Need advise on NVIS antenna choices.

[NL7UZ]

Hi All!

Thanks for taking the time to look and answer!

I live in a remote part of Alaska and would like to setup  dependable HF comms in the 50-200 mile range. I have been doing lots research and would like input from others. I am planing on using three primary ham frequencies (40, 60, 80), plus need to be able to use on the Alaska Public Fixed service frequencies around 2.5 MHz. I have autotuners for all points and can use full size dipoles for two sites, but need to find a good option for the boat. Not much I can find for high angle radiator that is small in size and can be tuned to 2.5 MHz. I don't think the marine antenna needs to be very efficient as the fixed sites can make up for some of its lacking. Can I use a loading coil and a short piece of wire on the top of the wheel house? Again, none of this is for DX!

Thanks All
DE NL7UZ Matt

[WX7G]

The optimum height for 7 MHz is 20 feet with a gradual decrease in high angle gain to a height of 40 feet. A good compromise height to cover 80, 60, and 40 meters is 30 to 40 feet.

The boat monopole antenna has an essentially lossless ground. If you'd like a short antenna for 2.5 MHz there is an off-the-shelf solution. A Hustler RM80 resonator, with a DX Engineering top hat, mounted on a 72 inch Hustler mobile mast will work. The 3:1 VSWR bandwidth will be about 5 kHz. Assuming the antenna height from ocean water to the top hat is 15' the radiation efficiency is about 5%.

[KQ6Q]

Keep mind that NVIS imples Near Vertical radiation/reception. A vertical has a NULL in the vertical direction, top hat or no.
For the boat, suggest something like an Icom AH-4 tuner with a horizontal wire, or at least a sloping wire. It needs to radiate UP.

My mobilehome setup at first had short verticals on metal roof for all HF bands, some with multiple resonators. From Long Beach, I found I couldn't work the northern california sections. I switched to a hamstick dipole on 40m, and now I can work near and far -  all the california sections, and all the sections in the SS. that's the kind of coverage you want on the boat. Use a horizontal antenna.

Fred, KQ6Q.

[WB6BYU]

It is possible that a vertical would work well enough in the boat since you probably can
cover a lot further distance on groundwave over salt water than over land.  Or a more
distant station might need to relay for you.  So, even though verticals generally aren't
optimum for NVIS, don't rule it out in that case.

How big is the boat?  How tall is the mast?  It may be that some sort of loop will do
better than anything else in that situation, but that greatly depends on the space
available.  The "Hamstick dipole" approach might work, but the tuning is so sharp
with a shortened antenna that the whips waving in the swell likely will be enough
to watch the SWR change.  It probably would be better to use a loaded dipole
with the ends of the wires secured to the boat - make the dipole as long as possible
and use enough loading coil at the feedpoint to get a match.  In fact, if you
use a link coil around the main loading coil to attach the feedline, you can set the
resonant frequency on the main coil and then fine-tune the SWR by varying the
number of turns on the link coil.  It will still be narrow banded, but you should be
able to make it work.

[NL7UZ]

Thanks for the ideas, the boat is 46 foot fiberglass hull with aluminum mast and stabilizer poles. There is a 23' Morad Marine HF, MF vertical mounted on the mast. The radio is a Yaesu 897 commercial marine Auto tuner built by SEA. The Autotuner is mounted to the top of the wheelhouse roof and is connected to 3' copper strap that is bonded below decks. There is several hundred square feet of copper strap bonding everything together. This arrangement puts out a nice signal on the upper bands, but I can't talk to my house 50 miles away. As far as ground wave goes, everywhere around here is 5000 to 8000 foot mountains coming straight out of the water, so we lose groundwave quickly. I thought I might be able to lay the 23' vert. on the horizontal to get my signal to burn the clouds as they say, but it is just to big and looks wrong. What about using a loading coil at the autotuner feedpoint and then running a essential wire on standoffs just above the boats wheelhouse? The tuner should be able to handle tuning a 23 foot wire to 160 meters.
I have been thinking about Mag. Loops, but it has to be compact enough and still work on 2 MHZ.
As far as the Hamstick dipole goes would a couple of 160 meter sticks with the tuner at the center work to tune 2-30 MHZ?

Thanks
Matt DE NL7UZ

[WB6BYU]

As far as the Hamstick dipole goes would a couple of 160 meter sticks with the tuner at the center work to tune 2-30 MHZ?

No.

The loading coils will act like big RF chokes on some bands, unless they are self-resonant in which case
you have a lot of small wire for the RF to flow through that isn't contributing anything.  If you put a tuner
right at the feedpoint you might get a pair of 80m elements to work down at 2.5 MHz.

I was thinking of running a large loop of wire over the top of the mast and down to each end of the boat,
then back to the tuner in the center.  A practical application might incorporate the stays as part of the
loop instead.  I had a friend who did this on a sailboat (I think the loop used the boom, boom stay, and
mast, with a feedpoint insulator between the mast and the boom) and reported it worked better than
the backstay as an end-fed wire.

A horizontal end-fed wire will work, but you need to make sure that the maximum current is in the
horizontal portion, which is not the case in many installations.  But you might be able to add a loading
coil to bring the current maximum away from the tuner (though getting that to work on multiple bands
might not be easy.)

The reason for trying the loop is that the current maximum is always going to be opposite the feedpoint
regardless of loop length, which is where you want it.  I don't know if your tuner has enough range to
match such a loop, but it would be worth a try.

