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 Feeding trapped inverted L question 
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Post Feeding trapped inverted L question
I have made and put up one of the trapped inverted L's http://www.users.icscotland.net/~len.paget/6_band_inverted_l_antenna.htm

Now my question is, would there be any benefit of some form of choke at the feed point. Right now i have got some random wires down and also some chicken wire under the grass as the Ground. I will be added more when it gets dark under the back hedge. Also does it matter it the radials run close to each other. Sorry if this is as clear as mud BTW.

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Sat Aug 07, 2010 5:00 pm
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 Feeding trapped inverted L question 
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Post Re: Feeding trapped inverted L question
I have used a inverted L for a few years now.Originally i just had a normal feedpoint at the base,i then used a 1:1 current balun at the feedpoint and tx & rx was greatly improved with lower noise levels on 80mtrs.I also use several ground radials of various lenth,ranging from 17ft to 50ft.


hope this helps.

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Sat Aug 07, 2010 6:26 pm
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 Feeding trapped inverted L question 
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Post Re: Feeding trapped inverted L question
A lot depends on how good a ground system you have. If it is really low impedance, it's unlikely that current would flow along the coax braid rather than into the radials; if the ground system is not so good, a 1:1 Current Balun will likely help.

Steve G3TXQ


Sat Aug 07, 2010 6:31 pm
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 Feeding trapped inverted L question 
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Post Re: Feeding trapped inverted L question
Thank you for taking the time to reply guy's

The ground system is a work in progress at the moment. I was thinking about the noise like M0BHN mentioned Steve as I am fighting a battle as are a lot of other's.

M0BHN did you use a home brew or a off the shelf balun ?


Many Thanks

John

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Sat Aug 07, 2010 6:44 pm
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 Feeding trapped inverted L question 
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Post Re: Feeding trapped inverted L question
I run this aerial using a 9:1 balun on the feed, can easily tune 80-10mtrs
2x radials both measure 16' in length, gets me over the pond and further at times.

Would benefit from a better ground system. Mines into shingle/gravel not good, but it works.

Phil


Sat Aug 07, 2010 6:58 pm
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 Feeding trapped inverted L question 
Silent Key

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Post Re: Feeding trapped inverted L question
G7BZD wrote:
I run this aerial using a 9:1 balun on the feed, can easily tune 80-10mtrs
2x radials both measure 16' in length, gets me over the pond and further at times.


Phil,

I'm curious why you would want a 9:1 impedance transformation at the feedpoint of a trapped Inverted-L which is designed to be resonant and approximately 35-50 ohms on all bands?

Did you really mean you run this aerial?

Steve G3TXQ


Sat Aug 07, 2010 7:27 pm
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 Feeding trapped inverted L question 
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Post Re: Feeding trapped inverted L question
Another excuse for an analyser, I measured the wire up and put the trap in, then stretched the wire from the
pole to the house, and YES i did stretch the wire. Have adjusted its length at least twice now.

Reason for the 9:1 is the fact thet my earth here is into sand/gravel, very poor at the end of the garden where the
pole is sited, I dug down over 2 foot and found no real soil, the radials dog leg all over the place, following the fence.
Before fitting the radials I had days where the atu on my ft1k wouldnt play ball on all bands.
One trick i do is to chuck the washing up bowl of water over the earth rod area each morning and evening.
seems to work better now.

Have spent complete weekends experimenting with and without trap/9:1 balun and added more radials, but have come to
the conclusion, that so far its as good as it will get.
Anyway its due for its pre winter GALE checks, so its all coming down in the next month or so and being checked etc.

Phil


Sun Aug 08, 2010 8:39 am
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 Feeding trapped inverted L question 
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Post Re: Feeding trapped inverted L question
M0GWR wrote:
Thank you for taking the time to reply guy's

The ground system is a work in progress at the moment. I was thinking about the noise like M0BHN mentioned Steve as I am fighting a battle as are a lot of other's.

M0BHN did you use a home brew or a off the shelf balun ?


Many Thanks

John


Home brew,using 8 to ten turns of copper wire (insulated) on a ferreite core.Plenty of info on buildining one on the web or indeed on this site.

cheers
Paul.

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Sun Aug 08, 2010 9:59 pm
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 Feeding trapped inverted L question 
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Post Re: Feeding trapped inverted L question
Antenna analyser is on its way, ordered tonight. so I may well find my inverted L gets re-hashed.

Wonder ? as Marconi did some real ground breaking experiments just down the road from me
"Alum Bay" what did he use for an earth/aerial ?
bet he didnt ring up the not so local store and buy an off the shelf aerial. Trouble is its not
an exact art, or is it ?

I remember the saying... if it aint broken dont fix it.
My inverted L works with a 9:1 balun, so do I bother to fix it ?

Phil


Mon Aug 09, 2010 8:29 pm
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 Feeding trapped inverted L question 
Silent Key

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Post Re: Feeding trapped inverted L question
Phil,

With the analyser you should be able to understand better what is going on.

With a perfect ground system the trapped Inverted-L should have a resistance at resonance somewhere in the range 20ohms to 35 ohms. With a moderate ground system that range will rise to 40 ohms to 60 ohms, which is a pretty good match to 50 ohm coax without any 9:1 UnUn.

My guess is that your Inverted-L is not resonant where you want it, and you are seeing much higher feedpoint impedances which the UnUn is helping to bring down to a better match to the coax.

With the analyser right at the feedpoint you'll be able to find the resonant frequencies and measure the resistances there; that will tell you something about how efficient your ground system is, and will allow you to get the antenna resonant on the required frequencies. Then you wont need the UnUn.

Steve G3TXQ


Mon Aug 09, 2010 9:30 pm
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