Have you asked on the Google JT65HF web page ? This is where you will find the originator of the mode

mel G0GQK

There is a page about operating DX from an island somewhere on the net and the writer found the same as yourself. After trying different things they found that you should not lay the radials on the sand, they should be above the sand. As an example I feed a vertical similar to yours four feet above earth and the raised radials are tuned to length and have no difficulty in making contacts

Mel G0GQK

Ah ! You can bet your granpaw had loads of fun and spoken to lots of people all round the US and the world and he may be up there now yakkin' away and giving them all 59

Mel G0GQK

Good grief I bet that fast morse code makes 'em tremble !The answer is that Big Bazooka they keep talking about.

Mel G0GQK

Best thing to do is to connect everything together squirt a bit of RF into it and see what happens

Mel G0GQK

Only use about 5 watts when you're tuning and when its done make a note of the settings... then next time you won't have to do it, it will be already tuned.

Mel G0GQK

Advise him to make a card to place on the top of the tranceiver ANTENNA NOT CONNECTED

Mel G0GQK

I also have a 718 but do my connecting a different way. I use the ACC1 socket on the rear of the tranceiver, that's what its for, and connect this to a simple opto coupler and from there to the PC. Set the 718 for no ALC and full power, check your handbook to see how, and then adjust the power with Windows, its easy. You ned a power/SWR mter and keep the power to 25 watts, no need for any more. I suggest you start on a simple software like Airlink Express or Digipan, learn how to operate properly then you can try other modes. The most popular is PSK 31, the whole of Europe awaits !

Mel G0GQK

I've seen an oak tree in England, where lightning is rare, struck  and split open and totally splintered from top to bottom.

Mel G0GQK

I can understand your problem and a yagi doesn't work at its best at such a low height. Have you considered using a multiband vertical, one which doesn't require radials laid under the soil surface ? I've used an old design Cushcraft R5 for about 16 years and it works well and gets me around the world using SSB and PSK

Mel G0GQK

Could you just imagine all those big hairy truckers saying "Romeo on that " ha ha ha

Mel G0GQK

If you use 300 ohm or 450 ohm feeder any length and a 4:1 balun and an ATU you can use it on 40, 30, 20, 17, 15, 12 and 10 metres

G0GQK

Its the other pieces of equipment which are getting in the way. If you did a connection between ACC1, and an opto coupler, and made the right settings as required, there wouldn't be any difficulty.

G0GQK

The reason it happens is because they're thickheads. If they took time to listen to advice, read what they should do to operate properly, this sort of behavious wouldn't occur.
I could almost guarentee that when the band conditions improve there are people, who in their need to contact DX stations, will wind everything up and cause chaos. When they do, tell 'em, and tell 'em firmly, they're pests !

G0GQK

Er.... but you were addicted to JT 65 where there is no human content at all.

G0GQK