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Author Topic: 160m Inverted L: Installation question  (Read 559 times)
EI4HQ
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« on: March 17, 2008, 02:11:25 AM »

Hi,

I'm installing an inverted L for top-band. The vertical section is approximately 57' & the horizontal section is circa 120' - it'll be cut to tune.

My question: From a performance point of view only, is the height at which the horizontal section end is supported, important?

I can easily support the end at a height of about 25', but I could go an extra 15' with some extra hassle - from a performance perspective is the extra 15' worth the extra hassle?

The antenna will be used almost exclusively on TX, I have separate RX antennas.

Rgds
Cormac, EI4HQ
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N3OX
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« Reply #1 on: March 17, 2008, 06:20:16 AM »

"From a performance point of view only, is the height at which the horizontal section end is supported, important? "

In the context of the choice you've described, only a little bit and only if you have a quite bad ground radial system.

Using a downsloping top section on an inverted L provides *some* cancellation of the radiation from the vertical section, lowering the radiation resistance.

I ran the numbers in EZNEC and having the end at 40 feet is about 1dB stronger than having the end at 25 feet when I use 30 ohms for the ground loss resistance, which is a lot... so for a really bad ground, the difference is  a dB.

Since you have a lot more to gain from improving ground system resistance by adding radials (perhaps 4dB), I'd just tie off the top section where it's convenient.  You won't be able to measure the difference once you have enough radials.

Dan





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

Monkey/silicon cyborg, beeping at rocks since 1995.
EI4HQ
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« Reply #2 on: March 17, 2008, 06:45:09 AM »

Dan,

Thanks for running the EZNEC model to give me a clear answer. I'm just getting to grips with EZNEC, and I haven't yet quite worked out how to put the capacitor, at the feed end of the inverted L, into EZNEC.

After some messing around, I decided that as I was putting the antenna up, I might as well do it properly. Hence, I've gone the extra mile and mounted the end at about 40 feet. I reckoned the extra height wasn't going to do any harm anyway :-)

Like you suggest, I can't really see any noticeable change on the antenna analyser, & the big issue is the earth - the analyser readings indicate I've alot to do there still... well begun however...

Rgds
Cormac, EI4HQ
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N3OX
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« Reply #3 on: March 17, 2008, 07:58:10 AM »

"I reckoned the extra height wasn't going to do any harm anyway :-) "

Certainly won't.

I'll be listening for you on 160 ;-)

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

Monkey/silicon cyborg, beeping at rocks since 1995.
KL7AJ
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« Reply #4 on: March 17, 2008, 09:17:57 AM »

Greetings, Cormac:

   The Inverted-L is a foyn antenna indeed.  I've had one up for ages.

    The radiation resistance is primarily determined by the vertical section, and the bigger this number is, the more efficient.  However, at 57 feet, you're probably up high enough to reach the point of diminishing returns.  As we say here all the time, "If it ain't broke, don't fix it!"

   Don't go out of your way to prune it to resonance, unless you really have a compelling reason to do so.  The more wire you have up there, the better.  This is known as the TCC (total copper content) factor.   It's my highly unscientific, yet indisputable method of figuring out how well an antenna's going to work.  The more copper, the better. Smiley
   
   Do your final tuning with a series capacitor if the antenna is too long.

Good luck!  (Hey, it's St. Patrick's Day, isn't it?)

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K4SAV
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« Reply #5 on: March 17, 2008, 11:27:58 AM »

Good decision on putting the wire higher.  I have almost the same antenna (tuned by a motorized capacitor) and I have been very pleasantly surprised at how well it plays.  No doubt I will be able to hear you on 160.

You indicated that you have work to do because of what the analyzer was reading.  That is surprising.  This antenna when tuned to resonance with the capacitor should be very close to 50 ohms.  If your analyzer is reading something very different, try checking it with a little power and an SWR meter.  Analyzers often don't read correctly on 160 because of RF pickup from local AM stations.

Being able to model this is very beneficial.  Here is some help with the EZNEC modeling.  Adding the matching capacitor at the bottom of the antenna is easy.  Bring up the "Loads" window and add a load on the wire where the source is located.  Change the % location number until the load appears just above the source. You can verify that by looking at the "View Antenna" window. If the values in the table don't come up as R,L, and C, look under the "Other" menu item and select "Change Load Type".  Select RLC.  Then enter the capacitor value.  

A loss resistor simulating near field ground loss should be added just below the source.  You can get the value for that resistor from a table showing ground loss versus number of radials and length. The ARRL Antenna Book has a table, as well as most other antenna books.  For example, with 32 quarter wave length radials, the value is about 8.5 ohms.

Modeling L's is sometimes tricky.  Here is another point often overlooked when modeling L's on 160.  EZNEC will give optimistic results for a long low horizontal wire over a Mininec ground.  The gain will be too high and the feedpoint resistance will be too low.  For the L you have, the horizontal wire, even though it is at 57 feet, on 160 meters it is a VERY LOW wire compared to the wavelength.  So if you want a more accurate answer, the antenna needs to be modeled over a Sommerfield-Norton ground (a high accuracy ground in EZNEC).  To do this you will need to model the radials since you can't connect a wire directly to a Sommerfield-Norton ground.  Manually enter one radial and then use the "Create Radials" menu function to automatically create the rest of the radials.  

After modeling my antenna using the information I gave above, and then measuring the SWR across the band, I was amazed to see that the measured numbers fell exactly on the calculated SWR line.

This problem doesn't exist when modeling verticals because all of the antenna is vertical and so you don't have a long horizontal wire over a Mininec ground.

Another very useful function of EZNEC is in determining the ratings for the matching network components, in this case just one capacitor. To do that, look under the menu item "Options/Power Level..." on the main window, and set the power level to whatever you want.  Then press the "Load Data" button and you can read the voltage and current levels on all the loads.  You should be in good shape with the dimensions you have chosen, but try lengthening your antenna in EZNEC and tuning it to resonance which will require less capacitance.  You will notice the voltage go up on the capacitor.  As you continue lengthening the antenna you will discover the limitation on what lengths can be used.  As the required capacitor becomes very small, the voltage will become so large that getting a capacitor to handle it will be very difficult.

When you get EZNEC working, you can do some experiments to determine if there are any benefits to be gained from changing the antenna in any way.  Good luck with your antenna project and with the EZNEC experiments.

Jerry, K4SAV
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KL7AJ
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« Reply #6 on: March 17, 2008, 12:51:47 PM »

SAV:

   It could also be that our friend is just sitting on VERY good ground conductivity.  Ireland is known as being "well watered".  If he is indeed measuring a feedpoint resistance much less than 50 ohms, he is most blessed indeed!

eric
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AB3CX
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« Reply #7 on: March 17, 2008, 03:11:09 PM »

I think so. I use a very similar L configuration on 160 meters with a wire clamped to the tippy top of my HyGain Hytower vertical. Since it's a quarter wave, there are maximum currents at the very end of the wire. When I have the end wire down low to the ground, while transmitting, RFI actually blows my ground fault interrupt circuit breakers in the home electrical panel, which tells me that there is alot of RF energy going right into the turf!!  Yes, I do have good RF and electrical grounds. I'd try to get the end as high as possible. Remember, an inverted L is just a bent vertical, nothing more.
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