A top loading coil will have to be even bigger than a base loading coil.  Instead of a small top hat, you might consider a number of wires (3 to 6) sort of like guy wires) attached at the top and sloped at an angle of 30-45 degrees to the vertical.  Make the wires about two thirds the height of the antenna.

I ran an EZnec simulation for you.  I assumed the diameter of the vertical was 1 inch and that the eight  3 foot radials in the top hat were number 10 wire.  I further assumed 16 radials each 55 feet long.  It looks like a base loading coil in the vicinity of 75 micro-henries is needed.  The feedpoint impedence looks like about 4 ohms.

The math is fairly simple.  Assume (worst case scenario) that the tower pivots on the ground at it's base.  How long must the tower be in order to just touch a point 17 feet above ground that is a horizontal distance of 37 feet from the pivot point at the base of the tower?  In other words what is the hypotenuse of a right triangle whose two sides are 17 and 37?  The answer is 40.7 feet.  This would apply to just the tower with no antenna on it.  The tower could be shorter than this but if it has a beam on it then the distance of 40.7 feet would apply to the square root of the sum of the turning radius squared plus the height of the antenna squared.   For example a beam mounted at a height of 39 feet could have a maximum turning radius of  just under 11.7 feet without hitting the wire.  A beam mounted at 40 feet could have a maximum turning radius of just under 7.6 feet etc. That extra foot of height dramatically reduces the maximum allowable turning radius of the antenna.  To continue the example, a tower with a height of just under 40.7 feet would have to have a beam with a turning radius of zero in order to clear the wire.  Perhaps a good isotropic antenna would fill the bill in this case!

I'd be very careful running that amplifier to an antenna that close to where people are likely to be.  I suggest you use Eznec to model the antenna you propose to use.  Eznec will enable you to estimate the amount of rf exposure at various points (which you can specify) relative to the antenna.  

It all gets back to the very fuzzy interpretation of what FD really is.  Is it a contest, a picnic, an emergency drill?  Is it a place to take chances or is it a place to stick with what's tried and true?  Is it a place to learn or a place to use what you've already learned?  Is it a public relations operation where you amass points based on how many folks sign your guest book?

The problem with FD is that it's different things to different folks.  You apparently don't fit in with the hot shot competitors in your group that see it as strictly a contest where the score is everything.  Don't worry about it!

The journey's the thing - not the destination.  Stay home and have fun doing what you want.  If you want to make your stay at home more realistic...open the windows and let the mosquitos in,  turn off the AC, run on battery power,  eat hot dogs and drink beer all within the comfort and privacy of your own shack!  The important thing is for you to have fun.  

Beauty is in the eye of the beholder.  I like my TS-480HX.  It takes up very little room on my desktop and has performed flawlessly for me during the two and a half years I've used it.  It will put out a solid 200 watts (which I practically never use) and runs very cool at 100. I operate mostly on digital modes with occasional forays into CW and SSB.  I could care less what it sounds like on AM.  

I wonder whether coming out with a new HF rig every few months represents a triumph of marketing strategy over genuine engineering improvement and innovation.

Consider using the Olivia digital mode as a way of working thru the noise.  It's not as fast as psk31 but is far more impervious to noise and fading.  Olivia is able to work 12 to 13 db BELOW the noise level.  

Jerry:

Thanks for your reply.  I agree about the predicted performance on 20 meters but I'm planning on a loaded vertical in the attic for 20 thru 10 with a remote tuner at it's base in order to get some decent low angle radiation.  My primary interest in the Fishook is for use on 80 and 40 for NVIS work - although it would be nice to be able to use it on the higher bands as well.  

According to the latest version of the ARRL Antenna Handbook, they recommend a 4:1 voltage balun at the feedpoint and a 1:1 current balun for an isolation choke.  My thoughts were that the isolation choke should be placed just above ground level so that the radiation from the feedline would be allowed to take place a la the Caolina Windom.

According to Eznec I don't really need a 4:1 balun at the feedpoint if I'm only interested in 40 and 80.  

I'm thinking of moving to an area with HOA restrictions.  The lot I'm considering backs up to a wooded "reserve" area.  The woods begins about 30 feet from the back of the house. What I'd like to do is bury some good quality coax (Davis RF Bury Flex) thru the back yard into the woods and hide an antenna in the woods. I'm considering an OCF dipole (OCFD) with a 4:1 voltage balun at the feedpoint up about 15 feet. The antenna would be approximately 35 feet on one leg and 102 feet on the other. I realize this would be a "cloud warmer." DX is not my concern.  

