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Author Topic: Hamstick NVIS?  (Read 5798 times)
KE7FD
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« on: January 26, 2012, 08:01:52 AM »

I'd like to get comments from actual users of hamstick style 80 meter NVIS antennas.  There's plenty of speculative opinions out there but I'd like to hear back from guys who have actually tried using 80m hamsticks for NVIS.  I'll either go with the traditional NVIS antenna found at DX Engineering or hamsticks depending on what I find out here.  I may try both eventually but only if both seem to compare fairly close.  My attempt at not reinventing the wheel.

Thx in advance,
Glen - KE7FD
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WX7G
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« Reply #1 on: January 26, 2012, 08:28:09 AM »

DX Engineering has an NVIS antenna?

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WB6BYU
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« Reply #2 on: January 26, 2012, 09:03:53 AM »

The local ARES team, and several individual members, got a nice package with
Hamsticks for 40 and 80m, a mast, etc.  It can work on 40m, but a wire dipole
is actually faster to deploy (because you don't have to fuss with tuning the
antennas) and works much better.

Nobody has ever used it successfully on 80m.  The main problem is getting
the antennas tuned and keeping them there.  In one case a branch waving
in the breeze 5 feet away shifted the SWR up to 3 : 1.  This is aggravated
by the fact that, if the whips aren't exactly matched, it increases the
common mode currents on the feedline.  That means that how the coax
is arranged affects the antenna tuning.  Each time you take it down to
adjust it and put it back up the tuning will also shift if the coax hangs
differently, so you might lengthen the antenna and still have it end up
resonant higher in frequency than before.  And, of course, the bandwidth
is pretty narrow, so you have to get it carefully tuned to the frequency
you want to use.

There are ways you can improve it, however.  The first would be to add
some extensions between the feedpoint and the base of the Hamstick.
If you want to keep everything in a small package I'd suggest one or
two of the DX Engineering 4' mast sections on each side.  That doubles
the length of the antenna, reduces losses, broadens the bandwidth
somewhat, and improves efficiency.  You could also try using the 40m
or 60m coils with longer whips and/or with capacity hats added on the
ends of the whips.

A 1 : 1 current balun (which could be coax wrapped around a ferrite core)
at the feedpoint will help to stabilize the tuning.  I've also considered
inductively coupling the coax to the antenna rather than direct feed:  put a
coil between the ends of the two Hamsticks and wind a link coil around it
that connects to the coax.  The series coil affects the tuning of the
antenna, and the number of turns on the link sets the feedpoint impedance
at resonance.  This gives better impedance matching (the impedance can
be low if the antenna is at all efficient) and also serves somewhat as a
balun.)


Every time we've tried the Hamstick dipoles on 80m, we revert to a wire
dipole instead, and it runs rings around the Hamstick, even when it is
draped across a hedge and no higher than 8 feet at the highest point.
For a quick comparison, make a set of dipole wires with large ring terminals
on the ends that fit over the same 3/8-24 studs that the Hamsticks
attach to.  Even if you have to make a half-size wire dipole with loading
coils you'll still come out ahead, and if you can put up a 20' mast for
the Hamstick you can almost always find a way to hang the wire dipoles
on the same mast.
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AI8P
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« Reply #3 on: January 26, 2012, 09:32:07 AM »

I have used the fixture that Hamstick sells to make an 80M dipole from 2 Hamsticks.

I used this in the Ohio State Parks On The Air contest last year.  It was on a 10 foot mast that had a tilt mount on my truck hitch.   Coax just taped to the mast sections - no balum, no coils of coax.

I carefully tuned it up before the contest and marked the whips.  I would pull into a parking lot, assemble the mast and throw it up, make the required 3 contacts, pull it back apart and race to the next park - could  be done in 10 minutes.

It worked fine and it was definitely quicker and easier than deploying a wire dipole - required less space in the parking lot (or beside the road) also.   

Your mileage may vary.

