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Author Topic: QRP antennas?  (Read 5079 times)
928GTS
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« on: March 25, 2008, 11:30:17 AM »


 Blurgh so my new account with my actual callsign is still being approved so I have to post on my old account,whatever. Either way I'm buying an Icom 703 and I'm wondering what good antenna solutions are there for 40M? I have an MFJ1840T telescopic and would that even be worth my time for quick drop down and setup up operating with a counterpoise attached to it? I don't care about DX with this setup,would I get ANY contacts at all with 10 watts? Who makes the best telescopic/hamstick antenna? Just wondering because what if I go to a place with no trees,y'know?

 So what should I do for an antenna if I have trees? I thought about an end fed dipole but I remember hearing something about leak from the feed line with these? Whats the deal? So what about those and what about standard dipoles? Are there much difference between them?

 Basically I'm looking for two solutions

A)Sit down on a beach with no trees anywhere and work SOMEONE(doesn't have to be into Europe though we can dream?)
B)If I'm near trees and want a REAL antenna that I can still stuff into a backpack. Dipole.....right?

I'm a bit blind here,help me? =)

KC2TAU - Mike
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NG0K
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« Reply #1 on: March 25, 2008, 02:28:26 PM »

I'm not familiar with your MFJ-1840.  

Get a telescoping fiberglass pole like a 31 foot tall Jackite.  Suspend wire from the top and feed it at the base with coax against 8 radials, each about 20 feet long just laying on the ground.  Use your IC-703 tuner.  

If you want an optimal signal/match on a particular band then go with a resonant 1/4 wave length.  You can use the same radials.  It's easy to swap wires if you want to change bands.  

Another antenna that works well is to use two poles and suspend a 50 foot length of light gauge wire between them horizontally.  Feed it at the end up at the top of the pole with brown twinlead feedline and use a 17 foot counterpoise hanging down along the pole.  (Sort of a upper corner fed inverted-L configuration that is a total of 67 feet long)  Run the twinlead away from the upper corner and attach a 4:1 balun and coax to your IC-703's tuner.  It will be a very close match on 20m and 40m plus it tunes easy on the other bands.  

Works great!  I worked the ARRL DX contest QRP using this setup in my back yard just for fun and worked several DX stations, even very weak ones, and worked VP6DX on 17m and 20m.  

73, NG0K
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73, Doug - NG0K
K5LXP
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« Reply #2 on: March 25, 2008, 07:10:01 PM »

Then, for 20M use that same Jackite pole to suspend an end fed half wave like a PAR end fed as an inverted vee.  No radials required, simple and easy to put up, and no tuner to hook up or adjust.

Mark K5LXP
Albuquerque, NM
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W3JJH
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« Reply #3 on: March 25, 2008, 08:02:10 PM »

Or put up an inverted-vee dipole fed with ladder-line and matched with a tuner such as an MFJ-971.  Guy a lightweight fiberglass pole with binder twine and plastic tent pegs from WalMart, and it will stay up in any wind that you'd want to be in on a beach.  A 66 ft dipole fed this way should work on all bands from 40 through 10 m.  A 132 ft dipole will work on 80 through 10.  If you don't need multiband coverage, then simply cutting the dipole for the middle of the band you want and feeding it with coax (and some sort of balun, even if it's just a few turns of the coax where it joins the dipole) should work well enough with the internal tuner in a '703.

Oh, and the Pioneering Merit Badge book published by the Boy Scouts is an excellent source for learning which knots really work for which purpose.
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W3JJH
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« Reply #4 on: March 25, 2008, 08:05:37 PM »

Mike,

You would do well to find a club in your area and attend their Field Day this June.  You'll see how experience hams set up and run a portable operation.

73 de W3JJH
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928GTS
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« Reply #5 on: March 25, 2008, 09:30:59 PM »

I've heard of problems with feed line radiation with end fed dipoles like the PAR,whats the skinny?
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N3OX
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« Reply #6 on: March 26, 2008, 06:35:15 AM »

I do wish PAR would add a wingnut ground post on their end-fed antennas.  I know they omit it for rhetorical reasons, but even an end-fed wire benefits from a real RF ground... it can be a bunch of very short radials.  My 40m antenna is an end-fed-half-wave vertical and I feed it against my radial system just like on any other band.  No reason not to if you have the radials available.

The nice thing about an end fed half wave, though, is that the capacitance of the coax shield and rig case to ground usually provide enough of a ground return for the antenna to work properly and reasonably efficiently.... lack of a ground radial system doesn't totally destroy the effectiveness of an end fed half wave like it would a quarter wave.

All in all, I think some Par End-Fedz or equivalent antennas would be a good choice, and I wouldn't worry about feedline decoupling too much when running 10W.  It just won't cause you that much of a problem.  If you're out in a portable situation and the feedline is on the ground, it just acts as your ground system.... yeah,  it has   current on it, but so would a set of radials.

If you use it from your dorm, you might try grounding the coax shield to some handy bit of metal or a resonant radial to spread out the current a bit... but I don't think you'll have too much problem with RF in the room just because you're not running a lot of watts.

