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Author Topic: 40M Inverted Vee verses Zepp?  (Read 2815 times)
W7VO
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« on: April 27, 2011, 03:01:44 PM »

Here is one for the guys with the antenna analysis software handy:

I had my 40M dipole break due to some ice build up last winter. The apex was at about 50 feet on my tower, and the ends were about 20 feet off the ground. Not quite an inverted Vee, but close... I have real estate for something longer here on both ends, so I figured that maybe a 5/8 wave double ended Zepp type (Dipole with times 1.28 length, or about 42 feet on a side) fed with a 4:1 balun might be a better choice here. (The impedance of the Zepp appears to be about 200 ohms)

Am I off track here, or would I be better off with the simpler dipole/inverted vee?

My main 40M antennas are a set of 3 full wave slopers, (which work great), but one can never have enough antennas!

Thanks;
 
73;

Mike, W7VO

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W5DXP
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« Reply #1 on: April 27, 2011, 03:23:08 PM »

... so I figured that maybe a 5/8 wave double ended Zepp type (Dipole with times 1.28 length, or about 42 feet on a side) fed with a 4:1 balun might be a better choice here. (The impedance of the Zepp appears to be about 200 ohms)

The length of an extended double zepp (EDZ) is ~1234/f = 1234/7.15 = 172 feet for 40m or 86 feet on a side. Your above 42 feet on a side seems to be for 20m. The feedpoint of an EDZ is about 175-j1000 ohms. A 4:1 balun would ideally reduce that impedance to 44-j250 ohms which requires a tuner. One way to resonant an EDZ is with ~0.2 wavelength of ladder-line as a matching section which results in a feedpoint impedance around 30 ohms not requiring a tuner.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W7VO
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« Reply #2 on: April 27, 2011, 03:58:52 PM »

Interesting, and doable in the space I've got here! What would the theoretical gain of this antenna be over an equivalent dipole? Enough to make it worth it?

Thanks again;

Mike, W7VO
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W7VO
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« Reply #3 on: April 27, 2011, 04:20:35 PM »

Just answered my own question. 3 dBd is the gain of a DEZ antenna. Smiley

The research I did in the ON4UN book showed that the length was supposed to be about 1.24 times the length of a 40M dipole, (which is about 33 feet on a leg). That is where the 42 feet came from. (Essentially the antenna goes from being a 1/2 wave dipole to a 5/8 wave dipole) I must of mis-read.....

I found a great chart for DEZ antennas here:

http://home.comcast.net/~n8itf/doubzepp.htm

Time to order some ladder line!

73;

Mike, W7VO
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WB6BYU
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« Reply #4 on: April 27, 2011, 04:39:25 PM »

Quote from: W7VO
The apex was at about 50 feet on my tower, and the ends were about 20 feet off the ground. Not quite an inverted Vee, but close...

With the ends lower than the center, it certainly qualifies as an inverted vee in my book.  But a 40m dipole
with a 30' drop between the center and the ends would mean the included angle was just a bit over 60 degrees,
making the wires too steep for good efficiency.


Quote
Am I off track here, or would I be better off with the simpler dipole/inverted vee?


As always, the answer is, "it depends".

The EDZ could be matched reasonably well with a 4 : 1 balun and about a 10uH coil in series with each
wire.  (Adjust as needed.)  If your primary interest is NVIS propagation for relatively local contacts, and
perhaps medium-distance contacts broadside to the antenna, then the EDZ might be a good choice.  But
for DX the inverted vee is likely to be better.  Here's why:

First, we'll assume that the EDZ is also installed with the feedpoint at 50' and the ends at 20', and the
comparison will be an inverted vee with the same support points.  (That means that the ends of the wires
are extended with ropes and tied off in the same place as the EDZ.)  The maximum current on the EDZ legs
will be 1/4 wave up from the ends, or about 32' (1/4 wavelength) off the ground.  At this height maximum
radiation will be upwards due to ground reflections.  By contrast the maximum current point is at the feedpoint
of the dipole - we have to reduce the effective height a bit due to the wires sloping downward, but that
still puts us around 40' or so.  While the inverted vee is still capable of reasonable NVIS coverage at this
height, it will also have better low angle radiation due to the increased height.

