So, I have a design, two dipoles working at UHF and VHF. We fed using a diplexer. I am having difficulties with the testing using network analyzer. VHF passes but UHF, specially at 400 MHZ, the VSWR shoots to 2. 1 or even 2.3.
Is there anything I am missing and replace to pass vswr for UHF too?
Thank you
Dear Meenakshi Kohli ,
You have to investigate and improve the matching between the source impedance and the input of the UHF port. You can measure S11 and see whether the resonance center frequency is lies at the minimum of S11. The minimum must at least lower than S11= -10dBs. May be you need to fine tune the path to the UHF antenna. Also, the radiation resistance may be smaller much deviated from the source resistance.
Best wishes
Dear Meenakshi Kohli ,
You have to investigate and improve the matching between the source impedance and the input of the UHF port. You can measure S11 and see whether the resonance center frequency is lies at the minimum of S11. The minimum must at least lower than S11= -10dBs. May be you need to fine tune the path to the UHF antenna. Also, the radiation resistance may be smaller much deviated from the source resistance.
Best wishes
Thank you Abdelhalim abdelnaby Zekry , The antenna is already fabricated. Do not have the simulation for this one. This diplexer looks like works well VHF but not with UHF.
Meenakshi Kohli I assume the two antennas are in proximity. The larger one (VHF) elements will impact the smaller one (UHF) and affect the input impedance. If possible, increase the physical separation of the two antennas. You may orient them orthogonal to each other as well.
Dear Meenakshi Kohli ,
To get an evidence about the performance of the diplexer loaded by the antennas, you can measure S11 as a function of frequency by using a network analyzer.
Best wishes
thank you Shiv Prasad Tripathy . hank you. I can try this, to increase the distance between the two elements. But then my UHF will be closer to the feed. would that affect in any way in the performance?
Meenakshi Kohli It should not matter, if the match is good. If I were to do the testing, I would proceed as below. There is no shortcut to troubleshooting.
1. Test each antenna independently to verify port impedance without diplexer. [This assumes they are separate fabrication entities and are thus independent objects.]
2. Test each antenna independently using the duplexer, with the unused antenna port terminated with a match. This will indicate diplexer issues, if any
Above tests give information on uncoupled antenna behaviour. If these are good, then test results with both antennas are due to coupling.
Ideally, the dual-band design should be done as a single design. Hope it helps!
/rgds
Dear Meenakshi:
This pdf might help you, as it talks about the practical design of VHF, UHF
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