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Author Topic: APRS bandpass filter?  (Read 4069 times)
KE7FD
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« on: August 22, 2012, 11:08:54 AM »

I know tis is similar to a question I posted a while back but due to a recent event (my mobile being totaled, not by me, however...), I'm thinking of putting a stand alone APRS radio (already have) in my [new] vehicle while using a dual band radio for normal use.  Why? Have you seen what APRS enabled (TNC) mobiles are going for? YIKES!  So, an over the counter dual band radio makes more sense and the APRS unit can just sit off in the corner of the trunk doing its thing.  Here's what I need to know:

Is there an over the counter narrow bandpass filter for 144.39MHz to put on the APRS unit OR is there a filter to attenuate everything below say 144.5 MHz that can be put on the feed line of the dual band to keep it from being desensed by the APRS radio at 144.39MHz?  I know filters can be built but homebrewed amateur gear tends to be rather clunky and I need to see if there's a compact prefabbed unit.

Glen - KE7FD
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W4OP
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« Reply #1 on: August 22, 2012, 05:52:34 PM »

Unfortunately, the size of a filter for that  that narrow a spacing (if it can even be realized) is well beyond anything  you would want to put in your car.

Dale W4OP
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SV1XV
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« Reply #2 on: August 23, 2012, 02:29:20 AM »

...if it can even be realized...
It can, think of a 2m repeater duplexer!
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K7RBW
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« Reply #3 on: August 23, 2012, 07:06:36 AM »

I think most people who run such a configuration, just take the hit and suffer through 1-2 seconds of de-sense every minute or two.

You probably wouldn't want your APRS unit to be digipeating packets while you're using your mobile rig as that might get annoying, but the interference you're worrying about is short and should be infrequent (i.e. probably not worth the hassle of filling your trunk with a big cavity filter). It'd be easier just to give each rig (i.e. the APRS radio and your comm radio) it's own antenna and install them as far away from each other as you can--perhaps on on the back fender and the other on the opposite front fender.
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SV1XV
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« Reply #4 on: August 23, 2012, 08:36:32 AM »

BTW, it may be possible to insert in the voice radio antenna coax a notch filter for the APRS frequency. You may manage to reduce the APRS signal to a level that does not affect the voice channel.
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W4OP
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« Reply #5 on: August 23, 2012, 09:41:05 AM »

...if it can even be realized...
It can, think of a 2m repeater duplexer!


Here in the U.S. (from memory) the repeater bandplan has a pair assigned that would require the user to TX as low as 144.520MHz. That's 130KHz above the APRS frequency-  well under the 600KHz spacing that duplexers are designed for. In addition, the passband of a duplexer is optimized for a small spectrum- not 145-148 MHz.
My company designs some pretty sharp notch filters in small packages- but I don't think you'll see anything with a significant  notchat 144.390MHz and yet still  have acceptable insertion loss for the repeater band, decent return loss and also capable of handling 50W unless it employs a COR bypass.

Dale W4OP
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AF6OF
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« Reply #6 on: August 25, 2012, 03:03:14 PM »

I generally like to use a low pass frequency in the receivers I build. I like the SMD low pass filters from Mini-Circuits. These of course are not tight notch filters, but they do prevent de-sense from on high ( there is a lot more above 2 meters than lie below it if you think about it: the bands above are infinite!) There are many articles on building 2 meter helical filters, but unless you have the equipment with which to test the filters, its hard to know if you are just wasting your time and adding insertion loss.

You may also want to consider using an active low pass audio filter, which can provide a major improvement in the s/n ratio, and consequently, improve marginal packet decoding.

Allen AF6OF
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