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Author Topic: qrp antenna ideas  (Read 1410 times)
KD5MMJ
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Posts: 21




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« on: January 28, 2001, 06:57:34 PM »

Howdy!  Right now Im a tech but in the process of upgrading.  I love to build electronics and the main reason of upgrading is building (qrp)radios rather than shelling fortunes for other equipment.

 What kind of antenna should i put up? I cant do a dipole because my qth has very few trees and those are short (max 15 feet). plus it isnt pleasing to my folks to have wire flying everywhere. also there are power lines to the left of the house and in front.

any ideas for a commercial antenna (no beams, or towers, i dont have a lot of money) or maybe a stealth or indoor antenna?
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WB6BYU
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Posts: 13488




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« Reply #1 on: January 29, 2001, 12:28:20 PM »

I've had good results with a wire loop around the edge of the roof
(often stapled to the inside of the facia boards, or tucked under
the shingles.)  Feed it with any sort of feedline to a tuner.  If you
can run the loop out to one or more trees, so much the better.
(The exact shape of the loop doesn't matter.)

A horizontal antenna up only 15' should still get you lots of QRP
contacts on 40 meters.  For the higher bands, a 16' to 20' vertical
on the roof - with switched matching networks at the base for each
band - is a good homebrew project, and capable of good results.
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WB2WIK
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Posts: 20636




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« Reply #2 on: February 01, 2001, 11:25:53 AM »

The wire loop is a great idea.  To make maximum use of the available yard area, with or without trees, you could also install a vertical, very low cost pole, in each corner of the backyard and run a wire around them (insulating the wire from the poles, of course).  For temporary installations, I've used bamboo poles that were literally "free," obtained from a carpet & rug distributor who receives their imported rugs from the far east rolled up on long bamboo poles that they throw away once they unwrap the rugs.  These poles are 16' long each, very strong, very lightweight, and very waterproof (bamboo is a weed that grows underwater, anyway...).  Make the loop as large as possible, pick a feedpoint for open-wire or ladder-line feed, and run the line into the shack.  If you're going to homebrew the transmitter anyway, and stick with QRP, the TX can be designed for any output impedance you wish, it doesn't have to be centered on 50 Ohms like most commercial designs.  If you use a robust, emitter-ballasted PA transistor, you don't need to design in "SWR protection" that folds back power when a mismatch occurs...let the mismatch be whatever it is, and just keep pumping power into it.  Use a 50-100W heatsinked transistor for a 5W (output) PA stage, it will take a lot of abuse...who cares if the PA dissipates 20W while putting out only 5W?  It's all low power, anyway...and a lot more fun to just "go," than to bother with countless minutes tuning up, playing with matching accessories, etc.  Good luck!
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AUG4EXAM
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Posts: 16




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« Reply #3 on: July 26, 2001, 02:49:15 PM »

wire loop around the house sounds like one of only a few solutions for me.  My question is:  What is zip wire?  Like lamp cord?  If so, does the insulation inhibit rf?

I rent a house, and the power line comes from the rear corner of the back yard to the middle of the house, making almost all but verticals impossible.  The only other hope I have for 40m and qrp is a T2FD I found in a book, "Practical Wire Antennas".  I hope to run it down the side of the property toward the corner where the power comes, but not all the way.

My exam is in two weeks, but I can't get antennas out of my brain.  Thanks
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N6AJR
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Posts: 9921




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« Reply #4 on: September 05, 2001, 07:46:01 PM »

You might try an inverted V as high and as long as you can. These work well with a tuner.

I made an antenna a while back with 1 coax feed line and then a pair of 80 meter wires(66 feet each side), 40 meter(33 feet each side), 20 meter (16 feet 6 inches each side), and a 10 meter ( 8 feet 6 inches each side) all joined with one side of each length dipole to the center of the coax and the other to the shield.

I mounted the various pieces of the antenna on little wooden blocks just inches from the roof of the house and ran the long 80 meter straight as far as I could then bent the ends down and then back around the edge of the roof. Fan them out in different direction (like one going from 6 o'clock to 12 o'clock, the next is from the 7 o'clock position to the 2 o'clock position). 15 meters tunes on the 40 meter as a 3/4 wave or something.  This is called a multiple dipole antenna, a fan dipole antenna, or sometimes a dipole array.  It always reminds me of the letter T with hairy armpits. Smiley

You would probably do better if you put the center up a couple of feet and drooped the other ends down like 4 little inverted V's joined in the center.  I did this on the back of the house roof and you can't see it from the street.  It worked well on most of the bands with out a tuner, but later I added a MFJ tuner I got used and cheep and it let me bring the swr's down to zippo.

