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Author Topic: Driven vs parasitic beams  (Read 9754 times)
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K6JEK
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« on: May 27, 2009, 06:29:38 PM »

My buddy Mike, W6THW, is in the process of putting up a 75M 3 element wire beam so the guys in Southern California can really hear him 500 miles away.  It's not enough, apparently, that he runs an RCA BTA 1R. 

I've been telling him he'd be better off with driven elements since it's low but I can't explain why.  I just remember somebody saying once (maybe on this forum) that if a beam is low you're better of with phased, driven elements than parasitic elements.

It will be 65 feet up which should give it a pretty high main lobe.  He's not calling CQ DX. He just wants to make sure the people  down south don't miss him when he keys up.

Can anyone help me with my understanding of the topic?
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K5UJ
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« Reply #1 on: May 27, 2009, 06:51:13 PM »

My thinking off-hand is that this might do okay because he only has to get 500 miles.   The beam should certainly be no lower than 65 feet.   Generally, the parasitic antenna of choice for low heights is the quad, but obviously a quad on 75 meters is for most of us pretty ambitious.   I'm not that wild about phased horizontal dipoles because I have not liked the gain I've read they have (around 3 or 4 dBd for two or maybe three elements compared to about twice that for a parasitic yagi of the same number of elements) and the people I know who have tried them have not seemed to have done all that well. 

My hunch is that ground is going to distort the expected pattern but the good news is 65 feet may be high enough to avoid a lot of ground loss from coupling to earth. 

If it were me I think I'd try a couple of phased 1/4 w. verticals with 60 radials on each but others with more than my textbook experience may differ.

73

rob / k5uj
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« Reply #2 on: May 27, 2009, 07:49:23 PM »

The advantage of a driven array is you can flip the pattern. If you are at 65 feet and only want to point South I think you can save the two extra runs of coax.
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Steve - WB3HUZ
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« Reply #3 on: May 27, 2009, 08:08:20 PM »

The all driven array may also lower the take-off angle a little. This may not make that much difference in this application. The problem with parasitic arrays at low heights is the coupling from the driven element to the parasitic elements is highly influenced by the ground. So you may end up with an array that works great or one that is crap. With an all driven array, the feed system is more complex, but, if you get it right, you will be guaranteed an antenna system with a good F/B ratio and near optimum gain for the given number of elements.

For this application, I'd go with either a two-element driven array or a parasitic Moxon array (also two elements). He will only see about one more dB of gain with three elements. He will get much better F/B with three elements versus two, but it sounds like forward gain is the most important requirement. The extra effort of three elements hardly seems worth it.

Stay away from verticals. The take-off angle would be way too low for this application and the amount of work to put in a good radial system makes them much more effort than a horizontal array.

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K6JEK
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« Reply #4 on: May 27, 2009, 11:20:57 PM »

The all driven array may also lower the take-off angle a little. This may not make that much difference in this application. The problem with parasitic arrays at low heights is the coupling from the driven element to the parasitic elements is highly influenced by the ground. So you may end up with an array that works great or one that is crap. With an all driven array, the feed system is more complex, but, if you get it right, you will be guaranteed an antenna system with a good F/B ratio and near optimum gain for the given number of elements.

For this application, I'd go with either a two-element driven array or a parasitic Moxon array (also two elements). He will only see about one more dB of gain with three elements. He will get much better F/B with three elements versus two, but it sounds like forward gain is the most important requirement. The extra effort of three elements hardly seems worth it.

Stay away from verticals. The take-off angle would be way too low for this application and the amount of work to put in a good radial system makes them much more effort than a horizontal array.


Thanks.  This is the explanation I was looking for.
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K1JJ
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« Reply #5 on: May 28, 2009, 02:12:20 AM »

Hi Jon,

Here's a short article on 75M 2el driven arrays that applies to what you axed:

http://amfone.net/ECSound/K1JJ21.htm

For a 600 mile path, a 75M array at about 65' would be ideal with the take-off angle matching up well for the job. Much will depend on the skip angle, though lower than 65' will increase ground losses and reduce the pattern sharpeness (in the vertical plane) and higher than 90' will mean too low an angle for that short path.  This applies to nightime. During the day, you will find an array at 130' will perform VERY well for a 300+ mile path due to high absortion of the higher angles.

One thing to remember is that the take-off angle is directly affected by the AVERAGE height of the antenna. So, if it's a 75M quad with the apex at 65', the antenna will act like a flat Yagi at only about ~35' high. (horizontal fed driven element)   Same with inverted vee elements - it's the average height that directs the vertical take-off angle.

For example, I needed to hang the apexes of my 2el 75M quads elements at 190' to equal the TO angle performance of a 2el flat Yagi at 135'.   

