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Discussion Starter #1
Isn't a log-periodic array already desined to do this sans motors and gears? :p
Yes / no. An LPDA isn't a Yagi-Uda because all of the elements are electrically connected. So although the shorter elements behave a little like directors and the longer elements a little like reflectors, they also contribute directly to the current flow.

The advantage of an LPDA is it provides reasonably consistent performance across a large bandwidth. However, a Yagi-Uda of the same boom length will provide higher gain using fewer elements but a narrow bandwidth.

So an adjustable Yagi-Uda should provide higher gain than a comparable LPDA.
 

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Hmm. I'll have to go back to some of my 25+ year old notes from my antennas and propagation course to remember for sure, but I believe you can get double-digit gain over a wide bandwidth with an LP. This is especially true if you sweep the elements forward.

Aren't the pattern characteristics of an LP more stable than with a Yagi-Uda, too?
 

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However, a Yagi-Uda of the same boom length will provide higher gain using fewer elements but a narrow bandwidth.
Yeah, I found that to be true too. LPDA is great when you need the wide bandwidth for vhf-lo and vhf-hi together. Since most now will only need vhf-hi (and uhf), a folded dipole type yagi can cover vhf-hi nicely in less space and a simpler cheaper build. (LPDAs require either double booms or insulated elements with phasing wires. Most (or all) of the yagi parts can be riveted straight to the boom)
 

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Problem with most Yagi's is they provide more gain than an LPDA, but don't cover the band.
K6STI's optimized 5-Element Yagi barely covered Hi-VHF band: 30-in boom, 7.5-9 dBi Gain:
http://imageevent.com/holl_ands/yagis/k6sti
Note that the 8-Element or 4-Element K6MEM Yagi designs only covered 1 to 3 channels.
Good luck trying to add more elements without losing Gain on Ch7 and/or Ch13:
http://imageevent.com/holl_ands/yagis

Although an LPDA can be readily designed to cover the Hi-VHF band, it needs
more elements and a longer boom to improve on K6STI's 6-Element Yagi.

Fol. 14-Element LPDA provides 10+ dBi Gain using a 100-in boom:
http://imageevent.com/holl_ands/zigzaglpa/lpda/lpda14el
Ditto for a Wedge Zig-Zag LPA:
http://imageevent.com/holl_ands/zigzaglpa/zigzagnoboom

Log-Yagi's (e.g. YA-1713 or Y-10-7-13) provide 10+ dBi Gain across the entire
Hi-VHF band, using only 10-elements and a 100-in boom.
 

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Problem with most Yagi's is they provide more gain than an LPDA, but don't cover the band.
Good luck trying to add more elements without losing Gain on Ch7 and/or Ch13:
Take a look at this simple old one, NEC file at post 313

http://www.digitalhome.ca/forum/showthread.php?t=107706&page=21

Same NEC file, rescaled 6 mhz upwards:

