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Author Topic: Extended Double Zepp Antenna  (Read 7893 times)
W4EGT
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« on: August 28, 2000, 04:03:06 PM »

I was flipping through the ARRL's Wire Antenna Classics and came across information regarding an "Extended Double Zepp" antenna.  I was wondering (for those of you that have built it - anyone?) if the feed point resistance (at the 1:1 balun) really is 50 Ohms as advertised or if I should be running through a tuner (or getting a 4:1 balun)?
The reason I ask is that I'm going to be building this antenna shortly for a side-by-side comparison against a dipole (that I'm also building soon) and want to make sure that the antenna will be as advertised.  It's not that I don't believe what I read, it's just that I don't want to bet a radio on my being 100% right!  Thanks!!!

73
tim
W4EGT
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AC5UP
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« Reply #1 on: August 28, 2000, 08:23:10 PM »

The Extended Double Zepp is easy to build, isn't fussy about height or proximity to other wires, can be trimmed to a broadband 1:1 match (zero SWR) and makes a dandy general purpose SWL antenna that 'hears' extremely well. It is not practical below 40 Meters for most of us, as a 40 Meter EDZ is 170' tip-to-tip, and the preferred working height is 1/2 wave above ground. (492 / f MHz)

Here's how I've built and 'Worked The World' with them:

1) Pick a band. (we'll use 20 Meters as an example)

2) Use the lowest possible frequency in MHz and divide 1500 by that number. (1500 / 13.9 = 108 feet) Add a foot or two for the rope-tie end loops and 'insurance' if you like.

3) Cut a 12 gauge solid insulated wire (THNN house wire works fine, the darker colors are less visible) to the 108 foot total wire length, then cut it in two equal halves. (you want to keep everything symetrical & balanced)

4) Decide on the mid-band frequency in MHz you're most interested in working, then divide 1200 by that number. 1200 / 14.150 = 85 feet for full coverage that favors the DX phone segment.

5) The 85 foot dimension is the tip-to-tip length of the horizontal flat-top section. You want to stay fairly close to the calculated length as you won't be pruning this.

6) Mark each wire at 42.5 feet (half of 85' each side) then secure the center dog-bone insulator at the marks by wire-wrapping a scrap of bare 12 gauge solid wire top & bottom on each side. Wrap tight and nothing will slip, there is no need to break or solder any wires at the insulator. The wire-wrap technique is strong, weatherproof, and less prone to breakage than using 450 ohm balanced line for the tuned feeder.

7) Buy a 5' or 10' length of 1/4" C-PEX plumber's tubing (cheap) then use a tubing cutter to make a dozen or more 1.5 inch long spacers from it.

Cool Arrange the two wires hanging down from the center insulator into something resembling an open-wire feeder using the C-PEX spacers and 7" black nylon tie-wraps.

9) Attach your 50 ohm coax feed line (any length) through a 1:1 current balun to the bottom of the tuned feeder. RG-8X works well, wrap 4-5 turns of coax semi-snug through a T-240 125u (Mix 61) or better ferrite toroid for a Reisert 'Choke' Balun. DO NOT use a 'voltage' balun or omit the balun, as it isolates the radiating section from the feed line and balances the antenna currents. The EDZ is a balanced antenna that works best when mechanically and electrically symetrical about the center insulator. Ignore this, and you'll risk RF in the shack or a weird radiation pattern.

10) Haul the antenna up to a comfortable working height where you can reach the coax feed point, then check the SWR at the absolute lowest frequency in the band. It should be dead flat. If you dare, use 13.9 MHz as your SWR test frequency (briefly!) then start trimming the bottom of the 12 gauge open-wire feeder until the SWR at the bottom of the band shows any SWR increase at all. Stop trimming and weatherproof the coax connection, then haul the antenna up to your final working height.

11) QSY to a quiet spot and start calling CQ, CQ, CQ...

The EDZ has a sharp SWR rise at the bottom of the band and a very gentle rise toward the top. The 20 Meter version can easily cover the entire band with less that 1.2:1 SWR and you'll note little-to-no SWR change when you haul it up to final height. A 15 Meter EDZ (carefully pruned) can resonate dead-flat on 15 Meter 'phone and 40 Meter CW and is small enough to fit the average attic. I've built EDZ's for Six and Ten Meters that were tuned in the back yard then installed in the attic with no SWR change or RF problems in the shack at the 100 watt level, but I do recommend 'semi-QRP' for any indoor installation.

It's a very tolerant antenna (provided you choke the coax at the feed point) and once you get handy at building them, you'll never go back to a dipole...

Note: You can also build loops using the same technique.

Use 1500 / f MHz for the total wire length, but substitute 1005 / f MHz for the length of wire in the loop instead of 1200 and DON'T cut it in the middle!

Also, if you should revert back to a dipole, instead of trimming the ends to bring it into resonance leave a foot or two of wire hanging from the center insulator and prune that instead. This won't make any noticeable difference in how the antenna works, but it's much easier than pruning the ends.

Wire is cheap, and antennas can be fun to experiment with! If you spend more than $10.00 on an EDZ (not counting the coax, balun, and supports) you were smart and bought a 500' roll of 12 gauge solid THNN 'cuz you figured out you can build six (or more) antennas with it and they're a good choice for Field Day...
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