Can orthogonal dual monopoles reduce signal strength in a cantenna?

[Andrew Thornett]

I recently I tried using two probes same length, same distance from bottom cantenna – it seems to me that this has adversely affected the signal power level detected on both probes – is that a sensible suggestion?

 

Andy

[Marcus D. Leech]

On 2025-11-20 18:09, 'Andrew Thornett' via Society of Amateur Radio Astronomers wrote:

I recently I tried using two probes same length, same distance from bottom cantenna – it seems to me that this has adversely affected the signal power level detected on both probes – is that a sensible suggestion?

 

Andy

Are both of them terminated in a proper receive chain, or do you use them one at a time?   An unterminated port in those situations may cause some
  unwanted reflection issues.

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[andrew....@googlemail.com]

By this question, Marcus, do you mean are both connected to filters and receivers? If so, then yes, both are independently connected to Nooelec SAWBird H1, KMR400 coax, RTL-SDR v3/4, and independent mini-PCs running ezCol from ezRA. Is it still possible one might affect the other? Currently both monopoles are made of straightened paper-clip wire(!)

 

Just struck me – does having two orthogonal monopoles change the resonant length of the individual monopoles? That might explain my apparent decrease in signal intensity.

 

Andy

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[Stephen Arbogast]

In my humble opinion it is  very likely going  to cause trouble to put  another conductor in your feed can.  Maxwell's equations  always win!!  Mother Nature  knows best.

[Alex P]

I'll finish the NEC Model tomorrow .....

Prof P

[Marcus D. Leech]

On 2025-11-20 19:31, 'Stephen Arbogast' via Society of Amateur Radio Astronomers wrote:

In my humble opinion it is  very likely going  to cause trouble to put  another conductor in your feed can.  Maxwell's equations  always win!!  Mother Nature  knows best.

I'm not sure there's anything "mother nature-like" about a circular waveguide microwave feed horn :)

It's instructive to look at Ku-band LNBFs with the "lid" off.  They have a pair of probes, orthogonal, and about 90deg separated in the circular waveguide.  These are then
  combined in-phase to produce circular polarization of the correct chirality.  Similarly for C-band LNBFs.     Plenty of other structures out there that provide for
  ortho-linear polarizations as well.  Like crossed Yagis, turnstile antennas, ortho-fed patch antennas (we have one of those).    There's also septum feeds that have a stepped
  septum down the middle and two probes on the same plane.

Now, do the two probes interact with each other?  Yes.

It will be interesting to see the NEC simulation.   I've used an ortho-feed approach in can feeds in the past with no noticeable issues, although I haven't tried to form
  either CP from those probes, choosing instead to treat them, as Andrew has, as separate receive chains.

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[Stephen Arbogast]

I am just  saying  EE  comes  from  Physics which comes from  Nature.

[Adrian]

Well the differences are that Nature has a paywall but nature is all around us just as it's study could be included in the Physic's Department but the laws of physics rule electromagnetic wave propagation!

[Marcus D. Leech]

On 2025-11-20 22:12, Adrian wrote:

Well the differences are that Nature has a paywall but nature is all around us just as it's study could be included in the Physic's Department but the laws of physics rule electromagnetic wave propagation!

I fully understand.  I was just being funny.  I thought. 

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[Stephen Arbogast]

Andy    this  is   is  cool  stuff.....    https://www.youtube.com/watch?v=6GfIDwwxfsM

[fasleitung3]

We are using a cantenna with two probes at 90° and the same distance as part of our sun-polarimeter, very much like Marcus has described.

We took care that the two probes are exactly at 90° and verified isolation from each other by measureng the cross polar isolation which was > 15 dB.

In our case we keep the two signal chains separate as we process the data later on digitally to calculate the 4 Stokes parameters.

My understanding is that if the cross polar isolation is sufficiently large the two probes will not influence each other.

Wolfgang

[andrew....@googlemail.com]

Is there a simple way I can measure cross polar isolation on my set-up?

Andy

 

From: 'fasleitung3' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: 21 November 2025 08:33
To: sara...@googlegroups.com
Subject: Re: [SARA] Can orthogonal dual monopoles reduce signal strength in a cantenna?