CM AGT 1.0 at 195 mhz, Autosegmentation 21 at 216 mhz
CM Lorkoe's 8 director, 1 reflector, folded dipole Yagi
CM Rescaled 6 mhz upwards
CE
GW 3 3 0 12.7098125 0 0 12.7098125 -2.310875 0.1824375
GW 4 3 0 -12.709813 0 0 -12.71039 -2.3108749 0.1824375
GW 8 13 -11.676 0 0 -11.676 16.0545 0 0.1824375
GW 9 13 -11.676 0 0 -11.676 -16.0545 0 0.1824375
GW 10 11 8.392125 0 0 8.392125 12.2233125 0 0.1824375
GW 11 11 8.392125 -12.223313 0 8.392125 0 0 0.1824375
GW 12 11 16.78425 0 0 16.78425 11.7064549 0 0.1824375
GW 13 11 16.78425 0 0 16.78425 -11.706163 0 0.1824375
GW 14 11 25.176375 0 0 25.176375 11.676 0 0.1824375
GW 15 11 25.176375 -11.676 0 25.176375 0 0 0.1824375
GW 16 11 33.5685 0 0 33.5685 11.6151875 0 0.1824375
GW 17 11 33.5685 0 0 33.5685 -11.615674 0 0.1824375
GW 18 11 41.960625 0 0 41.960625 11.6151875 0 0.1824375
GW 19 11 41.960625 -11.615674 0 41.960625 0 0 0.1824375
GW 20 11 50.35275 0 0 50.35275 11.6151875 0 0.1824375
GW 21 11 50.35275 0 0 50.35275 -11.615674 0 0.1824375
GW 22 11 58.744875 0 0 58.744875 11.6151875 0 0.1824375
GW 23 11 58.744875 -11.615674 0 58.744875 0 0 0.1824375
GW 24 11 67.137 0 0 67.137 11.6151875 0 0.1824375
GW 25 11 67.137 0 0 67.137 -11.615674 0 0.1824375
GW 28 7 8.392125 0 0 16.78425 0 0 0.4865
GW 29 7 16.78425 0 0 25.176375 0 0 0.4865
GW 30 7 25.176375 0 0 33.5685 0 0 0.4865
GW 31 7 33.5685 0 0 41.960625 0 0 0.4865
GW 32 7 41.960625 0 0 50.35275 0 0 0.4865
GW 33 7 50.35275 0 0 58.744875 0 0 0.4865
GW 34 7 58.744875 0 0 67.137 0 0 0.4865
GW 35 3 0 -0.72975 -2.310875 0 0.72975 -2.310875 0.20433
GW 36 5 0 0 0 0 -5.838 0 0.1824375
GW 37 5 0 -6.811 0 0 -12.709813 0 0.1824375
GW 38 1 0 -6.811 0 0 -5.838 0 0.1824375
GW 39 5 0 0 0 0 5.838 0 0.1824375
GW 40 1 0 5.838 0 0 6.811 0 0.1824375
GW 41 5 0 6.811 0 0 12.7098125 0 0.1824375
GW 42 5 0 -0.72975 -2.310875 0 -5.838 -2.310875 0.1824375
GW 43 5 0 -12.71039 -2.3108749 0 -6.811 -2.310875 0.1824375
GW 44 1 0 -6.811 -2.310875 0 -5.838 -2.310875 0.1824375
GW 45 5 0 0.72975 -2.310875 0 5.838 -2.310875 0.1824375
GW 46 5 0 12.7098125 -2.310875 0 6.811 -2.310875 0.1824375
GW 47 1 0 5.838 -2.310875 0 6.811 -2.310875 0.1824375
GW 48 3 0 -6.811 0 0 -6.811 -2.310875 0.0608125
GW 49 3 0 -5.838 0 0 -5.838 -2.310875 0.0608125
GW 50 3 0 5.838 0 0 5.838 -2.310875 0.0608125
GW 51 3 0 6.811 0 0 6.811 -2.310875 0.0608125
GW 52 11 -11.676 0 0 0 0 0 0.4865
GW 53 7 8.392125 0 0 0 0 0 0.4865
GS 0 0 0.0254 ' All in in.
GE 0
EK
LD 5 0 0 0 2.49e7 0
EX 0 35 2 0 1 0
GN -1
FR 0 1 0 0 216 0


Ya gotta appreciate the build simplicity, heh.
 

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The above 10-Element Yagi needs to be rescaled smaller by another 2-3 Mhz to
eliminate the SWR increase on the top end of Ch13. Otherwise it looks like a
10 dBi Gain antenna with mostly good F/B & F/R Ratios and "only" a 78-in Boom.

FYI: Earlier, kgb posted an optimized UHF 13-Element Yagi, using 2 Reflector Rods.
Here is a rescaled version for Hi-VHF band, presuming 1/2-in Type M Copper Tubing:
http://imageevent.com/holl_ands/yagis/hivhf13elyagikgb
Raw Gain starts at 8.5 dBi on Ch7, increasing to about 11 dBi on Ch13. Boom=115-in.
Note that 13-El maintains F/B & F/R Ratios across the entire Hi-VHF Band (vs 10-elem).
Perhaps there is a better design that maintains good F/B & F/B with flatter Gain curve....



