RF circuit help....

[IQ_imbalance]

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.

[Stretchyman]

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.

[chanito]

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.

[IQ_imbalance]

A VNA is in my future.  Going to have to make some educated guesses about the component values as they’re not marked but we’ll see.  Given that the only UHF I’m really interested in is toward the upper limits of the current configuration hopefully I’ve got plenty of room to tweak the VHF section without affecting UHF

[redhat]

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

+-RH

[IQ_imbalance]

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.....

[Stretchyman]

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.