I am in the final stages of building a 38' steel ketch and am currently
looking at the installation of a SSB system (Icom M802 / AT-140). I have
read back through the forum postings and gained some good knowledge, however
am wondering if anyone can help out with some the problems specific to a
ketch rig.

My boat (a Roberts Offshore 38) has a main mast 38' off the deck, and mizzen
28' off the deck. This gives me a total backstay length of about 42', and a
triatic about 14' long. My plan is to rig the main mast with twin backstays
(i.e. the backstay is split right from the top of the mast).

I realise that the longer the antenna the better, however the backstay runs
quite close the shrouds of the mizzen mast near the deck, and so I figure I
would probably need to place my lower insulator 6' off the deck (what is the
minimum clearance between the antenna and another shroud to avoid
coupling?). I've seen recommendations saying the top insulator should be 3'
from the mast head (any comments??). This would leave me with an effective
antenna length of about 33'. I guess what I am trying to gauge is what
effect all of the other rigging around it will have on performance (being a
steel boat all of this other rigging will be grounded). Is there a gain to
be made by insulating the triatic and linking this to the backstay
antenna??? What is your judgement on wether this will be an adequate antenna
system??? One suggestion that I have had is to rig a whip antenna off the
stern and use a change-over switch to manually select either the backstay or
whip depending on the frequency for transmission . . . I much prefer the
idea of just using the backstay. Any comments would be much appreciated.

On Tue, 2 Nov 2004 17:02:47 +1030, "David Swindon"
wrote:
One suggestion that I have had is to rig a whip antenna off the
stern and use a change-over switch to manually select either the backstay or
whip

==========================================

Having the whip antenna as a backup is not a bad idea for an offshore
boat, and you may find the performance to be just as good or better.
You could do the change over manually to eliminate the need for a
switch. The only drawback I can think of is the possibility that the
mizzen boom is long enough that it could contact the whip.

How about coming down from the main with a 3' insulated penant, then split
the twin backstays with insulators on each at the lower end of each. Using
your calculation, that would give you a total length of 66ft.

In my own SSB installation on my 38 ft cutter, with twin back stays, the
upper insulator is only about a foot from the other rigging (and grounded
twin). I haven't noticed any problem (so far).

Since my backstays, run only a couple inches from my radar arch, at the
lower end, I placed the lower insulator just above the arch.

As I side note: I don't fully trust radio insulators (not that I have ever
known of one to fail), so I like the idea of leaving the one backstay out
of the potential failure equation. My main mast can stand and continue to
carry full canvas, even if the insulated backstay were to fail.

Just my thoughts, FWIW.

Steve
s/v Good Intentions

Steve wrote:

How about coming down from the main with a 3' insulated penant, then split
the twin backstays with insulators on each at the lower end of each. Using
your calculation, that would give you a total length of 66ft.

In my own SSB installation on my 38 ft cutter, with twin back stays, the
upper insulator is only about a foot from the other rigging (and grounded
twin). I haven't noticed any problem (so far).

Since my backstays, run only a couple inches from my radar arch, at the
lower end, I placed the lower insulator just above the arch.

As I side note: I don't fully trust radio insulators (not that I have ever
known of one to fail), so I like the idea of leaving the one backstay out
of the potential failure equation. My main mast can stand and continue to
carry full canvas, even if the insulated backstay were to fail.

Just my thoughts, FWIW.

Steve
s/v Good Intentions



The main thing for good xmission is good counterpoise.
krj

"krj" wrote in message
...
The main thing for good xmission is good counterpoise.
krj

I don't think David (OP) is going to have a counterpoise problem, his boat
being steel hull. I think his problem is going to be the lead from the tuner
to the insulated backstay (6ft above the deck).

Myself, I have no external ballast and all through hulls are non-metalic..
I'm using a 3" foil strips in the bilge and my SWR is very good (I don't
remember the exact ratio).

Steve
s/v Good Intentions

In article ,
krj wrote:

The main thing for good xmission is good counterpoise.
krj

Counterpoise or RF Grounding System? These two items are NOT the same
thing.


Bruce in alaska
--
add a 2 before @

In article ,
"Steve" wrote:

I think his problem is going to be the lead from the tuner
to the insulated backstay (6ft above the deck).

