4) Jestream JTH2

It turns out I have had all three (now four). These gain antennas are (generally) all significantly better than the stock antenna, especially on 220 mhz. (Jetstream is untested at this point).

There are only three (now four) gain antennas for the Kenwood TH-F6A that cover all three of the radio's transmit bands (146, 222 and 440 MHZ):
Jetstream JTH2

I looked for a Jetstream JTH2 review on eham and found a mention of it in another review. Only the one, and it is a 0/5, the unit did not work on UHF. Need to do some testing!
http://www.eham.net/reviews/review/58567∞

4) Jestream JTH2 (new at Dayton 07)
It turns out I have had all three (now four). These gain antennas are all significantly better than the stock antenna, especially on 220 mhz.
The Jetstream JTH2 showed up at Dayton 2007. Ken and Oliver purchased them, at a price of $25 they are the lowest cost by a factor of two. Ken reports a mechanical failure, more details need to be gathered.

There are only three gain antennas for the Kenwood TH-F6A that cover all three of the radio's transmit bands (146, 222 and 440 MHZ):
If you read the eHam.net reviews for each of the above antennas you will find that the Diamond is the newest and has the best rating. I decided to try one of the new Diamonds after breaking my Maldol and putting a permanent bend into the wife's Comet.
I ordered the antenna after not finding it at the local HRO. It was out of stock and took awhile to get here. These antennas seem to be very popular. All three antennas are about the same price, so that is not an issue. The Maldol is very nice, but there is no support for the base of the antenna aside from the gold plated screw-on connector. The antenna elements are fairly small diameter and it is possible to bend them. This eventually led to the demise of my Maldol, after quite a few years of use. The Comet has a much thicker element. It also got bent. It does have support by resting the coil cover shoulder on the radio, so this is good.
The Diamond is a bit shorter than the other two. It is a quarter wave on 2m and 220 mhz, and a 1/2 wave on 440, according to package info.
The Diamond fits on the radio especially well. The lower coil cover shoulder touches the radio and adds support to the antenna (like the Comet).
The lower section seems like a spring. Stiff but flexible and somewhat thick (approx 1/8"). The upper section above the mid-coil is thinner and much more flexible.
RF testing will take some time. Mechanically this is definitely a nice antenna. I do recommend that bending any of these antennas significantly be avoided. This is difficult with the radio on the belt.
Read the eHam∞ reviews on this antenna for more comments.

There are only three gain antennas for the Kenwood TH-F6A that cover all three of the radio's transmit bands (146, 222 and 440 MHZ:

There are only three gain antennas for the Kenwood TH-F6A that cover all three of the radio's transmit bands (146, 222 and 450 MHZ:

1) Maldol MH-610
2) Comet HT-224
If you read the eHam.net reviews for each of the above antennas you will find that the Diamond is the newest and has the best rating. I decided to try one of the new Diamonds after breaking my Maldol and putting a permanent set into the wife's Comet.
I ordered the antenna after not finding it at the local HRO. It was out of stock and took awhile to get here. These antennas seem to be very popular. All three antennas are about the same price, so this is not an issue. The Maldol is very nice, but there is no support for the base of the antenna aside from the gold plated connector. The antenna elements are fairly small diameter and it is possible to bend them. This eventually led to the demise of my Maldol, after quite a few years of use. The Comet has a much thicker element. It also got bent. It does have support by resting the coil support on the radio, so this is good. Now on to the Diamond.
The Diamond fits on the radio especially well. The lower coil cover touches the radio and adds support to the antenna.

Diamond SRH320A Antenna for the Kenwood TH-F6A