[Elecraft] Insulated Backstay

[Don Melcher]

Mike

The 23' feet dimension is specified because that is the shortest length
the tuners like the SGC230 and Icom AT120/130/140 AH2/3, etc., will load
on 2 MHz and 2.182 MHz is the "distress" frequency on Marine MF - though
not monitored much outside the US any more.

I have done computer modeling of a typical cruising boat - a Valiant 40
- and found that actually the BEST antenna was to only insulate the
bottom of the backstay and load the whole rig, AS DISCUSSED in the
current editions of the ARRL Antenna Book. But I got tired of arguing
with people that this would actually work - "But it is a short!" - as I
was advocating it long before the Antenna Book, and gave up. And - in
some cases it didn't work very well - on a ketch rig or with a "split"
backstay. On a sloop, with a single wire from the mast to the stern - it
is killer. In a lot of cases, we simply ran a 6 gauge wire back from the
base of the mast or the forestay to the ground of the tuner and jumpered
all the fitting toggles with wire for good continuity - forming a big
"loop". The one insulator was because without it, there would be a lot
of leakage from the chainplate at the feed point to ground after the
stern gets coated with salt spray (but some installations worked OK
without it anyway - Brian may recall Bill Parks' Islander 36 Stray Cat
Blues about 2 or 3 races ago and his effort before that - Bill
introduced the idea to me when he was interviewing me for an article in
Latitude 38. It was in a book that was published some years ago on Ham
Radio and Cruising and the author discussed not using insulators - at
first I too said it wouldn't work - then I thought about it for a
while).

For an insulated stay. a length of 45' seems to be (a stretch of the
word) "optimal" in that it is efficient on the lower frequencies yet not
too long so it maintains a decent angle of radiation on the higher
frequencies. In reality - an insulated backstay is a pretty poor antenna
when you look at the pattern as on most bands there are two pretty deep
nulls about 120 degrees off the bow. Fortunately - they are fairly
narrow. But you won't be able to achieve that length on a W32. A 23'
whip as typically used on a power boat and most multi-hulls has an
almost perfect omni pattern and a good launch angle - of course -
assuming the boat is upright in the water - which is often NOT the case
under way.

IN short - no matter what you do - it is a compromise and you would have
to work REALLY hard to screw it up so it wouldn't work as long as you
follow good engineering practices. I have never seen an Icom tuner fail
to load anything on any frequency - not the same can be said for the
SGC230 (but that seems to be a different problem where it just won't
tune sometimes without being reset) or certain SEA tuners (the 1635 will
not load a 1/4 wave on 4.146 MHz - the check-in frequency for the PacCup
race - go figure - and that is EXACTLY what they cut the backstay to on
a Santa Cruz 52 - I received panic calls from three boats just before
they were leaving on past Pacific Cups who had that tuner installed by
someone because it - with the companion SEA235 radio - was light weight
- didn't bother to check if it would work). SEA's other tuners (like the
1631) do not appear to have this problem and like the SGC, will work
with any radio. 

As far as grounding - there are many methods - but the only one I can
say WILL work is bronze plates (drag is NOT an issue on a Westsail 32) -
I recommend 2 of the 3" x 12" - and I have close to 500 installations
now with no RFI problems running 150 watts using e-mail to back that up.

I am not saying other methods WON'T work - just that it is difficult to
predict the results and it can be a tremendous amount of work and
expense with poor results. One thing that has been determined is SOME
sort of direct sea-water contact is needed.

Don Melcher
W6CZ
HF Radio On Board
www.hfradio.com