 

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Rescaling above 10-Element Yagi by only 1% smaller "brackets" the SWR curve so it
comes closer to 3:1 at both bottom of Ch7 and top of Ch13, fixing the high SWR problem.
Although F/B and F/R Ratios aren't TOO bad, they fall far short of K6STI's 5-Element Yagi:
http://imageevent.com/holl_ands/yagis/hivhf10elyagilorkoe

Code:
CM Hi-VHF Yagi 10-Element, Folded Dipole, from Lorkoe, 17Jun2010
CM Rescaled 6 MHz [Plus 0.99] upwards. Autoseg=21 at 216 MHz. AGT=1.0.  78-in Boom.
CE
GW	3	3	0	12.5827144	0	0	12.5827144	-2.2877663	0.18061313
GW	4	3	0	-12.582715	0	0	-12.583286	-2.2877662	0.18061313
GW	8	13	-11.55924	0	0	-11.55924	15.893955	0	0.18061313
GW	9	13	-11.55924	0	0	-11.55924	-15.893955	0	0.18061313
GW	10	11	8.30820375	0	0	8.30820375	12.1010794	0	0.18061313
GW	11	11	8.30820375	-12.10108	0	8.30820375	0	0	0.18061313
GW	12	11	16.6164075	0	0	16.6164075	11.5893904	0	0.18061313
GW	13	11	16.6164075	0	0	16.6164075	-11.589101	0	0.18061313
GW	14	11	24.9246113	0	0	24.9246113	11.55924	0	0.18061313
GW	15	11	24.9246113	-11.55924	0	24.9246113	0	0	0.18061313
GW	16	11	33.232815	0	0	33.232815	11.4990356	0	0.18061313
GW	17	11	33.232815	0	0	33.232815	-11.499517	0	0.18061313
GW	18	11	41.5410188	0	0	41.5410188	11.4990356	0	0.18061313
GW	19	11	41.5410188	-11.499517	0	41.5410188	0	0	0.18061313
GW	20	11	49.8492225	0	0	49.8492225	11.4990356	0	0.18061313
GW	21	11	49.8492225	0	0	49.8492225	-11.499517	0	0.18061313
GW	22	11	58.1574263	0	0	58.1574263	11.4990356	0	0.18061313
GW	23	11	58.1574263	-11.499517	0	58.1574263	0	0	0.18061313
GW	24	11	66.46563	0	0	66.46563	11.4990356	0	0.18061313
GW	25	11	66.46563	0	0	66.46563	-11.499517	0	0.18061313
GW	28	7	8.30820375	0	0	16.6164075	0	0	0.481635
GW	29	7	16.6164075	0	0	24.9246113	0	0	0.481635
GW	30	7	24.9246113	0	0	33.232815	0	0	0.481635
GW	31	7	33.232815	0	0	41.5410188	0	0	0.481635
GW	32	7	41.5410188	0	0	49.8492225	0	0	0.481635
GW	33	7	49.8492225	0	0	58.1574263	0	0	0.481635
GW	34	7	58.1574263	0	0	66.46563	0	0	0.481635
GW	35	3	0	-0.7224525	-2.2877663	0	0.7224525	-2.2877663	0.2022867
GW	36	5	0	0	0	0	-5.77962	0	0.18061313
GW	37	5	0	-6.74289	0	0	-12.582715	0	0.18061313
GW	38	1	0	-6.74289	0	0	-5.77962	0	0.18061313
GW	39	5	0	0	0	0	5.77962	0	0.18061313
GW	40	1	0	5.77962	0	0	6.74289	0	0.18061313
GW	41	5	0	6.74289	0	0	12.5827144	0	0.18061313
GW	42	5	0	-0.7224525	-2.2877663	0	-5.77962	-2.2877663	0.18061313
GW	43	5	0	-12.583286	-2.2877662	0	-6.74289	-2.2877663	0.18061313
