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?Hi, Bill - thanks for the welcome note.

Going along with Dave's recommendation of attaching a couple of proper ferrites around the 6-ft coax at the feedpoint, try adding a couple of ferrites at the distant end of your 6-ft coax jumper. The metallic cross-section of VNA won't present very much loading at 6-ft from the DUT input port.
The VNA should be running on internal battery only.
Find a 6-ft USB jumper cable to run between the VNA and your laptop (also running only on internal battery).
Snap on a couple more of the ferrites to each end of the USB cable & you'll probably be at the best achievable distant from the DUT using a 6-ft coax.

Have fun with it - try different lengths everywhere - keep track and share your findings! <smile>

Cheers,
Marty

On Nov 15, 2024, at 5:00?PM, Bill WA2WIO <sla@...> wrote:

?Hello, Marty,



Thank you for joining in!



The question on the table is to formulate a viable VNA measuring strategy given all the previous discussion and specifics.



Obviously, my measurements will have to be made in the near field. I can't see how it is practical for anyone to make any measurements in the true far field.



· What is common practical and reasonable ham level practice to get useable measurements with the 1/2 wave dipole antenna as the only DUT?

o Would a few clamp-on ferrites correct for this?

· Perhaps I should make all these measurements from the rig side?

o Calibrating from the coax feed line connection point at the antenna in the attic?

o Leaving only the antenna as the DUT?

o Moving 30-40 ft away from the antenna has to be better, but is it worth the effort?



Thoughts?



Thank you,



Best,



Bill WA2WIO



-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of MARTIN HADFIELD via groups.io
Sent: Friday, November 15, 2024 7:45 PM
To: [email protected]
Subject: Re: [nanovna-users] Dipole antenna with Low Z0 feed point Impedance



Barging in here for a moment...



In general antenna theory, the "far field" starts at 10x lambda. That is where the E and H fields have become coherent.



Regards,

Marty Hadfield, CPBE

WA/OR/ID FCC ABIP Inspector




On Nov 15, 2024, at 4:15?PM, Bill WA2WIO <sla@...> wrote:



?Dave - W?LEV;



I hadn't thought of near field issues. I have had instances of interaction between my body and other objects with VNA measures before.



As I understand it, at 40m HF, using a 1/2 wave dipole antenna (66ft), far field starts at about 1/2 a wavelength from the antenna. Recommendations go so far as to recommend 2-3 wavelengths away to eliminate unwanted influences. This is not achievable as the rig is only about 30-40ft from the antenna.



Would a few clamp-on ferrites correct for this? Perhaps I should make all these measurements from the rig side? Calibrating from the coax feed line connection point at the antenna in the attic? Leaving only the antenna as the DUT? A much greater pain in the butt, but doable. Moving 30-40 ft away from the antenna has to be better, but is it worth the effort?



Thank you,



Best,



Bill WA2WIO



-----Original Message-----

From: [email protected] [mailto:[email protected]] On Behalf Of W0LEV via groups.io

Sent: Friday, November 15, 2024 12:40 PM

To: [email protected]

Subject: Re: [nanovna-users] Dipole antenna with Low Z0 feed point Impedance



In your measurement of the antenna only at the VNA end of your 6-foot

jumper, it would be a good idea to attach a couple of 43 material clamp-on

ferrites around the coax at the feedpoint to isolate you and the VNA/laptop

from the measurement. You will be in the near-field of the antenna and

your presence and the coax jumper will influence the measurement unless

isolated.



Dave - W?LEV



On Fri, Nov 15, 2024 at 5:31?PM Bill WA2WIO via groups.io <sla=

[email protected]> wrote:

Dave - W0LEV;

Thank you for the insight. I will place the CMC choke as you suggest.

I intend to calibrate in the attic using only a 6ft coax jumper to the

antenna feed point. I will calibrate out the jumper, etc leaving only the

antenna as the DUT.

Once the antenna is trimmed, I was going to retest with the 1:2 BalUn in

place.

Based on your suggestion, I would then add the CMC coax choke and test

again.

I would finally recalibrate at the rig and retest the whole system.

Does this all sound reasonable? Suggestions on process and procedure? What

am I missing?

