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Old 2010-09-04, 08:28 PM   #31
Dalek Prime
 
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Oops... that URL was antenneX.com, not antennax.... I mentioned the LPDA Notes, linked from http://www.cebik.com/content/bookant.html to antenneX, only because I found it more cohesive and complete for novices. But that's just me.
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Old 2010-09-06, 11:10 PM   #32
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This thread prompted me to try to model a log-periodic antenna. After much pounding of head against brick wall, I finally came across a webpage whose explanation and equations I THINK I understand, namely http://www.members.westnet.com.au/im...periodic_A.htm

I tried to implement the equations, using 4NEC2 SYmbols. The only variables/constants I have to select are...
  • LO_FREQ the lowest frequency
  • HI_FREQ the highest frequency
  • N the number of elements

The result looks like a log-periodic, but the gain is abysmal. Either I'm doing something wrong, or else 4NEC2 under WINE isn't working like it should. Here's the listing for a 7-element log-periodic, for 470..698 mhz, intended to be one wavelength long (approx 26 inches) at 470 mhz. Any comments...?
Code:
CM Symbolic log periodic NEC file
CE
SY SEG_COUNT=11 'Number of segments per element
SY MID_SEG=(SEG_COUNT + 1) / 2  'Middle segment
SY C=299.792458 'Speed of light in megametres / second
SY RAD=0.1' Radius
SY LO_FREQ=470  'Lowest frequency in megahertz
SY LAMBDA_MAX=C / LO_FREQ * 39.37       'Lambda of lowest frequency; also length of boom; also twice the length of longest element
SY HI_FREQ=698  'Highest frequency in megahertz
SY LAMBDA_MIN=C / HI_FREQ * 39.37       'Lambda of highest frequency
'*** Change N to change number of elements
SY N=7  'Number of elements
SY TAU=(LO_FREQ / HI_FREQ)^(1/(N-1))    'Tau constant for this antenna
'*** Number of Lnn SYmbols must equal N
SY L01=LAMBDA_MAX / 4   '1st element Y extent
SY L02=L01 * TAU        '2nd element Y extent
SY L03=L02 * TAU        '3rd element Y extent
SY L04=L03 * TAU        '4th element Y extent
SY L05=L04 * TAU        '5th element Y extent
SY L06=L05 * TAU        '6th element Y extent
SY L07=L06 * TAU        '7th element Y extent
'*** Number of Xnn SYmbols must equal N-1
SY X01=(LAMBDA_MAX * L01) * (1 - TAU) / (L01 - L07)     '1st X separation
SY X02=X01 * TAU        '2nd X separation
SY X03=X02 * TAU        '3rd X separation
SY X04=X03 * TAU        '4th X separation
SY X05=X04 * TAU        '5th X separation
SY X06=X05 * TAU        '6th X separation
'*** Number of XSnn SYmbols must equal N-1
SY XS01=X01     '1st X offset
SY XS02=XS01+X02        '2nd X offset
SY XS03=XS02+X03        '3rd X offset
SY XS04=XS03+X04        '4th X offset
SY XS05=XS04+X05        '5th X offset
SY XS06=XS05+X06        '6th X offset
'*** Number of GW lines must equal N
GW      1       SEG_COUNT       0       -L01    0.      0       L01     0.      RAD
GW      2       SEG_COUNT       XS01    -L02    0.      XS01    L02     0.      RAD
GW      3       SEG_COUNT       XS02    -L03    0.      XS02    L03     0.      RAD
GW      4       SEG_COUNT       XS03    -L04    0.      XS03    L04     0.      RAD
GW      5       SEG_COUNT       XS04    -L05    0.      XS04    L05     0.      RAD
GW      6       SEG_COUNT       XS05    -L06    0.      XS05    L06     0.      RAD
GW      7       SEG_COUNT       XS06    -L07    0.      XS06    L07     0.      RAD
GS      0       0       0.0254'Inches to metres.
GE      0
EX      0       N       MID_SEG 00      1.      0.
EK
GN      -1
'*** Number of TL lines must equal N-1
TL      1       MID_SEG 2       MID_SEG -300    0.      0.      0.      0.      0.
TL      2       MID_SEG 3       MID_SEG -300    0.      0.      0.      0.      0.
TL      3       MID_SEG 4       MID_SEG -300    0.      0.      0.      0.      0.
TL      4       MID_SEG 5       MID_SEG -300    0.      0.      0.      0.      0.
TL      5       MID_SEG 6       MID_SEG -300    0.      0.      0.      0.      0.
TL      6       MID_SEG 7       MID_SEG -300    0.      0.      0.      0.      0.
FR      0       38      0       0       470     6
RP      0       1       73      1510    90      0       1       5       0       0
EN
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Old 2010-09-07, 07:21 AM   #33
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Walter, you're trying to get to much with too little. The angle strikes me as too wide, the boom length is too short, and the elements, from what I understand, are too long for the UHF range you are modeling, LPCAD showing 1/2 wavelengths (longest about 13"). More elements on a longer boom would help.

