SatSig topic:

C band transmit reject filter Tx/RX Dish setup

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Jun 27^th, 2011 at 10:56am

Sir years ago my mates remove a part of our Tx/Rx standby dish,I don't know whats the function of it but it is connected between OMT and LNB. It has screws on the side and I think it was use for some adjustment. And now we are going to use the dish to transmit and receive, is it ok if we connect the LNB directly to OMT? And sir what's the use of that part that has been removed I have a picture of it on our other same antenna but I don't know how to post a picture here.

Thanks,

ken u.

Jun 27^th, 2011 at 12:35pm

Given the limited info provided thus far, my first guess would be transmit reject filter. Email a photo to eric@satsig.net, he'll insert it for you.

//greg//

Jun 27^th, 2011 at 1:56pm

Thankyou sir greg.

I will email it to sir eric.

Image inserted by admin...

The white transmit reject filter (with row of adjustment screws) is connected between the LNB and the OMT. Its purpose is to stop transmit energy leaking across into the LNB and overloading the LNB or damaging the LNB. In the image above it would appear that the transmit path (lower black rectangular waveguide into circular waveguide) can be configured for either polarisation. If co-polar operation is chosen then the use of the filter is absolutely critical.

I don't know if there is a polariser to the right, if so it would have its pins/vanes/ slots at 45 deg. A polariser would change the Linear polarisations at the OMT into Circular at the feed and vice versa.

Jun 30^th, 2011 at 6:56am

Sir can I still transmit/receive even without that transmit reject filter?

thanks,

ken u.

Jun 30^th, 2011 at 4:53pm

Without the transmit reject filter, which serves to protect the LNB from unwanted transmit power leakage across the OMT or diplexer, you run the risks:

1. When you transmit this will overload the LNB and this will interrupt your receive path. The effect may range from just a mild increase in bit error rate to complete loss of the receive signal.

2. When you transmit this will not just overload but actually damage your LNB, possibly burning out the front end low noise amplifier transistor.

3. If you are operating co-polar (i.e. transmitting and receiving on the same polaristaion) then the transmit reject filter is doing more than just protecting the LNB. It is reflecting a great deal of the transmit power to redirect it up the feed. In this case omitting the transmit reject/diplexer filter will destroy the LNB and may also seriously damage/destroy the transmit power amplifier due to incorrect termination, causing reflections back down to the transmit amplifier.

Best regards, Eric.