Also, inverted vee elements that slope down at 45 degrees per side need to be at 150' peak height so that their average height is at the optimum 135' height of a flat Yagi - to equal the performance.  Same concept applies to lower heights. Bottom line is to get those ends up as high and flat as possible to improve f-b and forward gain.  The lower the ends, the more the legs cancel out each other  - until they come close together and become a non-radiating transmission line.

I have found that inverted vee elements work very well compared to flat elements, as long as they are mounted higher (apex) than flat elements and they are cut somewhat shorter to compensate for the increased capacitance causing effective longer length.

The best way to tune a Yagi is to use a reference dipole to compare against - and tune the band looking for stations off the back. You will find a sweet spot band area where the f-b is the highest. Then add or cut wire to bring the Yagi elements to the desired spot with the best f-b. When tuning a driven array, you can do the same thing by adjusting the phase delay length of the coax.

The Huzman said it all about the ground effects when using a parasitic array when it is closer to the ground. (like 1/4 wave high) A driven array appears to work better due to the forced-direct  feed of the elements. The parasitic elements of a Yagi appear to couple together poorly when closer to the ground, thus the vertical pattern takes on a "fat and lazy" look.   

The 75M driven array at 65' average height described in the article above would be the perfect match for your friend.

Good luck.

Tom, K1JJ
 

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« Reply #6 on: May 28, 2009, 10:30:13 AM »

Hi Jon,

Here's a short article on 75M 2el driven arrays that applies to what you axed:

http://amfone.net/ECSound/K1JJ21.htm

...

I think this is the antenna Brent uses or used.  There are a couple of guys in the NE who are loud out west if I can hear anyone.  Brent is one of them.  QIX is another but I think that's because of parasitic elements in his heating system.
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« Reply #7 on: May 28, 2009, 12:06:29 PM »

My force fed inverted Vees at 60-70 feet apex don't work worth a crap. Ends are at about 20 feet. Rarely better than the reference dipole in the woods over the stream bed. I think the ends need to be at least 40 feet.
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K1JJ
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« Reply #8 on: May 28, 2009, 02:50:03 PM »

My force fed inverted Vees at 60-70 feet apex don't work worth a crap. Ends are at about 20 feet. Rarely better than the reference dipole in the woods over the stream bed. I think the ends need to be at least 40 feet.


Yep.  Bottom line is to get those ends up as high and flat as possible to improve f-b and forward gain. 

Think of it this way - it's a matter of degree:  The lower the ends, the more the legs cancel out each other  - until they come so close together (extreme case) they become a non-radiating transmission line.  With the apex at 65' and the ends at 20', the average effective height is only 42' PLUS the leg cancellation and poor pattern.  That's less than a 45 degree angle per leg, too.

That array wud come to life with the ends pulled out at 40' or higher.



As a side note:

This effect is even worse when using double extended Zepps, two half waves in phase, lazy H's and the like. (using more than 1/2 wavelength per dipole) Modeling 1/2 wave elements as moderate inverted vees works well. My 40M 3el wire Yagi for SW has a tremendous f-b and forward gain performance. However, model the same antenna using full wave elements as inverted vees and it works no better than shorter 1/2 wave els due to leg cancellation and pattern distortion.   

Two half waves in phase arrays, etc, MUST be erected as flat as possible to obtain the desired advantage of using longer elements.  This applies even if the array is at 1/2 wavelength above ground. (optimum height for DX/low angle)   Over the years I was disappointed too many times until I modeled it to see what was going on.  When using inverted vees for a Yagi or driven array, 1/2 wave els are the way to go.  If the els can be erected flat, then full wave els will give us about 2.5db more gain. (sharper frontal lobe)

T
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Steve - WB3HUZ
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« Reply #9 on: May 28, 2009, 02:57:00 PM »

Quote
Two half waves in phase arrays, etc, MUST be erected as flat as possible to obtain the desired advantage of using longer elements. 

Another way to look at it is that they are no longer in phase, since the proximity to ground screws up the current distribution.
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K1JJ
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« Reply #10 on: May 28, 2009, 03:00:32 PM »

Quote
Two half waves in phase arrays, etc, MUST be erected as flat as possible to obtain the desired advantage of using longer elements. 

Another way to look at it is that they are no longer in phase, since the proximity to ground screws up the current distribution.


I've always thought that to be the reason for the deterioration in pattern too.  Though, I see the same effect at heights of 1/2 wave or more above ground. Also another possibility: Is the inverted vee leg proximity to each other inducing out of phase currents through mutual coupling? Normally the legs are in perfect colinear when flat. The coupling between them is nil



Is this a possibility?