Dennis

PS I also had a 40M hamstick on my bed rail that I was using mobile - when the mast was up it completely screwed up the SWR on the 40M, so I couldn't use them both without  re-rigging.
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KE7FD
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« Reply #4 on: January 26, 2012, 09:57:50 AM »

To the post asking about DX Engineering, they sell the parts to build one as outlined at:
http://www.dxengineering.com/techarticles.asp?ID={6EA88D7E-7C98-4D86-A86E-D3F9AACF85AE}&KeyWords=True

g
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KD5RGJ
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« Reply #5 on: January 26, 2012, 10:06:02 AM »

I used a pair of hamsticks for a dipole as my first antenna. With a tuner I even worked 80-10.
I even worked stations in central and south America.
KD5RGJ
SPENCER HUDSON
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KE7FD
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« Reply #6 on: January 26, 2012, 10:27:49 AM »

Dale, it sounds like you've done what I'm asking about.  We have a group that meets weekly on 80m, from the east coast marching across the country to the west over the course of a few hours, with the specified requirement to reach only a few hundred miles out at most.  It's a bit too cold (for me) right now to get too involved with testing but I wanted to get the details worked out now so when spring arrives I can deploy an NVIS antenna to hook up my my chapter of the group.  I've got some hamsticks but never had much success with those although admittedly, if we take our time tuning them up better, anything is possible.  Unfortunately, when real world emergencies dictate the mood, having time to tweak this and that just are a luxury we would not have.  It sounds like dipoles of some sort will be the way to go.

BTW, the NVIS system popularized by US Military forces, the AS-2259/GR, looks to be a quarter wave doublet offset by 90 degrees.  DX Engineering and others have given good write-ups on the unit and why the departure from typical gain antenna considerations are key don't really apply for this antenna.

Thanks to all and any others who comment.

Glen - KE7FD
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KE7FD
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« Reply #7 on: January 26, 2012, 10:33:43 AM »

Again, the challenge isn't working DX but "local" stations.  This notion easily escapes most hams who are are wrapped up in contests and pushing the limits of antennas and amplifiers;  In an emergency situation you often just want to jump over a short distance where VHF and the sort is known not to work, and it needs to be inclusive within that range without dead spots as is common with "skip".  Someone a thousand miles away is still going to have to pick up the phone to get the information back to your intended destination.  NVIS fills this gap.

Glen - KE7FD
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KE7FD
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« Reply #8 on: January 26, 2012, 10:45:38 AM »

Dale, you said:

"A 1 : 1 current balun (which could be coax wrapped around a ferrite core)
at the feedpoint will help to stabilize the tuning.  I've also considered
inductively coupling the coax to the antenna rather than direct feed:  put a
coil between the ends of the two Hamsticks and wind a link coil around it
that connects to the coax.  The series coil affects the tuning of the
antenna, and the number of turns on the link sets the feedpoint impedance
at resonance.  This gives better impedance matching (the impedance can
be low if the antenna is at all efficient) and also serves somewhat as a
balun.)"


My preferred approach to this is just use our SGC tuner at the feed point or the base of the 15 ft mast.

Glen
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WB6BYU
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« Reply #9 on: January 26, 2012, 11:02:02 AM »

Quote from: KE7FD

BTW, the NVIS system popularized by US Military forces, the AS-2259/GR, looks to be a quarter wave doublet offset by 90 degrees.  DX Engineering and others have given good write-ups on the unit and why the departure from typical gain antenna considerations are key don't really apply for this antenna.



The problem with the AS-2259/GR is that it was required to operate up to 12 MHz.  To
do that the wires had to be short enough so the main lobe was still overhead.  As a
result the efficiency on 80m is rather low, even using the special mast that has a very
loss coax line running down the center (designed to connect directly to a rig with a
built-in tuner - we used it with a PRC-104.)

