[Russ L]

Hello all,
I have learned that radio antennae choice is fraught with compromise, not unlike choosing a suitable sailboat.
The long wire horizontal radio antennae is considered a good performer, however it is directional. There is little practical radiation from the ends, so your best transmission is to the sides (beam to) and above. The long wire should be at least a half wave, and should be that distance above ground, so getting it 30 feet above the ground plane on a sailboat is easily done. The lead is part of the antennae length and since a tuner has better performance "making the wire longerî we should use a shorter than ideal antennae. So to maintain any kind of performance we need the tuner near the end of the long wire (top of mizzen).
The long wire horizontal antennae is not the best choice for a sailboat in transit or swinging on the hook.
I believe a dipole antennae to be impractical for the spring stay. It has the advantage of a coaxial lead but it is usually tuned for a specific band. The coaxial lead will usually fall from the middle of the antennae, as such it will foul the mainsail. The dipole consideration may explain the insulator in the middle of the spring stay, Bill.
A vertical antennae is omni directional. The broadcast area is greater so you need to optimize antennae efficiencies. This is your best choice for a system on the move. (Think VHF radio.)Ö and a boat offers a great ground plane.
A backstay antennae is kinda in between the two but much closer to vertical characteristics. However, a mainmast backstay antennae on a ketch rig with aluminum masts and wire shrouds surrounding it would be severely limited and not come close to the efficiencies that we require of a near vertical antennae.
Which leads me to the recommendation of a wire topping lift antennae on a ketch rig. It is near vertical and clear of the worst interference.
The best arrangement of this configuration should be a topic of much discussion. Two possibilities are: tuner up high on the mizzen or down near the boom. The upper placement of the tuner allows for a short lead length but is a top-loaded antennae and I have no idea of the effect on efficiencies of top-loading. The lower tuner placement allows for better sailing performance, but the lead along the boom lowers the antennae efficiency.
The details are the easy part. Isolating the antennae can be commercial or shop made items (as the load is low compared to standing rigging). The power loading of the antennae lead is not high so good quality wire is up to the task. There are many steel sloops cruising around with the tuner inside and the lead traveling under a side deck and out to the backstay. Topping lift movement (whipping around when the sail lifts the boom) can be countered with a section of bungee lashed to the wire & boom. I did this as a matter of course on the main (cutter rig) to limit chafe on the sail.
Cheers, Russ