ASTROPLANE best vertical antenna ever?

[bob85]

Build one with a 3/4wave mast, remove the hatted 1/4wave and turn it upside down Homer, let us know how it performs

Half of the problem with visualising currents with these type of antennas is how NEC & other mom software displays the currents as one current distribution per conductor,

you don't see the transmission-line mode currents and antenna mode currents that Barkley & the arrl article i posted years ago talks about,

you don't see antenna mode impedance and transmission-line mode impedance in parallel at the feed-point, just the result of the two impedances in parallel,

you don't see the radiating current on the short leg of a j-pole or similar antenna
but it is there & you see the result in the pattern as more gain in the direction of the short leg the wider / more you unbalance the stub,

how the currents are displayed does not help visualize what's going on with these antennas.

 

[bob85]

DB
I have never seen the same distribution on any sigma style ssfm j-pole,

The electrical shortening caused by bringing the 1/4wave elements close to the monopole & the resulting drop in feedpoint impedance that smart folk tell us is due to transmission-line mode currents flowing in parallel with the antenna mode impedance of the monopole,
does not make the monopole anywhere near 1/4wave shorter electrically,
according to what i have seen does not change the current distribution significantly like odd current distribution we see in the astroplane mast.

 

[The DB]

Half of the problem with visualising currents with these type of antennas is how NEC & other mom software displays the currents as one current distribution per conductor,

I agree with this. When I first saw the "blue wire" on AP models, at first I thought that there was no current flow on said wire, but it turns out that their was in fact current flow, just the same amount in both directions.


The DB

 

[HomerBB]

Who's on first?

 

[freecell]

 

[nfsus]

Hows this look? I just got the nanovna in the mail. If I have it set up right, this is the reading on my home made astroplane

 

[Handy Andy]

Homemade - eh?

​

Kudos!

 

[nfsus]

Yeah homemade. Why?

 

[nfsus]

I've got a 2.5meg bandwidth of 1.7 or lower set. That's cool and all but I'm wondering about the reactance now. I've not used it enough to interpret my info. I know I need to adjust the feed point to change the ohms to 50

 

[freecell]

the length of the vertical element above the feedpoint controls the reactance and the position of the horizontal (fiberglas) insulator between the two curving elements connecting to the loop below controls the impedance match. sliding the insulator up or down will change the match. the reactance @ -6.65 ohms tells us that the vertical element is (capacitively reactive) a bit too short for resonance (X=0) at the selected frequency. slightly lengthening it will cancel the capacitive reactance indicated by novavna.

remember, you're shooting for R=50 / jX=0 at your preferred target frequency. 27.185 mhz. is the center of the band. then take another look at your swr / bandwidth curve. i would be interested to know what the band edges are at 1.5:1 when you're done.

 

[nfsus]

I put it up before I had the vna so I'll take it down maybe tomorrow evening and start adjustments. I'll let you know and screenshot stuff

 

[Marconi]

In response to DB's post in the link below, I plotted one of my AstroPlane models and made 6 iterations of 12" inches each to the Antenna Mast length starting at 162" inches. This is 15" inches less that a typical 1/2 wave Antenna Mast at 27 mhz might be...in this case with the AstroPlane.

ASTROPLANE best vertical antenna ever?

This chart is the 1st page in the PDF file below.

Next is the model for the Astro Plane with overlays.

Next is the model for the A99 I did for comparisons purposes.

 

[freecell]

"162" inches......This is 15" inches less that a typical 1/2 wave Antenna Mast at 27 mhz might be."

a half wavelength @ 27.185 mhz. is 18 feet, slightly shorter for conductors with a greater diameter to length ratio. @ 13.5 feet or 162 inches the conductor is resonant as a 1/2 wave somewhere between 36 & 37 mhz..

162 inches or 13.5 feet is 54 inches or 4.5 feet shorter than a 1/2 wavelength @ 27.185 mhz..

 

[Marconi]

Thanks FreeCell for clearing this up.

 

[freecell]

you're welcome! thank you for posting the previous pdf..