Every HackRF One shipped to a Kickstarter backer will come with a telescopic antenna. It took me some time to find an antenna that I like. I wanted something with a wide frequency range below 1 GHz and that would mechanically suit HackRF One. Most of the telescopic antennas I like have a BNC connector instead of SMA, so I ended up having one manufactured to my specifications with an SMA connector.
I just received samples of the telescopic antenna, and I am very pleased with it. The length (including the connector) fully collapsed is 20 cm. The length fully extended is 88 cm. This is long enough to reach frequencies below 100 MHz as a quarter wave antenna and short enough to reach frequencies above 1 GHz as a 5/8 wave antenna. It has an elbow allowing it to be bent to a vertical or horizontal position, and it can be rotated without loosening the SMA connector.
It may be a little weird to ship HackRF One with an antenna that only covers a fraction of the operating frequency range. For that reason and also due to cost, I do not plan to include the antenna with the HackRF One retail package to be sold after the Kickstarter rewards are delivered. Every Kickstarter backer is getting one of these for free with each HackRF One, however, because I think it is the most generally useful antenna you could have. Since I am happy with the way the antenna turned out, I am having more manufactured and plan to make it available post-Kickstarter as a separate item.
Personally I've used telescopic antennas like this for short range operation over a much wider frequency range extending from 27 MHz to at least the 2.4 GHz band. When you are only operating over a short distance (like 10 meters or less), you can usually get away with using an antenna that is sub-optimal for your operating frequency. If I had to choose just one antenna to travel with, it would be this telescopic antenna.
The PROTOTYPE backers will receive a couple of additional antennas. One of them is a Log Periodic Antenna covering frequencies from 850 MHz to 6500 MHz.
The LPA is made by Kent Britain (WA5VJB), my favorite antenna designer. It nicely covers the upper reaches of HackRF One's range of operating frequencies, and it is directional. I'm in the process of soldering on the SMA connectors now. I will include a short SMA cable with each one. You can use the SMA cable for the antenna, or you can use it for clock synchronization when you receive your second HackRF One from the final production.
The other antenna I'm including with the PROTOTYPE rewards is a small 2.4 GHz dipole like the kind you might find on Wi-Fi or Bluetooth equipment. It is very portable and durable, and it performs well for its size. Like most Wi-Fi antennas, it has an RP-SMA connector which does not match the SMA connector on HackRF One, so I'm including an RP-SMA/SMA adapter. If you have any other antennas with RP-SMA connectors, such as the popular high gain Wi-Fi antennas, you can use them with this adapter on HackRF One.