Waveguide WG22 for 24GHz and 47GHz?

Some of you may have seen WG22 around at round tables, rallies, etc, and wondered  if it might be of any use. Well the answer is yes!

24GHz

On 24GHz, WG20 is commonly used. It is available new at moderate but not outrageous  cost - eg about GBP 5 a foot and GBP 9 a flange from Mainline, in the UK.

WG22 has a recommended frequency range of 26.5 to 40.0 GHz, with internal dimensions of 7.112 x 3.556 mm. Cutoff is calculated as 21.08 GHz - when the broad dimension equals half a wavelength. So 24GHz will propogate OK, except that the waveguide wavelength gets a bit long: at 24.192, the waveguide wavelength works out at 25.2mm (compared to about 15mm in WG20).

Closer to cutoff, the loss goes up, so that at cutoff, it is theoretically infinity. In WG20, minimum theoretical loss is about 16dB/100ft at 24GHz. In WG22, being closer to cutoff, the loss is somewhat more - about 26dB/100ft minimum. But provided you are only using short lengths, this is not a problem.

There are lots of components around in WG22, some of which work well at 24GHz  - eg nice calibrated attenuators, as well as the usual bends, twists and flexiguide.  Some things which are less likely to work are couplers and isolators, which will have been designed for frequencies well away from 24GHz.

G4KNZ has successfully built a DB6NT LNA amplifier on WG22 instead of WG20 (the probes into the guide need to be changed in position and length).

One problem found was trying to design a filter using cavities defined by iris plates with round holes. To make a filter with a bandwidth of 100MHz or so, the iris hole sizes have to be bigger than the height of the guide. In WG20, they are well under the guide height. So use rectangular holes or a post type filter instead.

A transition to WG20 can be made from a piece of WG20 with a flange on one end, cut, tapered and filed down to fit into a WG22 flange at the other end. Make the taper reasonably long enough - eg a few wavelengths.

Given that WG22 bits have been bought as cheap as 10 pence an item, this could be a cheaper way of getting going on 24GHz.

47GHz

Normally, WG23 or WG24 would be used. But, for example, in the UK, from Mainline, WG23 costs over GBP 30 a foot - the smaller guides and flanges are a lot more expensive because it is hard to make them with the accurate tolerances specified. Flanges are dear too - about GBP 13 each for WG23! WG23 has internal dimensions of 5.69 x 2.845 mm.

So what about WG22?

The upper recommended limit of WG22 is 40GHz. Above 42GHz, overmoding can occur, and by 47.1GHz, several modes are possible. In practice, G4KNZ has found short lengths working well at 47 to 48GHz, with no noticeable loss due to overmoding. Bends seem fine too.

Components such as attenuators, though, do not now seem to work so well, whereas they did on 24GHz. Perhaps the energy sneaks past in a different mode. Again, things like couplers and isolators are not likely to work, having been designed for much lower frequencies than 47GHz.

One nice feature is that the WG22 square flange mates with a WG23 flange, with the same fixing hole positions. And the step between the two guides does not seem to give excessive loss, though it would be easy to make a taper. With a calibrated attenuator, a source and detector, all in WG23, G4KNZ found it was not possible to measure the loss of a few inches of WG22 placed in the WG23 system  (at 48GHz).

From K&S, come lots of model making brass sections, rods, etc. Included are various sizes which can be used as waveguide. For example, K&S size 264 has internal dimensions 5.55 x 2.38 mm, and external dimensions of 6.35 x 3.18 mm. What's more, it only costs about GBP 2 a foot.

The internal size is fairly close to WG23, and would be a good choice for the 47GHz band.

One idea tried by G8KMH, is to fit this inside WG22. It is a loose fit (WG22 internal is 7.112 x 3.556 mm), but it can be centred with packing, eg a bit of copper foil wrapped round each end. Then it can be soldered in place. The K&S section has thin walls and easily collapses - so be careful not to force it in the WG22. If the piece of WG22 already has flanges, then you end up with a very strong assembly with flanges what will mate with WG23 with now much less of a step (than to WG22), and a guide which will not overmode.

Again, with WG22 bits seen as low as 10 pence a piece in the UK (with flanges), it has to be worth considering!

G4KNZ.