The long and short of it, from what I understand so far of LPDAs, is that they will suffer tremendously on gain if the angle is either too wide, or too narrow.

Thoughts or corrections, anyone?
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Old 2010-09-07, 04:32 PM   #34
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The following calculator says 470-698MHz with a 660mm (26") boom & 11 elements gives about 8.1dB.

http://www.changpuak.ch/electronics/lpda.html

One thing to check is whether you are doubling your element width. L01 should be around 1/4 C/470MHz.
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Old 2010-09-07, 05:37 PM   #35
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Thats a nifty calculator. Is it just for double boom type lpdas ?
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Old 2010-09-07, 11:22 PM   #36
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Quote:
Originally Posted by ericball View Post
One thing to check is whether you are doubling your element width. L01 should be around 1/4 C/470MHz.
From the symbolic xode in my model...

Quote:
SY LO_FREQ=470 'Lowest frequency in megahertz
SY LAMBDA_MAX=C / LO_FREQ * 39.37 'Lambda of lowest frequency; also length of boom; also twice the length of longest element
...
SY L01=LAMBDA_MAX / 4 '1st element Y extent
So that should work. Note the "twice the length of the longest element" refers to the combined length of the left and right sides.
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Old 2010-09-07, 11:49 PM   #37
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Quote:
Originally Posted by Dalek Prime View Post
The long and short of it, from what I understand so far of LPDAs, is that they will suffer tremendously on gain if the angle is either too wide, or too narrow.
I made a separate variable BOOM_LENGTH, and pushed it down to 6 inches, leaving everything else unchanged (SYmbols are wonderful). The gain is more like it (6.0 to 7.5 db across the entire UHF band), but the SWR is still bad. That was the 60-second tweak. Now I'll try with more elements, and see what happens. Is there a recommended angle?
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Old 2010-09-07, 11:53 PM   #38
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Quote:
Is there a recommended angle?
Sweep forward ? Most Ive read recommend less than 30 degrees, more like 15 degrees is common.
For vee booms, what Ive read is less than 60 degrees on the vee separation.
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Old 2010-09-08, 06:00 PM   #39
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Quote:
Originally Posted by Walter Dnes View Post
Is there a recommended angle?
Quote:
Originally Posted by 300ohm View Post
Sweep forward ? Most Ive read recommend less than 30 degrees, more like 15 degrees is common.
For vee booms, what Ive read is less than 60 degrees on the vee separation.
Sorry, that's not what I meant by "angle". I meant the alpha angle in the diagram on http://www.amateur-radio-wiki.net/in...yeh_lpda-2.jpg
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Old 2010-09-08, 11:15 PM   #40
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Achieving a low-SWR with an LPDA (or even analyzing) can be a bit of a challenge.
Running 4nec2 with TL Cards simulating a Zig-Zag feedine (like W-G antennas's) means you
ASS-U-ME that the real antenna will actuallly have the impedance in the TL Cards....
But probably not, esp if using a nearby metal boom....4nec2 can investigate sensitivity...

One could TRY to model the metal boom and zig-zag feelines using many-many wire
statements, but based on my experience, I think the quad-precision NEC4 engine is
needed to handle the mix of big & tiny wires, which apparently cause problems using
the double-precision NEC2 engine.....

Impedance of a Twin-Boom LPDA can be analyzed in LOGPERIO3.XLS spread sheet:
http://www.ycars.org/EFRA/Module%20C/LOGPERIO3.xls
Here are some Hi-VHF & UHF examples I pulled from my archive:
http://photos.imageevent.com/holl_an.../LPDAHiVHF.XLS
http://photos.imageevent.com/holl_an...en/LPDAUHF.XLS

Here's an alternative, very similar spread sheet calculator:
http://hamradio.pl/sq9jdo/_Technika/Anteny/LPDA.XLS
[On my system, GoogleDocs allows you to "Choose another application"....]

Note that at the highest frequencies, only the forward most elements provide significant
contribution to the overall Gain. Hence it is important to pick a high-frequency design
frequency that is 10-20 percent HIGHER than the max operating frequency.

