Scatter Forager - wide pattern bi-directional "Slot" antenna - Page 3 - Canadian TV, Computing and Home Theatre Forums

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post #31 of 68 (permalink) Old 2010-07-24, 05:26 PM Thread Starter
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Question

Here comes the silly questions.

Does the distance at the feed gap change the feed point impedance of the antenna? Would narrowing at the feed point improve the antenna's impedance?

It is my understanding that the entire balance of the element lengths in relation to the size of the ground plane/passive elements/reflector sets the impedance value. Is that correct?

This is a confusing animal because, in the case of a slot antenna, there is no ground plane/passive elements or reflector to tune the aperture to. It's all one piece with a hole in the middle [the hole is the driven element?] and it's all electrically bonded together with the feed points.

Hmmm! heh!

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post #32 of 68 (permalink) Old 2010-07-24, 05:56 PM
 
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When I was playing with nec simulations just about any dimensional change to the antenna changed the impedance - some dimensional changes were more sensitive than others.

As well since this is a broadband antenna the impedance will change with frequency as well. A smith chart is really handy to understand the characteristics of impedance changes with the different frequencies, this is what I look at while playing with the nec simulations.

Impedance match is not necessarily the holy grail as I've chased the perfect impedance match and ended up with an antenna with less gain than the one with an impedance mismatch even after factoring in the loss of the SWR. Or I've found a perfect match only to reduce the bandwidth of the antenna.

The slot antenna has two dimensions - the slot dimension and the outside dimension. These will affect the impedance. I guess only nec simulations will give you an idea of what exactly happens when these dimensions are changed.

------

The best analogy of moving the feedpoint in the slot to change the impedance would be to compare this to a gamma match and how that works.

------

I don't pretend to understand how all this works

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post #33 of 68 (permalink) Old 2010-07-24, 06:36 PM
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Basically yes. Changing anything or changing the relationship of any pieces relative to each other changes impedance.

Narrowing or widening the feed gap is a very common way of adjusting impedance.

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post #34 of 68 (permalink) Old 2010-07-25, 12:29 AM
 
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Some basic slot theory

The ideal slot theory is usually presented in terms of dipole theory, on the basis of the "Booker extension" of "Babinet's principle". The earliest reference in my notes is by Booker from 1946, "Slot aerials and their relations to complementary wire aerials."

Slots and (strip) dipoles are complementary by interchanging the electric and magnetic fields. To use the results for round dipoles for strip dipoles, use twice the diameter for the width of the strip. For horizontal polarization, the dipole will be horizontal while the slot will be vertical. For half-wavelength the impedance of the dipole is (72 +j42.5) Ohm, while the equivalent slot is (363 -j211) Ohm (i.e. the dipole is inductive longer than the resonant length, whereas the slot is capacitive). Wire dipoles have low impedance at odd half-wavelengths, while slots have low impedance at even half-wavelengths. It is possible to combine them for bandwidth improvement.

Moving the feedpoint off-center is a technique sometimes used for enhancing bandwidth. As usual, the theoretical results are based on simplifying assumptions, such as infinitely thin dipoles, sheets of infinite extent, perfect conductors, etc.

An interesting development is the use of a parasitic wire with a slot:
T. Morioka, S. Araki, K. Hirasawa
Slot antenna with parasitic element for dual band operation
Electronics Letters, vol. 33, no. 25, 4 Dec 1997, pp. 2093-2094.

Hopefully there are better web pages on the subject, but here is one:

http://www.antenna-theory.com/antenn...rture/slot.php

HTH
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post #35 of 68 (permalink) Old 2010-07-25, 11:37 PM
 
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Thanks for the insightful explanations. I did not think of the posibility of bandwidth changes due to feed points. I will have to try some more experiments with my test unit.
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post #36 of 68 (permalink) Old 2010-07-26, 11:22 PM Thread Starter
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Xauto rescaled the slot size to "18" by 1/8" attached balun to center" [see post #13.]

The top portion above your balun was 9" and the remaining 3" below the balun, so Xauto basically rescaled to the total slot length to acheive 9" above the balun and 9" below the balun, hence the avg. UHF frequency should be the same as your antenna.

Moving the balun down renders the longest portion of the slot as the being the determining factor of the frequency of the full slot [dipole equivelent] length. If you look at 300ohm's post #27, you'll see that moving the balun down below center seems to discard the bottom portion of the slot and redirects the vertical reception angle.

I would think that making the slot to the designated dipole length and then dividing it in half by having the balun centered would make more sense than moving the balun off-center.

I made one as you had described using a 4" dryer vent tube with a 17" x 3/4" slot and the [4:1] 75 Ohm balun is centered at 8.5". I'm not sure what the actual gain figure would be, but at 25 feet height I get most of my available UHF channels at 100%. The lowest at 14.1RF & highest at 50.1RF are very strong. This cylinder design absolutely seems to be 100% omni.

I believe nec2 is capable of analyzing cylinders, but not sure about slotted cylinders.