The TWO current peaks are in the middle of each leg for full wave els, so more apt to see each other when sloped and screw up the overall phase.

With the ONE current peak at the center of the 1/2 wave dipole,  maybe it's more immune to this stray coupling.



T
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« Reply #11 on: May 28, 2009, 03:15:01 PM »

Yep. Also more wire to couple to the ground compared to a half-wavelength dipole.
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K1JJ
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« Reply #12 on: May 28, 2009, 03:18:40 PM »

Yep. Also more wire to couple to the ground compared to a half-wavelength dipole.


Yes. I've found that a full wave element has to be 20' higher than the 1/2 wave element to equal the same TO angle on 75M.  ie, There is a price to pay for using longer elements.

T
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« Reply #13 on: May 28, 2009, 08:07:42 PM »

Well I'm done in the woods for now. I have a spot of P.I. on my leg that will take a month to clear. I need to rub some paint thinner on it. I scratch in my sleep. Maybe XYL needs to lace me into a straight jacket at night.
I need to get the second element in the woods and convert the ant I have in the yard to 40m. Or sit on the beach and inspect.
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K1JJ
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« Reply #14 on: May 28, 2009, 08:25:36 PM »

Well I'm done in the woods for now. I have a spot of P.I. on my leg that will take a month to clear. I need to rub some paint thinner on it. I scratch in my sleep. Maybe XYL needs to lace me into a straight jacket at night.
I need to get the second element in the woods and convert the ant I have in the yard to 40m. Or sit on the beach and inspect.

It's good you used a reference dipole to see the real performance truth.

So many fool themselves into thinking an array is working. But once it is compared against a dipole that is in the clear and at the same height, the reality sets in.  Front to back is usually impressive and easy to get. But since a good working 3el Yagi is only about 4-5 db better than a dipole, unless the Yagi is working perfectly, disappointment is often the result when compared against the reference dipole.

BTW, on the poison ivy...  I get a little bit every year and have a little right now. When you get the first signs of a rash, I've found the best remedy is to first wash and scrub it hard with a facecloth, dishwash detergent and hot water. Soap it up. The dish wash detergent cleans out the ivy oil well. (wash also if you think you have rubbed against it)   After the washing, then scrub the area again with rubbing alcohol. You want to scrub the skin aggressively, even if it means to the point of irritation to clean the ivy oil out.

Continue to wash the affected area every 3 hours or so with the alcohol for a couple of days. This will help dry it out.  You will find the rash heals quicker and the little welts turn a bright red within a few days and disappear.  Forget Calamine lotion and other remedies that cover up and smother the rash. You want to clean out the oil and keep the "wound" clean.  I find the itching to be almost non-existant when the area is kept clean with alcohol.  Tom Vu told me the skin cells get drunk and lose their mind.

BTW, there was a time in the 50's and 60's when I got PI so bad I looked like a swollen monster.  After a few years of this, the doc gave me three ivy immunity shots every spring. They were actually the ivy extract and my body was getting used to it, just like when people get injected with rattlesnake venon over time and build an immunity.  So now, I barely get it unless I handle it aggressively.  Before the shots, I'd get it badly every spring no matter how careful I was.

T
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« Reply #15 on: May 28, 2009, 08:54:16 PM »

I have an area on my lower left leg where I had it real bad and very sensitive to it. I was digging a hole for a mail box down the beach and across the street there is a pile of the stuff. I stepped into the hole to push a rock loose and went against some roots and got it real bad. I forgot about dish soap.
Paint thinner also works the same way to draw out the oil I bet a wire brush would help if you could stand the pain. That pink shit just makes it worse.
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K1JJ
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« Reply #16 on: May 28, 2009, 09:10:20 PM »

I bet a wire brush would help if you could stand the pain.


 Grin Grin Grin    Just scrape the skin away and the PI has no choice but to go wid it!   Grin Grin Grin
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« Reply #17 on: May 29, 2009, 02:07:52 AM »

The California central coast must be the poison oak capital of the world.  It flourishes everywhere.

Conventional wisdom out here for poison oak is wash first with cold water. The theory is you don't want the pores to open up and accept more of the oil.  The preferred product is Technu.  I always keep it on hand. It's not cheap but worth it.

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Steve - WB3HUZ
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« Reply #18 on: May 29, 2009, 12:15:35 PM »

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The preferred product is Technu.

That stuff works great. If you apply it within an hour or so of exposure, you'll won't even get a rash. I first used it when I worked in landscaping back in the early 80's. I think it was only available as an industrial product then.
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« Reply #19 on: May 29, 2009, 12:31:47 PM »

Another good technique is to wear a full body condom when hiking or landscaping.   You know what they say about an ounce of prevention.

T
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There's nothing like an old dog.
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