Thank you,

Best,

Bill WA2WIO

-----Original Message-----

From: [email protected] [mailto:[email protected]] On Behalf

Of W0LEV via groups.io

Sent: Friday, November 15, 2024 12:08 PM

To: [email protected]

Subject: Re: [nanovna-users] Dipole antenna with Low Z0 feed point

Impedance

Your coaxial CMC should be installed just after the matching transformer

between that and the coax. This is to isolate the coax from being part of

the radiating structure. If you use the outer surface of the coax as a

radiating element, yes, it should go 1/4-wavelength down the coax from the

matching transformer.

Dave - W?LEV

On Fri, Nov 15, 2024 at 1:57?AM Bill WA2WIO via groups.io <sla=

[email protected]> wrote:

Thank you all for your considered and detailed suggestions and teaching

points.

To summarize what I believe I have learned:

· I am running QRP <> 3 watts (uSDX+ HF), does not leave much

power to loose.

· Serious attic structural limitations do not allow for changing

antenna leg angles or height.

· I am operating on 2 bands: 40m and 20m. I operate in both the

CW

and SSB portions of the bands.

· SWR without the ATU-10 tuner is =>6:1 at the rig, across all

bands (MFJ-872)

o The ATU-10 will bring it down to <2:1.

o This will keep the rig happy, but actually waste >> significant

power.

· VNA at the rig end w/o ATU shows no resonance anywhere except

at

17mhz. (sweep 1-30Mhz).

· Antenna is certainly not actually balanced.

o Antenna needs trimming, regardless.

o My first step will be to go up into the attic, disconnect the feed

line, and use my VNA to confirm just what the Z0 actually is.

o I also need to verify the feed line from rig to antenna is viable.

o Trim the antenna until resonant at 7.15Mhz (narrow sweep 7.0-7.4 MHz)

o Assuming I am correct that Z0 is around 25 Ohms and complex, use a

1:2

BalUn to get closer to 50 Ohms.

o Check 20m resonance (narrow sweep 14.0-14.5 MHz)

· If I get this far, I will add a CMC Coax Choke, 15T 4” on pvc

form about 33ft (1/4 wave) from the antenna feed point.

· The goal is to get <> 2:1 across a reasonable portion of both

bands without the ATU-10.

· Add in the ATU-10 ?

Does this strategy make sense? Am I foolish to waste my time tuning and

matching, as you say? I have made several contacts, but my RS(T) is way

down in the mud. Noise level is >>S4-5 (a story for another day?). I do

not

see any alternative.

Thoughts?

Thank You,

Best,

Bill WA2WIO

-----Original Message-----

From: [email protected] [mailto:[email protected]] On Behalf

Of Joe

Sent: Thursday, November 14, 2024 6:03 PM

To: [email protected]

Subject: Re: [nanovna-users] Dipole antenna with Low Z0 feed point

Impedance

WHAT?? All that power being lost????

Seriously, a 2:1 SWR will only loose 11.1% of the power. So a 100 watt

station will give you 88.9 watts of ERP (Less cable loss and plus

antenna gain).

Really not worth loosing time on the air.

73, Joe, K1ike

On 11/14/2024 4:23 PM, W0LEV via groups.io wrote:

My.....my........ For heaven's sake......! What concerns about a

meager

2:1 SWR!! Just go with it, get on the air, and enjoy.

Dave - W?LEV

On Thu, Nov 14, 2024 at 7:15?PM DP via groups.io <dpoinsett=

[email protected]> wrote:

Hi Barry K3EUI

You and others make an excellent point about the practical

acceptability

of of a 2:1 SWR. I agree that for the casual ham using commonly

available

equipment, it's probably not a big deal especially on the lower HF

frequencies.

You are correct that by adding an inductor across the antenna

terminals

and altering the antenna dimensions, the feedpoint impedance can be

transformed to 50+j0 ohms, but the cut-and-try approach will be no

easier

than any of the other suggested methods. Also, just adding an inductor

will

not automatically increase the impedance. Depending in the initial

antenna

impedance, it may go up or down.

If we stay with Bill's 25-ohm example, the antenna length and angle

between each leg could be adjusted to get a 25-j25 feedpoint impedance

at

say 7.15 MHz. Adding a 1.1 uH inductor across the antenna terminals

will

then get you to 50+j0 ohms. Other combinations could work, too. Of

course

whatever combination he comes up with will only work at the design

frequency.

Dave NU8A

--

*Dave - W?LEV*

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Dave - W?LEV

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*Dave - W?LEV*





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Dave - W?LEV