开云体育

I am trying to determine the values of variable capacitors in DB Products Low Pass Filters to see if I can convert an 800 Mhz LPF that I bought in a flea market to a 450 Mhz LPF by just changing the inductors between the capacitors.

Here is the setup. I bought two DB4333B type low pass filters for 850 Mhz for a very reasonable price.? The actual model number listed on the cases is 020252-004.? The frequency range is 851-869 Mhz.? There are N females on both input and output.? Coincidentally, there is not a model number in the Spectra catalog that shows N(F) to N(F).? The specific frequency range is also not listed in the catalog, but are wider. I assume this is some kind of OEM model number.? It is labeled Decibel.

The inductors in the 800 Mhz LPF between the four capacitors are bare, straight, and no turns.?

I would like to move the cutoff frequency to convert these to the same as a UHF DB4332B to use on an isolator for my amateur UHF repeater.??

I already have a couple of DB 4332B LPF's for UHF (450 -512 Mhz). The inductors look like 2 1/2 turns on the input and output and 3 1/2 turns between the two inner capacitors.

I tried to measure the capacitors in both of these LPF's, but considering the capacitors are in the pf range, it was not that easy to deal with stray capacitances.

Does anybody know the capacitor values in the the DB4333-2450 and DB 800 Mhz LPF's?

If the capacitors between eh 450 and 800 Mhz are the same value or even close, I should be able to lower the LPF cut-off by increasing the number of turns in the filter to the same number as the DB4332-A.

If the capacitors between 450 and 800 Mhz are different, I will just have to cut and try and see what shows up on the spectrum analyzer.? I did not want to solder and unsolder inductors to the expensive piston type trimmers and damage them with heat.

The wire looks like #12 to me and is covered with shellac in the UHF and plain old copper wire for the 800 Mhz device?? I have measured the diameter of the coils in the 450 Mhz LPF and have it written down, someplace??

I am just trying to avoid buying additional DB4332B LPF's or equivalent replacement capacitors. That would turn my "reasonable price" into a "not so reasonable price."?

Thanks.

Micheal Salem N5MS

开云体育

Mike:

Thank you for your emails. I agree it would be great if the capacitors at 850 Mhz were adjusted to something less than half of their range, I could just install the same size coils as in the 450 Mhz LPF and retune the capacitors.

Looking at an old 1994 DB Catalog, the curves for the VHF, UHF, and 800 Mhz LPF's are identical. The 800 Mhz LPFs' are not in the 1993 Db Catalog.

I did considered padding the capacitors if their values were different. I assumed Q of the circuit would change with the size of the capacitor and that would affect the response of the low pass circuit.?

I can not see any component identification on the capacitors in either LPF that shows their manufacturer or value. Just looking, they kind of look like Johanson. I have pictures inside both of the LPFs.

I have a capacitance meter with tweezer type probes. It was difficult to get a consistent readings. Besides, since I am not unsoldering the coils, I am actually measuring some overall capacitance value for all four of the capacitors connected in parallel which I expect would not be affected by the inductances of the coils because the measurement frequency is pretty low. I can't say it is that accurate since it is not a very expensive capacitance meter.? It did read something.

I was just hoping that someone might have replaced a capacitor after a lightning strike or something similar, and knew the value.

I may just start by reproducing the UHF coils and install them in the 850 Mhz LPF, then use the spectrum analyzer and see how low I can bring the knee of the filter and pad it if I can't reach 450 Mhz.? The UHF coils look like they are expanded and mashed as part of the tuning.

Micheal Salem

?

About 3 years back I had one DB4332-B UHF low pass filer but needed two, so decided to reverse engineer it and build a clone. The variable piston capacitors are Johanson 0.8 - 10Pf rated at 150V. Back then I found several sellers on ebay offering them, and there was one selling them in 4 packs for $25 plus some shipping. The trimmer cap model I used was PC25J100. The model is stamped onto the top solder side of the cap, so usually not possible to see in the original LPF. Just checked on ebay but original listing is gone, however looks like listing #256220267439 is the same guy still offering those trimmer caps in packs of 4 still at the same price.

The 3 coils are made of #18 wire, one side being 2T, the middle one 3.5T and the last one again 2T. Coil diameter is 7/32 or about 5.55m, stretched out same as the original coils.

I also managed to find on ebay someone selling the identical metal boxes (Pomona model 2413) for $10-15 new old stock in the original bag and with connectors on each end, but these instead had one SO-239 and one PL259, which both where very easy to replace with NM/NF. I then also had to drill the four holes to mount the trimmer caps and assemble everything. The clone LPF was then checked on a VNA and compared to the original DB4332 and it had basically identical characteristics as the original one, and of course also tuned up just fine.

About a year later after that I needed two additional UHF LPFs and did build two additional clones that also worked perfectly. The attached picture shows the original LPF at the top, and one of my clones in the blue box at the bottom.

About a year later after that I needed two additional UHF LPFs and did build
two additional clones that also worked perfectly. The attached picture shows
the original LPF at the top, and one of my clones in the blue box at the
bottom.

Nice clean work there.

I'll make a suggestion, and it's not specific to this project, but one of those "makes perfect sense" things that applies in many cases where you have high-level RF through multiple inductors in series such as in an LC low-pass filter:

Wind all of the inductors on one contiguous piece of wire. You greatly reduce the risk of a solder joint failure due to overheating or thermal cycling with the wire carrying most of the current rather than the solder joint doing it.

--- Jeff WN3A