Ku-band LNB with external reference: good for interferometry ?

[Kimmo]

I am planning to set up a two-element correlating radio interferometer.

The goal is to detect, and also to resolve the Sun.

The receiver will be RSPduo, which is a two-channel phase coherent receiver.

I have two LNAs and long Yagis for the amateur radio 23cm band.

At that wavelength, the distance between the antennas would have to be about 32 meters, to have a resolution of about 0.5 degrees, the diameter of the Sun.

Not very practical. 

However, at a frequency of about 12 GHz, at the Ku band, the corresponding distance would be only about 3.5 m. 

There are external reference Ku-band LNBs (low-noise block downconverter), which receive a separate, external 10 MHz signal to lock the phase-locked loop (PLL) oscillator.

The idea is of course to feed the same 10 MHz signal to both LNBs, so that they are phase coherent. The IF signals of the LNBs are then measured with the RSPduo.

Would this kind of setup work ?  Mainly, would the LNBs be phase coherent at a level which is sufficient for interferometry ?

If it works, can you recommend any 10 MHz signal source ?

Those LNBs are quite expensive, but there are used equipment on the market.

Cheers, Kimmo

[Marcus D. Leech]

On 2025-12-06 12:10, Kimmo wrote:

I am planning to set up a two-element correlating radio interferometer.

The goal is to detect, and also to resolve the Sun.

The receiver will be RSPduo, which is a two-channel phase coherent receiver.

I have two LNAs and long Yagis for the amateur radio 23cm band.

At that wavelength, the distance between the antennas would have to be about 32 meters, to have a resolution of about 0.5 degrees, the diameter of the Sun.

Not very practical. 

However, at a frequency of about 12 GHz, at the Ku band, the corresponding distance would be only about 3.5 m. 

There are external reference Ku-band LNBs (low-noise block downconverter), which receive a separate, external 10 MHz signal to lock the phase-locked loop (PLL) oscillator.

The idea is of course to feed the same 10 MHz signal to both LNBs, so that they are phase coherent. The IF signals of the LNBs are then measured with the RSPduo.

Would this kind of setup work ?  Mainly, would the LNBs be phase coherent at a level which is sufficient for interferometry ?

Yes, my friend uses 21GHz LNBs with external reference for a specialized interferometer for the Moon.

If it works, can you recommend any 10 MHz signal source ?

If you search on "10MHz OCXO" on Amazon, you'll find a number of sellers that sell modestly-priced 10MHz OCXO clock modules.

Those LNBs are quite expensive, but there are used equipment on the market.

Cheers, Kimmo

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[Eduard Mol]

Marcus, 

Sorry for going a bit off topic but I am curious what 21GHz LNB unit your friend is using?

Op za 6 dec 2025 om 18:14 schreef Marcus D. Leech <patchv...@gmail.com>

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[Marcus D. Leech]

On 2025-12-06 15:34, Eduard Mol wrote:

Marcus, 

Sorry for going a bit off topic but I am curious what 21GHz LNB unit your friend is using?

A couple of semi-custom units made by Orbital Research.  A friend of mine, Ian McEachern, actually bought Orbital Research a few years back
  after selling his other company, so, I had an "inside track".

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[Eduard Mol]

Ah okay. Not something the average amateur could easily obtain… 

Op za 6 dec 2025 om 21:42 schreef Marcus D. Leech <patchv...@gmail.com>

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[batem...@gmail.com]

Hi Marcus and others,

I've had some experience with Ku band LNBs with a 10 MHz reference, the issue we had with an interoferometer was the phase drift between the 2 units with temperature. Any comments on this issue ? 

Best regards, Tim 

[Marcus D. Leech]

On 2025-12-11 20:28, batem...@gmail.com wrote:

Hi Marcus and others,

I've had some experience with Ku band LNBs with a 10 MHz reference, the issue we had with an interoferometer was the phase drift between the 2 units with temperature. Any comments on this issue ?

Without having a complete idea of the hardware involved, and the setup, it's hard to say.   Things like differential heating of the coax cables at the IF would be an issue, and it depends
  on how the 10MHz reference is used, and the PLL architecture, and other things.

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[fasleitung3]

We operated a Ku-band interferometer with two 1.2-m dishes placed on a 10-m baseline in east-west direction. It is dismantled at the moment as we are working to refurbish and relocate it.

It uses two PLL-type LNBs which were modfied to accept an external reference as descibed by a french amateur here: http://f1chf.free.fr/LNBPLL/inside.pdf

In order to minimize phase drift both the cables suppying the reference and the IF output were of the same length and exposed to the same environmetal conditions. Temperature effects therefore partially compensate.

We encountered temperature effects again with our L-band interferometer. While the cables were of equal length, carefully phase matched, not the same length was exposed to sunlight and we saw quite some drift. We corrected that later on.

Best regards,

Wolfgang

[Peter East]

A big problem in length/phase/temperature matching conventional cable lengths in the field, is the sudden phase transition that occurs in the usual PTFE dielectric between 10 and 20 degrees Centigrade. It annoyingly occurs around room temperature and only small temperature differences in this region can produce unacceptable phase differences. You can purchase special cables that use more stable dielectrics that minimise this effect but certainly at much higher cost. There is still an expected temperature/length change  of course but this is easier to compensate.

