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5/8 Wave Dipole CB Radio
- Thread starter radio59
- Start date
I believe an end-fed 5/8 is about 150Ω feed point impedance, so a center-fed 5/8 shouldn't be any higher, possibly even lower.
I'll venture an inspired guess that a ¼λ piece of 75Ω RG-11 would transform it to dang near a 50Ω impedance and you'd see gain over the ½λ dipole.
A double extended Zepp is a set of .64λ wires fed in the center, & about 23' each for 27.2mHz.
It should be about a 140Ω at the feed point so with a piece of ¼λ RG-11 you should have less than a 1.4:1 and about 5dB gain over the center fed ½λ dipole, great for DXing in the horizontal.
Don't forget the velocity factor of the 75Ω coax when calculating for the ¼λ .
I'll venture an inspired guess that a ¼λ piece of 75Ω RG-11 would transform it to dang near a 50Ω impedance and you'd see gain over the ½λ dipole.
A double extended Zepp is a set of .64λ wires fed in the center, & about 23' each for 27.2mHz.
It should be about a 140Ω at the feed point so with a piece of ¼λ RG-11 you should have less than a 1.4:1 and about 5dB gain over the center fed ½λ dipole, great for DXing in the horizontal.
Don't forget the velocity factor of the 75Ω coax when calculating for the ¼λ .
That 23 feet is one '.64' wave length, so two of them would be 46 feet. Half of that doublet would be 23 feet, the full length would be 46 feet. Take about 6 inches off each side and you'd have a 5/8 wave on each side.
You can forget that 5 dB gain over a 1/2 wave dipole, never happen. There would be no difference in gain because of changing polarization. All that you would be changing is the 'shape' of the 'other half' of that antenna from several radials to a single one. You won't see a difference in input impedance either, it will be the same as with a 5/8 wave vertical 'groundplane', something less than 72 ohms (lots of factors will determine that, height above ground, etc.). If you're lucky, you might see something like 1.2 - 1.5 dBd, or 3.3 - 3.6 dBi of gain (both of those gain figures are the same BTW, deals with how you convert isotropic to 'real world' gain figures). That also means that using a 2:1 impedance transformation device should get you in the ball-park. (That's turning a 2:1 balun around backwards from usual since it's converting 'down' instead of 'up'. It will also mess up the 'balance' part of that balun. That's a PITA ain't it?) An electrical 1/4 wave 'Q' section would also get you in the ball-park, and wouldn't affect that balanced to unbalanced thingy. If you can get the input SWR down to something around 1.5:1, there would be no practical difference from a 1:1 SWR. It's your time so spend it changing SWR as you think fitting. Or, you could feed the whole mess with parallel feed line and a tuner and just not worry about it at all, which is one of the benefits of doing things that way! 'Course, using parallel feed lines is certainly different than using coax, can't run them both the same way.
- 'Doc
You can forget that 5 dB gain over a 1/2 wave dipole, never happen. There would be no difference in gain because of changing polarization. All that you would be changing is the 'shape' of the 'other half' of that antenna from several radials to a single one. You won't see a difference in input impedance either, it will be the same as with a 5/8 wave vertical 'groundplane', something less than 72 ohms (lots of factors will determine that, height above ground, etc.). If you're lucky, you might see something like 1.2 - 1.5 dBd, or 3.3 - 3.6 dBi of gain (both of those gain figures are the same BTW, deals with how you convert isotropic to 'real world' gain figures). That also means that using a 2:1 impedance transformation device should get you in the ball-park. (That's turning a 2:1 balun around backwards from usual since it's converting 'down' instead of 'up'. It will also mess up the 'balance' part of that balun. That's a PITA ain't it?) An electrical 1/4 wave 'Q' section would also get you in the ball-park, and wouldn't affect that balanced to unbalanced thingy. If you can get the input SWR down to something around 1.5:1, there would be no practical difference from a 1:1 SWR. It's your time so spend it changing SWR as you think fitting. Or, you could feed the whole mess with parallel feed line and a tuner and just not worry about it at all, which is one of the benefits of doing things that way! 'Course, using parallel feed lines is certainly different than using coax, can't run them both the same way.
- 'Doc
It seems rather odd this over-size antenna design would weather the test of time and still be in use if it didn't deliver considerably more gain & performance than a standard dipole.
Well, according to the following it's not 5dBd but 5dBi (~3dBd) gain:
N8ITF's Build Your Double Extended Zepp Antenna Page
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http://www.k0ru.net/images/40medz.jpg
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6 METER EDZ EXTENDED DOUBLE ZEPP SIGNAL SQUIRTER
But wait! - Here his EZNEC Pro/2 shows 9.51dBi (~7.36dBd!!) on 40m for a 40m (7mHz) Double Extended Zepp (~1/3 way down the page):
http://www.nfarl.org/articles/The_40M_Extended_Double_Zepp_K4DLI.pdf
We saw what acted like about 6dBd gain in comparison to the previous year's ½λ dipole (about the same as with ½λ dipole + amp) located in the same position between the same two 80' pine trees, so when I wrote 5dBd I was being a bit conservative.
Here's where I read it should be about 140Ω impedance: http://www.qsl.net/n/n4xy//PDFs/ANTs/ant_edz.pdf
I read this before I built the 40m E.D.Z. for Field Day, and it worked both local & DX stations as well with ~130w radio power as did the previous year's simple ½λ dipole using a 500w SB-200.
Again, I think the easiest & least expensive way to feed it would be a ¼λ x velocity factor (about 7') of 75Ω RG-11, and cut the wire ~21'-23' per side, - depending on the VF of the wire.
One could always experiment with hanging a 3/16λ length of end-shunted 450Ω-600Ω balanced line in series on each side at ¼λ out from the feed point to play with re-phasing the pattern, though it doesn't seem necessary when considering the rather impressive performance without it.
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http://www.zk2v.com/n4gg1.pdf
Use the formula to figure any frequency you like for the antenna.
As stated feed it in the center with a 1:1 current balun.
Does it work? Get it up in the air high enough and it screams.
Leave it low to the ground and the DX is Amazing.
Hard to beat a few $ worth of wire and the cost of a balun.
Use the formula to figure any frequency you like for the antenna.
As stated feed it in the center with a 1:1 current balun.
Does it work? Get it up in the air high enough and it screams.
Leave it low to the ground and the DX is Amazing.
Hard to beat a few $ worth of wire and the cost of a balun.
