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Author Topic: Short Inverted-L's  (Read 5748 times)
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N1ESE
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« on: November 22, 2007, 08:51:44 AM »

I am very antenna challenged at this QTH.  My current antenna is a 40M  resonant coax-fed dipole at roughly 35', it is broadside to the North and South.  I've worked the world with it on 40M and, with the "tooner" in my Elecraft K2, I've been able to match it fairly well around 3.600 for several 80M QRP contacts down into Virginia, New York, South Carolina and even out West and Northwest into Chicago and Ontario Canada.  However, I need to upgrade the feedline to at least LMR-240 in the spring before I can run at a decent power level on AM.  Currently, it is fed with RG8X which can only handle about 350W continuous duty cycle.  LMR-240 can supposedly handle 1.49KW continuous duty cycle.  Although, I am also looking at LMR-195 which is rated at 1KW continuous.  All these numbers based on 30MHz.  My reasoning for staying with smaller coax is because I have no center support for the dipole and need the lightweight stuff.
 
My main goal is to be able to run enough power to talk to the northern East Coast boys on 75M with as good audio as I can muster up.  Most of them all come in here with really nice signals especially K1KBW and KF2VM.  The local boys such as W1IA and Timtron are always booming in here at 20 to 40 over according to my meter on the NC-183.
 
To get to the topic of this thread, I may be able to put up an Inverted-L but it would be quite short.  The vertical leg would be roughly 35' long and the horizontal about 40' to 50' long.  The vertical leg would also be fairly close to the house and a metal fence.  Could I get this to play on 75 and 40 any better than my current dipole? 
 
In either case, I'll need a high power transmatch for either antenna.  May try a homebrew or buy a Palstar 1200W (AT1KP) unit or a 1KW Dentron.
 
Would greatly appreciate any suggestions.
 
Thanks
 
- JT
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The Slab Bacon
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« Reply #1 on: November 22, 2007, 09:34:07 AM »

first of all lose the coax!! use balanced feeders for multibanding. If you have room for a 40m flat top, you have room for the same antenner that I use. It is a 60' folded dipole with 12" spacers, but broken at the top center so it doesnt make a complete loop. Fed with 14ga crappy brown stuff. I have been using this antenner for years with very good results, and I am not piss weak!! No one has any trouble hearing me. It works somewhat well on 160 as well, but it rocks on 80 and 40, and will work fairly well all the way up to 10m.
I have made a sketch of it, but my scanner is crapped out so I cant post it yet. I will try to scan it in at work this week and post it. You will however need a robust tuna capable of handling heavy feedline currents, probably better with a HB unit if you want to run high power.

Inverted Ls and other antennas with a large vertical portion dont tend to do well for close in rag chewing. You really want a flat top with good high angle radiation.

                                                                    The Slab Bacon
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N1ESE
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« Reply #2 on: November 22, 2007, 09:39:09 AM »

first of all lose the coax!! use balanced feeders
You are right, I don't know why I keep forgetting about balance feeders.  Duh.  I'll replace the whole damn thing in the Spring.
 
- JT
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KB2WIG
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« Reply #3 on: November 22, 2007, 09:39:21 AM »

I'd throw away the coax, and feed it with ladder line...   If you can , make the dipole longer...  There are a few tuna designs floating around. Try searching here on amfone  for them...   or try   cebik com ........  he has got as lot of fine stuufff on his site...........     klc  
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W8EJO
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« Reply #4 on: November 22, 2007, 10:11:46 AM »

I agree with Slab Bacon. An L will give you more low angle radiation which is great for DX but not as good as a horizontal doublet for your stated goal.

I don't know how much total horizontal space you have but if NE US ragchewing from NH is your goal, you need to go horizontal. 35' high is an OK height for this.

Try to get at last 90 - 100' horizontal (of course 130' is better), center fed with ladder line/open wire, ends as high off the ground as possible. As to tuner, an old Johnson Matchbox or a HB link coupled type will do the job. You may have to fiddle with feed line length to get a match on all bands of interest.

Now, having said all this, if you are limited by horizontal space you could go with an inverted L as a compromise antenna dictated by your QTH. 45'x45', center fed (again with open wire) will work. Radials won't help much if at all. I use a 90'x90 version, no radials, at my MI cabin with good results on 160, 80 & 40.

LB Cebik discusses the center fed L here:

http://www.cebik.com/wire/ltv.html
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N1ESE
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« Reply #5 on: November 22, 2007, 10:24:22 AM »

90' I won't be able to do as much as I agree it'd be better.  My current 40M halfwave is about as long as I can get and even this is pushing it as it isn't in as much open air as I'd like.  A 60' folded dipole like Slab mentions would be ideal as it'd get it into a little more open space.
 
- JT
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w3jn
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« Reply #6 on: November 22, 2007, 10:30:29 AM »

W7FG has whole antenner/balanced feedline kits.  Superb quality - you couldn't make the antenna for less money.

Investing in W7FG balanced line was the single best addition I ever made to my station.  I once tried an inverted L and it worked horribly - hard to tune and noisy.  Many have had good luck with inverted Ls and my results were probably just my local situation, but you'll never go wrong with a balanced fed flat top as long as you can get it.
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