SSB Antenna for a Ketch

[David Swindon]

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.

 

[Steve]

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

 

[krj]

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

 

[Steve]

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

 

[Bruce in Alaska]

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

 

[Bruce in Alaska]

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

 

[Bruce in Alaska]

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

 

[Doug Dotson]

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

 

[Doug Dotson]

A number of issues have been raised here that deserve some further comment.

Regarding RF coupling to the standing rigging, don't be overly concerned.
You will be coupled to the rigging no matter where you place the antenna.
Some geometries will doubtless be worse than others, but it will be
difficult to predict in advance. Moreover, such coupling is not
necessarily a bad thing. It is just a difficult thing to model and thus
more of an unknown.

This is my experience.

Regarding antenna length, more is not necessarily better and may be worse!
But whether worse or better, changing the length of an antenna may make it
different. For example, if you are crossing oceans and want reliable skip
communication over great distances, you want low radiation angles. A
quarter-wave or 5/8-wave vertical will be your best choice. That would be
about 16 feet in length at 14 MHz. Make your antenna 32 feet long and you
have a half-wave vertical with very little low-angle radiation at 14 MHz,
but at 7 MHz and below, low-angle radiation will be plentiful.

I think the OP mentioned an automatic tuner. From my experience
both the ICOM and SGC tuners require at least 23'. Not sure how a
longer antenna fairs. Shorter will definitely not tune well.

 

[Bruce in Alaska]

Regarding antenna length, more is not
necessarily better and may be worse! But
whether worse or better, changing the
length of an antenna may make it
different. For example, if you are
crossing oceans and want reliable skip
communication over great distances, you
want low radiation angles. A
quarter-wave or 5/8-wave vertical will
be your best choice. That would be about
16 feet in length at 14 MHz. Make your
antenna 32 feet long and you have a
half-wave vertical with very little
low-angle radiation at 14 MHz, but at 7
MHz and below, low-angle radiation will
be plentiful.

Also be aware that autotuners CAN"T tune antennas that
are within 50Khz of 1/2 wavelength. So inlight of this
one must pick a length of antenna that puts the 1/2
wavelength point on a frequency band that will never be used for
transmitting. Also understand that short antennas preform
very BADLY, as the Input Capacitance on the L tuner model
is increased. So your preformance below 4 Mhz will be drastically
reduced with antennas of less than 50 ft of electrical length.
If your using MF Frequencies for comms of less than 400 miles,
which is what they are there for, you will need a longer
antenna.

Bruce in alaska

 

[Bruce in Alaska]

I think the OP mentioned an automatic tuner. From my experience
both the ICOM and SGC tuners require at least 23'. Not sure how a
longer antenna fairs. Shorter will definitely not tune well.

As I posted eslewhere, autotuners have some very specific flaws
that keep them from having optimum preformance. 23' isn't near
long enough for ANY reasonable comm's below 8Mhz. The tuners
get get VERY lossy as input capacitance is increased, in order to tune
short antennas.

Bruce in alaska

 

[Doug Dotson]

I've been able to do reasonably well on 40 and 80 meters with my
23' whip. I do get more side effects like twinkling lights and sometimes
the LectraSan activates itself

Doug, k3qt
s/v Callista

 

[David Swindon]

So your preformance below 4 Mhz will be drastically

reduced with antennas of less than 50 ft of electrical length.
If your using MF Frequencies for comms of less than 400 miles,
which is what they are there for, you will need a longer
antenna.

Bruce in alaska

Theres been some really good discussion here. In my experiance cruising we
used a whole range of frequencies as sometimes we were communicating with
boats in the same area, and other times with boats back in home port. With
regard to the need for a longer antenna for short range (definatly required)
what are your thoughts on tying the triatic into the backstay as part of the
antenna system (the triatic is 14' long - although as the mizzen is shorter
than the main mast, the angle between the triatic and backstay is only about
30 degrees).

 

[Gary Schafer]

As Bruce says, "tuners get very lossy with short antennas". But that
is not the only problem with short antennas. The antenna and ground
system become very lossy with short antennas. Below 1/4 wavelength the
radiation resistance of the antenna drops drastically. It can be less
than an ohm. That equates to very high losses. The antenna system in
those cases may be only a few percent efficient.

It is far better to have a longer antenna that gives a much higher
radiation resistance even if it may not be the optimum length as far
as radiation pattern is concerned. If you can't get the power to the
antenna the radiation pattern doesn't much matter. You still won't get
out very well.

On a typical boat the radiation pattern is going to be far from ideal
with whatever length antenna you have due to all the surrounding
objects on the boat.

The difference in radiation patterns between a 1/2 wavelength and 5/8
wavelength antennas are minimal. About the only real difference is the
feed point impedance they present.