If you have a specific interest in one direction and can install the EDZ broadside to that direction, it might
be an improvement at some distances.  Likely it would also be better for NVIS at distances out to 100 miles
or so (when the ionosphere ever gets around to supporting such modes again...)  But for general use I think
the inverted vee (with a flatter angle than the one you had) will be better overall.  Whether it is better than
your slopers for DX will depend on ground losses, etc.



[edited to use coils instead of capacitors]
« Last Edit: April 27, 2011, 04:44:19 PM by WB6BYU » Logged
N4JTE
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« Reply #5 on: April 27, 2011, 05:08:15 PM »

Having used a EDZ on 40 for many years, 186 ft. long, it is extremely important to get it up at least 1/2wl and be flatopped to get close to the 3dbd of gain. The gain is acheived by narrowing the lobes substantially and this won't occur with the ends at only 20 ft. The impedance at the correct height is way above 200 ohms but can be matched exactly to 50 ohms by using a 1/4 or 3/4wl ladderline stub shorted at the end and tapped futrher up the line to achieve a 50 ohm coax match. Makes for a great 80 meter antenna also easily tapped for good match.
Regards,
Bob
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WA8JXM
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« Reply #6 on: April 27, 2011, 08:50:36 PM »

Antennas do not really have "gain", they just redirect energy.  If you get 1.8 db of gain, you lose that somewhere else.  Such gain works well where you can rotate and direct the antenna but for a wire antenna you gain a bit and lose a bit.   

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WB6BYU
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« Reply #7 on: April 27, 2011, 09:31:01 PM »

A quick check with EZNEC suggests that the EDZ is 1dB better at 45 degrees above the horizon,
but 1dB worse at 20 degrees.  So the inverted vee would be slightly better for DX, but the
differences aren't all that large.

On the other hand, the EDZ is about 4dB better straight up due to the antenna gain and the
difference in ground reflection due to height.

But off the end of the antenna, the EDZ signal drops rapidly:  at 50 degrees above the horizon (which
would correspond to distances of about 200 miles I think) the EDZ is 15dB weaker than overhead,
and the inverted vee beats it by about 10dB.


Which confirms that the inverted vee is a better choice for an all-round antenna, while the EDZ can
give some improvement in specific directions at the expense of others.  The choice depends on
your specific operating preferences and the coverage of your other antennas.
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W7VO
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« Reply #8 on: April 28, 2011, 11:43:00 AM »

I fully realize that antennas do not have "gain" so to speak, but I was interested in an antenna that would get me a better shot into EU (which is N from here), so directional "gain" and take off angle is important. I can run either an inverted vee or the double ended extended zepp with the space I have in an E/W configuration.

Sounds like I might be better off with the Vee, but I will have to put the zepp together and try it. I have the wire and ladder line. Maybe I will end up putting both up!

One more question: At the end of the ladder line, a 4:1 balun or a 1:1?

Thanks;

Mike, W7VO
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W5DXP
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« Reply #9 on: April 28, 2011, 01:55:01 PM »

One more question: At the end of the ladder line, a 4:1 balun or a 1:1?

If you are using the ~0.18WL ladder-line matching section, definitely a 1:1 choke-balun at the ladder-line/coax interface. If you run ladder-line all the way to the shack, which balun is best depends upon the length of the ladder-line.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WA4FNG
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Posts: 162




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« Reply #10 on: May 02, 2011, 05:09:58 AM »

Quote
...but I was interested in an antenna that would get me a better shot into EU

If you're looking for some directivity and low angles, why not try a half-square. Easy to feed at the top corner with coax. It's essentially 2 qtr wave verticals sections separates by 1/2 wave. Feed at the top corner, you won't need radials for this.
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W7VO
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« Reply #11 on: May 03, 2011, 10:32:33 AM »

Actually, the 40M half square is starting to look pretty good right now. Smiley It would take up a lot less horizontal space than the zepp, but I need to figure out how to get the far end up in the air enough for the end vertical component to work. It would have to be about 45 feet up to work. Time to do some homework with a tape measure.....

Thanks again for all the comments and suggestions. Plenty of food for thought.

73;

Mike, W7VO
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