A $300 2m repeater duplexer and cavity resonators-hardware shop special

A $300 2m repeater duplexer and cavity resonators-hardware shop special

Introduction

I have been experimenting with cavity resonators for a 2m repeater for some months now. I had been using aluminium tube and brass fittings. However, with 150mm tube, the optimal antenna should be about 50 mm diameter, with a 3:1 ratio of diameters for impedance and performance. However, the mechanical arrangements and conduction between them became problematic. If I wrapped all the joins with aluminium foil, it worked well, but is not practical.

In this post I will outline a new technique that uses shim copper (0.1mm) for all the RF parts. The aluminium tube is lined with copper and soldered to copper shim top and bottom plates, in turn attached to the main aluminium plates. The antenna goes back to a conventional fixed lower section, with a movable section to adjust tuning.

The basic premise of the design is still that it can be made from readily available parts at low cost, and with basic workshop facilities. As such, a six cavity 2m duplexer can be made for about $300 over a weekend or two.

The basic design

The basic design of each half the duplexer has not changed much from my earlier attempts, shown below.

The aluminium tube is 150mm diameter and 600mm long. The top and bottom plates are 150 x 6mm, 480mm long. Threaded rod is used to hold the assembly together. Plumber 25mm threaded pipe is used to adjust length. In this prototype, only one cavity is working with a single input to the probe. In this prototype, the threaded rod moved the whole antenna to adjust length. That arrangement became problematic when the diameter of the antenna was increased to 50mm, the optimum for this size cavity.

The main cost is the aluminium tube at about $150 for 6m, enough to make ten cavities.
Aluminium is used as it is much cheaper than copper, as well as lighter. Further, it is difficult to get large diameter copper tube, over $1000 for a 6m length.

I was looking for alternatives and was browsing a non-ferrous metal supplies website, particularly sheet cooper; http://www.georgeweston.com.au/index.php/copper/copper-sheets. I saw that 0.55mm copper sheet was available and seemed quite thin, thus bendable ($136 for an 1800x900 sheet, $17/kg, not bad as I had been paying $7/kg for scrap copper)). I bought a sheet, but noticed they had 0.1mm, 600mm wide in a roll on the shop desk. I bought one metre thinking I might use it for something. At home, I noticed the price, $35 per metre or about $75/kg! However, I have found a cheaper supply in Melbourne: http://www.georgewhite.com.au/products/product_listing.asp?categorycode=CS-SH, with the shim price about $25/kg.

I have built one cavity with copper shim lining the outer aluminium pipe. While this is possible, it is difficult for one person to do. To make it a little easier, I ran some thin lines of non-corrosive silicone to hold things in place while I solder the join. The ends have turned-over copper shim to give a copper end and to solder to the copper top and bottom plates. A special-purpose soldering iron bit would help with the inside seam. I am not sure copper-lining the cavity wall is worthwhile compared to aluminium. I am currently experimenting with this.

The copper shim on the the antenna/probe was more success. I used PVC plumbing parts and tube, with the brass screw, to construct the basic antenna. The two parts are show below.

The assembled antenna/probe soldered to base plate of 0.5 mm copper. Contact between the fixed and movable section, just cuts in fixed section, over-laying each other, with zip tie holding them tight and two fingers turned back to hold zip tie,

The copper-lined aluminium tube is in the background.

 The Q of each component is important, however size matters! The smallest bits may warrant silver-plating, whereas there is little benefit in plating the larger pieces.

With that in mind, I thought I would try silver-plating the probes, as they are tiny, by area compared to the antenna, tube or top and bottom plates. I could get them silver-plated, but that would take time and money. With a bit of lateral thinking, a second-hand silver tray seemed a good source. I had a quick look on eBay and bought a reasonably sized silver-plated tray for $6 plus $10 post. The tray arrived a couple of days later but was a bit heavier/thicker than I envisaged, making cutting and  working harder. However I fashioned a probe, on the right in the photo, as best I could to match the other one, made from copper sheet. The probes are about 5 mm from the antenna per the QST/ARRL design.

The assembled filter and the impressive SWR, much better! I fixed a few things, so all the improvement may not be from the one silver-plated probe.

For ball-park, I ran the analyse on a old commecial 2m cavity, 100 mm diameter, and I am not too far out; most pronising.

In my tests with two probes, I have had to put a 50 Ohm terminator on the second probe. Without it, just a dimple. When I started, I only had one probe and was able to get usable SWR. Not sure what is happening, other than the probes work well!

Discussion

Thermal stability

While I have not mentioned it so far, I recognize issues such as temperature and mechanical stability. PVC has a thermal expansion double or triple that of brass or copper, which in turn is worse than Invar. I simply can't afford Invar, plus it is not easy to get; old off-frequency filters being a possible source. For the moment I will continue with PVC fittings for the antenna adjustment rod, but will be looking for better ways of doing it. I want to build 6 m band cavity filters where I will need something other than PVC, given the longer length.

However, the duplexers are to be used in my club's repeater room on the Gold Coast, Queensland, Australia, where the temperature is fairly stable (maybe 15 to 40 degrees); they don't need to do the +/- 40 degrees of snowline North America.

Aluminium flashing rather than copper shim

I have contemplated using aluminium flashing instead of copper shim, 0.3 mm thick at a number of widths, cheap and easy to get at hardware stores. The aluminium flashing could be used for the probe/antenna. It could even be used to line 150 mm or 200 mm PVC drainage pipe, half the price of aluminium pipe at about $80 for 150 mm 6 m length. However, that price difference is probably not enough for the hassle involved. It may be different for 200 mm, as aluminium is not readily available at that diameter, but I have not checked further. I think 150 mm will be ok for 6m cavity filters.

An advantage of all aluminium is less electrolytic corrosion. The Gold Coast can get pretty humid and marine, but again, I think the repeater room is dry and stable enough for it not to be a problem. In a shed on a mountain, that may be a different issue.

Bandpass vs notch

The filter is a bandpass filter. I could make it into other types of filter, such as the various types of notch filter using an inductor or capacitor between the probes. I will do that later; a good bandpass is sufficient or proof of concept for now.

Similarly, I have not been concerned with losses through the filter. Losses and selectivity are a compromise with the degree of coupling, along with the Q of the filter (ref??). On RX, I am less concerned, just use a pre-amp; selectivity is crucial compared to losses. On TX, I am not sure I even need the sharpness of a three cavity filter.

Conclusion

Close enough for me for now. I have a design that seems to work quite well. I won't do any more major changes on this filter, rather call design closure and make another one, I have enough copper shim to do that, plus I can make a tidier new one.