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2 identical monopoles spaced 70mm apart using 2 connectors

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akhilpaulv

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Hi Sir/Madam,

Please go through the details given below. I request you to help me to improve my design

I have designed 2 identical monopoles spaced 70mm apart using 2 connectors in order to work in the UHF band. Its center frequency is 370 MHz. I have measured individual monopoles and got the VSWR <1.5. When taking the combined measurement, the VSWR is greater than 3. My aim is to make both the ports transmitting or receiving at the same time. Kindly advice me the steps to improve the VSWR equal to or less than 2
 

The wavelength at a frequency of 370 MHz is about 80cm so cm is definitely a too short distance between them. They will interfere one each other, not only from the vswr point of view, but also the geometry of the radiation pattern will be strongly distorted. I think you have to change geometry, increasing the distance between them.
 
is any way to reduce interference without adjusting the distance between them?? how its affect the radiation pattern of the antenna???
 

As these antennas are very close to each other do they act more or less as a single antenna structure. Your both radios can as well share one antenna via a T-junction and result will be the same.
Improve isolation at that short antenna distance can be done with a very big(infinite) grounded metallic shield standing between the antennas. That will affect all antenna properties as shape of antenna ground is changed.
Monopole antennas are poor selection if isolation between antennas are important. 90 degree crossed dipoles or loops, are common solutions if two antennas must exist close to each other with some isolation maintained.
Designing antennas with opposite direction of circular polarization do also work.
 

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