GW	44	1	0	-6.74289	-2.2877663	0	-5.77962	-2.2877663	0.18061313
GW	45	5	0	0.7224525	-2.2877663	0	5.77962	-2.2877663	0.18061313
GW	46	5	0	12.5827144	-2.2877663	0	6.74289	-2.2877663	0.18061313
GW	47	1	0	5.77962	-2.2877663	0	6.74289	-2.2877663	0.18061313
GW	48	3	0	-6.74289	0	0	-6.74289	-2.2877663	0.06020438
GW	49	3	0	-5.77962	0	0	-5.77962	-2.2877663	0.06020438
GW	50	3	0	5.77962	0	0	5.77962	-2.2877663	0.06020438
GW	51	3	0	6.74289	0	0	6.74289	-2.2877663	0.06020438
GW	52	11	-11.55924	0	0	0	0	0	0.481635
GW	53	7	8.30820375	0	0	0	0	0	0.481635
GS	0	0	0.0254		' All in in.
GE	0
EK
LD	5	0	0	0	2.49e7	0
EX	0	35	2	0	1	0
GN	-1
' FR Freq Sweep choices in order of increasing calculation time (fm holl_ands):
' FR 0 0 0 0 470 0		' Fixed Freq
' FR 0 29 0 0 470 12		' Freq Sweep 470-806 every 12 MHz - OLD UHF BAND
' FR 0 34 0 0 410 12		' Freq Sweep 410-806 every 12 MHz - Even Wider Sweep
' FR 0 39 0 0 470 6		' Freq Sweep 470-698 every 6 MHz - PREFERRED FOR UHF
' FR 0 77 0 0 470 3		' Freq Sweep 470-698 every 3 MHz
' FR 0 153 0 0 470 1.5		' Freq Sweep 470-698 every 1.5 MHz
' FR 0 71 0 0 300 10		' Freq Sweep 300-1000 every 10 MHz - WIDEBAND SWEEP
' FR Hi-VHF choices:
' FR 0 15 0 0 174 3		' Freq Sweep 174-216 every 3 MHz
FR 0 29 0 0 174 1.5		' Freq Sweep 174-216 every 1.5 MHz - PREFERRED FOR HiVHF
' FR 0 43 0 0 174 1		' Freq Sweep 174-216 every 1 MHz - Hi-Rez
' FR 0 26 0 0 150 6		' Freq Sweep 150-300 every 6 MHz - WIDEBAND SWEEP
' FR Lo-VHF choices:
' FR 0 19 0 0 54 3		' Frequency Sweep every 3 MHz for Ch2-6 + FM
' FR 0 28 0 0 54 6		' Wide Freq Sweep every 6 MHz for Ch2-13
' FR 0 35 0 0 54 1		' Frequency Sweep every 1 MHz for Ch2-6
' FR 0 36 0 0 75 1		' Frequency Sweep every 1 MHz for Ch5 + Ch6 + FM
' FR 0 64 0 0 54 12		' Super Wide Freq Sweep 54-810 every 12 MHz
' RP choices in order of increasing calculation time:
' RP 0 1 1 1510 90 90 1 1 0 0	' 1D Gain toward 0-deg Azimuth - SIDE GAIN
' RP 0 1 1 1510 90 0 1 1 0 0	' 1D Gain toward 90-deg Azimuth - FORWARD GAIN
' RP 0 1 1 1510 90 180 1 1 0 0	' 1D Gain toward 270-deg Azimuth - REVERSE GAIN
' RP 0 1 37 1510 90 0 1 5 0 0 	' 2D (Left only) Azimuthal Gain Slice
RP 0 1 73 1510 90 0 1 5 0 0 	' 2D Azimuthal Gain Slice - PREFERRED
' RP 0 73 1 1510 90 0 5 1 0 0 	' 2D Elevation Gain Slice
' RP 0 73 73 1510 90 0 5 5 0 0 	' 3D Lower Hemisphere reveals antenna (use Fixed Freq)
' RP 0 285 73 1510 90 0 5 5 0 0	' 3D Full Coverage obscures antenna (use Fixed Freq)
EN