For 40m and 80m operation it is much easier to just use half wave dipoles installed
the same way.  This is much more suited to ham use (unless you are planning to use
30m), and allows operation without a tuner.  It also reduces losses in the coax on
those bands.  Yes, that makes the antenna somewhat larger:  you can use a coil-loaded
80m dipole if you need to reduce the size, but that may require more tuning adjustment
than a full-sized one.  I've never had a problem finding space to string up a portable
80m dipole, even in a campground, though I haven't operated from a crowded trailer park.

The other things that the DX Engineering article fails to mention are (1) that you have
to pick the band to suite current conditions - sometimes 40m and even 80m will NOT
work for NVIS because the critical frequency drops too low.  and (2) 160m works great
for NVIS, especially when the other bands go out.  (The last few winters we have been
using 80m during the day and 160m at night due to the low sunspots.  40m didn't open
for NVIS for about 3 years.)

I have a couple of wire dipole kits that I built for the ARES team.  One uses a dipole
for 80m and one for 40m / 160m (that is about the same length as the 80m dipole)
on a common feedpoint.  This allows operation on all 3 bands without any changes.
The other is a single wire antenna with color-coded Anderson connectors to switch
sections in and out.  Basically a 40m dipole with 80m extensions, and a loading coil
that can be connected across the joint to tune it for 160m.  The single wire dipole
is better for a quick setup, the multiple wire one better for long-term operation.

Using a tuner is fine, but you have to evaluate your coax losses to see if it is an
efficient approach.  The good news is that even very low power (or inefficient
antennas) can still radiate - we did hear a signal from one of the Hamstick dipoles
driven by a FT-817 even though the rig shut itself down due to high SWR, but
that was with an S1 noise floor and no other signals on the band.
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KE7FD
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« Reply #10 on: January 26, 2012, 12:00:37 PM »

Thanks Dale. Well, enter the ham operators arch nemesis, Compromise.  There are some parameters of this situation that to date we've not been very successful getting others to change such as the band of choice: 80m (75).  I can see us using an 80m dipole in some circumstances but not all (I'm not fixated on the AS-2259/GR design, I only point it out as a popular choice).  Heck, when I was a novice we had a guy in our high school radio club who worked a station a few hundred miles away using his Heathkit oil filled Cantenna dummy load, or should I say coax?  This whole project is wrought with compromise and I need to set aside temptations of being a purest; I'm still interested in real experiences.  That being said, I expect that we'll never be found using saturated lengths of spaghetti or bed coils.  There's going to be compromises forced on operators in real world events, what actually ends up being used boils down to choosing the compromises we can live with. Some of the sites I expect we might operate from have room for the AS-2259 as stated but not a full size 80m dipole without putting emergency aid workers or victims at risk; we can't take over all the space to set up an antenna.  We can't be as choosy as we might be on Field Day and will have to co-exist with all sorts of other things going on.  More compromises.

You bring up good points and I'd now be interested in hearing from users of the AS-2259 which uses shortened legs; might be interesting.

Glen
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KB4QAA
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« Reply #11 on: January 26, 2012, 08:55:06 PM »

DX Engineering has an NVIS antenna?

Yes, they do.  The drawback is that it can only be shipped "Ground".   Arrrr arrrr!
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W5DXP
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« Reply #12 on: January 27, 2012, 05:00:59 AM »

I'd like to get comments from actual users of hamstick style 80 meter NVIS antennas. 

There was a magazine article in one of the ham rags on that idea. Here's something from the web:

http://www.w0ipl.net/ECom/NVIS/nvis.htm

What I would worry about with dual 80m hamsticks is the radiation efficiency. With 100w input, one is not likely to radiate more than one watt. A 24' dipole with high-Q loading coils would probably radiate +10dB (10 times) as much RF as two 80m hamsticks.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KE7FD
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« Reply #13 on: January 28, 2012, 10:06:39 AM »

Cecil, very good site there.

Thx,
g
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WB6BYU
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« Reply #14 on: January 28, 2012, 02:27:36 PM »

Quote from: W5DXP
... Here's something from the web:

http://www.w0ipl.net/ECom/NVIS/nvis.htm



Clearly W0IPL and I have some different experiences working NVIS (not to mention different ideas
how a transmission line works), though the general approaches are similar.

