3cm Waveguide double-slot antenna

Omnidirectional, horizontally polarised waveguide antenna for the 3cm band that can be used for ATV or beacon operation. This is not the beginners project as it requires some special machining tools, or CNC equipment . On the other side the software and calculations can be useful for the lower band antennas (23cm or 13cm) where hand-tool can be used to fabricate waveguide antenna.

All calculations are done using the HLSSA software running properly also in the DOS window. Using the  WR-75 size waveguide, 10 slot per side with the central frequency of 10.450 MHz, gain of 12 dBi was calculated.

Actual antenna dimensions are then modified to meet the calculated ones. The calculated distance Slot-Short (Lwg/4) was 11.29 mm, but I prefer to use 3 x Lwg/4 and adding the 2.3 mm length for the short-end cap give us a total length of 36.17 mm. If the short-end tuning is required, it is easy to insert the sliding short-end peace when you are using 3 x Lwg/4 length. With only Lwg/4 length, there is not so much room for tuning the antenna for the best RL.

For easy tuning and troubleshooting there is no coaxial to waveguide transition on the antenna. Instead, the WR-75 size flange was attached. To test or measure and tune the RL/SWR of the antenna the commercial or previously checked and measured coax to waveguide transition should be used not to affect the RL of  the antenna itself.

If you are planing to use the antenna indoor or on the location protected from the rain fall or snow than no radom is required. If you are planing to use the antenna positioned outside, on the mast, than good microwave radom should be used to protect the antenna from the watter but also from the bugs that just like to settle inside the waveguide. The radom should be at least 6cm in diameter not to degrade the basic antenna characteristics. Do not use the heat shrink tubing or similar ideas other then microwave radom. Anything in the near field region of the antenna can spoil the gain, diagram and overall characteristics. If you are constrained to use this kind of antenna protection, than insure that the same is inserted in the HLSSA program during the antenna designing.

The last attached photo can give you some idea how to install the antenna without radom. When installing the antenna, pay attention to position the antenna vertically. 3dB vertical beamwidth is quite narrow !! More slots will give you more gain but the vertical beamwidth will be narrower. If your antenna is positioned very high , on the hill comparing to the close stations, the close stations may suffer the narrow bandwith problem. If the antenna is slanted, this problem will affect some users even more.
Another fact is that the omnidirectional diagram is not the perfect, to be honest, far away from the perfect becoming more like cross diagram with the regions with the difference of 3dB comparing to the maximum antenna gain. So if you want to cover your preferred areas, turning the antenna can give you sometimes even 3 dB more gain. This problem can be solved by using the "wings" attached to the antenna, already published on the web.

3cm Waveguide to SMA transition

Not so easy to find now days and if new, sometimes quite expensive, the waveguide transition became the target of many home-brewers. So many different dimensions and designs are published with the same idea of making a good waveguide to coax transition.
If you have the possibility to measure the RL and SWR than almost any design can be tweaked for the best results, but if you can't measure it than you have to rely on the data provided by the author or constructor. The transition that I built, based on the Paul W1GHZ article was easy to copy, without any fancy tool and tuning equipment. I prefer to use his design because there are only two measures that you have to take care of while all others dimensions are defined by the standard waveguide size.

To build this transition we need a peace of WR-75 or WR-90 waveguide with the flange, SMA female connector, and a peace of the brass or copper sheet. Cut the peace of the required waveguide length, but not shorter than 1 lambda. Drill the hole for the SMA connector from the short end of the waveguide (distance C). Distance is measured from the end of the waveguide to the center of the pin hole. Hole should be 4,2 mm in diameter if you are using standard SMA connector with the teflon insulation. Find the female SMA connector with the longer pin/teflon insulation and put the same in the prepared hole on the waveguide. From the closer waveguide end cut the excess teflon inside the waveguide so only center pin will remain like on the picture below. The diameter of the center pin is 1,27 mm and the only thing you need to do is to cut the pin exactly according to the length D. This is the pin length in the waveguide!! Finally, cut the peace of the brass or copper sheet to form short end cover for the waveguide (AxB dimensions).

W/guide       A        B       C      D

WR-75     19,05  9,525  5,26  5,49

WR-90     22,86  10,16  5,46  5,89

(all measures in mm)

Now when we have all parts ready, it's time to put all this together. Of course, we will need to solder the short end peace to the waveguide and then the SMA connector in the previously prepared hole. This can be done by using the high power soldering iron or gas torch with not so tidy result. I prefer to use the iron, the same one I am using for tone transfer PCB. With the highest temperature set after 5 to 6 minutes the parts will be hot enough to work with them. Using the pliers, rotate the waveguide vertically with the flange sitting on the flatiron and with the other set of pliers position the short end on the waveguide end. Use the standard soldering iron to solder the short end. While the waveguide is still hot, position the SMA connector on the place and use the same technique to solder the SMA.

After the job is done, leave the waveguide to cool down for a few minutes. I make a several transitions using the same technique and the results are quite good. The RL obtained is -20dB (SWR 1:1.2) which is not bad for no-tune version. You may get lucky to get better results but if you are looking for lower SWR than you have to tune the transition. This can be done using different techniques: by changing the short end distance or even by adding some tuning screws. AD6IW give me a hint how to do this easy way. Making the hole for the SMA bigger (lonitudinally) we are able to change the distance from the pin to the short end by moving the SMA along the waveguide. Find the position where the SWR is lower and then solder the SMA. This will be good enough for all amateur applications.

After all, one hour job can give you a nice waveguide transition. To bulid or to buy, it's up to you...