Replacing Terra TRT 250D transponder in RV-4

My Terra TRT 250D transponder has stopped working properly. I'm thinking I should replace it with a new transponder with ADS-B out capability. Problem is, I have an RV-4 and panel space is very limited. The Terra 250 D occupies a non-standard hole 3-1/8" wide by 1-5/8" high.

Can anyone recommend a transponder that will require minimum alteration of my existing panel?

(I'd include a photo if I knew how.)

Have you checked out the Trig transponders? 2 1/4 in head with remote mount body.

rvator51 said:

Have you checked out the Trig transponders? 2 1/4 in head with remote mount body.

Click to expand...

To be more precise, the Trig control head is only 1.7" tall by 2.25" wide. It has spacers that make it fit a 2 1/4" hole if needed.

Sandia may work:

https://sandia.aero/product/stx-165-panel-mounted-transponder/

Great products

+1 on the Trig TT-22 (do not buy the TT-21, it's not legal for ADSB in the US). For a gps position source, it can read Garmin's ADSB+ output from a 400W, 500W, 650, or 750 series IFR GPS. Alternatively, GRT's $500 gps can serve as a position source for the Trig.

Thanks for the Trig and Sandia suggestions, both of which I was aware. I hoped that there might be something else out there that my research missed. The lesson, for me at least, is that non-standard sizes for items in the panel complicates replacement.