As of 2023 there are many badly performing clones on the market. V2/3GHz NanoVNA uses parts like ADF4350 and AD8342 which are costly and clones have been cutting costs by using salvaged or reject parts.
See official store and look for V2 Plus4/V2 Plus4 Pro versions only to avoid getting a bad clone. We have stopped selling V2.2 versions since October 2020, so all V2 hardware that are not Plus or Plus4 are not made by us and we can not guarantee performance.
I need to measure a 1/2 wavelength at 433MHz of RG-6 coax for impedance matching a yagi. Not the sharpest pencil. I can't seem to find a how to out there but there must be one, I would think.
Any help much appreciated.
Hi Johnathan,
Pretty simple. First, determine what a full wavelength would be at your desired frequency. Divide 300 by the frequency in MHz, in this case 300 ÷ 433 = 0.6928 meters, or 69.28 cm. That would be the full wavelength in free space, so a ½ wavelength at 433 MHz would be 69.28 ÷2 = 34.64 cm.
The trickier part is accounting for the velocity of radio waves in the RG-6 cable, since radio wave propagate slower through the cable. Although RG-6 is nominally 75 ohms, the velocity factor can vary according to manufacturer, primarily due to the dielectric of the spacer between the center conductor and shielding. Some RG-6 cable with polyethylene dielectric has a velocity factor of 0.66, whereas others with foam dielectric has a velocity factor of 0.81. Depending on the manufacturer and age and condition of the cable, it can probably vary anywhere between these two values. I've even seen some RG-6 speced out at 0.83 for the VF.
To determine the length of the matching stub, you need to multiply the length in free air (34.64 cm) by the velocity factor. If you are using cable with PE dielectric and a velocity factor of 0.66, your matching stub would be 34.64 * 0.66 = 22.86 cm, whereas if you had foam dielectric with a VF of 0.81, the stub would need to be 34.64 * 0.81 = 28.06 cm.
Here's where the NanoVNA comes in handy. Many models have an option to measure the length of the cable in question. Usually you input the velocity factor into the NanoVNA and the device will output the approximate length of the cable. That means you have to know the VF. However, the inverse approach would be to use a known length of the cable and input a VF of 1.00 (100%) and let the NanoVNA tell you the length of the cable. Compare the output of the NanoVNA estimated length to the actual length, and determine the actual velocity factor for that piece of cable. That should account for any differences due to manufacturer, dielectric material, age of cable, etc.
Good luck.
Ken -- WBØOCV
________________________________
I take it this is for a balanced T-match or similar Yagi feed. That
isn't hypercritical for length.
Connect a slightly long 1/2 wave to the measurement port with the free
end open and cut flush. Do an S11 sweep at half frequency while looking
at impedance (216.5 MHz). You want the lowest impedance at 216.5 MHz.
Since that is a 1/4 wave open stub at 216.5, it will also be a half wave
at 433 MHz. There is actually less error this way than doing a short or
open line on 433 MHz.
That will be close enough unless you have some long adapters in line.
Are you using F connectors on the 75 ohm line or what?
73 tom
On 3/21/2025 5:52 PM, Jonathan Peakall wrote:
> I need to measure a 1/2 wavelength at 433MHz of RG-6 coax for
> impedance matching a yagi. Not the sharpest pencil. I can't seem to
> find a how to out there but there must be one, I would think.
> Any help much appreciated.
>
--
This email has been checked for viruses by AVG antivirus software.
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Hi Ken,
Thanks for the reply. I have been doing the math as you suggest and
using different VF's but somehow it just isn't working out, hence my
desire to use the nVNA to measure the actual VF or wavelength.
Sadly, it turns out only have one adapter to go from F to SMA to
connect to the nVNA. I could kludge something but then that will surely
alter the results. I'll see about ordering another adapter. Kind of
silly to get an adapter I probably use once but there it is. I suppose I
could also order some RG-6 with a known VF but then that would be even
more. I could also order the correct stub from M2 antennas but what is
the fun in that?
Thanks for the help.