You can pick up a used tuner for cheep on ebay or sometimes at ham fest and sometimes here on Eham in the classifieds.  If you want you could probably add the WARC bands and use the 468/f (in mhz) formula to find the overall length, then cut it in half and use one piece on each side of the center, I never tried this though, but you might want to if you use those bands, but you could probably just pull them in with a tuner too.

I guess it works because just like me, RF is lazy and if you are on or near 40 meters with a signal, most of the signal will go to the 40 meter antenna and the rest of the antenna looks like the wrong impedance to it or something...any how it works, probably better (in my humble opinion) than the straight long (130 foot ) inverted v in most instances...( it picks the wire that looks resenonant at what ever frequency you choose).

Any how, it costs about $5.00 for some wire(any kind will do that is strong enough to stretch that length, I used some enameled hook up wire, but you could use lamp cord, copper weld , or pull the conductors out of a piece of romex and use that..almost any thing that is long enough will do.  Make sure the ends are where folks can't touch them, because the potential in the rf energy there can be awsome. Then you need to spend $20 or so for a piece of coax the will reach from there to the back of your radio.  Solder the ends of the dipoles to the coax the seal it real good so the water doesn't get in and rot the coax cable. (use some bath tub caulk type stuff then tape it well when it sets). I have used this antenna with darn near a KW, with 16 gauge wire and never had a problem.

Here are the rules for all antenna, at least the rules that count.  I didn't make them up, they have been around a long time.

1. Any antenna is better than no antenna at all.

2. A longer antenna is better than a short antenna (usually).

3. The higher the antenna the better (usually).

4. The cheaper the antenna the better (always).  

5. If you have an Idea for an antenna, try it, it might work, (Mr. YAGI's did).

6. Making antennas is a lot of fun.




Good luck and enjoy,

Tom   N6AJR
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N6AJR
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Posts: 9921




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« Reply #5 on: September 05, 2001, 07:48:52 PM »

PS.. no reason why you couldn't put the multi dipole in the attic inside, as long as you don't have a lot of electrical wires and metal ducts to contend with.  Just seperate them as much as possible and run the as far as you can then bend them or zig zag them.

tom
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KT8K
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Posts: 1490




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« Reply #6 on: December 11, 2002, 12:43:55 PM »

Watch out for metal screen ridge vents in your roof.  I have them, and my first dipole effort - stretched over the peak, performed *very* poorly.  It was fine after I lifted it a few feet above the roof, however.

Now I'm thinking about seeing if I can split the ridge vent screen in the middle and connect some feedline to it to make it into a dipole!  When you've got lemons ... make lemonade!
73 es best rx de kt8k - Tim
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K9ZMD
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Posts: 173




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« Reply #7 on: December 25, 2002, 06:10:57 PM »

Hello Ramiro,

     First, good luck with the upgrade & welcome to HF.
     WB6BYU suggested wire around the eaves, and N6AJR mentioned placing antenna wire on short stand-offs from the roof.  In true accordance with N6AJR's rule number one, those antennas will work, but first consider any choice at all that gets your antenna wire away from all the metal and wiring in your house.  Here's why:
    I have an 80 meter dipole entirely on my roof.  It snakes around the concrete roof tiles on 1/2" plastic standoffs.  It isn't even close to running in a straight line, more like a squared-off figure "S".  It isn't even horizontal, as it has to follow the roof up and down the gables.  
    It has its good points: (1) No one trips on it or bumps into it.  (2) It is invisible to the naked eye from nearly everywhere in the world.  (3) It does work & I have fun; from California, I have worked 46 states and 5 continents in the past year using 20-45 Watts with digital modes.  (4) It works multiband, 80 - 6 meters because I center feed it directly with an LDG RT-11 in the attic.  (5) It is educational, as I am learning a lot about suppressing RFI in my home.
    The bad points are what need careful consideration: (1) It is very close to nearly everything, so it doesn't get out as well as any dipole suspended at the same height & in the clear.  (2) It is very close to nearly everything, so it does get RF into all of the domestic wiring.  I have had QSLs from the XYL for reception on the TV, stereo, telephone, intercom, outdoor thermometer, and kitchen stove.  My best QSL came from sheriff's deputies after my home security system burped on RFI & sent out a silent panic alarm.  (3) After those two, you want more?  How about regular holiday greeting cards from companies that market those Ferrite beads used for suppressing RFI?  
    Punch in my call sign on qrz.com & see the house picture.  73 from CC&R Country.

Gary, K9ZMD
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AB1AW
Member

Posts: 13




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« Reply #8 on: January 16, 2003, 08:18:56 AM »

You might want to consider an Isotron antenna to get onto the lower bands (e.g. 80m, 40m). I use an 80m Isotron on 80m w/ 2 watts (PSK31) and have decent results. The Isotron is about the size of a bird feeder and can mount on your deck/porch or just about anywhere.

There is a review of the Isotron on eham.net and elsewhere in the QRP forum.

Mike -- AB1AW
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