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Author Topic: egg beater antenna phasing harness  (Read 2270 times)
KD0OCY
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Posts: 17




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« on: January 06, 2013, 10:31:37 AM »

I am looking at building an egg beater to work the birds. Problem is I don't know where I can find 100ohm coax.  I was curious if I could substitute 75ohm coax (i have a ton of rg6) if I was to modify the length. If it is feasible, would it be as simple as adding 25% to the length or would it be more complicated than that?

Any input is appreciated!
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W4OP
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« Reply #1 on: January 06, 2013, 11:15:32 AM »

While I have not looked at the eggbeater, I always assumed (?) it was a pair of 1 lambda loops oriented at right angles and fed 90 degrees out of phase with one another. While 100 Ohm coax may be hard to find , 93 Ohm coax is very common- RG-62/U. You must obviously take the VF into account when you make the phasing line. Because the dielectric in RG-62 is a polyethylene tube with a spiral of polethylene to keep the center conductor centered, I would expect some variation in VF- so you would be smart not to  accept the factory spec but rather to qualify the cable yourself.

Dale W4OP
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WB6BYU
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« Reply #2 on: January 06, 2013, 12:26:14 PM »

Since loops are balanced antennas, the ideal feedline would be 100 ohm balanced
line.  (Assuming that the loops are dimensioned to give a 100 ohm feedpoint.

Conveniently you can make such a line by using two pieces of 50 ohm coax in
parallel:  connect the shields together at each end and use the center conductors
as a balanced feedline.
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W9GB
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Posts: 2650




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« Reply #3 on: January 06, 2013, 01:02:33 PM »

Quote from: KD0OCY
I am looking at building an egg beater to work the birds. Problem is I don't know where I can find 100ohm coax.
Jerry, K5OE built a number of DIY versions, a decade ago.
http://wb5rmg.somenet.net/k5oe/
K5OE Eggbeater
http://wb5rmg.somenet.net/k5oe/Eggbeater_2.html

Dale is correct, RG-62/U is readily available at 93 ohms Z.

RG-62/U is the standard coaxial cable used by IBM as part of their System Network Architecture (SNA) from 1970s to 1990s.  The cable is used between the IBM 3274/3174 terminal controller and the IBM 3270 terminals used with IBM mainframes (360 series).
MILLIONS of miles of this coax cable were installed from mid-1970s to mid-1990s in computer data centers, and then slowly removed for UTP cabling since 1995.  
My last surplus RG-62/U was given to Jerry, K5OE in late 1990s.
http://wb5rmg.somenet.net/k5oe/Texas_Potato_Masher_2.html

Pres, N8UG at The Wireman; Joel at RF Connections; and Digi-Key sell it by the foot.
http://media.digikey.com/pdf/Data%20Sheets/General%20Cable%20PDFs/RG_62U_Type.pdf

Quote
If it is feasible, would it be as simple as adding 25% to the length or would it be more complicated than that?
Sigh.  Learning the Basics (Radio, AC Theory, Electronics) is important ... It's complicated when you don't.
« Last Edit: January 06, 2013, 01:17:10 PM by W9GB » Logged
KD0OCY
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Posts: 17




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« Reply #4 on: January 06, 2013, 03:38:13 PM »

Since loops are balanced antennas, the ideal feedline would be 100 ohm balanced
line.  (Assuming that the loops are dimensioned to give a 100 ohm feedpoint.

Conveniently you can make such a line by using two pieces of 50 ohm coax in
parallel:  connect the shields together at each end and use the center conductors
as a balanced feedline.

Thanks for that! I ran across that suggestion somewhere but the way they worded it didn't make much sense. Let me make sure I have it correct.

After connecting the braid, the one center conductors in the two cables is connected in place of what would have been the braid of the rg62 coax?  I'll be trying that out!


/\/\/\ dont think i worded it well either.
« Last Edit: January 06, 2013, 03:52:19 PM by KD0OCY » Logged
WB6BYU
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Posts: 13457




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« Reply #5 on: January 06, 2013, 04:48:10 PM »

I would do this with TWO pieces of feedline, one from the feedpoint of each
loop to the common balun.

1 piece is 1/2 wavelength long
1 piece is 1/4 wavelength long

You may be able to get by with a single piece if the conductors are close
together at the feedpoint, but trying to do the same thing with dipoles
where the two sides are an inch apart or so, it just doesn't work well.

So the two center conductors of each assembly connect across the loop.
The shields are tied together at each end.  Then the two free ends are
each connected in parallel at the 1 : 1 balun.

This assumes that the loop impedance is 100 ohms, which might or might
not be the case.  You adjust the loop impedance by changing the aspect
ratio (height vs width) of the loop:  tall and skinny lowers the impedance.
A square loop is likely to be in the 120 - 130 ohm range.  You can get a
good SWR with such a loop when the currents are unequal in the two loops,
giving a less omnidirectional pattern.  It may work well enough, but that
just goes to show that SWR doesn't really tell you that the antenna is
working as intended.

If you want to do it with 75 ohm coax instead, design the loops for an
impedance of 150 ohms (slightly wider than it is tall) and use two pieces of
75 ohm coax to make 150 ohm balanced line.  The two 150 ohm loads
connected in parallel gives you 75 ohms for the feeder to the shack.
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