I've been playing with EZNEC and have christened my design the "fishook."  The fishook places the two legs of the dipole at an angle of about 70 degrees. This would enable me to place the feedpoint relatively close to the edge of the woods while the two sides of the dipole go back into the woods.  So far things look pretty good - on EZNEC...but now for the real world!

Does anyone out there have any experience with OCFD's mounted close to the ground?  Has anyone tried to bend the two legs of the OCFD?  Has anyone experimented with varying the position of the feed point?

Take a look at Fldigi at w1hkj.com.  It's a multimode software program that (unlike MultiPSK) has an extremely well thought out and friendly GUI.  

I downloaded MultiPSK and it just sat on my computer for months before I worked up the nerve to tackle it!  It was just plain scary.

MultiPSK is a good program but requires a large investment in learning to use it.  The author of MultiPSK seems to have no interest in improving the user-interface, which he certainly has every right to do.  I quickly abandoned MultiPSK in favor of Fldigi when it came out.  IMHO the learning curve on Fldigi is much, much faster and easier.

I think the general consensus is that there is some optimum way of deploying a given number of wavelengths of wire in terms of how many ground mounted radials to make and how long to make them in terms of fractions of a wavelength.  I have seen several articles discussing this topic.  The general rule seems to be that a larger number of relatively short radials is more efficient than the same amount of wire deployed with a smaller number of long radials.  Of course the 5-BTV is a multiband antenna so the answer to the question will necessarily involve a compromise.

I have used 5-BTVs having 30 radials each about 25 feet long on 80 thru 10 meters and they have worked satisfasctorily.  Certainly such a system is bound to be more efficient on 40 meters than on 80 meters.

I think it boils down to how much space you have and how much time, money and effort you want to spend putting down radials.  If you have a favorite band I think it pays to optimize your design for that band and live with the radial system on the other bands.

I suspect that your deploymnet of 60 5 meter radials should work rather well.


Tim, N4UM

I think the general consensus is that there is some optimum way of deploying a given number of wavelengths of wire in terms of how many ground mounted radials to make and how long to make them in terms of fractions of a wavelength.  I have seen several articles discussing this topic.  The general rule seems to be that a larger number of relatively short radials is more efficient than the same amount of wire deployed with a smaller number of long radials.  Of course the 5-BTV is a multiband antenna so the answer to the question will necessarily involve a compromise.

I have used 5-BTVs having 30 radials each about 25 feet long on 80 thru 10 meters and they have worked satisfasctorily.  Certainly such a system is bound to be more efficient on 40 meters than on 80 meters.

I think it boils down to how much space you have and how much time, money and effort you want to spend putting down radials.  If you have a favorite band I think it pays to optimize your design for that band and live with the radial system on the other bands.

Tim, N4UM

Contact Dave, W1HJK.  He can be reached at
w1hkj@w1hkj.com.  You might also want to check out the NBEMS group on Yahoo for assistance.  When all else fails, read the manual.  It's a long one!

Dave, W1HKJ, has just announced the latest versions of two programs comprising NBEMS (Narrow Band Emergency Messaging System.) Fldigi 3.0 is a user friendly general purpose multi-mode digital communications program that handles a broad variety of digital modes -including several new ones; Thor, MFSK32 and MFSK64. Flarq 4.0, a very easy to use error-free file transfer program using ARQ has also just been released.  Both programs run under Windows 2K, XP and Vista as well as under Linux.  Check it out at w1hkj.com.

Tim, N4um

Dave, W1HKJ, has just announced the latest versions of the two programs comprising NBEMS (Narrow Band Emergency Messaging System.)  The two programs are Fldigi 3.0, a very easy to use multi-mode digital communications program and Flarq 4.0, an ARQ based error-free file transfer program that works in conjunction with Fldigi 3.0.  Fldigi has been designed from the ground up to be both flexible and user-friendly.  The latest version of Fldigi includes several new modes, Thor, MFSK32 and MFSK64.  Both Flarq and Fldigi will run under Windows XP, 2K and Vista as well as Linux. Check it out at w1hkj.com.

Tim, N4UM