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Author Topic: QRP antennas?  (Read 5692 times)
WB6BYU
Member

Posts: 13335




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« Reply #15 on: March 28, 2008, 05:22:30 PM »

I agree with Dan - start with a simple dipole.

I built a kit of dipole wires for each band (80 through 10m, pre-WARC)
plus a length of coax cable with a center insulator on the end to use
while operating QRP portable.  Each time I stop I attach one or more
sets of wires to the center insulator (depending on what bands I want
to operate) and hang the thing from a tree or prop it up with a stick.
I've made lots of 40m contacts with the antenna from 5 to 20 feet off
the ground.  Since the 40m wires also work on 15m I can cover all
5 bands with 4 sets of wires.  The dipoles have been installed in many
different configurations: bent, sloping, etc.  On at least one or two
occasions the feedpoint has been BELOW the level of my tent.  This
gives me a lot of flexibility to accomodate available supports and
conditions:  near salt water I can install it as a sloper for vertical
polarization, or I can get creative and use one of the 40m wires to make
a full wave loop for 10m, etc.  I can even use a single wire for a
vertical and the rest of the wires for radials (though I've never had a
case where I would use a vertical instead of a sloper.)

Even for single band use, I still find a dipole is the most practical
portable antenna to carry.  Doesn't need a tuner.  Hardly weighs
anything (using #24 stranded hookup wire and RG-174 coax.) And
cheap, too:  the materials cost just a few dollars.  I set the antenna up
in a local park and tuned it once (about 30 years ago) and I still just
plug it in and operate without any adjustments.

If I were only interested in 40m there are two other types of antenna
that I would consider.  The first is a full-wave loop (which I can put
together using the wires in a non-standard configuration.)  I've used
these a few times with good results, especially if you can hang it
with the two top corners high in the air and fed at the bottom to make
maximum use of your coax.  (I only carry 25' of feedline.)  This might
be an excellent choice if you are going to be near the beach since
you can get either horizontal or vertical polarization just by hoisting up
a different part of the loop.  A big advantage of the loop over a vertical
wire is that the loop doesn't require radials.

The other antenna I'd consider is an end-fed wire using a home-brew
matching network ( a simple L network with a small coil and a variable
capacitor.)  With a half-wave wire this is the equivalent of the PAR
antenna, but if you have tall trees you may find that a full wave wire
gives better results when you can get it up in the air (with much of the
wire running horizontally.)  Once the matching network is tuned once it
shouldn't need readjustment as long as you always use the same piece
of wire attached to it.
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N9ESH
Member

Posts: 27




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« Reply #16 on: March 28, 2008, 07:11:37 PM »

I’ve had real good luck with end fed halfwaves. For 40M, it consists of 66.6 ft. of 22 GA wire and a simple matching circuit. You can use a PAR matching box and substitute your own 22 GA wire such as the Teflon coated stuff. The substituted wire fits better in a backpack. A good matching circuit description can be found here and built into an Altoids tin if you don’t want to use the PAR box:  

http://www.natworld.com/www/ars/pages/back_issues/2007_text/0107_text/KI6SN.html

I’ve used this antenna in the back woods of Yellowstone with my KX1 with good success.

For use at the beach, use a Parafoil kite to hold up the end fed halfwave. Here is a description of a Parafoil kite from the Adventure Radio Society.

http://www.natworld.com/www/ars/pages/back_issues/2001_text/0601_text/kite.html

I like end fed halfwaves because they are easy to put up and take down and don’t take much room in a backpack.

Jim/N9ESH
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928GTS
Member

Posts: 22




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« Reply #17 on: March 28, 2008, 08:07:07 PM »

Okay,this has me thinking which is good because then I'll have the best chance of spending my money in the best place. So basically to build a dipole I'd need an SO-239 connector,some wire and some sealant.

Now basically lets say I used double cored speaker wire(you know,how it has literally two wires when it comes off the roll?)and so I cut it a bit longer than the  frequency I want it to resonate on(just in case I need to trim)and then take one wire and split the conductors and connect one conductor to the pin and then the other to the shield and then do the same with the other wire? Naturally you'd want to insulate both connections when you're done but am I on the right track?
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WB6BYU
Member

Posts: 13335




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« Reply #18 on: March 29, 2008, 03:11:55 PM »

My approach was to make a center insulator (a small piece of plastic
will do:  my original one used a slice of radiator hose) and attach that
to the end of a piece of coax cable.  This way I have just one connector
(at the end where it plugs into the rig) rather than another set at the
antenna feedpoint.  This saves a bit of weight, presuming that you
aren't going to use the coax for any other antenna.

