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I got this low pass filter at a hamfest for $10. When it's connected up it seems to double my SWR, which effectively halves the bandwidth of my antenna.
It is useable at the point where the SWR is lower, but where the SWR gets to around 1.5 without the low pass filter, it becomes 3+ with it so I my bandwidth is reduced from around 2MHz to around 0.7MHz
Is this just what low pass filters do or is there something wrong with it?
Here is a photo with the cover removed. I only used it with the cover on.
That looks like an unusual configuration, not the classic series inductors and capacitors to ground LPF configuration.
Presumably you are measuring with the SWR meter before the filter, out of interest what is the SWR after the filter?
If the output of your radio or amplifier is not clean the SWR can look higher because the filter is doing it's job, harmonic energy is being reflected back by the filter instead of gettting to the antenna.
If you are using a CB amplifier check with and without the amplifier.
The input of an RF low pass filter generally only looks like 50ohms if the output is also connected to a 50ohm load.
The coils and caps in the filter do change the impedance somewhat when a mismatched load is connected.
If you have a dummy load check if you get 1:1 SWR with and without the filter.
It may be be perfect but it should be pretty close, not worse than 1.1:1
Is this just what low pass filters do or is there something wrong with it?
YES!
Are you transmitting within the filters frequency parameters e.g. if it is 30MHz, is your transmission under that ?
Are you sure it is "LOW PASS" and not "BAND PASS"?
have a look half way down this page:- http://www.electronics-tutorials.ws/fil ... ter_2.html
Jon G4FUT
Even if the voices on my head aren't real, they do have some pretty good ideas
There is a sticker on the lid saying "low pass filter".
I'm thinking that maybe its a corrosion/dry solder joint issue due to age. I'm going to try reflowing all the solder joints and cleaning up all the contacts.
Is this filter a homebrew one or a commercially made one? If it is a commercially made filter I would do a search on the internet using its model number to find a circuit diagram and the technical specification to identify the type of filter and its frequency range.
A low-pass filter really should not make a lot of difference to the VSWA of the antenna system, I suspect like Jon has pointed out, it’s out of its frequency range, for the VSWR to increase that much.
I would also give the filter a visual inspection and look at the solder joints to make sure they are of sound condition, if you spot any that look suspect then I would rework that joint with a soldering iron.
Also look around the SO239 sockets on the input and output to make sure that there is no decolourisation of the insulator which would mean over heating of the socket, which may cause the insulator to break down. If you do find any signs of overheating on the SO239 sockets replace them.
Whatever you do, do not alter the inductances inside the filter, as this will affect the operation of the filter.
It looks like a bespoke LPF with shunt LC resonators providing sharp rejection at a chosen frequency.
At a guess I'd say it was designed for use around CB frequencies and it is designed to provide extremely high rejection of the second harmonic (54MHz?)
above this second harmonic frequency the filter rejection will degrade because of the topology it uses. I'd expect the rejection to be very poor in the 75MHz region. Above this it would get better again.
The discs are capacitors but I guess you knew this already. They all look to be shunt caps. The inline discs that look like they are in series are really just shunt caps to ground.
Unless you want high rejection of maybe 54MHz or thereabouts I'd say the filter is not much use although it looks like it is rated for very high power. Maybe to allow a big amplifier but not interfere with services on 54MHz?
