Scatter Forager - wide pattern bi-directional "Slot" antenna - Page 2 - Canadian TV, Computing and Home Theatre Forums

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post #16 of 68 (permalink) Old 2010-07-22, 12:49 PM
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Pattern for drawing post #8:

Not a particularly wide beamwidth for the gain. Its wider on the back side.

VHF high gain ranges from 4.34 dbi 19.6 SWR at channel 7 to 4.51 dbi 4.8 SWR at channel 13. Also a linear graph.

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post #17 of 68 (permalink) Old 2010-07-22, 01:14 PM Thread Starter
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Quote:
Not a particularly wide beamwidth for the gain. Its wider on the back side.
Yes, in real life it does seem perform better [wider but at a cost of slightly lower gain] from what should be considered the backside [the mast mounted side]. I also noted that the antenna does better if it's mounted lower from the tip of the mast. Maybe the mast is acting as a director? or the 45° mesh screen edge bends forward are giving some directivity toward the front side? I don't think the mast should have much influence, but the forward mesh edges could add some focus toward the direction of the bends which may explain the forward beamwidth and gain values.

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post #18 of 68 (permalink) Old 2010-07-22, 01:34 PM
 
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You don't need an antenna with a positive gain value to pick up strong stations. For example, my portable LCD tv with the telescopic antenna extended about 12 inches will pick up CBC 5.1 here even though tvfool says it is 56 miles away and a NM of 8.3. If all things are accurate, then even if my antenna has a gain of -7db it will still pick up the station because the NM value is still positive.

At 60 degrees your antenna has a gain of -5db. You'll still pick up strong stations. You're in an area blessed with some good OTA signals (well at least compared to me in Kitchener )

Beamwidth is a measurement that indicates when the gain of the antenna drops 3db from its maximum gain.

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post #19 of 68 (permalink) Old 2010-07-22, 02:07 PM Thread Starter
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On the horizontal plot shown in the previous post, the beamwidth is maxed when the gain reaches 0 dB. anything above 0 dB can be made useful with some amplification.

In my location, gain balance is important because too much raw gain is risking co-channel problems, especially with amplification. I seem to be in a 'too close' but 'too far' situation from many market sources where a lower gain antenna may be the solution.

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post #20 of 68 (permalink) Old 2010-07-22, 03:45 PM
 
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I`ve found through testing reflectorless GH`s and the Centipede2 , if you`re within 2" of a steel mast it will act as a deflector making the backside stronger. At 3" away from the mast i`ve found a slight diff. on 1 or 2 channels. Most are even signals front to back .
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post #21 of 68 (permalink) Old 2010-07-22, 05:25 PM
 
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Regarding the slot antenna....
I don't know how the design found the feed point to be 1/3 of the way along the slot. I tried a feed point midway, but did find that the 1/3 position worked better. We would need Stephen Hawking to explain that one.
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post #22 of 68 (permalink) Old 2010-07-22, 06:27 PM Thread Starter
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I still have the antenna shown in post#8 mounted at my shop. It's still giving awsome performance. When I have some more time, I'll try moving the feed point down to the 1/3rd distance to see what happens.

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post #23 of 68 (permalink) Old 2010-07-22, 10:36 PM Thread Starter
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If I were transmitting with a 19.6 SWR , I'd probably blow something in the radio!

Two way radio SWR should be less than 1.0, by adjusting the physical antenna length to match the reflector/ground plane. How can you adjust the OTA antenna's SWR? I would think making the aperture longer if the ground plane is larger or making it shorter if the ground plane is smaller.

Would improving SWR necessarily improve reception gain figures?

So, with that though in mind I got my cutters out, I increased the length of the aperture of post#8 from 16" to 18" and I didn't notice any difference.

Any thoughts of what I could try?

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post #24 of 68 (permalink) Old 2010-07-22, 11:07 PM
 
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VSWR is caused by impedance mismatch between the antenna and your receiver. Perhaps moving the feed point as suggested will improve the VSWR.

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post #25 of 68 (permalink) Old 2010-07-22, 11:33 PM Thread Starter
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Yeah! That's the next step. I'll move the balun down 1" at a time until I'm about 1/3rd down on the aperture.

