Glenn, I can understand your confusion.
Marine SSB sites & suppliers: they all keep copying the same old info about the 100 sq feet of copper.
Another "famous" independent research
by Gordon West showed that a short connection to an underwater bronze (only) was very efficient:
HAM radio operators do not stick to the compromise solution of what is called a "random wire antenna and automatic antenna tuner". There are tuned antenna solutions usually dedicated to 1 frequency band that do not need an antenna tuner. There is a huge amount of theoretic and practical data and designs in the HAM radio world, I wull dare to say that the HAM world is a lot more expert than your average "marine SSB site".
Now how to help you with clearer advice?
To be able to work multi-frequency with a sloping wire antenna (isolated part of a backstay or dedicated sloping wire) you will need an automatic antenna tuner. (unless you want to use frequency-dedicated antennas hence 1 per frequency to be able to skip the automatic antenna tuner - this IS possible but more complex on board of a sailing boat )
You will also need a good Radio requency grounding system to make the antenna system efficient AND to help the antenna tuner find a tune.
1. the simplest solution
: isolated backstay, GTO-lead from the base of the isolated stay to the automatic antenna tuner (total length isolated stay portion + GTO lead typically 30 - 42 feet) and a KISS grounding device connected to the ground lug of the antenna tuner
2. The cheapest solution
: a marine-grade wire sloping down from a spare halyard to the antenna tuner (same length as 1.), a short stretch (< 10 feet if possible) of 3 to 5 inch wide copper tape to an isolated (not bonded to other-) underwater bronze like a seacock, bronze plate,...
3. Probably the best solution
is combining the grounding solutions:
- connect tuner ground lug copper tape to an underwater bronze (dedicated bronze plate: expensive) or unbonded existing (eg a seacock, bronze strut supporting propeller
- connect also to a maximum of existing metalware: SS lifelines
, pulpit, pushpit, aluminum
toerail if you have one)
- run about 4 long stretches of copper tape under your floorboards from aft to the front, try to seperate them maximally (medium cost) OR
run a number of so-called radial wires (isolated copper wire 16 gauge or marine grade tinned copper wire: CHEAP) under the floorboards or on deck
(I run them along the toerail where they are protected and not hindering). Just any length (as long as possible) or pre-cut for your target frequencies following this formula:
300/(frequency in Mhzx4) gives you the length in metres.
I would suggest 2 radial wires (one each side of the boat) per target frequency. Bundle them together. In essence this is what the KISS thing is....
Even applying only one of 3 solutions should give you good enough results.
Because on top of that the seawater provides the best "take-off" platform to enable long-range HF communication.
Hope this made it a bit clearer?
A HAM operator