[VA7CPC]

FWIW --

With all that aluminum above decks, the "vertical null" pattern of your vertical whip may be distorted, and it might work OK as an NVIS antenna. 

Working stations 50 miles away is often troublesome.  They're too far away for ground wave on 40m/20m.  At that distance, to use the ionosphere, you will have near-vertical incidence, and need 180-degree reflection. 

That means that you can't use frequencies greater than the "critical frequency".  For a map of critical frequency (near-real-time), see:

http://www.ips.gov.au/HF_Systems/6/5

According to that map, right now (11 PM PDT), 80m may be usable for NVIS in Alaska; nothing higher (in frequency) will work.

There's a good reason that the marine emergency calling frequency was set at 2 MHz -- it gives decent _short-range_ results a lot of the time.  Before setting up new antennas on the boat (always a trial!), try using 160m with the 23' whip. 

_If_ the antenna tuner can handle it, and you can arrange a base-station antenna for 160m, it's worth trying.   You might get through with ground wave, you might get through with NVIS sky wave -- it really doesn't matter.

                 Charles

[WA3SKN]

For NVIS operations, I like a Loop, mounted less that 1/4 wavelength high... and with a 46 ft boat you just might be able to make one!  I also prefer ladder line, but it might not be the best choice for marine use,  It would be great for multiband operations, though!
What is the beam of the boat?

-Mike.

[WA3SKN]

Oh, and a vertical is a poor choice, but an inverted Vee might do!  Again, mounted low.

-Mike.

[K8KAS]

A good friend lives on a 38 foot sailboat in the winter and uses a Hamstick 40 meter dipole / 2-- 8 foot 40 meter mobile whips and a center feed point, this is fed with balanced line to a balanced tuner. he hauls the Hamstick dipole up 35 or 40 feet on the mast with the halyard. He puts out one of the best signals from South Fla period.
I have found good results NVIS with any dipole, etc less than a 1/4 wl high on 75/40/30 meters, from my experience your don't need anything fancy just an efficient horizontal antenna below 1/4 wl on the bands you need the high angle signal. For NVIS from the boat just keep the antenna low 15 to 20 feet and you should get some nice high angle.   73 Denny K8KAS

[WX7G]

The reason I didn't recommend a dipole is that the efficiency is so low that the vertical can outperform it. Running a NEC simulation of the vertical I describe vs a 14', 160 dipole shows that the vertical has more gain at take-off-angles (TOA) below 86 degrees. That means that the vertical will be better at NVIS distances beyond 13 miles.

The sims included expected loading coil losses.

[WA4PTZ]

If you are seeking a multiband antenna then NVIS is not what you want.
NVIS is a directional system designed for a single band. It is like a yagi pointing
up, toward the sky, which is why a NVIS is referred to as a"Cloud Burner".
A 80 meter dipole or loop would be a better choice of antenna, since you have an
auto-tuner.
73,
Tim WA4PTZ

[WB6BYU]

If you are seeking a multiband antenna then NVIS is not what you want.
NVIS is a directional system designed for a single band. It is like a yagi pointing
up, toward the sky, which is why a NVIS is referred to as a"Cloud Burner".
A 80 meter dipole or loop would be a better choice of antenna, since you have an
auto-tuner.

I think there is some confusion here.  There is no contradiction between NVIS and "multi-band".  In fact,
since the operating frequency for NVIS has to change to keep below the Critical Frequency, most
NVIS installations NEED a multi-band antenna, even if it is just a set of dipoles for each band.


I did some modeling experiments with a loop antenna.  Guessing a maximum height of 25 feet, a
minimum of 10', and a total length of 40' (to fit the length of the boat) then a triangular loop of
#14 wire fed in the bottom center should give you a suitable pattern.  Efficiency is low - down
10dB or more, and improves if the top of the antenna can be made higher.  The antenna is
highly inductive with low radiation resistance (mostly due to wire losses) in the 2.5 MHz range,
and some tuners may have trouble if they are designed for short wires (which are capacitive.)
Adding a capacitor in series with each side of the feedpoint might help the matching on one or
more bands, but they need to be rated for a few thousand volts, and salt spray may be a problem.
Ideally it would be fed in a balanced manner with the tuner insulated from ground, but that might
not be convenient in your situation.

[WX7G]

Can I use a loading coil and a short piece of wire on the top of the wheel house?

Yes that will work. A screwdriver antenna with a wire along with an antenna tuner. For example, a Tarheel 200HP screwdriver with a 12' wire will tune down to 2.5 MHz. The 'wire' can consist of a 48" Hustler mast extension with a 108" whip (MFJ sells these) on top. The input impedance at resonance will be around 10 ohms. The tuner or autotuner will match this to 50 ohms.

Tune procedure. With the tuner bypassed drive the coil until minimum VSWR is found. This will be about 5:1. Now switch in the tuner and tune.

[KY5U]

Take a look at the SGC tuners.  I did marine electronic work and used SGC tuners with varying lengths of wire.  I believe 25 feet was the minimum.  Also if you have a tuner, check out the Shakespere 390 23' vertical (fiberglass).  The 390 would tune 2285 kHz on the old AM ship to shore rigs.  Also, make sure you have a good ground plate and tune the rig with the boat in the water and configured as you will be when you're under way.