 

We are using a cantenna with two probes at 90° and the same distance as part of our sun-polarimeter, very much like Marcus has described.

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[b alex pettit jr]

And the Answer IS :

The second feed was modeled as a 1/4WL monopole loaded with 50 ohms Z

On Thursday, November 20, 2025 at 07:36:39 PM EST, 'Alex P' via Society of Amateur Radio Astronomers <sara...@googlegroups.com> wrote:

I'll finish the NEC Model tomorrow .....

Prof P

[andrew....@googlemail.com]

Sorry Alex – your graphs look great – but I don’t understand them (I’m not very clever) – does this mean that 2^nd monopole affects 1^st or not?

Thanks for doing all this work.

Andy

 

From: 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: 21 November 2025 13:34
To: 'Alex P' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] Can orthogonal dual monopoles reduce signal strength in a cantenna?

 

And the Answer IS :

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[fasleitung3]

You will need a test signal generator and a highly polarized antenna. We use a commercial log-per antenna for that purpose.

You measure the power with the polarization aligned to one port and then turn the test antenna or the feed horn by 90°. The drop in signal ist the cross polar isolation.

Wolfgang

[b alex pettit jr]

On Friday, November 21, 2025 at 10:26:47 AM EST, andrew.thornett via Society of Amateur Radio Astronomers <sara...@googlegroups.com> wrote:

Sorry Alex – your graphs look great –– does this mean that 2^nd monopole affects 1^st or    not 

[Dave Typinski]

Hi Wolfgang,

Could one obtain a useful measure of cross polarization discrimination by using
a 2-port VNA to perform an S21 sweep between the orthogonal feeds, then
correcting the results to account for S11 at each feed?

Using an antenna test range is the better method, but I wonder what options
exist for situations in which a test range is not practical. For example,
measuring the cross polarization discrimination of a large array of orthogonal
HF band dipole elements.
--
Dave

On 11/21/25 10:45, 'fasleitung3' via Society of Amateur Radio Astronomers wrote:
> You will need a test signal generator and a highly polarized antenna. We use a
> commercial log-per antenna for that purpose.
> You measure the power with the polarization aligned to one port and then turn
> the test antenna or the feed horn by 90°. The drop in signal ist the cross polar
> isolation.
> Wolfgang
>
>
> Am Freitag, den 21.11.2025, 09:32 +0000 schrieb andrew.thornett via Society of
> Amateur Radio Astronomers:
>>
>> Is there a simple way I can measure cross polar isolation on my set-up?
>>
>> Andy
>>

>> *From:*'fasleitung3' via Society of Amateur Radio Astronomers
>> <sara...@googlegroups.com>
>> *Sent:* 21 November 2025 08:33
>> *To:* sara...@googlegroups.com
>> *Subject:* Re: [SARA] Can orthogonal dual monopoles reduce signal strength in

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[andrew....@googlemail.com]

Thanks Wolfgang

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[andrew....@googlemail.com]

OK Thanks Alex

Andy

 

From: 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: 21 November 2025 16:49
To: andrew.thornett via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] Can orthogonal dual monopoles reduce signal strength in a cantenna?

 

 

 

On Friday, November 21, 2025 at 10:26:47 AM EST, andrew.thornett via Society of Amateur Radio Astronomers <sara...@googlegroups.com> wrote:

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[Marcus D. Leech]

On 2025-11-21 17:20, andrew.thornett via Society of Amateur Radio Astronomers wrote:

OK Thanks Alex

Andy

 

IF the probes are perfectly at 90deg, the coupling between them should be quite minimal, but experience would suggest (and I haven't looked at the equations) that
  there's an unpleasant square-law that comes into play as they deviate from 90deg from each other.  

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[b alex pettit jr]

BLUE =  Feed Beam & Gain Pattern at Feed #1  with a single Feed ( Feed #1 )

RED  =   Feed Beam & Gain Pattern at Feed #1  with the addition of Feed#2  

=    The addition of a second Feed perpendicular to the first ( #2 ) does not influence the performance of the original Feed  ( #1 )

Prof P

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