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  #9  
Old 05-07-2007, 11:57 AM
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Quote:
The feed systems are 50 ohm direct feed - dual driven elements.
So the driven element is a dipole?

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Old 05-07-2007, 12:32 PM
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In a beam generally yes unless it is a quad and then the driven element is the loop. All direct feed means is that the coax is connected DIRECTLY to the radiating element,whatever it happens to be, without ANY form of matching system.

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Old 05-07-2007, 12:52 PM
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Quote:
but someone told me about a yagi using a "direct feed", no gamma.
I figured BS.
OK, it was BS as I thought.

I mentioned that I have a beam with the driven element being a dipole,could this be what he meant, I was told the coax was hooked to the yagi with a single driven element on a direct feed, gaining 5 db over a gamma. :frky

I figured why waste time telling him he couldn't gain 5db with anoter antenna just like it stacked.

This thread made me consider there was a method of feeding a yagi like he was trying to describe.

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Old 05-07-2007, 02:38 PM
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No 74in it is not BS. Mosely uses direct feed in some of their yagi antennas.

Check it out: Mosley Antennas!
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  #13  
Old 05-07-2007, 04:10 PM
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74IN where is the BS? We have told you and provided links to antenna,yagis,beams, whatever you want to call them and they all have DIRECT FEED, NO GAMMAS or any other matching device. The only BS I see is a claim of 5dB over an identical antenna using a gamma feed.

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Old 05-07-2007, 04:29 PM
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The driven element of a yagi type beam is a dipole of some sort. It may be 'folded', or loaded (traps), but it's typically a 1/2 wave length, or dipole.
As other elements are added to the driven element, director(s), and reflector(s), the input impedance of the driven element is lowered. Depending on the number of 'other' elements, the input impedance can get down to something around 20-30 ohms. Depends on the number of elements, their spacing, (and at times the color of the sky I think). That's why a gamma (or beta, or delta) match is then used, they tend to raise the input impedance back to something around 50 ohms. Feeding a multi-element beam directly can still be done but usually a balun is needed to get that 50 ohms back (but then you may not consider it directly fed anymore).
There is no 5 dB difference between matching systems, that's about the only B.S. about the whole thing (unless it's MY antenna, and MY antennas are always 5 dB stronger than anybody elses! You believe that too, right?).
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Old 05-07-2007, 04:49 PM
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Marconi, I can't get the link to work.

The BS is, as I see it, is to simply eliminate a gamma and hook the coax to a one piece driven element.

Maybe I'm missing something.

For some reason adobe has gone to heck on this computer, so I couldn't see the pdf link above.

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  #16  
Old 05-07-2007, 04:58 PM
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Quote:
2. What does it mean to say that a certain Yagi design uses "direct feed?"

Although many Yagi designs in current use have feedpoint impedance running from 20 to 30 Ohms, it is possible to design a high performance Yagi that shows a feedpoint impedance of 50 Ohms. In this case, we do not need a matching network, since the feedpoint impedance is the same as the characteristic impedance of the most common coaxial cables.





However, we do have some constraints when using a direct feed driven element, as shown in Fig. 1. Regardless of the construction methods used for the other elements, the driven element must be insulated from the boom. The driven element must be split at the center to create a gap similar to what we find in a common wire dipole. The size of the gap is not critical at 10 meters and might range from 1/4" to 1".

We connect the inner conductor of the coax to one side of the element, and the braid to the other side. Hence: direct feed.
This is how I made my six element beam (minus the balun).

This is how I understand one to be direct fed.

This is NOT how the guy on the radio described "direct fed".

This is the source of my confusion. :?:

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