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Author Topic: Twin-lead Marconi Antenna ?  (Read 1210 times)
KA6KBC
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« on: September 06, 2006, 08:19:11 AM »

Hi All,

Thanks in advance for any comments.   Over the weekend I took up a 40 meter Antenna Project I found in one of my old books (William Orr's Radio Handbook).   In theory for 40 meters the Antenna is about 32.5 Feet long (@ 7.2 MHZ - 234/F) and is about 50 ohms so it can be feed directly with Coax.   My first tests look awful, but later I found I missed a major item.   I needed a counterpoise (Ground system) to go along with the new antenna.   Even with that I did not have much luck getting the Antenna to match very well.   I'm looking for more information on the design and construction of this Antenna using Twin Lead, but have not found much.   Has anyone build/used this type of Antenna ?    Any tips or write ups I can review ?

Thanks Again,

Bill - KA6KBC  
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K8KAS
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« Reply #1 on: September 06, 2006, 10:42:39 AM »

I don't understand your twin lead question, do you want to feed it with twin lead or make it out of twin lead. A 32 foot vertical wire fed with coax with one 32 foot elevated counterpoise will work fine, or use the 32 foot vertical wire with 10 to 20 randem length radials laying on the ground will also work (MORE ARE BETTER), with either ground system you should see under a 2:1 VSWR. This system will offer a nice wide bandwidth and you need nothing fancy to make it work well.
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KC8VWM
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« Reply #2 on: September 06, 2006, 11:16:53 AM »

I would check your feedline connections. 32'.5" should yield excellent VSWR results on 40m.

In fac,t a Marconi antenna doesn't require any counterpoise at all because in therory it uses the ground below it as the physical "other half" of the antenna.

Adding a counterpoise of equal length on the other half would make this a dipole or inverted L antenna design.

If you are using twin lead as the radiator, then I am suspecting that you are attempting to build a folded dipole ? in which case the twin lead wire length you indicated for 40m doesn't apply.

73
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KA6KBC
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« Reply #3 on: September 06, 2006, 01:03:55 PM »

Hi,

Actually the Antenna is made of twin lead and feed with coax.   I have only seen a limited amount of info on this design - First in Bill Orr's Radio Handbook and then in an article at the link below:

http://home.comcast.net/~fuzzbeast/160MAN_D.htm

Both mention good grounding is needed.

Thanks,

Bill - KA6KBC
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KB4QAA
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« Reply #4 on: September 06, 2006, 02:08:00 PM »

Hmmm, interesting article.

It's a Marconi, it's a Folded Monopole, it's a Long Wire, it's an L-shaped Vertical.   Work it against ground, radials, counterpoises, against itself.

It's a dessert topping, it's a floor wax!

There are so many options presented that it can be easy to get confused over what is important.  The single biggest thing I can fault it for is interchangeably using the terms "ground" and "radials" as being synonimous.  

Your single best bet is to lay out some radials/counterpoises, say 4, and see if that helps.  You might try laying one of them directly under the radiator as a mirror.   The article suggests a 4:1 Balun may be needed.  Alternatively, you might try preventing radiation from your coax by installing a 1:1 line isolator, or make a choke balun by winding 6 turns of the coax around the diameter of a coffee can and taping together.

Have you tried feeding both sides of the twin lead from the same point as the article suggests?

If you can't make this work, you might try putting up a real Marconi and compare the difference.

Good luck, let us know how it goes.

73, bill

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N3PM
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« Reply #5 on: September 06, 2006, 06:50:44 PM »

 I used a folded monopole (made from the windowed twin lead) on 80M a few years ago. It worked OK, but I couldn't see any real difference when compared to a single wire hung from the same tree. There were 90 to 100 radials in my 50x100 side yard. Some theories say the higher radiation resistance of the folded monopole results in better efficiency. I think the efficiency comes from the what's under the antenna.
 Right now I use a single wire 40M vert, hung from a tree, over 30 random length radials, from 15 to 50 feet each. Fed directly with co-ax, no choke, no beads, no problems at 5 watts.
 
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W8JI
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« Reply #6 on: September 07, 2006, 04:03:04 PM »

Bill,

You need a ground system with that antenna. Contrary to what Orr says, you need just as good a ground as any other monopole needs.

The fact you fold the element does NOT decrease losses. Orr was totally wrong about that antenna. He admitted it in a letter, but never got around to changing the text in his books.

For decent efficiency you need about 15 radials that are buried or about four elevated radials minimum. Either will be about the same and within a few dB of optimum.

Of course if you want a really good system you would need 40 radials or so. Then you would be near 90% or more efficiency.

73 Tom
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K4SAV
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« Reply #7 on: September 08, 2006, 02:43:11 PM »

If you are having trouble matching this antenna, it's probably because you don't have a 4:1 balun at the base. The feedpoint impedance should be somewhere around 130-140 ohms. If you use a 4:1 balun you should be able to get under 1.6 to 1 across the full band.

The other choice is feed it as a single wire monopole, then you don't need a balun. It is then a good match for a direct coax connection.  You should be able to get under 1.5 to 1 across the entire band. To try this quickly, just tie both wires of the twinlead together at the bottom.

The reason for doing this antenna as a folded monopole which requires a balun, was based on some faulty reasoning that it raised the radiation resistance of the antenna, thereby increasing the efficiency.  It does not.  It raises the feedpoint resistance, which is a different parameter, and the efficiency is the same as that of a single wire monopole. W8JI pointed out this in his post.  He just didn't give you as much detail.

Jerry, K4SAV
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