An antenna for 27 Mhz (or any other frequency) has to be an electrical conductor. From there, it can be of almost any 'shape', wire, or a metal rod which will stand up by it's self. Using simple wire means that you have to provide the support for that wire, bamboo for instance, or any other non-conducting device or material.
1/4 wave antennas, which are usually the shortest usable length without adding a coil to the antenna, is dependent on on the frequency of use. A very rough means of finding that length is by dividing '234' by the frequency in Mhz, which will give you length in feet. It will also be slightly too long, which means that it will have to be "trimmed" to the correct length, which is usually referred to as 'tuning' the antenna.
In making an antenna, your imagination is very useful!
- 'Doc