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Author Topic: Short distance communications on 20M  (Read 12422 times)
WA3SKN
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Posts: 6496




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« Reply #15 on: July 28, 2011, 08:12:14 AM »

I am not sure there is any single band for 24/7 communications, however... I would try 40 meter NVIS setup for this.  A horizontal loop mounted no higher that 1/4 wavelength should do for the house.  The vehicle might be more of a problem to get horizontal polarization on 40 though.  Are you using while driving, or are you stationary when operating?  A "tilted vertical" might give you enough to be useful.

-Mike.
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WB6BYU
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Posts: 17053




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« Reply #16 on: July 28, 2011, 10:44:33 AM »

Quote from: 4671HYBRID

Do you have any wire dipole kits that you'd recommend or are they pretty much all the same? And why don't you need to tune them, is it because they're a specific length that corresponds to the frequency?



I make my own - they are very inexpensive if you can scrounge some wire.  It doesn't have to be fancy: I've
taken a 8' scrap of multi-wire telephone cable and connected all the conductors in series.  A computer networking
cable with a bad connector on one end may yield enough wire for this purpose when you separate the conductors.
A shop that rewinds motors probably can sell you some magnet wire - it kinks more than stranded wire, but will
still work.  You'll just have to look around and see what you can find.

My solution is make a center insulator with bolts and wing nuts to connect the wire elements.  My original one
used a slice of radiator hose as the insulator, but for heavier kits I'm using bits of broken PVC privacy lattice.
(This is another opportunity to improvise from materials at hand.)  The center insulator is attached to the
feedline (typically RG-174 for backpacking, or RG-58 for vehicle transport.)

Then I prepare separate dipole wires for each band.  (Color coding helps.)  Each time I set up the antenna I
choose which bands I plan to operate and attach the corresponding wires to the center insulator, hoist the
center in the air somehow, and tie the wire ends off to convenient bushes or clumps of grass with some
nylon twine (or dental floss, etc.)

I took the kit to a local park and adjusted the wire lengths for resonance on each band in a typical setup
(or a bit worse than that), and 30 years later I can still use the same wires without a tuner.  True, the
resonant frequency shifts around a bit each time I install it, but it has been close enough.

The kits I've built recently for the local ARES team are heavier duty and incorporate a few refinements.
The 80m band is wide enough, and subject to enough detuning from having the ends close to the ground,
that I added the ability to make some tuning adjustments to it.  I tied the ropes to the wires about 3 to 5
feet in from each end.  Then I can string up the antenna normally and clip the very end of the wire back
and forth along the standing wire and string to get about 200kHz of tuning range.

Using multiple wires I can operate multiple bands without any adjustment (convenient for day and night
operation) but requires more wire.  If the latter is in short supply, I have another kit that is basically
a 40m dipole cut to resonance, with the option to clip on an extension for 80m.  Again the wires are
tuned to length and no tuner should be needed.  I've some designs that use switches between the
wires, but my model uses color-coded Anderson connectors.  I've also added loading coils that I can
connect across the junction, giving me 3 three bands of operation on the same wire.  (60m operation
can be added as well - in fact it may come close enough if you use the 40m wire on one side and the
80m wire on the other.)  You can get by with just twisting the joints or using alligator clips.  (Make
sure the wires are properly secured to each other first, before making the electrical connection.)

The design philosophy is to make everything simple to use, while still permitting flexibility to meet
unusual circumstances.  I've used the 40m dipole wire to make a full wave loop for 10m (adding one
clip lead to tune it for SSB and two for CW), as well as pressing the longer wires into use as a
parallel feedline for a long run.

I've never seen such a dipole kit offered commercially.  There are some that use bare wire on a
spool that is unwound to the desired length, and at least one with large tape measures for the
elements, but they are all big, bulky and inconvenient compared to the kits I build for myself.
(You do have to learn how to wind up the ropes and wires so they don't tangle - this is what
makes the antenna quick to install.)


For NVIS operation, height isn't as important:  you can get your antenna TOO high, but that isn't
likely for a portable station like yours.  (You're fine if you keep it under 40' on 40m, 80' on 80m, etc.)
Height does minimize ground losses, but a 20' to 25' mast should be adequate.
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KF9OK
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Posts: 1




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« Reply #17 on: July 29, 2011, 12:31:33 PM »

For the base station, if you have real estate to work with the antenna is no problem at all.  If you're not concerned with DX, an NVIS dipole can be very close to the ground and still work very well - remember that the signal is going UP, bounding off the ionosphere then coming back down.  You'll usually get less noise and less interference from DX signals by keeping antenna height low to minimize DX.  The other trick is to put a "reflector" wire underneath the dipole which will help with antenna efficiency.  Dipoles as low as 1 or 2 meters can work.

As previously mentioned the lower frequency bands are what you need, 80 to 40 meters.

Mobile antenna is a little more of a challenge as there are few antennas designed for this kind of signal pattern on the market.
http://www.stealth.ae/plugins/custompages/detinf.php?id=372&id_categories=136
Here is one, designed as a roof rack and antenna for an SUV type vehicle for NVIS use on HF.  Could also homebrew an antenna from hamsticks or the like mounted horizontally from a small mast or something. 
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K2OWK
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Posts: 1273




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« Reply #18 on: July 29, 2011, 01:17:40 PM »

Just a quick mention. If you need a horizontally polarized mobile antenna, back in the day there was a mobile antenna called a halo which was a horizontal multi loop mounted to a mobile mount, and if I remember correctly was available for 80 and 40 meters. I would imagine it should be available someware. Do a Google search and see what you com up with.

