7Z) wrote:
If you sweep with your calibration plane at the antenna, and at the VNA, I would like to see those results...
73, and thanks,
Dave (NK7Z)
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources Here's an example of a 6BTV with 100 ft of RG-8X.
Blue trace is cal at the NanoVNA
Orange trace is cal at the end of the Coax.
You can pretty clearly see the loss of the coax increasing with frequency.
Also attached is a plot of the measurement of two 100 ft cables, (new and old) with the cal at the NanoVNA end, and the far end of the coax just open.
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On Sun, Sep 19, 2021 at 09:15 AM, Jim Lux wrote The other thing that can trouble you unexpectedly is that for a lot of
antennas, the coax is part of the antenna system (either by design, or
happenstance).
kk7xo is measuring a J-Pole. Those antennas are famous for radiating on the outside surface of the shield and need good RF chokes. Something to consider when calibrating at the end of the coax (or not). Roger
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On 9/19/21 10:00 AM, Dave (NK7Z) wrote: Hi Jim,
THANK YOU!? I also have a Rig Expert AA-600.? If I sweep at the shack, I get a 2:1 bandwidth that is far wider, than if I sweep at the antenna feedpoint.
Why does the nano not show this same effect?? What am I missing?
Is it because the calibration the nano provides, removes the effects of the feedline, while the AA-600 does not, because it does not have a calibration option? Sure, think about how putting, say, a 3dB pad would change the 2:1 bandwidth - it would make it wider, because it pushes the entire trace down 6dB. You can see that on the plot I just posted - look at the resonance around 20 MHz, the blue trace (with coax) is wider at -14dB than the orange trace.
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Thanks again! I missed that in your post... Thanks again!!
73, and thanks,
Dave (NK7Z)
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources
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On 9/19/21 10:15 AM, Jim Lux wrote: On 9/19/21 10:00 AM, Dave (NK7Z) wrote:
Hi Jim,
THANK YOU!? I also have a Rig Expert AA-600.? If I sweep at the shack, I get a 2:1 bandwidth that is far wider, than if I sweep at the antenna feedpoint.
Why does the nano not show this same effect?? What am I missing?
Is it because the calibration the nano provides, removes the effects of the feedline, while the AA-600 does not, because it does not have a calibration option? Sure, think about how putting, say, a 3dB pad would change the 2:1 bandwidth - it would make it wider, because it pushes the entire trace down 6dB.
You can see that on the plot I just posted - look at the resonance around 20 MHz, the blue trace (with coax) is wider at -14dB than the orange trace.
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I'm using a choke on the antenna feed. It may not be enough inductance, but it's better than nothing. Once I finish my blog post I'll put up a link in this thread. By the way, Roger, I'm using your component fixture to help with the design.
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The definitive article on the Jpole was written by N3GO some years ago. From a J to a Zepp The truth and its consequences Gary E. O'Neil, Raleigh, N.C. (N3GO) 73 -Jim NU0C On Sun, 19 Sep 2021 16:49:46 +0000 (UTC) "KENT BRITAIN" <WA5VJB@...> wrote: Terry Turner, W5ETG, now a silent Key, was the first to develop the twinlead J-Pole back in the 1960's.? ?That puppy has certainly traveled far!? ?The hard part today is finding Twinlead! hihi
On Sunday, September 19, 2021, 11:19:29 AM CDT, kk7xo via groups.io <kk7xo@...> wrote:
Thanks.? The reason I'm doing this is because I made a blog post about how to design a 2-meter twinlead J-Pole using SimSmith and the NanoVNA.? One of my readers asked me to post a final sweep of the SWR.? I have a sweep, but it is at the end of 15 feet of LMR-100 and I know it will look better than it really is, and I didn't want to cheat.
I will post the result in RL, and hope the reader can deal with that over SWR.? I think probably he can.
Just for fun I think I will sweep SWR with the calibration plane at the end of the coax and compare.
Albert KK7XO
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Here is a link to my blog post:
By the way, that J-Pole article Jim posted is great. I'd like to link to it from my article. Is it available online? Like maybe the ARRL site?
Albert KK7XO
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On Sun, Sep 19, 2021 at 11:05 AM, <kk7xo@...> wrote: By the way, that J-Pole article Jim posted is great. I'd like to link to it
from my article. Is it available online? Like maybe the ARRL site?
Albert KK7XO
Here you go >>> Roger
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Roger Need
Jim Shorney