The antenna, being in the attic, is already influenced by very close-by
objects. The best you can do is isolate it from the measurement setup -
the NanoVNA, the laptop, and anything the laptop is connected to (charger
and house wiring). The ferrites on the coax will accomplish that.
As far as far field is concerned, the usual criterion is:
Far Field Distance (about) = 2[D^2] / Freespace wavelength
While this is only an approximation, it's pretty good.
D = largest dimension of the test article.
For 40-meters, D would be 280-feet. The attic is clearly in the nearfield,
but so be it. You are a small portion of a wavelength at 7 MHz, so don't
bother isolating yourself. The attic takes care of the "near field" and is
not an option in your installation.
Dave - W?LEV
On Sat, Nov 16, 2024 at 12:15?AM Bill WA2WIO via groups.io <sla=
[email protected]> 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*
--
Dave - W?LEV
--
*Dave - W?LEV*
--
Dave - W?LEV
--
*Dave - W?LEV*
--
Dave - W?LEV