The fol. LPDA Analysis addresses not only Impedance, but also optimization
and the use of the (optional) "matching stub" on the back of the LPDA:
http://www.salsburg.com/Log-Periodic.pdf
Some alternative LPDA Analyses:
http://www.supernec.com/lpda.htm
http://www.arcticpeak.com/antennapages/LPDA.htm
FYI: The "matching stub" was discussed earlier in this thread:
http://www.digitalhome.ca/forum/showthread.php?p=883858

Mayes & Carrel's original Patent for a Swept Forward LPDA summarized how
the optimum swept forward angle depends on other LPDA design parameters:
http://www.freepatentsonline.com/3108280.html

Analysis of a Swept Forward LPDA with variable sweep can be found here:
http://showcase.netins.net/web/wallio/LPDA.html

I avoided the Impedance calculation in vicinity of a metal boom problem by simply
eliminating the boom entirely....the NO-BOOM ZIG-ZAG LPA, both a "flat, stacked"
version and a higher performance WEDGE:
http://imageevent.com/holl_ands/zigzaglpa
It was also simple to construct....for indoor/attic use....

PS: Unfortunately, no doubt because it must TRANSMIT on Hi-VHF and UHF TV Bands, we
can't buy an inexpensive VSWR meter, such as is readily available for Ham Radio bands.....
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Old 2010-09-09, 09:31 AM   #41
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"Note that at the highest frequencies, only the forward most elements provide significant contribution to the overall Gain. Hence it is important to pick a high-frequency design frequency that is 10-20 percent HIGHER than the max operating frequency."

I have to learn how to quote in this forum... Yes, absolutely, Holl_ands. And populate at the front, so it has something to work with.

Walter, start with modelling the Silver Sensor you have. It's a good working example of the type of LPDA in the UHF range you're modelling. Note that the longest element is nowhere near 26". Anyways, just take it's measurements, and enter those in your program. Then you can tweak it for better performance with, say, more elements on a longer boom, and where those elements are. I'll also take the measurements of my UHF LPDA (commercial grade) with eleven elements (470-890 MHz), and post them here for you to work with as well

PS. As to the angle Walter, the gain was next to zero in LPCAD twice when I modeled what you gave me, as I adjusted for boom length, limiting myself to the seven elements as a constant. One went wide, one went narrow. Both were negligible gain. I forget what diameter elements I used though, and that may have taken it's toll in terms of spacing. That's what I get for rushing a model, and not thinking it through.
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Old 2010-09-09, 11:24 AM   #42
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Quote:
Walter, start with modelling the Silver Sensor you have. It's a good working example of the type of LPDA in the UHF range you're modelling.
No need for extra work. Heres Ken Nists Silver Sensor model converted to 4nec2.

http://www.wuala.com/300ohm/Documents/silvera.nec
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Old 2010-09-09, 12:21 PM   #43
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Excellent! Thanks 300ohm!

Here's the basics for my Blonder Tongue, give or take:

# of elements: 11
All elements are ~ 0.33" (diameter) Hollow tube aluminium, btw.
Longest element (exposure from boom) ~ 5.75"
Shortest: ~ 2"
Total boom length ~ 2', spaced fairly evenly at ~ 3" between elements on ONE side, half that if counting between opposing elements.

Note LPCAD only asks for back element diameter, and shrinks it to the front. All my elements are, in reality, the same diameter.

Rough gain in LPCAD: 8.5 dBi.

Epilogue: I'm a bit surprised by my antenna's actual build dimensions. While the elements line up on the angle as they should, the dimensions are not what LPCAD ie. theory, would suggest. It's almost like the company went with my "free and easy" build methods mentioned earlier. ie they spaced equally, and cut the rods to fit the angle for that spacing. Perhaps in reality, it is the angle that is all important for a working LPDA, given reasonable spacing of decent elements?
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Old 2010-09-10, 09:23 AM   #44
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What would something like a corner reflector do, if anything, for a LPDA? Anyone tried this? Or a parabolic dish, focused to the front element? Come to think of it, how does one know where a corner reflector is focused, even on a yagi? I get focusing a parabola, but a corner reflector?
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Old 2010-09-10, 11:29 PM   #45
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Quote:
Originally Posted by holl_ands View Post
Note that at the highest frequencies, only the forward most elements provide significant contribution to the overall Gain. Hence it is important to pick a high-frequency design
frequency that is 10-20 percent HIGHER than the max operating frequency.
Is that because the frontmost element has to act like a yagi-director, i.e. be shorter than the shortest λ / 2? While we're at it, should the rearmost element be slightly longer than the longest λ / 2 in order to act as a reflector at that wavelength ?

Quote:
Originally Posted by holl_ands View Post
I avoided the Impedance calculation in vicinity of a metal boom problem by simply eliminating the boom entirely....the NO-BOOM ZIG-ZAG LPA, both a "flat, stacked" version and a higher performance WEDGE: http://imageevent.com/holl_ands/zigzaglpa
A 45 element antenna that's 10 and a half feet long? Ouch!

Out of shear curiosity, is a narrow-band LPDA possible? Say 6 mhz of bandwidth? Has anyone tried modelling such an animal, assuming it's even possible in the first place?
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