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Author Topic: RF circuit help....  (Read 1295 times)

Offline IQ_imbalance

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RF circuit help....
« on: June 12, 2020, 0219 UTC »
Hi all.  I purchased a diplexer so i could run a VHF and a UHF antenna into the same feedline.  Seems to work well, but I didn't notice that there's a gap between the two bandpasses (VHF 47-230 MHz, UHF 470-862 MHz).  Unfortunately there's a 350 MHz system i'd like to receive, so I'm wondering if i can tweak the circuit to fix this...ideally by shifting the VHF bandpass from 47-230 MHz to 47-400 MHz (or thereabouts). 

I did my best to trace the circuit (damn tiny surface mount components!!!)  Can anyone make heads or tails of this, and suggest somewhere i might start to try and figure out how to modify the VHF bandpass?  I'm fine w/ the UHF settings, so hopefully i just have to do something to the VHF leg.

« Last Edit: June 12, 2020, 0320 UTC by IQ_imbalance »
LOG/NE-SW unterminated BOG/DJ-130/800Mhz Yagi
AFEDRI SDR-Net ICF-SC1 SDS-200 various RTL-SDR
Central MD

Offline Stretchyman

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Re: RF circuit help....
« Reply #1 on: June 12, 2020, 0807 UTC »
Plenty of online progs to simulate the filters. You can then rebuild the full simulation in LTSpice, again free!

Ideally fully test with a VNA, the nano ones being $60?

There you go!

Str.
'It's better to give than receive' so why RX when you can TX!

         15W, 60W and 100W models available.

                   Buy one from me, NOW!


                                              ;)

Offline chanito

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Re: RF circuit help....
« Reply #2 on: June 12, 2020, 1435 UTC »
If it is receive only, you might try a Mini Circuits power combinerider.


Find your range and connector style here [size=78%]https://www.minicircuits.com/WebStore/Splitters.html?[/size] Then look on eBay for one at a fraction of the new price. Last couple I bought were around $20.
« Last Edit: June 12, 2020, 1442 UTC by chanito »
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Offline IQ_imbalance

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Re: RF circuit help....
« Reply #3 on: June 12, 2020, 1450 UTC »
A VNA is in my future.  Going to have to make some educated guesses about the component values as theyre not marked but well see.  Given that the only UHF Im really interested in is toward the upper limits of the current configuration hopefully Ive got plenty of room to tweak the VHF section without affecting UHF
LOG/NE-SW unterminated BOG/DJ-130/800Mhz Yagi
AFEDRI SDR-Net ICF-SC1 SDS-200 various RTL-SDR
Central MD

Offline redhat

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Re: RF circuit help....
« Reply #4 on: June 12, 2020, 1734 UTC »
You can model a filter using various software as stretch mentioned.  I use the SVC program from tonne software.  Its a good freebie.  Using it you can play around with values to get the response you want, then build it :)

http://tonnesoftware.com/svcfilter.html

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Offline IQ_imbalance

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Re: RF circuit help....
« Reply #5 on: June 16, 2020, 1641 UTC »
My NanoVNA finally arrived.  As expected, the diplexer chops down on the 200-400Mhz sensitivity. The top scan is my DJ-130, the middle scan is the UHF (~800-900 MHz) yagi, and the bottom is the combined feed from the diplexer.  What's curious is that the amount of attenuation around 300 MHz doesn't appear too massive (within a log at least) yet i've lost a 380 MHz system.   Still playing around with it.....
LOG/NE-SW unterminated BOG/DJ-130/800Mhz Yagi
AFEDRI SDR-Net ICF-SC1 SDS-200 various RTL-SDR
Central MD

Offline Stretchyman

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Re: RF circuit help....
« Reply #6 on: June 16, 2020, 1740 UTC »
Well done, you've got the right equipment now.

No longer costing /$50K+ anyone can now own a VNA, such a versatile piece of equipment. Filters and antennas demystified in an instant!

Str.
'It's better to give than receive' so why RX when you can TX!

         15W, 60W and 100W models available.

                   Buy one from me, NOW!


                                              ;)

 

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