On 3/22/2025 2:59 AM, kjsejkora@comcast.net wrote:
Tom, Yes, it is.Your instructions seem quite clear except i do not know what am S11 sweep is. I will look around and see if I can figure that out.I will not be able to work on it today. We are getting the neighborhood together to trim the trees and brush on our very rural little road. Chainsaw and burn pile for me today. Thanks very much for your help and I will report back my results
-------- Original message --------From: Tom W8JI <w8ji@w8ji.com> Date: 3/22/25 8:59 AM (GMT-08:00) To: NanoVNAV2@groups.io Subject: Re: [nanovnav2] Measure 1/2 wave of 75 ohm coax
I take it this is for a balanced T-match or similar Yagi feed.
That isn't hypercritical for length.
Connect a slightly long 1/2 wave to the measurement port with the
free end open and cut flush. Do an S11 sweep at half frequency
while looking at impedance (216.5 MHz). You want the lowest
impedance at 216.5 MHz. Since that is a 1/4 wave open stub at
216.5, it will also be a half wave at 433 MHz. There is actually
less error this way than doing a short or open line on 433 MHz.
That will be close enough unless you have some long adapters in
line. Are you using F connectors on the 75 ohm line or what?
73 tom
On 3/21/2025 5:52 PM, Jonathan Peakall
wrote:
I need to measure a 1/2 wavelength at 433MHz of RG-6 coax for
impedance matching a yagi. Not the sharpest pencil. I can't seem
to find a how to out there but there must be one, I would think.
Any help much appreciated.
Virus-free.www.avg.com
Either measure the phase shift of a length of cable, or the resonant
frequency of a stub, and trim to length!
https://groups.io/g/nanovna-users/topic/1_2_coax_wavelength/84244505#:~:text=Very%20easy%20%2D%20cal%20the%20vna,a%20multiple%20of%20half%20wavelengths
.
On Sat, 22 Mar 2025, 15:59 Ken Sejkora via groups.io, <kjsejkora=
comcast.net@groups.io> wrote:
Jonathan,
What do you have for adaptors to get the 75-ohm cable to the port on
your VNA? There is one port used for SWR/impedance measurements. You
want to set the VNA to show the impedance and have it calibrated as
close to the 75 ohm F connector as possible. Don't use a long string of
adaptors past the open, short, load calibration point.
A typical 1/2 wave CATV type coax on 433 MHz is 11.6 inches (29.4 cm)
long. You should be in that general area with most foam cables, although
it varies with the exact cable construction. You probably would want to
start at 15 inches and snip back while watching the VNA for the
impedance null of the cable you are trimming to move to 216.5 MHz. This
will let you snip with sharp current to trim the open end line while
watching the VNA and move the line impedance null up to 216.5 MHz.
As far as antennas go, the T match is a great feed method. It is far
better for element current symmetry and balance than something like a
simple gamma. But connection lengths and connectors will always cause
errors in the 1/2 wave cable target no matter how perfect you cut the
line itself. This is because of unavoidable connection dimensions.
Fortunately the error caused by your test adaptors is in the same
direction as connection errors in the real world feed, plus phase error
is not hypercritical anyway in this application, so you will get close
enough unless you use a string of physically long adaptors. A half inch
or inch of cable error is not even going to be noticeable to the antenna
feed on the 70 cm band.
What do you have for adaptors to get from cable F to the VNA's open,
short, load calibrated point?
73 Tom
On 3/22/2025 12:40 PM, Jonathan Peakall wrote:
> Tom,
>
> Yes, it is.
>
> Your instructions seem quite clear except i do not know what am S11
> sweep is. I will look around and see if I can figure that out.
>
> I will not be able to work on it today. We are getting the
> neighborhood together to trim the trees and brush on our very rural
> little road. Chainsaw and burn pile for me today. Thanks very much
> for your help and I will report back my results
>
>
> -------- Original message --------
> From: Tom W8JI <w8ji@w8ji.com>
> Date: 3/22/25 8:59 AM (GMT-08:00)
> To: NanoVNAV2@groups.io
> Subject: Re: [nanovnav2] Measure 1/2 wave of 75 ohm coax
>
> I take it this is for a balanced T-match or similar Yagi feed. That
> isn't hypercritical for length.
