Tunable monopole cantenna probe

[Andrew Thornett]

Here is my attempt at making a tunable cantenna monopole probe......not sure how well it will work!

Andy

[Marcus D. Leech]

On 2026-01-16 12:39, 'Andrew Thornett' via Society of Amateur Radio Astronomers wrote:

Here is my attempt at making a tunable cantenna monopole probe......not sure how well it will work!

Andy

I've done similar in the past.

Keep in mind that tuning a circular waveguide feed involves not just the probe length, but probe position, also.  I've seen designs with a kind of moveable "sled"
  for the probe connector, and a way to tighten it in place once it's tuned.

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[andrew....@googlemail.com]

My solution to that problem is to use one of these attached – this is stainless steel and cost me same price as the aluminium tubing…..

More details of this build on https://www.astronomy.me.uk/designing-lro-aluminium-stove-pipe-cantenna-h-line-16-1-2025

I would like to say a BIG THANK YOU to everyone (Wolfgang and others) who have sent me figures and taken time to publish online their own work, and from which I was able to develop this idea. I am hoping to get rid of that horrible experience which is best explained in the poem below.

Andy

 

Here’s a light-hearted poem that should feel uncomfortably familiar to anyone who’s ever trusted a pair of side-cutters a bit too much:

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The Trimming of the Probe.

The Bard of Lichfield, 16/1/2026.

I made the probe a trifle long,
By theory sound, by practice wrong,
For wisdom says (and books agree)
“You can’t add back what you snip away.”

So, in the cantenna’s hollow tin,
I set the monopole within,
Then fired the NanoVNA,
To watch the dip in S-one-one play.

A snip—just that much—nothing more,
A hair’s-breadth sacrifice I swore.
The trace improved! The curve sank low!
I grinned. The maths said “Told you so.”

Another snip. A gentler bite.
Even better! Pure delight.
The resonance slid into place,
A textbook dip! A thing of grace!

“Just one more trim,” I calmly said,
Hubris blooming in my head.
The cutters closed. A click. A pause.
I checked the screen—then froze.

The dip had gone. The match was wrecked.
The curve now mocked me, unimpressed.
Where once perfection briefly lived,
Now stood a graph that would not forgive.

I stared, aghast, at copper short,
A millimetre too far—of course.
No solder spell, no RF plea,
Could resurrect that length from me.

So there it sits, that wounded probe,
A lesson etched in metal robe:
That NanoVNA tells no lies—
But side-cutters act with finality.

And still I’ll build another one,
Make it long, and start the fun…
For hope springs eternal in the shed,
Right up until the probe is dead.

To view this discussion visit https://groups.google.com/d/msgid/sara-list/d4250a6a-235d-4c1d-ab85-907d858c358c%40gmail.com.

[Stephen Arbogast]

Andrew,

I am  an old  hippie from  the  1960's...   was never good  at poetry..    I  like   Physics....    Some how   learned to  play  guitar....   Here is  a reminder  of a  guy    on  Boulder    Mall   Mark  playing    Ripple    The  Grateful  Dead......     https://www.google.com/search?q=mark+ripple++boulder&client=ubuntu-sn&hs=xCP&sca_esv=4ba3af865432350d&channel=fs&sxsrf=ANbL-n6CrkoG9qmGnmi_w70BaRP5TJAa8A%3A1768630552540&ei=GClraYjdILCcw8cP18OEkQ4&ved=0ahUKEwiIycqe9pGSAxUwzvACHdchIeIQ4dUDCBE&uact=5&oq=mark+ripple++boulder&gs_lp=Egxnd3Mtd2l6LXNlcnAiFG1hcmsgcmlwcGxlICBib3VsZGVyMgUQIRigATIFECEYoAEyBRAhGKABMgUQIRigAUjAOlCdFFjRNHABeAGQAQCYAXegAbAHqgEDMS44uAEDyAEA-AEBmAIKoALbB8ICChAAGLADGNYEGEfCAgYQABgWGB7CAggQABgWGAoYHsICCxAAGIAEGIYDGIoFwgIIEAAYogQYiQXCAgUQABjvBcICCBAAGIAEGKIEmAMAiAYBkAYGkgcDMi44oAftKLIHAzEuOLgH1gfCBwUwLjUuNcgHHYAIAA&sclient=gws-wiz-serp#fpstate=ive&vld=cid:f5c5c0c1,vid:K3pEOHd-28o,st:0

[Alex P]

 From Marcus : "  Keep in mind that tuning a circular waveguide feed involves not just the probe length, but probe position, also. 

 I've seen designs with a kind of moveable "sled"   for the probe connector, and a way to tighten it in place once it's tuned."

============================================================================================

Two Dimensional Adjustments are Required  As Shown Below

( The back <> feed spacing was manually adjusted while monitoring the  VNA before fastening into final position )

Alex Pettit

[Jim Sky]

For what it is worth, I spent some time using the big calibration horn at Greenbank.  It had a number of interchangeable feeds all of which used the same shape probe (these were rectangular transition feeds).  Probes looked like cones with a ball on top, much more massive than you might expect. I assumed it was a design meant to broaden the response.  Not sure how the N socket fed it, but I assume there was space beneath the cone 'skirt'. It just made me wonder if a knob at the end of the probe might be beneficial.  Jim S

.

[ja...@ganssle.com]

I’m working on a feed using 6” duct and a 12-gauge copper wire probe. Cutting 2 mm from the wire dramatically improved its response, as shown in these two VNA plots.

 

Jack

N3ALO

 

From: sara...@googlegroups.com <sara...@googlegroups.com> On Behalf Of Jim Sky
Sent: Saturday, January 17, 2026 11:38 PM
To: Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: [SARA] Re: Tunable monopole cantenna probe

 

For what it is worth, I spent some time using the big calibration horn at Greenbank.  It had a number of interchangeable feeds all of which used the same shape probe (these were rectangular transition feeds).  Probes looked like cones with a ball on top, much more massive than you might expect. I assumed it was a design meant to broaden the response.  Not sure how the N socket fed it, but I assume there was space beneath the cone 'skirt'. It just made me wonder if a knob at the end of the probe might be beneficial.  Jim S

.

 

On Friday, January 16, 2026 at 12:39:10 PM UTC-5 Andrew Thornett wrote:

Here is my attempt at making a tunable cantenna monopole probe......not sure how well it will work!

Andy

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[b alex pettit jr]

Typical calc for a 1/4 WL antenna is  WL / 4 * 0.95 

211 /4 * 0.95 = 50.11mm .. 

The diameter of the wire creates a bit of 'capacitance hat' effect & in my case, using a  4 mm dia brass tube 

changes that to 49mm

FYI, on my folded 20 meter dipole ham antennas, 

I used to tune/trim the lengths to 1cm :  That equals 0.1mm on a 0.211m feed

Alex

===============================================

On Sunday, January 18, 2026 at 05:34:45 AM EST, <ja...@ganssle.com> wrote:

I’m working on a feed using 6” duct and a 12-gauge copper wire probe. Cutting 2 mm from the wire dramatically improved its response, as shown in these two VNA plots.

 

Jack

N3ALO

.