Sorry, drookit, but feedback in this form is purely audio being picked up by the mic and "looping" back through the audio amp causing the amp to go into overdrive ... the aerial doesn't figure in the equation at all.
The symptoms the OP describes is most likely caused by one of three faults.
1. The TX switching transister is not doing it's job properly and allowing the RX circuit to stay active during TX.
2. The contacts in the microphone are not breaking the audio output properly (when the GND wire switches to the RX position in the PTT switch this activates the speaker whether external or internal and likewise disconnects the speaker when the PTT button is pressed.)
3. There is a bad connection somewhere in the rig (dry joints are very common) or possibly a short in the mic cable.
Open the back of the mic and check that the wires are not frayed, also check that all the soldering is nice and neat. Check the same at the plug end also. A damn good squoosh of WD40 into the slider of the PTT button, whilst pressing it in and out, often cures problems related to the PTT switch (crackling, noisy audio, audio that sounds like you are in a wind tunnel etc.)
As for it being because the speaker is plugged into the PA socket, this is very unlikely as most rigs would require the user to switch the unit into PA mode before the PA speaker will kick in.... But sometimes in older rigs the PA/CB switch can get grotty (WD40 will cure that also.)
The rig (guessing from the fault described) sounds like it is an old DNT or possibly a YORK. But further guesses would be useless as no one has mentioned the make or model of the units involved, I very much doubt it'll be an Amstrad 901, or a Audioline. Most likely it'll be a cybernet chassis based rig. (The little old midland 99's and 104's did this occasionally also.)
Hope this helps.