We are assuming that the raw feed point value is 300ohms. Maybe the 4:1 balun is not going to perfectly balance the impedance at any point. I believe that aperture antennas are normally geared to have 50ohm output for wifi and higher frequency use.

With some tweeking the 2.9-5.9 SWR may be improved somewhat on UHF, but VHF's SWR it way high. Though it still is doing a great job on VHF's RF7 & RF12 regardless of the high SWR.

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post #26 of 68 (permalink) Old 2010-07-23, 06:45 AM
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Quote:
Would improving SWR necessarily improve reception gain figures?
Yes, high SWR affects digital reception way more than analog reception. Holl_ands has a long discussion of why some where in this forum.

Quote:
We are assuming that the raw feed point value is 300ohms. Maybe the 4:1 balun is not going to perfectly balance the impedance at any point.
Impedance varies with frequency. The SWR is the impedance mismatch from 300 ohms (which is a setting in the 4nec2 program). The further away from 300 ohms, the higher the SWR, ie the Impedance mismatch. Because Impedance consists of real and imaginary resistances, it will take time to figure out the best impedance value for this antenna. Then if you wanted to, build a custom coax balun for it to connect to a 75ohm coax downlead. (Most ham coax is 50 ohms).

The SWR mismatch also effects Net Gain.
A quick way to look at the SWR figures to subtract from Raw Gain :
Code:
SWR     Mismatch Loss dBi
-----   -----------------
1.0     0
1.5     .18
2.0     .51
2.5     .88
3.0     1.25
3.5     1.60
4.0     1.94
4.5     2.25
5.0     2.55
Quote:
Two way radio SWR should be less than 1.0
No, no, SWR is a ratio. 1 to 1 is the best you can get. If your meter had a zero setting, then that would have been 1:1, which would correspond to 1.0 in our context. A lot of older equipment used general purpose meters, whose scales would run from 0 to 10, 25, 50, 100 etc.

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post #27 of 68 (permalink) Old 2010-07-23, 07:01 AM
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Quote:
I still have the antenna shown in post#8 mounted at my shop. It's still giving awsome performance. When I have some more time, I'll try moving the feed point down to the 1/3rd distance to see what happens.
Moving the feedpoint down to 5 inches above the bottom of the slot, the gain peak is 4.38 dbi 5.32 SWR at 488 mhz with a bumpy curve downward to 1.63 dbi 2.60 SWR at 698 mhz.

The vertical pattern (horiz is about the same) is modified to this :



So the peak gain of the antenna is 36 degrees upward, both front and back. That may not be a bad thing, depending on how the direction of the signal is coming in.

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post #28 of 68 (permalink) Old 2010-07-23, 08:34 AM
 
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EDIT : Post #20 Should be " Director " NOT " Deflector "
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post #29 of 68 (permalink) Old 2010-07-24, 11:37 AM Thread Starter
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Does 4nec2 determine the actual feed point impedances or does 4nec2 assume that the antennas impedance is already designed at 300Ohms?

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post #30 of 68 (permalink) Old 2010-07-24, 12:08 PM
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Does 4nec2 determine the actual feed point impedances or does 4nec2 assume that the antennas impedance is already designed at 300Ohms?
You input the Characteristic Impedance as a global setting. It can be any figure so 4nec2 is fine for ham or wifi etc usage also. Since common TV cable is either 300 ohm twin lead or 75 ohm coax and the common TV baluns sold are 4:1, 300 ohm characteristic impedance is the figure to use for TV antennas. The 300 ohm standard was set in the 30's/early 40's. Many at that time wanted a 600 ohm standard for TV, as 300 ohm twin lead required higher tolerances to manufacture. 75 ohm twin lead was just impossible to make, heh.

Quote:
Then if you wanted to, build a custom coax balun for it to connect to a 75ohm coax downlead.
It looks like a 6:1 coax balun (450 ohms to 75 ohms) is the best compromise for use with the antenna in post #8.
SWR is still high at 2.8 SWR at 470 mhz to 4.5 SWR at 698 mhz, but much lower than with a 4:1 balun.

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