Band pass filter for 50mhz - Page 1
Forum section: electronics / rf-microwave

author: Jesset1996.5
author_rank: Regular Contributor
timestamp: December 30, 2017, 12:30:11 am
content: I am working on a band pass filter for my 6m repeater of which does not use duplexes due to there large size and cost, It uses antenna spacing instead.The antennas are just far enough for it to work but i need to reduce the db of the adjacent channel on the receiver a little. I have found a filter design that should work but i am a little stumped on the values of the capacitors. It says that they are polarized but i know for a fact that polarized caps do not go into the pf or nf range. Couldn't i just use a variable cap and a fixed cap together to make it somewhat adjustable?I am new to RF circuits so any help is appreciated.the specs are as followsPart ValuesPart BesselL1 0.0017 uHC1 5370 pFL2 38.61 uHC2 0.2417 pFL3 0.0003 uHC3 35068 pF

author: rhb
author_rank: Super Contributor
timestamp: December 30, 2017, 12:54:11 am
content: Download Elsie from:www.tonnesoftware.com/elsie.html

author: ThomasDK
author_rank: Regular Contributor
timestamp: December 30, 2017, 12:55:36 am
content: Those component values look weird.Normally the inductors would be 1000 times higher in value, and the capacitors 1000 times smaller.0.0003uH = 0.3nH. A typical capacitor has more inductance than that...

author: Jesset1996.5
author_rank: Regular Contributor
timestamp: December 30, 2017, 01:15:04 am
content: Quote from: ThomasDK on December 30, 2017, 12:55:36 amExactly my thoughts, i can get the parts but im curious if there is another design that will work similar and have common value parts.

author: KJDS
author_rank: Super Contributor
timestamp: December 30, 2017, 02:09:53 pm
content: If it's 50 ohms in and out then the filter should be symmetrical.As it isn't, then either Elsie is unreliable or else you've set some impedances incorrectly

author: matseng
author_rank: Frequent Contributor
timestamp: December 30, 2017, 02:25:12 pm
content: Quote from: Jesset1996.5 on December 30, 2017, 12:30:11 amFrom what I learned the cap with one half as an arc (instead of just two straight lines) are just the US version of a regular non-polarized capacitor. Normally the polarized caps either have a + sign marking the positive side, or a hollow rectangle as the positive side line.

author: rhb
author_rank: Super Contributor
timestamp: December 30, 2017, 02:41:50 pm
content: Attached are plots from Elsie. I'll leave further work to the OP.

author: dmills
author_rank: Super Contributor
timestamp: December 30, 2017, 06:31:02 pm
content: The OP may well do better to do something with a shorted 1/4 wave stub made of something seriously low loss, say a bit of LDF450 or something.This generally makes a better notch then an LC network at those sorts of frequencies without having quite the beer barrel sized inconvenience of a cavity.73 Dan.

author: denverpilot
author_rank: Regular Contributor
timestamp: January 01, 2018, 05:08:10 pm
content: Quote from: dmills on December 30, 2017, 06:31:02 pmTypical 6m cavities aren’t anywhere near “beer barrel” sized. You don’t need that high a Q with the transmitter and receiver as far apart as most 6m repeaters are.Ours are about 6’ TALL but they’re not that big around.There’s a number of DIY 6m cavity designs out there, but I’m with you or whoever said it, just add a T and cut a quarter wave shorted coax line and use it as a notch.The only downside is if you’re located at a very RF “hot” site, then you usually don’t want a coax notch filter acting as a receiving antenna and helping mix products in your transmitter. That will not make you any friends on a tower site.