I think the biggest problem in the dorm would be the other way around... noise coupling onto the antenna because the feedline is poorly decoupled from the antenna... but the ONLY WAY to successfully decouple the feedline from a PAR End-Fedz is to rig up a good RF ground connected to the shield of the SO-239 connector at the antenna... would be easier if they gave a ground connector.  

I wonder if they could sneak a single green PowerPole recessed in the housing without anyone freaking out that the antenna all of a sudden had a place to hook up a ground ;-)


73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AA4PB
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« Reply #7 on: March 26, 2008, 02:48:35 PM »

All end-fed antennas, even those that are 1/2 wavelength long, require a counterpoise of some type. With the typical installation, the coax shield and everything attached to it acts as the counterpoise or the other half of the antenna. Is that bad? It depends. If you have a good deal of coax up in the air where its radiation can contribute to the antenna pattern then it'll probably work pretty well. If the coax is very short or laying on the ground then it won't be too efficient.

Personally I think you are better off using a center fed dipole or inverted V if you can. That way the antenna does the radiating and the coax very little.
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928GTS
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« Reply #8 on: March 26, 2008, 04:12:25 PM »

How high of a pole would be good for an inverted V?
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W3JJH
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« Reply #9 on: March 26, 2008, 08:33:15 PM »

Higher is better.  I use a 33 ft MFJ pole when I have to, but I prefer to use a military surplus 40 ft mast.  These provide good results on 40 m and up.  I'd like to have 60 or 70 ft for 80 m, but that's not easily portable.

Wonder Pole makes a 40 ft telescoping pole that seems to offer the best trade-offs among cost, height, and weight.  <http://www.wonderpole.com/wp640_630.html>
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928GTS
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« Reply #10 on: March 26, 2008, 09:02:17 PM »

Excellent,thanks.

I'm looking for a good dipole to buy and I was thinking the RadioWavz G5RV Lite but I've been put off by some reviews saying the quality is bad. What are some good dipoles that will tune 20 and 40 meters and aren't any longer than 75'?
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928GTS
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« Reply #11 on: March 26, 2008, 09:21:27 PM »

What about those slinky antennas? Are they just too much of a compromise? Do they serve a decent purpose for indoor/confined space work?
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W5ESE
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« Reply #12 on: March 27, 2008, 06:54:13 AM »

Mike, you should spend alot of time reading about
antennas.

Here's a good start.

http://www.cebik.com/fdim/fdim9.pdf

73
Scott
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W3JJH
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« Reply #13 on: March 27, 2008, 07:59:10 AM »

"Practical Wire Antennas" by John D. Heys, G3BDQ, is a good reference to have on hand. You can order a copy from <http://www.arrl.org/catalog/index.php3?category=Antennas%2C+Transmission+Lines+%26+Propagation>.   Scroll down almost to the bottom of the page.
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N3OX
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« Reply #14 on: March 27, 2008, 01:17:24 PM »

"Do they serve a decent purpose for indoor/confined space work?"

No, they're touchy and terrible.  If you need a loaded antenna you should do it with good copper wire coils of the right inductance at the right places in the antenna.  

Unfortunately it seems like it's the fashion to do everything *but* that... linear loading, distributed helical loading (like a slinky) etc, etc.  They all work worse than winding a couple of good copper wire loading coils.

Slinkies are terrible.

"What are some good dipoles that will tune 20 and 40 meters and aren't any longer than 75'?
 "

The easiest way to do this is to have two antennas, a 20m half wave dipole and a 40m half wave dipole, or to run them in parallel with a common feedpoint (often called a fan dipole around here)

A 40m dipole is under 70' long and a 20m dipole is under 35' long.

Buy some center and end insulators and wire and cut and tune your own resonant half-wave dipole antennas (inverted vee installation is fine).  You'll thank yourself later.

A good fraction of the commercial G5RV offerings rely on losses in the feedline to make the SWR low, that's the last thing you want.  A real G5RV with good low loss feedline works just fine but few of them are set up that way... and it's sort of a minefield of bad antennas with a few good ones peppered in.  YOu can never trust a user review of a multiband wire antenna; everyone loves ham radio too much to give a bad review to their only antenna... so they all work GREAT even if they're really kind of crappy.

A good old half-wave dipole will tend toward 90%+ efficiency and be a very predictable performer.  Start there for 20/40.

- - - - - -

That said, if you want to do all the bands between 40m and 10m a  75 foot "doublet" fed with ladder line or real open wire line to a good tuner can also be very efficient. (Doublet is the term for an arbitrary length center fed wire... a dipole typically refers to a resonant half-wavelength long antenna)

The SWR on the feedline for these antennas is very high but the feedline has tiny losses even in those conditions.  

- - - - - -

My strongest vote would be to build half-wave dipoles first.  It will get you started on the road to knowing how antennas work, and it's actually really important to know how they work even if you're going to buy commercially made ones!

73,
Dan

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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