Re: examples of effect of feedline loss on apparent VSWR
That's pretty much what I am seeing, although my feedline loss is 1.4 dB, giving a total of 2.8 dB up and backj.
|
Re: examples of effect of feedline loss on apparent VSWR
On 9/19/21 11:47 AM, W0LEV wrote: Jim reminds me that loss matching and loss broadbanding are both accepted
approaches in RF design. Of course the antenna is the worst place to
introduce losses in the system, but I've seen it used to broadband several
of the ionosonde LPDAs. In designing discrete "RF boxes", loss matching is
also excellent for stabilization between gain stages. And....these days,
gain is cheap. No kidding, it is the core of the famous terminated folded dipole, especially the ones made of stainless steel cables.? If you're an embassy, or state operator, and you've got that 10 mW in to 10kW out amplifier from Harris, lossy broadbanding is a much better way to deal with things like ALE or frequency hopping than trying to run a tuner and wear out the relays. Hams are somewhat unique in having a "output power" limit (as opposed to a radiated power or EIRP limit), but no requirement on efficiency, and, as well, various rules that tend to reduce the market for inexpensive amplifiers with 40dB of gain in one box.
|
Re: examples of effect of feedline loss on apparent VSWR
Again, thanks for sharing!
73, and thanks,
Dave (NK7Z)
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources
toggle quoted message
Show quoted text
On 9/19/21 11:37 AM, Jim Lux wrote: Attached are some plots for a antenna with nominal impedance 52 ohms at resonance of 10MHz, with zero and 1 dB loss in the feedline.
I chose the 52 ohms so that it wasn't a perfect match to 50 ohms so there's *some* mismatch (-30dB) at resonance
|
Re: examples of effect of feedline loss on apparent VSWR
Jim reminds me that loss matching and loss broadbanding are both accepted
approaches in RF design. Of course the antenna is the worst place to
introduce losses in the system, but I've seen it used to broadband several
of the ionosonde LPDAs. In designing discrete "RF boxes", loss matching is
also excellent for stabilization between gain stages. And....these days,
gain is cheap.
Dave - W?LEV
toggle quoted message
Show quoted text
On Sun, Sep 19, 2021 at 6:37 PM Jim Lux <jim@...> wrote: Attached are some plots for a antenna with nominal impedance 52 ohms at
resonance of 10MHz, with zero and 1 dB loss in the feedline.
I chose the 52 ohms so that it wasn't a perfect match to 50 ohms so
there's *some* mismatch (-30dB) at resonance
--
*Dave - W?LEV*
*Just Let Darwin Work*
|
Re: Sweeping an Antenna from the Shack
On Sun, Sep 19, 2021 at 09:23 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)
I posted both sweeps to my blog. A link to it is in another post I made to this thread. I don't know if it will be above or below this post because I don't know how groups.io orders these replies. :)
|
Re: Sweeping an Antenna from the Shack
I don't think there would be a problem with hosting the PDF somewhere with proper source credit. I grabbed it off the net some years ago. The original web article is no longer up and the Internet Archive Wayback Machine is down at the moment due to internet outages so I don't know if it is archived there. The moral of the story is if you see something useful to you save a copy of it. You can't depend on it being online next month, next year, or 20 years from now. :) 73 -Jim NU0C On Sun, 19 Sep 2021 11:20:24 -0700 "Roger Need via groups.io" <sailtamarack@...> wrote: 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
|
examples of effect of feedline loss on apparent VSWR
Attached are some plots for a antenna with nominal impedance 52 ohms at resonance of 10MHz, with zero and 1 dB loss in the feedline.
I chose the 52 ohms so that it wasn't a perfect match to 50 ohms so there's *some* mismatch (-30dB) at resonance
|
Re: Sweeping an Antenna from the Shack
Great! Thanks, Roger. I added that to my reference section.
de KK7XO
|
Re: Sweeping an Antenna from the Shack
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
|
Re: Sweeping an Antenna from the Shack
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
|
Re: Sweeping an Antenna from the Shack
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
|
Re: Sweeping an Antenna from the Shack
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.
|
Re: Sweeping an Antenna from the Shack
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
toggle quoted message
Show quoted text
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.
|
Re: Sweeping an Antenna from the Shack
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.
|
Re: Sweeping an Antenna from the Shack
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
|
Re: Sweeping an Antenna from the Shack
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
toggle quoted message
Show quoted text
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.
|
Re: Sweeping an Antenna from the Shack
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
toggle quoted message
Show quoted text
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
|
Re: Sweeping an Antenna from the Shack
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.
|
Re: Sweeping an Antenna from the Shack
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
toggle quoted message
Show quoted text
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
|
Re: Sweeping an Antenna from the Shack
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
toggle quoted message
Show quoted text
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
|
Jim Shorney
Roger Need
