Extend TIU antenna?

You could try the following, from page 162 of The DCS Companion 3rd Edition:

TIU Modification 

If everything appears to be in good operating condition and there are still sporadic problems with TIU to DCS Remote communications, it could be that the TIUs are physically placed differently than was suggested earlier (refer to Part III - DCS Implementation, TIU Channel Assignment and Placement earlier in this book). If this is the case, there's a relatively simple modification that can be made to the TIU that can increase its range for communicating with the DCS Remote. 

Note that this modification may void the MTH warranty for the TIU and should only be attempted by knowledgeable individuals or MTH authorized technicians. 

To perform the modification, proceed as follows: 

• Disassemble the TIU as described earlier in this section and locate the transceiver board. It is the small circuit board that's plugged into the left, front part of the main TIU board when viewing the TIU from the channel input side. 
• While gently but firmly holding down the transceiver board, raise the antenna (black wire attached to the transceiver board) so that it is standing straight up. Do not under any circumstances lift up on the transceiver board! 
• Locate where the wire would touch the top half of the TIU's case and drill a 9/16" hole in the top half of the case at that spot. 
• Replace the top half of the TIU's case with the antenna wire protruding through the hole. Ensure that the transceiver board does not lift up while replacing the case! 
• Reassemble the TIU as described earlier in this section. 

The range of TIU to DCS Remote communications should be improved.

This and a whole lot more is all in “The DCS Companion 3rd Edition!" This book is available from many fine OGR advertisers and forum sponsors, or as an eBook or a printed book at OGR’s web store!

I've added an antenna to a TIU that was a 1 wavelength length total, that ends up being 13.13 inches.  I just added to the TIU antenna with the required wire length.  I also used a tube with a base to stand the whole affair vertically.  It worked quite well.

Thanks for the replies guys.

So gunrunnerjohn, just how did you attach the wavelength wire to the TIU wire, did you solder it?

I did this female connection to the end of the existing TIU antenna that I stuck through a small hole in the case.  On the antenna extension, I did a mating connector.  The wire was flexible, but I just had a tube with a base that supported it vertically.  I did it so I could easily take the mods off.