Using Cellular Multicouplers in place of Maxtrac Front End Filters.
Since the Toko and Murata filters for the Maxtracs are very hard to find anymore, and because they don't have very tight skirts and aren't that great on a busy hilltop, this is a good option. Below is the Allgon unit used in some old Ericsson analog cellular systems. This is not the best choice as the filter is quite wide (33MHz) and it doesn't tune up to the 900 HAM band well. It is possible to still use it with great success if you want to take the time to modify one. Here is what the unit looks like disassembled with the filter removed and a sweep of the stock unit.
Ericsson also used Cellwave filters in many of their sites also. We will cover them later.
Now, the hard part. If you are as "frugal" as I am and are intent on using one of these Allgon units, you will have to remove all the small TORX screws around the perimeter of the end OPPOSITE the tuning screws. Then, remove the FIRST center screw at each end closest to the connectors at each end. The entire plate should come off easily along with all the resonating rods, except the one rod at each end attached to the connectors.
Hard to see the settings, but the bandpass is from 821 to 854 MHz.
Now, to remove the two rods at either end, simply pull them off of the wire coming from the connector by pulling it sideways, away from the connector. The wire from the connector is NOT soldered to the rod. It is pushed thru it, insulated from it by a small piece of teflon tubing in the rod. Once they are out, unscrew the other rods from the plate and cut 3/32" off of each one from the end opposite the mounting screw, including the two end rods. A band saw would be best for this. I used a grinder and took the time to flatten and shape the ends flat again. Not the best method, but it turned out OK.
Reassemble the filter and set it up for your needs. This filter is too wide to keep out the pagers and commercial repeaters between 928 and 935 if you tune the bottom for 902. If you set the bottom at around 895 MHz, and there are no cell towers in the same area, it should work OK as the upper bandpass limit will be at 928 and all the commercial stuff between 894 and 902 is mobile uplinks so it usually isn't a problem. Another option is to do what I do and put TWO in series, one tuned with the lower limit at 902 and the other tuned with the upper limit at 903. This makes a VERY tight repeater input bandpass with deep skirts on either side and works very well on busy hilltops!
Sweep after shortening the resonator rods in the filter and retuning.
You can't see the display in these shots, but the lower edge of the photo on the left is at 902 MHz and the upper edge is at 935. The sweep on the right is the same sweep zoomed in on the lower roll off. Settings are...
center freq = 902.000MHz....10db/div...2MHz/div. Loss was between 2-3db.
Notes on tuning. You MUST sweep these things! Trying to use a power meter on one frequency will only get you in to trouble. You will find that the outer adjusting screws are more of a "shaping" adjustment and the inner adjusting screws are more frequency related. Knowing that really helps in the tuning of these things. The entire process took me about 90 min not knowing what I was doing the first time!
The next test will be to see if it will handle any TX power so it could be used in place of the maxtrac front end filters completely! The Toko and Murata filters are nowhere as good or tight as one of these. This is just a little bigger and more trouble, but for repeater receiver filtering, this is far better. I will get some of the Cellwave filters that are more common and easier to tune and post pictures of them here also soon!
I have been able to get away with placing a good quality preamp between the duplexer and these filters. The Cellwave filters are only about 20MHz wide so you can usually get away with just one filter, but using a retuned cellular duplexer, N6CA preamp and a Cellwave filter, I still got 0.17uV sensitivity at the duplexer antenna port!