As far as antennas greater in length than a quarter wavelength, they
start to produce multiple lobes in the pattern. Which on a boat may
not be a bad thing. As you mention, sometimes higher angles are
desired depending on the distance trying to be covered.

A longer antenna on a typical boat is most always going to be more
efficient than a short antenna even if the longer antenna produces
multiple pattern lobes.

Regards
Gary

 

[Bruce Gordon]

As Bruce says, "tuners get very lossy with short antennas". But that
is not the only problem with short antennas. The antenna and ground
system become very lossy with short antennas. Below 1/4 wavelength the
radiation resistance of the antenna drops drastically. It can be less
than an ohm. That equates to very high losses. The antenna system in
those cases may be only a few percent efficient.

It is far better to have a longer antenna that gives a much higher
radiation resistance even if it may not be the optimum length as far
as radiation pattern is concerned. If you can't get the power to the
antenna the radiation pattern doesn't much matter. You still won't get
out very well.

On a typical boat the radiation pattern is going to be far from ideal
with whatever length antenna you have due to all the surrounding
objects on the boat.

The difference in radiation patterns between a 1/2 wavelength and 5/8
wavelength antennas are minimal. About the only real difference is the
feed point impedance they present.

As far as antennas greater in length than a quarter wavelength, they
start to produce multiple lobes in the pattern. Which on a boat may
not be a bad thing. As you mention, sometimes higher angles are
desired depending on the distance trying to be covered.

A longer antenna on a typical boat is most always going to be more
efficient than a short antenna even if the longer antenna produces
multiple pattern lobes.

Regards
Gary

Exactly, Doug says he does fairly well on 80 and 40 Meters with
a 23' whip and an autotuner. We take him at his word, but if he
would figure out how to increase that to 50' or 75', there is a GOOD
chance that he would do better, and even in poorer band conditions.
It doesn't take much power or antenna to communicate if the band is
open, to where you want to talk, on the frequency that your using.
Try that if the bands isn't so hot and the time of day is against
you, with a Very Marginal antenna system. Yea, I know most pleasure
boaters have never heard of Marine Frequencies below 4Mhz, but up
here in Alaska we have been using 1.6Mhz and 2.0Mhz - 4Mhz Marine
frequencies for years, and very suscessfully, even on Poor Band Years.
I have used 3261Khz for Maritime Comms for 35 years, and worked my
Fleet Vessels, with 65% completion of Comms, rate on a daily basis.
You will not get that kind of connectitvity, with a 23' antenna,
on your Maritime Mobile Stations. In the "Good Old Days", the previous
generation of Alaskan RadioMen used to work 1630Khz consistantly
every night for intercompany Comms.

Just because SGC says their tuner only needs 23ft of wire, doesn't mean
that you can actually talk to anyone with that type of system. By the
way SGC didn't do the design of that autotuner, themselves, they stole
it from SEA, and didn't even change the "CopyWrite Statement" in the
firmware code. All autotuners on the market today, come from the
original design work of Bill Schillb, an engineer for Motorola MF/HF
Systems, at the time. He worked out the basics of the tuning code and
the hardware design. When he left Motorola and came out west to
Seattle, he landed at Northern Radio for s short while, and while there
passed on the basic technology to Bill Forgey, who was Chief Engineer at
Nothern at the time. Both Bill's left Northern Radio just before it
went under, with Forgey taking the whole Design Team with him, and with
Dick Stephens started SEA. (Stephens Engineering Asscoiates) Dick was
the Chief Engineer at Northern before Bill, and his mentor. Bill Forgey
along with Mark Johnson (an ex Northern Tech) designed the first truely
Marine Radio Autotuner which was the SEA1600, using the basics that Bill
Schillb had imparted, and on which, they improved and expanded. The
first autotuner that had Frequency Memory and Instant Band Switching was
the SEA1612, and the B version is what SGC copied for their 23x series
tuners. Icom, Kenwood, Furuno, and the rest are "Johnnie Come Latelys"
in the world of Marine Autotuner design, and basically they reverse
engineered the SEA design and firmware, for their systems.

I was closely associated with these folks as a Traveling Radio Tech for
Northern Radio, before leaving to become Comm Supt. for the largest
Salmon Canner in Alaska. I count all these folks a close friends, even
after all these years, and also while working the Dark Side (I was a
Field Agent for the FCC for five years) of the industry. I also did a
pile of beta testing over the years for SEA, that included most of their
designs for autotuners. Some of the prototypes are still in use today,
in various places in alaska. I designed and installed the first Marine
Autotuner feeding a Dipole Antenna, and that system is still in use
today. Bill, Mark and I designed and built a 1Kw Maritime Mobile Coast
Station that has 8 Control Points, and uses a SEA1612B Autotuner, and
one of a kind Dipole Antenna for MF/HF Frequencies from 1630Khz
thru 30Mhz at the 150W PEP level, and 1Kw on 4Mhz, 6Mhz, 8Mhz, 12Mhz
16Mhz, 22Mhz Marine Frequencies, using another special Dipole Antenna,
from Morad Electronics. This system is still in use today as well, and
has been around for more than 10 years.