>
> Connect a slightly long 1/2 wave to the measurement port with the free
> end open and cut flush. Do an S11 sweep at half frequency while
> looking at impedance (216.5 MHz). You want the lowest impedance at
> 216.5 MHz. Since that is a 1/4 wave open stub at 216.5, it will also
> be a half wave at 433 MHz. There is actually less error this way than
> doing a short or open line on 433 MHz.
>
> That will be close enough unless you have some long adapters in line.
> Are you using F connectors on the 75 ohm line or what?
>
> 73 tom
>
>
> On 3/21/2025 5:52 PM, Jonathan Peakall wrote:
>> I need to measure a 1/2 wavelength at 433MHz of RG-6 coax for
>> impedance matching a yagi. Not the sharpest pencil. I can't seem to
>> find a how to out there but there must be one, I would think.
>> Any help much appreciated.
>
> <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient>
> Virus-free.www.avg.com
> <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient>
>
>
> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
>
--
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The magnitude of s11 can be directly translated to VSWR, dB return loss, or
mag of reflection coefficient. It’s all the same measurement, just a
question of how you want to display it. Full s11 has two parts, magnitude
and phase, or resistance (R) plus reactance (jX). So you would have two
traces on the sweep.
On Sat, Mar 22, 2025 at 10:29 PM Jonathan Peakall via groups.io <jpeakall=
madlabs.info@groups.io> wrote:
As mentioned previously, the easiest and most accurate method is to measure
an open-terminated short at alfthe frequency. Then you don't have to be
concerned about the correct Vp or cable variations. Don't overthink it.
Dave - WØLEV
On Mon, Mar 24, 2025 at 5:59 AM Bob W0EG via groups.io <morrisnc7=
gmail.com@groups.io> wrote:
> The magnitude of s11 can be directly translated to VSWR, dB return loss,
> or mag of reflection coefficient. It’s all the same measurement, just a
> question of how you want to display it. Full s11 has two parts, magnitude
> and phase, or resistance (R) plus reactance (jX). So you would have two
> traces on the sweep.
>
>
> On Sat, Mar 22, 2025 at 10:29 PM Jonathan Peakall via groups.io <jpeakall=
> madlabs.info@groups.io> wrote:
>
>> Tom,
>>
>> Yes, it is.
>>
>> Your instructions seem quite clear except i do not know what am S11 sweep
>> is. I will look around and see if I can figure that out.
>>
>> I will not be able to work on it today. We are getting the neighborhood
>> together to trim the trees and brush on our very rural little road.
>> Chainsaw and burn pile for me today. Thanks very much for your help and I
>> will report back my results
>>
>>
>> -------- Original message --------
>> From: Tom W8JI <w8ji@w8ji.com>
>> Date: 3/22/25 8:59 AM (GMT-08:00)
>> To: NanoVNAV2@groups.io
>> Subject: Re: [nanovnav2] Measure 1/2 wave of 75 ohm coax
>>
>> I take it this is for a balanced T-match or similar Yagi feed. That isn't
>> hypercritical for length.
>>
>> Connect a slightly long 1/2 wave to the measurement port with the free
>> end open and cut flush. Do an S11 sweep at half frequency while looking at
>> impedance (216.5 MHz). You want the lowest impedance at 216.5 MHz. Since
>> that is a 1/4 wave open stub at 216.5, it will also be a half wave at 433
>> MHz. There is actually less error this way than doing a short or open line
>> on 433 MHz.
>>
>> That will be close enough unless you have some long adapters in line. Are
>> you using F connectors on the 75 ohm line or what?
>>
>> 73 tom
>>
>>
>> On 3/21/2025 5:52 PM, Jonathan Peakall wrote:
>>
>> I need to measure a 1/2 wavelength at 433MHz of RG-6 coax for impedance
>> matching a yagi. Not the sharpest pencil. I can't seem to find a how to out
>> there but there must be one, I would think.
>>
>> Any help much appreciated.
>>
>>
>>
>> <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient>
>> Virus-free.www.avg.com
>> <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient>
>> <#m_-3871016103530779751_m_8830373384050118290_DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
>>
>>
>
>
--
*Dave - WØLEV*
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