I recommend target heights of 20 to 25 feet - that gives you a bit more efficiency (~2dB) as well as
somewhat less variation in antenna tuning due to the ground characteristics.   The low angle response
of a dipole antenna doesn't change that much over this range of heights - for example on 80m the
response at 30 degrees is about 5 +/- 1dB down from that at 90 degrees for antennas less than about
40 feet high.  (Or 30 feet over very good ground.)  There just isn't enough difference in low angle response
of the antenna itself compared to the high angle response to make any practical difference in the ability
to copy a local station through the noise.  But the gain due to height isn't a lot, and an antenna at 10'
will still work if that is what you can manage.

A couple thoughts on the issue of higher noise with an increase in height:  first, if interference from
distant stations is a limiting factor, you're probably not on the optimum band for the path and need
to move to the next lower band.  But since the antenna patterns themselves don't show any
relative difference, my guess is that it is due to vertically polarized pickup on the feedline due to
common mode currents.  I haven't experienced such problems (though I've certainly seen high
common mode currents with simple dipoles) but it is a good reason to use a balun at the feedpoint
of any such antenna.  (Our ARES trailer has a problem with interference from the battery chargers
that causes similar problems, and a balun reduces the pickup into the receiver.)

Note that this is INDEPENDENT of whether or not you use a tuner:  it is about preventing the coax
shield from being part of the antenna and picking up local noise and/or vertically polarized or low
angle signals (which may also be reduced by having less of the feedline running vertically, e.g. having
the antenna closer to the ground.)  I think this is one of the biggest problems with tuning up
a very narrow bandwidth antenna such as the Hamstick Dipole:  the tuning becomes highly
dependent on how the feedline is positioned.

A good estimate of antenna performance is the average height of the ends and the center:  raising
either one always increases signal strength when the other remains fixed (up to some reasonable
maximum: I've only analyzed the data up to about 50' on 80m.)  There isn't a lot of difference
between a vee or an inverted vee, but the losses are slightly lower with the feedpoint elevated than
with the ends elevated.

If you are operating from a base camp or other area with lots of people, this is another reason to
keep the ends of your antenna up in the air where they are out of the way.  Having 12' or 16' sectional
masts available to hold the ends up makes it much easier to coexist with other users.



Quote
What I would worry about with dual 80m hamsticks is the radiation efficiency. With 100w input, one is not likely to radiate more than one watt. A 24' dipole with high-Q loading coils would probably radiate +10dB (10 times) as much RF as two 80m hamsticks.


Here is where we get to compromises, and an analysis of what you expect to need to be able to do.
With proper choice of band NVIS can work well with very low powers - certainly those of us who
operate QRP have shown this many times.  As I mentioned above, I've heard signals from an FT-817
in SWR shutdown feeding a Hamstick dipole on 80m, and another station on 160m who simply used
his 80m dipole at whatever power level the rig cut itself back to.  Neither was particularly strong,
but both could be copied.

If you are only operating for a short time and have 100W available, the efficiency isn't as much of
a problem.  You can stick up a Hamstick dipole, use the tuner to get a match, make your contacts,
and move on.  Even a 20dB loss in antenna + feedline might not prevent you from making contacts.

But if you are going to provide communications for an extended period and have to make the best
use of limited battery power, or are working over marginal paths, then improving the antenna to a
full size dipole can make a big difference, as it would allow you to use lower power (and fewer
repeats) to get your message through.

This is where you have to have a clear understanding of the circumstances you plan to be operating
in and plan accordingly.  We still carry the Hamstick dipole kit in the ARES trailer (though I'm proposing
that we get some extensions to make it more efficient) because we might encounter a situation where
that is the only option.  But, even in my years with Search and Rescue, I haven't seen one yet.

« Last Edit: January 28, 2012, 02:29:57 PM by WB6BYU » Logged
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