The important jobs of a center insulator are:

1) insulation (to keep the wires from shorting together)
2) strain relief for the wires
3) strain relief for the feedline.

If you want to use a connector, one simple method is to mount it to
a small piece of plastic sheet, such as plexiglass.  Drill two holes in
the sheet to tie the wires through (for strain relief).  Drill another two
holes to mount a small bolt with a wing nut.  Run a short piece of
wire from the coax connector to the heads of these screws.  This
allows you to tie the wire to the insulator, then put the bare end under
the wingnut and tighten it down, thereby connecting to the coax.  You
can add additional wires as desired with no changes to the insulator:
just tie on the new wires and stick them under the wingnuts.  If you
don't use a connector at the antenna, drill a couple holes that are a
snug fit for the coax and pass it through them, giving strain relief, then
put lugs on the ends of the shield and center conductor and connect
them under the heads of the bolts.  Some sort of weatherproof goo
on the end of the cable is a good idea.

Remember, you never want mechanical stress on an electrical connection.  
You want all of the mechanical stress to be on the wires and where
they are tied to the insulator, with a couple inches of slack between
that point and where the electrical connections are made to the coax.

RG-58 coax is fine for car camping, but RG-174 is much lighter for
backpacking.  Just don't use too long of a piece:  it shouldn't be much
of an issue on 40m, but I lose 1/4 of my power in a 25' length of RG-174
on 10m.
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928GTS
Member

Posts: 22




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« Reply #19 on: March 29, 2008, 10:29:03 PM »

Excellent,thanks for clearing that up. So what coax should I use? I've heard praise for RG-8X and good old RG-58,whats the deal?
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928GTS
Member

Posts: 22




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« Reply #20 on: March 29, 2008, 10:40:05 PM »

Also is something like LM-240 overkill for this application?
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WB6BYU
Member

Posts: 13335




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« Reply #21 on: March 30, 2008, 10:42:13 AM »

Coax choice is a tradeoff between cost, weight, and losses.  For lengths
up to 100 feet on the lower HF bands running 100 watts, there is little
to be gained by using anything larger than RG-58.  You may want to
consider a good grade of RG-8X for use up to 10m.

Loss is a function of length and load impedance.  Losses go up as you
go higher in frequency, so cable choice is much more important at VHF
and above.  If you are running the coax at a high SWR then you need
better feedline to keep your losses low.  On 40m a 100' length of RG-58
coax has about 1dB of loss.  That is relatively insignificant.  Changing
to RG-213 (which is stiffer, heavier, and more expensive) gives less
than 0.5dB improvement.  (On 10m the same length of RG-58 will
have about 2.5dB loss.  Using RG-213 will cut that about in half, which
may be worthwhile.)

I find that I rarely put up a portable antenna more than 20' high, so I
only carry 25 feet of feedline.  (For backpacking weight is critical, and
the coax is the heaviest part - even with RG-174 coax.)  You may
decide that 50' to 100' is more convenient for your operating
preference.  Certainly at 50' there is no practical advantage to using
anything bigger than RG-58.  It mostly depends on your personal
choices of weight, volume, and cost.

If I'm hauling my station in the car and operating 40/80m, I'd probably
take 75' to 100' of RG-58.  That gives me more flexibility where I set
up the station relative to my antenna supports.  When weight and space
are critical I carry 25' of RG-174 coax (0.1" diameter) and set up my
tent to make it reach.  It is all a matter of personal preference.
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KG4YMC
Member

Posts: 297




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« Reply #22 on: April 03, 2008, 12:40:29 PM »

hi, I have the g5rv made by radio waves , the l04 foot model and it has been working great. the quality is ok . i suggest useing  a marine quality rope, that is uv resistant . I also have mine not strung super tight as we get high winds down here. I USE AN MFJ TUNER MOBILE MOD. AND NCG. TRIBANDER MADE BY PANASONIC . TEN WATTTS.  IT WORKS GREAT ON 40 , JUST HAVE TO DODGE FOREIGN BORADCASTERS, BUT YOU SHOULD BE ABLE TO MADE GOOD CONTACTS. ONELY PROBLEM I HAVE IS WIFES COMPUTER MONITOR, BUT THINK FLATSCREEN ARE QUIETER, GOOD GROUND , GOOD ANTENNA , LISTEN , AND YOU WILL DO WELL, I AM STILL LEARNING H.F. TO , SO HAVE FUN AND DON'T BE INTIMINDATED BY THE BIG GUNS, YOU HAVE THE ADVANTAGE OF KEEPING YOUR POWER ON FREQ. AND NOT SPLATTERING ALL OVER ... REAL HAMS CAN QRP .. HOPE TO HEAR YOU SOON, WITH THAT SAID , WISH I HAD A NOTCH FILTER, SPLIT , AMP.. AND MORE BANDS AND MOSTLY MORE MONEY HI HI ... 73S    TERRY KG4YMC
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W4FID
Member

