I am an antenna expert
Well it's a hobby. Let me get to my answer first. You should not replace the antenna. You should buy a new ELT with the matching antenna. I will tell you why I suggest that. As you may know this is an older ELT's that transmits on 121.5 MHz and 243 Mhz. BOTH of these are no longer monitored, so to speak. The new frequency is 406 Mhz is monitored by satellites whose job is to find you and aviator and sailors, and very very accurate. It transmits your unique identity and can also optionally send GPS position. However without GPS the accuracy is very good. The old system no longer monitored was not accurate or timely. Better than nothing, but new technology is light years better.
Seriously I am not a pointer Pointer 3000-1 expert (or is it Pointer 3000-10), but pretty sure either way this is an older ELT. If you REALLY want the advantage of safety and accurate position location
get a new ELT. Also ELT's are TSO'ed "systems" and the antenna is part of that. You can't mix and match any other antenna. Again old ELT might as well replace the whole ELT and antenna.
- ACK 406/121.5 MHz E-04 ELT FAA TSO
- ARTEX 345 406
Pointer ELT for 406 is their 8000 series. It is a new design so it does not use the 3000 series mount hardware or antenna. Pointer is more expensive than above models, so no advantage going Pointer.
Most ELT antenna even the new 4O6 Mhz models have antennas about 20-23" long. ARTEX has a 15" ELT antenna option. The driving factor is the 121.5 Mhz which is still part of the new ELT's. The 121.5 is for that last mile to "home" to airplane with Direction Finders. The ideal antenna length for 121.5" is about 18". If it was just for 406 Mhz the antenna could be 6" for a 1/4 or quarter length antenna. So 18" would be approx 3/4 wave for 406 Mhz. Some ELT antennas are shorter by using a base loaded coil in the antenna to trick the RF signal into seeing the antenna as electrically longer. That is why your antenna may be shorter?
You say your current antenna is 12" ? You are not going to get much shorter (but I am not 100% sure). Most ELT antennas I see are in the 20" ball park, they can be shorter, however the antenna has to match ELT's which are TSO'ed systems, and you MUST use the exact antenna part number that it was designed to use or an approved substitute by the manufacture.
WHY YOU NEED A NEW ELT - The new 406 ELT's are VERY accurate and transmit your unique identification (that is registered to you and your plane). The 406 beacon will be detected faster, more often and with precision. The 243 is not officially monitored any more and the satellites are not over head often and can not locate the signal with any precision. Also you can transmit your GPS position as an option. I was told by ELT manufacture that even without GPS data it is very accurate, way better than the older ELT's even on their best day. You might be lucky and get someone monitoring 121.5 Mhz but the range can be as short as a few 1000 feet to 5 miles in ideal line of sight condition. It is only 200 milli-watts transmitter. We transmit typically with our COM radio 5 to 10 watts. Your old ELT has 25 to 50 times less power. If you keep with the Pointer you really are checking a box to fill a regulation but it is pretty useless. These ELT's were to be phased out and no longer legal to use. The GOV backed down from the mandate, at least for now. I recall all new ELT installations going forward require the 406 ELT. You are repairing your old one so that is OK I believe?
The old ELT's date back to original FAA specs from early 1970's. This is old technology. One big reason to switch are false alarms. It is a huge pain, loss of big money (which you might be liable for) and resources to chase down false ELT alarms. With the new 406 has precision location (faster to find) and an ID with contact info. They can call you or at least find the location quickly wasting less time and money. Also new ELT's have newer battery technology that lasts up to 5 yrs for the one I know of.
DRAG and CLUTTER, HIDE THE ANTENNA - If you are worried about "drag" and antenna clutter on your sleek RV sky scooter, you can mount in the baggage area next to the plexi glass. Not as good as outside but not totally bad. Say you put it on the outside, top back and flip over? The antenna would be in the dirt. If you have it on the belly it is not ideal either unless you flip.