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post #37 of 68 (permalink) Old 2010-07-27, 07:10 PM
 
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Quote:
I believe nec2 is capable of analyzing cylinders, but not sure about slotted cylinders.
http://www.digitalhome.ca/forum/show...&postcount=726

X-731 from bokakob is a slotted cylinder. Seems it works.
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post #38 of 68 (permalink) Old 2010-07-27, 10:10 PM
 
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Quote:
Originally Posted by ota_canuck View Post
I would think that making the slot to the designated dipole length and then dividing it in half by having the balun centered would make more sense than moving the balun off-center.
Fig. 7-20 (page label 92, pdf page 96) of this pdf shows some effects of moving the feed point.

http://www.cwc.tf.uni-kiel.de/lectur...2009-04-23.pdf

Quote:
I believe nec2 is capable of analyzing cylinders, but not sure about slotted cylinders.
These two messages from the NEC-list archives may be instructive:

http://www.devilsfoot.com/Nec/Archive1997/0064.html

http://www.devilsfoot.com/Nec/Archive1997/0069.html

The second is from Jerry Burke of LLNL.
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post #39 of 68 (permalink) Old 2010-07-31, 07:14 PM
 
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ota canuck
Since I have a 50' roll of 1" x 2" mesh I gave this ant. a try . The dimensions I used was 20" x 28" , 14" x 4" slot , 45 deg. on the outer 2" , mounted 3" off the mast . No height tuning done , just a quick test . It pulled in VHF 12 and my only local digital 19 abit stronger than my SHGH setup . While the other channels were weaker it did pickup RF50 (54.1) . Not bad at all performance wise , unfortunately it doesn`t fit in with my line of "Alien builds" . See profile for pics. The SH on top of the SF isn`t active BTW. Impressive enough to do more testing another day .
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post #40 of 68 (permalink) Old 2010-08-02, 01:03 PM Thread Starter
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Question antenna bonded or non-bonded mounting?

Re-tests of the antenna that I described in post#6 & have shown in post#8.

Seeing that I believe that the screen portion of this design is a ground plane interacting with the aperture, I wondered if there was a need to isolate the mounting of this type of antenna from the mast.

I've done some testing with the idea of bonded and non-bonded mounting onto the grounded mast for this aperture style antenna. I have found no difference in performance, whether this antenna design is bonded or not bonded to the grounded mast. Mounting points are 2" from the top of the outer edge and 2" from the bottom of the outer edge. The feed points must be at least 1" away from the grounded mast.

I'm not sure if nec models can confirm the effect of bonding [or isolating] this antenna design to the grounded mast.


Two notes of interest from these bonding trials:

- plastic standoffs may not be required for mounting.

- the mast side [back side] of the antenna seems to become more directional than the front side that has the swept forward bends.

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post #41 of 68 (permalink) Old 2010-08-05, 08:04 AM
 
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I too did some testing with bonding the screen to the mast. I found some slight differences .
I did the testing off the shed , feedpoint about 10' off the ground , no pre-amp , pointed right trough a tree 100' away at digital ch 19 SunTv , 32.8 miles away. The idea was to use a rather weak signal for testing.
Using a LG tv , the meter reads up to 32 bars.
Not bonded to the mast at 3" spacing , front was 15.5 bars / back 16.5 .
2'" spacing was front 16.5 / back 16.5 . 1" spacing was front 17.5 / back 16.5 .
Bonded showed 1" spacing front 16.5 / back 15.5 . 1 bar stonger "not" being bonded. At 3" spacing , reading was reversed. Turning the screen around didn`t make any diff.
Very minor differences but shows some tweeking can be done.
I also found it had a narrower beam width than a GH .
As far as mounting , if going for non-bonding, some thought would have to be put to that for longativity . Plexy dont work well in a clamping method .
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post #42 of 68 (permalink) Old 2010-08-05, 06:05 PM Thread Starter
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Yeah! Not sure how the mast being bonded to the antenna will affect the grounding requirements. Maybe it's making the whole setup a lightning rod.

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post #43 of 68 (permalink) Old 2010-08-11, 11:43 PM Thread Starter
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Revised the #8 post design,--> by folding the screen in the center, I have reduced the 4" feed gap to about 2 1/2" and the mast is resting inside the 'V' with a clamp at the top and a clamp at the bottom. [therefore the aperture is now 2 1/2" x 16"] The screen is bonded to the mast. The feed point connection clearances on each side of the mast are about 3/4" away from the mast. I get equal front and back coverage, but less beamwidth than with the 4" gapped flat screen design.

See the 'Top View' example in the code box:
Code:
\____. .____/ \/

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post #44 of 68 (permalink) Old 2010-08-12, 11:53 AM
 
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My First Attempt...currently Untested

I just built it...err...cut it up last night. Based on OTA's 4221 equivalent (post #8), but made the mesh wider....purely experimental right now.

I haven't tested it, to hot to make an attic connection. Maybe the next time I go over to my parents, I'll give it an extreme test (sitting against an inside wall, like I first tested the M4, SBGH and Fractal).

Will let you all know how it turns out.







*EDIT*

First test - FAIL

Well I tried this antenna briefly in my other extreme location, which I have tested all my other antennas in...which is my livingroom, out the room's window. It failed at even picking up CHCH digital. I also tried my reference M4 antenna, and it too failed to pick up any digital channels. So I can't put full blame on the antenna.

It did, however, pickup CHCH analog, and produced a really decent picture.

So I will have to keep testing this over several nights, and different locations (both in my house, parents and in-laws). Then compare the results to my other antennas.

I've also changed the balun from the one pictured, to the typical Home Depot one that I have used on several of my other builds. Maybe that will make a difference too.

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post #45 of 68 (permalink) Old 2010-08-12, 06:53 PM Thread Starter
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It may not be possible to use that style of balun, because it's a dipole based on the width and length of the aperture, that type of balun may not be suitable, because it in essence cuts the aperture into two sections. Looking at the feedpoint connection on your picture, not only is the feed point reduced to 1/2", but the uncut wire being used to connect that balun may be cutting the aperture into two separated smaller apertures. As far as the extra screen width, I would think that may actually help with VHF.

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