Posts: 133




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« Reply #23 on: April 06, 2008, 02:32:43 PM »

I have a 703 and love it. Camper, picnic table, motel room and I'm good to go.

Since you are asking about 40 M in particular and seem to be realistic about the expectations ...... easiest way. Get a painter's pole from Home Depot. They are about $25 or 30 for one that telescopes from about 6 ft down to 15 to 20 feet extended. GET A FIBERGALSS ONE; not aluminum. That's enough height to do what you need and can be bungie corded to a picnic table, fence post, car bumper/luggage rack etc.

Get a back to back mobile whip dipole center. It will have 3/8-14 threads and either a PL-259 or lug contact for the feed. About $15. Put a 40 M hamstick on each side. About $16 each.

Now the choices.

1)     Use the center hardware with a PL-259 and tune the stick for the middle of either the CW band or the middle of the phone band, feed it with coax,  and work near where it's resonant.

2)     Use the center hardware with lugs and tune the sticks to the middle of the 40 M band and feed it with ladder line. You'll need a balun and tuner at the rig. Operate either CW or SSB.

3)     Use the center hardware with a PL-259 and tune one of the sticks for the middle of the CW band and one of the sticks for the middle of the phone band. Orient the center hardware so the hamstick is vertical. Choose/use whitchever you like for whatever mode you want to operate. Use a wire about 33 feet long (may/probably also will need somepruning) sloping downward to some convienient tie off as the counterpoise. Radiation will be favored in the direction it runs. CAREFUL IT WILL HAVE HIGH VOLTAGE ON IT ESPECIALLY AT THE END. Feed it with coax,  and work near where it's resonant.

40 M is a good choice since it's almost always open to somewhere on some mode and you can get a QSO almost always even with low power. Prior to the 703 i had the MFJ travel radio for 40 with the CW board in it and had a great time.

Same strategy also works for 30 and 20 M. As the sun fills in 17, 15, and eventually 10 will be a blast too.

Too many bands means too much stuff to carry and too many hamsticks runs the cost up too high. But for quick/simple/cheap and a band or two it's great. OR if you have the $$ the Buddipole is wonderful.

73 John  W4FID
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W4FID
Member

Posts: 133




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« Reply #24 on: April 13, 2008, 04:04:48 AM »

For several years I had back to back mobile whips as a 40 M dipole up about 18 feet on a piece of conduit that washeld to the post of my deck railing with hose clamps. Bohemian engineering at its finest! My town house was taller and aluminum sided. Trees were easily three time that height. Ran an MFJ 40 M travel radio (10 watts SSB and had the CW board in it). I worked anyone / everyone I heard. 300 miles on SSB in the day was routine. Longer on CW and at night was easy. So it can work. No; it's not a dipole at 65 feet. Yes you will make QSOs and have fun.
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WB6BYU
Member

Posts: 13335




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« Reply #25 on: April 20, 2008, 10:25:16 AM »

One of the local ARES members has a 40/80m Hamstick dipole for
portable operation.  The last time he set it up he put on the 40m
whips and slipped a pair of 80m dipole wires between them and the
mounting plate.  This gave him a wire dipole on 80m off the same pole,
and he was amazed at the difference: wider bandwidth, stronger signals,
better contacts.

The 40m Hamstick dipole isn't as touchy as 80m, but in my experience
there almost always is room for a 40m dipole if you are operating
outdoors.  


OK, I admit, when camping in a pasture once I had to run a dipole
down the top of a barbed wire fence about 4 feet off the ground and
it didn't work very well.  And I couldn't string it up while riding the
train, which I might have been able to do with a Hamstick dipole on
a mast (especially with the help of one of the porters who was a ham.)
So there ARE a few situations where the wire dipole may not be the
best choice.  But in my experience, not many.
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