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Author Topic: Filters For Dummies  (Read 4536 times)

Offline XmasScotty

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Filters For Dummies
« on: August 13, 2019, 2057 UTC »
I've been using a inexpensive import FM transmitter and an amp for a few years after my other transmitter.. well.. that is a bizarre story in itself, and best to be told at a different time and thread. Anyhow, back to the subject at hand. Recent circumstances have dictated that I seriously consider getting around to getting a filter for what I'm currently running.
My question is two-fold:
Part 1: After sitting in on the local two meter net, the topic of filters was brought up, and an Elmer was pontificating about the pros and cons (mostly pros) of cavity filters, and I am leaning that way. Is this a good way to go?
Part 2: If so, what do I need to know in purchasing said filter(s)
Part 3: Before the amp or after the amp (or both?) as far as application?


Offline Josh

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Re: Filters For Dummies
« Reply #1 on: August 14, 2019, 1854 UTC »
Cavs are a pain in size and adjustment but are low loss, and are typically used to be able to rx at the same time tx is ongoing on the same antenna some 600kHz away from the rx freq in the case of 2m. I suppose a lump sum lowpass filter would be all one needs for fmbc.
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Offline Brian

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Re: Filters For Dummies
« Reply #2 on: August 14, 2019, 1908 UTC »
Low pass after the amp.

Offline JimIO

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Re: Filters For Dummies
« Reply #3 on: August 14, 2019, 2358 UTC »
You're supposed to say: "I'm just asking for a friend".    8)
There's little to no benefit filtering before the amp. You don't say what you got. What makes you think you need to add a filter? If you do then look for a 5 or 7 pole FM low pass filter that will handle whatever power the amp is.

Offline ThaDood

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Re: Filters For Dummies
« Reply #4 on: August 15, 2019, 1800 UTC »
Roll you own filter and build it, use this calculator site,    http://www.calculatoredge.com/electronics/ch%20pi%20low%20pass.htm
A site like this worked for me, but I've added a better twist to the ones that I've built in the past. All the CAP's are variable to fine-tune the filter to be very sharp.
I can't decide upon what's worst, young and stupid, or old and chemically dumbed down.

Offline ThElectriCat

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Re: Filters For Dummies
« Reply #5 on: August 17, 2019, 1622 UTC »
One of the simplest FM filters which can be reasonably practical for the homebuilder are made from coax.
They rely on the fact that a quarter wave stub can either look like a short or an open circuit depending on whether it is open or shorted respectively. 
Get a coax tee, and connect a quarter wave of good quality coax to the tee solder the far end of it together (conductor and shield) and put the tee inline with your feedline.
This is, in fact, electrically very similar to a cavity filter, but with a little bit more loss and lower Q, but it is much cheaper and more homebrew friendly.

Remember that a quarter wave will be a normal quarter wave in free space multiplied by the velocity factor of your coax, which you should be able to look up (usually 0.6 to 0.8)

Make sure to include the length added by the coax tee and measure as accurately as possible.  A big advantage of these filters is that they are a short circuit at dc. When I worked in broadcast, I would put them in between the exciter (usually a 20 or so watt transmitter) and the 30 Kw power amplifer, that way DC spikes caused by transmitter problems wouldn't kill the solid state exciter. 

some things to be careful of.
make sure your transmitter is ac coupled, if the plate/collector/drain voltage is present on the output, this filter won't work
if you are transmitting more than a watt or so, hook up the filter and run it, make sure its not getting warm. if you have it cut for the wrong frequency, your transmitter will see a very bad vswr, and the filter will dissipate as much power as the loss in the coax allows.
In another life, I could have been a telephone engineer.