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ROS Problème on H4


Pascal CASTRATARO 2021/03/16 15:59

Hello, who could help me on a ROS problem with NanoVNA H4.

I have a problem with understanding with Nanovna H4. I will expose it as simply as possible in my tongue and I will translate it, hoping that this is understandable.

When I check the SWR of a 1/4 wave mobile antenna that I made (small brass rod) on a PL259, I still get the same ROS 1.1 centered on the frequency 433 MHz.
On 446 MHz, I have 2 ROS. But that I set "the brass stem" in add or less, this 1/4 wave that I carved for the 446 MHz (300/446 and /4 = 17 centimeters or 0.56 ft)
the minimum is still on The 433 MHz and not 446 MHz, as if it were impossible to move the frequency. Looking at the curve on my Nanovna (I made a picture) but also on a ROSMmeter
the hollow of the ROS (1.1), is always in the same place on 433 MHz. Who could explise me that? Thank you :)

Terry Nixon 2021/03/16 17:21

I hope I am not saying what others have said, but a 1/4 wave antenna at 2 meters (146 MHz)is an effective 3- 1/4 waves on the 440 MHz. band.  That is why you are getting that response.  It is correct and verifying what I am telling you.

Pascal CASTRATARO 2021/03/17 12:26

I do not understand, what I would like is to move the frequency from 440 to
446 Mhz. Wouldn't that be possible with a 1/4 wave?

Le mer. 17 mars 2021 à 04:45, Terry Nixon <TLNixon357@gmail.com> a écrit :

Terry Nixon 2021/03/17 05:38

In order to do that you will have to somehow shorten the antenna slightly.

Ken Sejkora 2021/03/17 09:04

Here’s an online calculator to give an approximate length of a quarter-wave antenna based on frequency:

https://m0ukd.com/calculators/quarter-wave-ground-plane-antenna-calculator/

Perhaps you didn’t incorporate the velocity factor into the equation when you calculated the target antenna length.

If your current resonant frequency is 440 MHz and you want it to be 446 MHz, then your current antenna is 1.35% too long. For example, if your current antenna length is 16.5 cm, then you would need to shorten it by 16.5 cm * 0.135 = 0.22 cm = 2.2 mm.

You might try clipping off 1 mm, and then rechecking the resonant frequency with the NanoVNA. That should move the resonant frequency up to around 443 MHz, but the NanoVNA will be a better indication.

One other thought – are you measuring the resonant frequency with a suitable groundplane under the vertical element? That will also affect the NanoVNA results.

Ken -- WBØOCV


From: Pascal CASTRATARO
Sent: Wednesday, March 17, 2021 07:27 AM
To: NanoVNAV2@groups.io
Subject: Re: [nanovnav2] ROS Problème on H4

I do not understand, what I would like is to move the frequency from 440 to 446 Mhz.  Wouldn't that be possible with a 1/4 wave?

Le mer. 17 mars 2021 à 04:45, Terry Nixon <TLNixon357@gmail.com> a écrit :
I hope I am not saying what others have said, but a 1/4 wave antenna at 2 meters (146 MHz)is an effective 3- 1/4 waves on the 440 MHz. band.  That is why you are getting that response.  It is correct and verifying what I am telling you.

Pascal CASTRATARO 2021/03/17 16:15

As I have explained, whether I shorten or extend my antenna, the middle
point, therefore, the minimum value of TOS 1.1 is kept at 433 MHz. It is
impossible to have the minimum value of 446 MHz as the frequency I am
interested in.

Le mer. 17 mars 2021 à 14:05, Ken Sejkora <kjsejkora@comcast.net> a écrit :

Pascal CASTRATARO 2021/03/17 18:34

Here is the curve in photo that I have with the NanoVNA

[image: P1100036.JPG]

Le mer. 17 mars 2021 à 16:16, Pascal CASTRATARO via groups.io <pcast.fr=
gmail.com@groups.io> a écrit :

Pascal CASTRATARO 2021/03/17 10:39

for Ken Sejkora
14h04 # 1296 ( https://groups.io/g/NanoVNAV2/message/1296 )

I don't do 1/4 wave. This is a car antenna with a magnetic base

David Eckhardt 2021/03/17 19:05

Remember, you don't need to have 1:1 SWR. Anything less than 1.8 or 2:1 is
good to go. Nor will it affect the pattern or antenna performance.

Dave - WØLEV

On Wed, Mar 17, 2021 at 5:39 PM Pascal CASTRATARO <pcast.fr@gmail.com>
wrote:

> for Ken Sejkora
> 14h04 # 1296 <https://groups.io/g/NanoVNAV2/message/1296>
>
> I don't do 1/4 wave. This is a car antenna with a magnetic base
>
>
>

--
*Dave - WØLEV*
*Just Let Darwin Work*

Pascal CASTRATARO 2021/03/17 21:53

Yes, I know, but it's more like a question of understanding. I have other
antennas and it won't cause me any problems.
I can get 1.1 at 433 MHz, I don't understand why I can't get 1.1 at 446 MHz.



Le mer. 17 mars 2021 à 20:18, David Eckhardt <davearea51a@gmail.com> a
écrit :

Jim Lux 2021/03/17 15:03

On 3/17/21 1:53 PM, Pascal CASTRATARO wrote:
>
> Yes, I know, but it's more like a question of understanding. I have
> other antennas and it won't cause me any problems.
> I can get 1.1 at 433 MHz, I don't understand why I can't get 1.1 at
> 446 MHz.
>
Without changing the length?


 433 to 446 is about 3% difference in frequency, so a dipole that's
resonant at one will be about VSWR=1.4 at the other.

(same for a monopole over a ground plane)

Jim Lux 2021/03/17 15:06

On 3/17/21 3:03 PM, Jim Lux wrote:
> On 3/17/21 1:53 PM, Pascal CASTRATARO wrote:
>>
>> Yes, I know, but it's more like a question of understanding. I have
>> other antennas and it won't cause me any problems.
>> I can get 1.1 at 433 MHz, I don't understand why I can't get 1.1 at
>> 446 MHz.
>>
> Without changing the length?
>
>
>  433 to 446 is about 3% difference in frequency, so a dipole that's
> resonant at one will be about VSWR=1.4 at the other.
>
> (same for a monopole over a ground plane)


with a pretty picture

Pascal CASTRATARO 2021/03/17 23:11

no by changing the length.

For the rest, yes I understood. But why modifying the lenght, I can not
have 1.1 in 446 as I have 1.1 in 433?

446 actual ROS = 2

Le mer. 17 mars 2021 à 23:03, Jim Lux <jim@luxfamily.com> a écrit :

Jim Lux 2021/03/17 15:18

On 3/17/21 3:11 PM, Pascal CASTRATARO wrote:
> no by changing the length.
>
> For the rest, yes I understood. But why modifying the lenght, I can
> not have 1.1 in 446 as I have 1.1 in 433?
>
> 446 actual ROS = 2


are there other internal components?  That would be my guess - I've had
antennas that I *thought* were just a whip connected to the coax, but
no, there were LC matching networks in the base, nicely encapsulated. 
The other odd possibility is that there is something that happens to be
resonant near 446 in the vicinity and/or there's something radiating
interference.

Pascal CASTRATARO 2021/03/17 15:40

I'm the one who made this wave antenna.

I would rather look at the second theory but the problem is that it has tried it in several places and it is always the same. There is also another possibility is that the NANOVNA and my ROS Meter is down but I do not believe much.

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