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I assume when you say “use other antennas” you are referring to the ATU, not the attached whip.
The Talking House system is certified for Part 15.219 use with the Talking House ATU. To use an external antenna tuner other than the Talking House ATU would not satisfiy the requirement of the certification and thus not satisfy the Part 15.219 rule.
I am aware of one instance where someone tested a different external antenna/tuner and the result was less than desireable compared to the ATU. I do not have all the details of the experiment so your own result could be different.
You could try one of the base loaded, 3 meter antennas described on various websites. Tuning would be trial and error, adjusting the coil to resonate the antenna. Matching impedance could be accomplished by adjusting the coax connection tap point from the coil bottom up. The process requires patience as tuning and impedance matching would most likely interact.
Everyone leans on using FCC certified systems to obtain that warm, fuzzzy feeling of security. But, it is legal to use a non-certified sytem so long as it satisfies the rules. Certification is required for the mass production and selling of transmitters, not the operation of such. It just so happens that the Talking House system is one which is certified for Part15.219 use with a separation between the transmitter and antenna using their ATU.
To separate the Talking House transmitter from an antenna/tuner other than the Talking House ATU would not satisfy the Part 15.219 3 meter rule as the Talking House certification which allows the separation requires using the ATU. Yes, we all feel that is silly but that’s the way it works.
However, if you could prove the field strength of your system meets Part 15.209 it would be legal regardless.
Short answers are few and far in between.