Re: Testing return loss of 24 GHz antennas and transitions

Paul G8AQA

Hi Neil,

I have a quantity of 2mm tuning screws salvaged from 23GHz equipment. They have a locking nut and square section lockwasher. Let me know if you want any.

Paul G8AQA

On 20/03/2021 11:03, Neil Smith G4DBN wrote:
I can certainly measure return loss up to 10 GHz in coax with a reasonable degree of precision, using some good SMA directioinal couplers or waveguide cross-couplers, and on 3.4, my NanoVNAv2plus4 and PocketVNA work very well and seem to agree with each other and with measurements using some HP directional couplers. I've made waveguide couplers up to 47 GHz, it's just that coax at 24 G is stretching me a bit.

It's simple enough to run a simulation in OpenEMS to find out the best balance of position, length, diameter and profile of the probe to get the desired return loss and bandwidth. The bandwidth does seem to come out right in the real thing, although defining the exact end of the PTFE insulation where it is trimmed does lead to a bit of uncertainty about the centre frequency.

I try to avoid tuning screws for aesthetic reasons, but that does lead to a lot of filing of the probes. I might relent, perhaps using sapphire rods mounted on screws, or threaded screws made from Rexolite, so there is no issue with contact corrosion. On some of the aluminium 3.4 GHz Super-VE4MA horns I made with welded backplates, I added a threaded tuning rod also made from aluminium.  That seemed a reasonable solution to prevent corrosion.

Using a fairly fat probe certainly helps to increase the bandwidth of the transition, at the cost of a lower peak return loss, but I guess anything over 17 dB or so is good enough?

Neil G4DBN

On 20/03/2021 04:49, Alan Beard via wrote:
Hi Neil,

This is why I was looking at broadband waveguide to coax transitions
last year as I wanted a "no tune" design, a 23cm feed horn for my 2m dish.
Preferably with enough bandwidth to be useful at 13cm.
Also, I don't care which face the coax connector is on.

I ended up with this DUBUS design, which I built.

Also, for 3.4GHz I have a Cross-bar transition (circ 1950). But nowhere can
I find any design details ie. how broadband is it?


On Fri, 19 Mar 2021 20:09:18 +0000, Neil Smith G4DBN wrote
I'm just embarking on machining some coax-input feedhorns and
waveguide transitions to use with these Wavelab units on 24 GHz, but
I don't have anything I can use to check the return loss of the
transitions or horns. I can make up a 20 dB cross-coupler in
waveguide, but while I can sort-of use that with my spectrum
analyser as detector, assuming my best 50 ohm load looks vaguely
resistive at that frequency, I can't think how to check a coax-fed feedhorn.

Without access to a 24GHz-capable VNA and the right connectors, obviously.

It is just a question of modelling it to get close and then trimming
for maximum smoke on TX and use cold sky versus moon/sun noise to
verify that it is working right?      Is there some magic bit of
testgear I can make or obtain to get the thing tuned spot on and
remove any uncertainty?

I'm thinking of using a 2.92 mm connector with a hermetic seal
feedthru and a solid silver probe, because why not. They seem to be
cheaper than
3.5mm or high-end SMAs. That much Sterling silver is about 10p.

It would be SO much easier with a waveguide launch PA/LNA and w/g
relay, wouldn't it...

Neil G4DBN


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