I don't usually "Toot my own horn", but I do have considerable practical
experience in this field, as well as a long history in the industry.



Bruce in alaska onetime Fed, and long time Radioman..........

--
Bruce (semiretired powderman & exFCC Field Inspector for Southeastern Alaska)
add a <2> before @
Bruce Gordon * Debora Gordon R.N. Bruce's Trading Post
P.O. Box EXI Excursion Inlet South
Juneau, Alaska 99850 Excursion Inlet, Alaska 99850
www.btpost.net www.99850.net

 

[Bruce in Alaska]

So your preformance below 4 Mhz will be drastically

Theres been some really good discussion here. In my experiance cruising we
used a whole range of frequencies as sometimes we were communicating with
boats in the same area, and other times with boats back in home port. With
regard to the need for a longer antenna for short range (definatly required)
what are your thoughts on tying the triatic into the backstay as part of the
antenna system (the triatic is 14' long - although as the mizzen is shorter
than the main mast, the angle between the triatic and backstay is only about
30 degrees).

Is that the rigging that goes between the mizzenmast and the mainmast
near their tops?

Bruce in alaska

 

[Doug Dotson]

I've seen quite a few rigged this way. Must be worth something.

Doug
s/v Callista

 

[Doug Dotson]

A 50' or 75' whip would be a bit ungainly

Doug
s/v CAllista

 

[David Swindon]

Is that the rigging that goes between the mizzenmast and the mainmast
near their tops?

Bruce in alaska

Yes, thats the one

 

[Gary Schafer]

Well do I have egg on my face!

Gary, you are correct, of course, in stating that there is not a lot of
difference between the vertical radiation patterns of half-wave and
quarter-wave antennas. Surely not the differences I was alluding to.

And so my statements to the contrary were just plain wrong.

While I was writing half-wave, I was thinking of something longer, like
3/4 wave. I should have been more careful and I do apologize.

My point, however, is just as valid. Many sailboats sport 45' backstay
antennas and that is close to 3/4 wavelength in the 15 MHz range. A 3/4
wave antenna has maximum vertical radiation at 45 degrees! I would say a
16- or even an 8-foot whip would be very competitive with such a
backstay antenna at the lower radiation angles needed for transoceanic
communication.

At higher marine frequencies, 3/4 wavelength is obviously even less than
45 feet.

Of course, the 3/4 wave will be efficient and easy on the autotuner.

I'll try to keep my brain in synch with my typing, henceforth.

Chuck

Hi Chuck,

That 3/4 wavelength antenna pattern you are looking at I will bet is
for a horizontal antenna 3/4 wave high. The pattern for a vertical
antenna is different. Also when you see antenna patterns that show
main lobe radiation angles you need to look closely at them to see how
many db down the signal really is at the desired angle. It does not
disappear entirely at any angle. Although there sharp notches in the
pattern at times where the signal is highly attenuated it is rare that
the signal is completely eliminated at that small angle.
Also with longer antennas, multiple lobes are created rather than a
single lobe as seen with a shorter antenna. Many times those multiple
lobes can be a help in filling in angles that may be otherwise missed.
Sometimes the nulls can work against you too.

With a sloping antenna such as a backstay, while the radiation angle
may be raised in one direction because of a long antenna it also is
lowered in the opposite direction because of the higher angle lobe.

On a boat you usually have little control of where the antenna goes
and the angle at which it runs.

The lowest radiation angle may not always be the best for the path you
are trying to work either. For very long distances low angles are
usually better but medium and shorter distances may be better with a
little higher radiation angle.

A note about the low frequencies:
If you are working surface wave communications below 3 mhz a vertical
antenna is essential. Only vertical polarization works in that mode
and is very reliable night and day over the given range.
Horizontally polarized signals cancel out and you get no surface wave
with them. AM broadcast stations are an example of this type of
propagation. Surface waves follow close to the earth on the low
frequencies. On higher frequencies they are quickly attenuated. I am
sure that Bruce can attest to the reliable communication on the low
band.

Doug's 23 foot whip may work very well on the higher bands as it is
more vertical than a backstay and probably more in the clear. But it
will not be a good performer on the lower bands.

Another note on short antennas: That 23 foot whip that Doug uses is
less than an 1/8 wavelength on 4 mhz.
A quarter wavelength vertical has a radiation resistance of around 36
ohms. Shorten it to an 1/8 wavelength and the radiation resistance
does not drop in half but goes down to around 6 ohms! That antenna
radiation resistance is in series with the ground system resistance
which is usually quite high. It may be in the order of 20 to 30 ohms
in many cases. Guess where most of the power goes.

Regards
Gary