[arm-allstar] DRA818V VHF and DRA818U UHF Transceiver Modules -- Low Pass filter required!

[Doug Crompton]

It looks like the minicircuits 440mhz filters are pretty effective at the 2nd harmonic but the 2 meter ones not so good. These would be good filters for 440 low power but they still cost over twice as much as the module so unless you have one laying around that kind of defeats the low price point. 

http://www.minicircuits.com/pdfs/VLF-530.pdf

http://www.minicircuits.com/pdfs/VLF-490.pdf\

http://www.minicircuits.com/pdfs/VLF-180.pdf
73 Doug
WA3DSP
http://www.crompton.com/hamradio


Date: Sat, 29 Nov 2014 18:09:32 -0600
From: w0anm73 at gmail.com
To: arm-allstar at hamvoip.org
Subject: Re: [arm-allstar] DRA818V VHF and DRA818U UHF Transceiver Modules -- Low Pass filter required!

I use a lowpass filter on my dra818u module. However, I don't have the test equipment to check the harmonics. I use this filter my rc gear.
It's a Mini-circuits 15542 low pass filter.

73,
Chris, w0anm
On Nov 29, 2014 12:12 PM, "Doug Crompton" <doug at crompton.com> wrote:



I only ordered UHF but I suspect they might have the same problem. I will definitely check on a spectrum analyzer when I get them. I think these modules are designed as part of an overall transceiver and the microprocessor, display, output amplifier and filtering are missing. For most people it would not be hard to add the proper filtering but few have the RF tools to measure the results. So we would have to come up with a foolproof filter design that can be easily duplicated. At the same time you don't want it to be physically large as that would defeat at least one of the advantages of theses modules to begin with. I have a large amount of double sided copper PC board and I would probably build a little box out of that and put the filter inside along with the module. It would be a matter of winding a couple of coils on some common small diameter material like a pencil and a couple of chip capacitors. Here are some links I found with a quick search on 2 meter filter design.

http://ve2zaz.net/LowPass_Notch_Filter/144LowPass432Notch.htm

http://www.radiomods.co.nz/lowpassfilterpage2m.html

http://www.g4ztr.co.uk/amateur-radio/construction/2m-low-pass-filter/

http://dutchrfshop.nl/ham-radio/2-meter-band/print-lowpass-filter.html?___store=en&___from_store=nl

So as Dave pointed out until we come up with some real measurements on these, and possible ways to fix the problem, I would be careful about putting them on anything but a dummy load. If the 2nd harmonic was 6db down running them at the 1/2 watt output level they would be putting out 125mw at twice the frequency. Enough to cause interference out to several thousand feet at least.

I wonder if anyone has looked at some of the cheap handhelds that come in from China on a spectrum analyzer?  I guess they are type accepted though so they should be OK.

73 Doug
WA3DSP
http://www.crompton.com/hamradio

 		 	   		  

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