[Martin Flynn]

Just starting down this road - considering heliax, which is LDPE

Is that stable enough, and if not, what should I be considering?

Cheers!

Martin

Sent from my iPhone

On Dec 12, 2025, at 8:23 AM, Peter East <pwe....@gmail.com> wrote:

A big problem in length/phase/temperature matching conventional cable lengths in the field, is the sudden phase transition that occurs in the usual PTFE dielectric between 10 and 20 degrees Centigrade. It annoyingly occurs around room temperature and only small temperature differences in this region can produce unacceptable phase differences. You can purchase special cables that use more stable dielectrics that minimise this effect but certainly at much higher cost. There is still an expected temperature/length change  of course but this is easier to compensate.

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[Peter East]

Heliax aims to get over the phase transition effect and to some extent balance the dielectric and copper temperature changes, but for interferometer work,  pairs of cables are best kept close in temperature.

Before choosing cable it is  advisable to check the cable length/temperature specification even for heliax.

[Dave Typinski]

This is new to me. Do all common coax dielectrics do this? How much of an
effect is it? I've not seen this relationship specified on coax spec sheets,
but perhaps I haven't been looking closely enough.

Operating in the HF band, I'd guess it's not a concern, but it would be nice to
know more about this and actually quantify the temperature to phase relation
instead of guessing. Any pointers to reliable reference material that discusses
this?

Thanks!
--
Dave

[fasleitung3]

This is also new to me.
I found this comprehensive paper about it:
https://www.elspecgroup.de/fileadmin/whitepaper/WP1_PTFECoaxialCables_EN.pdf

In particular page 14 of the part 1.3 ist of interest.
Best regards,
Wolfgang

> --

[Marcus D. Leech]

On 2025-12-12 12:43, 'fasleitung3' via Society of Amateur Radio

Astronomers wrote:
> This is also new to me.
> I found this comprehensive paper about it:
> https://www.elspecgroup.de/fileadmin/whitepaper/WP1_PTFECoaxialCables_EN.pdf
>
> In particular page 14 of the part 1.3 ist of interest.
> Best regards,
> Wolfgang
>

I did not know this, either (the step-function in thermal properties).

I'll note that the (relatively) cheap RG6 cables we use are all foamed
HDPE, rather than PTFE.

[fasleitung3]

I just checked on the cable types commonly we use: Aircell7 , Ecoflex
10, RG213 and RG214. All are (foamed) PE as well.
So the PTFE issue may not affect us, but PE may have some other
characteristics which may have other issues as far as phase stability
is concerned.
There is no day without learning something new!
Wolfgang

[Marcus D. Leech]

On 2025-12-12 15:12, 'fasleitung3' via Society of Amateur Radio

Astronomers wrote:
> I just checked on the cable types commonly we use: Aircell7 , Ecoflex
> 10, RG213 and RG214. All are (foamed) PE as well.
> So the PTFE issue may not affect us, but PE may have some other
> characteristics which may have other issues as far as phase stability
> is concerned.
> There is no day without learning something new!
> Wolfgang

All cables will have some thermal issues, which means phase drift. The
"exciting" thing about this PTFE
  foam dielectric is that it has a considerable "step" right around
room temperature, which is unpleasant.

Tom Landecker at DRAO in Penticton, years and years ago, developed an
active phase-measurement system to correct temperature induced
  phase effects on the cable plant for their 7-element synthesis
array.    The VLA have been using buried fiber-optic cables for decades,
but it
  wouldn't surprise me to find that the still do active cable-plant
phase compensation.

If you stop learning things....check for a pulse.

Marcus

[Peter East]

I am afraid phase tracking of RF cables is a bit of a minefield.

Cable suppliers have gone out of their way to supply expensive cables that attempt to temperature track, ensuring a low, closely linear temperature coefficient; they supply comprehensive specifications. Professionals usually go for these but still maintain the environment temperature.

Cable dielectric temperature coefficient generally overcompensates the copper temperature/length coefficient. Normal LDPTFE tracks pretty well either side of the 20 deg transition.

PE dielectric is fairly linear but massively overcompensates the copper coefficient so that small temperature differences can still cause large phase tracking errors.

There are various heliax versions that offer improved performance, but check the temperature specifications.

So for amateurs with a limited wallet, I would stick with identical low density PTFE cables and either ovening them, burying them deeply, working on sub-zero or better, +30C days as possible workarounds. Dynamic calibration is always worthwhile.

Don't know about fibre-optic cables

[duncan campbell-wilson]

I would bury the  cables in the ground  a meter down inside a conduit  where the diurnal delta T is small on a daily basis and drifts with the seasons.

The residual exposed cable  is the above  ground where  one might heavily lag the cables as per pipe lagging and enclose the lagging in PVC pipe for protection. Never tried the latter.

 A thought. Rgds Duncan

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