73s

K2OWK
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N1LO
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Posts: 1101


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« Reply #19 on: July 29, 2011, 01:27:13 PM »

see my antenna page at www.qsl.net/n1lo for a photo plan for building lightweight doublets. You can substitute rg-58 or even rg-174 for the parallel line.

I use 40' Spiderbeam telescoping masts and set the antenna around the 33' height. I have a drive-on mast base that holds the mast.

It's far simpler and faster to use the clip-on wire method I described earlier.

Mark
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WB6BYU
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Posts: 17053




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« Reply #20 on: July 29, 2011, 06:40:43 PM »

Quote from: K2OWK
...back in the day there was a mobile antenna called a halo which was a horizontal multi loop mounted to a mobile mount, and if I remember correctly was available for 80 and 40 meters...


I don't think you remember correctly.  The halo is a half wave dipole bent into a circle.  2m is easy.  6m for mobile use
is about as large as I would want to try, and may need some additional bracing.  But an 80m halo would be around
40 feet across - a bit large for convenient mobile operation.
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K2OWK
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Posts: 1273




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« Reply #21 on: July 29, 2011, 09:35:00 PM »

If I remember correctly the lower frequency's 40 and 80 used a coil load similar to the type in last months QST magazine. The ham who hand built that beautiful mobile antenna, but without as many loops. I may be wrong, but I still think they had some on 40 and 80 meters. because these antennas were horizontally polarized as were most base antennas on these frequencies.

73s

K2OWK
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KC9TNH
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Posts: 304




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« Reply #22 on: August 01, 2011, 06:59:05 AM »

Here's a trick that I use for an 'instant' big signal on 80m mobile/portable.
Thanks for the tip, I'll have to order some wire from the States or see if our warehouse has any over here.
Mark's tip is a practical personalized application of what military folks have been using (as well as guys who like to roam the desert in jeeps for fun) for some time but I like the convenience of it.

My $.02 is not to get hung-up on the wire guage, if you find 22 or even 18AWG speaker wire, you can make this work. And with speaker wire if you buy 75-feet you're really getting 2x 75-ft lengths after you strip it apart. I personally know a guy who built a similar setup to Mark's while deployed (twice) and regularly talked from Mosul to Baghdad and this thing is only running at whatever bumper height they could extend it out to where the heck they were going to tie it off.

For entertainment, a slightly dated but interesting exploration of NVIS can be found HERE.  Heck, in Viet Nam sometimes antennas were simply buried. (Kept them from being shredded by harassing mortar fire and, besides, they didn't need to go that far but got around a terrain issue not unlike yours - a really small upside down ice-cream cone which is also harder to DF.)

For what it's worth, my OCF dipole N1LO graciously contributed advice for pre-build, is up at 18-feet and has thus far given me a very nice mix of 30 or 17m CW 10K miles away, and being able to talk inside my 300 mile circle on 40 or 75m reliably. NVIS kinda requires a 'systems' approach where you ask yourself specifically the who (where) when (time of day) and what (capability you have at hand).

It is a very cool tool to have in the bag.
 Grin
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
W8JI
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Posts: 9749


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« Reply #23 on: August 01, 2011, 07:11:07 AM »

There is a lot of pure garbage on the Internet about NVIS.

The ideal height of an NVIS antenna is about 1/8 to 1/4 wave above ground. That would be 35-70 feet on 80 meters, and 17 to 35 feet high on 40 meters. As a horizontally polarized antenna is lowered below 1/10th wave above ground, efficiency rapidly starts to decrease.

Contrary to myths, co-phasing antennas or using loops really provides no gain advantage, although a closed large loop does work better on harmonics for matching impedances.

My NVIS 80 meter antenna is a simple dipole at 40 feet above ground, because that is how high I could install it. Even so I can work Europe and all around the world with it, although it is really a killer for short distances on 75 meters. Lowering height would not change pattern or wave angle, it would just make everything at every distance weaker by the same amount.   :-)





 
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VK5DWC
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Posts: 1




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« Reply #24 on: September 17, 2017, 04:50:44 AM »

Just a hint in operating mobile, the placement of your antenna on the vehicle, and the direction you are heading, has a great impact on how far you can get. For instance, if your antenna is mounted on the trailer hitch, as mine was when I use to operate mobile, pointing the vehicle in the direction you wish to operate, can increase your range, because the maximum signal follows along the path of your ground plane (your vehicle). Personally, I have noticed an increase of several S units, just by turning the car into the direction of the signal. A misconception with vertical antennas, is they are omni directional. This is only true if you have an omni directional ground plane. Mobile antennas can be quite directional.

I used an FT857, with an external analogue S meter mounted on the dash of my car. My antenna was a home brew helical whip wound on a 12 foot fishing pole, mounted on the trailer hitch of my car. I used an SG237 autocoupler to match it. I have worked DX on 80 through to 6 meters (mostly on 20 meters, with about 180 countries), and interstate on 160 meters, all with the same antenna.

With the Buddypole at your home, try elevating it, and mounting it in an L configuration, with the horizontal element pointing in the general direction you will be travelling. That's where most of the signal will go, and the polarity will match the mobile.
« Last Edit: September 17, 2017, 05:07:58 AM by VK5DWC » Logged
KC4ZGP
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Posts: 1637




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« Reply #25 on: September 17, 2017, 06:57:17 AM »


Hello there.

First thing to do is get out of Africa.

Kraus
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WA3SKN
Member

Posts: 6496




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« Reply #26 on: September 19, 2017, 02:40:59 PM »

2011 post.
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