One of the things that most folks just don't get, is the "Classic"
RF Thru Bulkhead Insulator. For Metal hulled vessels and houses
these are one of the most forgotten items of the installation.
On Military Vessels, MF and HF Antenna Feedlines are routed thru
no less that 6" Ceramic ThruHouse fittings. On most Commercial
SOLAS Vessels, they use the same standard size ThruHouse fittings.
If it were me designing for this vessel, I would look at using
a vertical wire seperate from any backstay or shroud, and a 28 Ft
unloaded vertical whip mounted on the front of the MizzenMast.
Since the hull is metal the Autotuner can be mounted anywhere
inside the hull, so the vertcal wire can be placed just about anywhere,
where it will have a straight shot to the bottom of the whip.
This would give you around 24ft of wire under a 28Ft whip for
52ft of total antenna length. That is very respectable in any
installation senerio. If you really want to do this right and have
GREAT MF capability, move the unloaded whip up to the MainMast and
Feed it with a wire that comes from the tuner to an insulator on the top
of MizzenMast, than on to another spring dampened insulator on the
MainMast just below where the whip is mounted. That will give you,
24Ft Mizzen wire, plus say 30ft between Masts and then 28ft whip, for a
total of 82 plus any jumpers or around 85ft. Now your talking with
significant MF Antenna radiation effecincy. By doing it this way you are
not comprimising any of your Mast Supports, and will have a very
effective MF/HF Antenna System.


Bruce in alaska
--
add a 2 before @

In article ,
Wayne.B wrote:

Having the whip antenna as a backup is not a bad idea for an offshore
boat, and you may find the performance to be just as good or better.
You could do the change over manually to eliminate the need for a
switch. The only drawback I can think of is the possibility that the
mizzen boom is long enough that it could contact the whip.

Another drawback to any switching of antennas external to the autotuner
is that you defeat the purpose of the autotuner antenna memory function,
in that you now have two different antennas the tuner is trying to tune
for the same frequency, and the tuner can only store one set of
data/Freq tuned. Makes for a lot of Relay chattering and the relay
points have ALWAYS been the weakest link in autotuners. Something most
noncommercial users never think about. Don't try that on a SGC Tuner,
or you will have it InTransit to the factory, more than installed on the
boat, especially in installations on metal hulled vessels with short
(less than 35ft) antennas on MF Frequencies.


Bruce in alaska
--
add a 2 before @

Don't get hung up on the length issue. I use a 23' whip and get as good
as perfomance and any of my friends with insulated backstays. I believe the
specs say that 23' is the min length that can be tuned by the AT-140. Any
length beyond that doesn't gain that much additional performance.
Doug, k3qt
s/v Callista

"David Swindon" wrote in message
...
I am in the final stages of building a 38' steel ketch and am currently
looking at the installation of a SSB system (Icom M802 / AT-140). I have
read back through the forum postings and gained some good knowledge,
however
am wondering if anyone can help out with some the problems specific to a
ketch rig.

My boat (a Roberts Offshore 38) has a main mast 38' off the deck, and
mizzen
28' off the deck. This gives me a total backstay length of about 42', and
a
triatic about 14' long. My plan is to rig the main mast with twin
backstays
(i.e. the backstay is split right from the top of the mast).

I realise that the longer the antenna the better, however the backstay
runs
quite close the shrouds of the mizzen mast near the deck, and so I figure
I
would probably need to place my lower insulator 6' off the deck (what is
the
minimum clearance between the antenna and another shroud to avoid
coupling?). I've seen recommendations saying the top insulator should be
3'
from the mast head (any comments??). This would leave me with an effective
antenna length of about 33'. I guess what I am trying to gauge is what
effect all of the other rigging around it will have on performance (being
a
steel boat all of this other rigging will be grounded). Is there a gain to
be made by insulating the triatic and linking this to the backstay
antenna??? What is your judgement on wether this will be an adequate
antenna
system??? One suggestion that I have had is to rig a whip antenna off the
stern and use a change-over switch to manually select either the backstay
or
whip depending on the frequency for transmission . . . I much prefer the
idea of just using the backstay. Any comments would be much appreciated.

"David Swindon" wrote in message
...

My boat (a Roberts Offshore 38) has a main mast 38' off the deck, and
mizzen
28' off the deck. This gives me a total backstay length of about 42', and
a
triatic about 14' long. My plan is to rig the main mast with twin
backstays
(i.e. the backstay is split right from the top of the mast).

We use a tuner mounted on the rail of the pushpit. The antenna is tied off
to the rail beside the tuner and basically looks like and inverted 'L'. We
use a flag hoist on the mizzen and then the antenna is run to the peak of
the main mast. The mainmast end, ends about three feet from the main and the
antenna is adjusted to clear the mizzen about the same as it runs down to
the tuner. The overall length is 28 feet.

Leanne
s/v Fundy