Portable Bullhorn Megaphone
There have been some complaints about the build quality and durability of this portable megaphone, but I personally didn't notice any issues. However, as it is advertised as being "rugged", it would be worth thinking carefully if this is the portable option for you if you're likely to be dropping it anytime soon.
This portable megaphone offers plenty of bang for the buck, and is able to clearly amplify voice over quite a distance (it worked fine for me on a large scale camping trip, but perhaps the others on the camp site didn't appreciate that!). The built in siren is also a nice to have feature, as it is an easy, repeatable way to get people's attention.
The sound quality itself is good enough, but I would not advocate use in a motor sports event or anything else with loud low to mid range backround noises, as this megaphone will not get loud enough. Also of note is that it is not a "push to talk" model, but it is actually an on/off switch mechanism. Fine for the occasional announcement, but you could end up shouting into the wind if you forget to flick the switch.
Some people have said this is too heavy for a portable megaphone, but it features a shoulder strap for easy access. Depending on how long you want the megaphone to be portable, it may not last long enough for you, and you would need to take some spare batteries with you!
Overall, a good portable megaphone, but you may need to scale up a bit for louder/longer carry of your voice.
Feel free to read more about the portable megaphone over on Amazon.