开云体育

-------- Original message --------From: "MARTIN HADFIELD via groups.io" <mdhadfield@...> Date: 11/15/24 4:45 PM (GMT-08:00) 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, CPBEWA/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.ioSent: Friday, November 15, 2024 12:40 PMTo: nanovna-users@...: Re: [nanovna-users] Dipole antenna with Low Z0 feed point ImpedanceIn your measurement of the antenna only at the VNA end of your 6-footjumper, it would be a good idea to attach a couple of 43 material clamp-onferrites around the coax at the feedpoint to isolate you and the VNA/laptopfrom the measurement.? You will be in the near-field of the antenna andyour presence and the coax jumper will influence the measurement unlessisolated.Dave - W?LEVOn Fri, Nov 15, 2024 at 5:31?PM Bill WA2WIO via groups.io <sla@...> 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