 

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Hi-VHF 10-El Yagi - Optimized by Lorkoe
Hi-VHF 10-Element, 78-in Boom Yagi with Folded Dipole, Optimized by Lorkoe, Rescaled for Hi-VHF Band.
Heh, Lorkoe didnt optimize anything. He was given the old antenna for free. I just drew up the model from his measurements.



Actually, with the sizes involved, a corner reflector yagi for VHF-HI isnt unreasonable. Shouldnt be much sweat getting 13.5 - 14 dbi peak gain with 7 or 8 directors on it.
 

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Discussion Starter #9
Yeah, I found that to be true too. LPDA is great when you need the wide bandwidth for vhf-lo and vhf-hi together. Since most now will only need vhf-hi , a folded dipole type yagi can cover vhf-hi nicely in less space and a simpler cheaper build.
VHF-Lo is about 70% of an octave* (a full octave if you include FM), VHF-Hi is a little less than a third of an octave - which is within the capabilities of a Yagi-Uda. For reference, the original ch14-83 UHF was almost an octave, while the current ch14-51 UHF is down to 57% of an octave.



* an octave is a doubling in frequency, from music
 

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300ohm: When you modeled Lorkoe's 10-Element Yagi, did you apply a Boom Correction?
Nope, so again there may not be a channel 13 drop off in real life with the antenna as is. Originally, Lorkoe thought he had a channel 12 cut yagi, common in his area. After I drew up the model, it turned out to be a pretty nice broad band vhf-hi antenna, especially considering its probably 20+ years old. For Lorkoe purposes, hes certainly good to go for channel 12 over his present 7 dbi Radio Shack LPDA, heh.

Also note, the model is a bit off, done for ease of modeling. In the real antenna, only the front director and rear reflector are in the center of the boom. The rest of the directors and the top of the folded dipole element are resting on the boom. I see why this was done for packaging the real life antenna, but Im a bit unsure of its effects, although the effects have to be small. And then of course we have all the holding clips which werent modeled either. And we are talking basically 1 channel difference only.

Im very leery about narrow frequency cut antenna designs. Too many things to go wrong in the build, heh.
 

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Out of curiousity, in practice, do the elements of an LPDA usually measure up in a straight line, at the ends? That is, will each element end connect to a ruler or other straight edge, given the angle?

The reason I ask is that, if they do, that knowledge would greatly simplify the construction of any LPA, provided the upper and lower specs are met in the construction. Then it all comes down to the angle meeting those needs.

As a hypothetical, assume a UHF TV antenna. Given the lengths of the upper and lower, the other elements in between would fit along that straight line (alternating, of course) at periodic intervals determined by the angle line. Naturally, the spacing would vary, as they do in theory, but could visually be determined by where the element actually meets the angle line. All that's left to do is determine how many elements you want then. The more the better for LPDAs, on average.

Does this sound about right, or am I simplifying it too much?
 

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So, in theory, LPAs ARE actually easier to build than I first thought. Of course, my free and easy style doesn't guarantee a perfectly, or even good, working LPDA, but it's good to know I'm on the right track, thanks again to you Holl-ands. Your help is always very appreciated.

Can I ask something else? The thickness of your wire LPDA; does it vary? I ask because I don't yet understand why, on LPDAs with standard dipoles, they often make them thicker further in. Why is that, may I ask? Does it simply conduct the signal better, when you increase dipole thickness going back?

I'm trying it mainly for the UHF spectrum right now, as they are smaller to construct. Eventually, I'd like to try my hand at one with loops to the side, ala the loop yagis, but for now, I'll keep it simple.
 

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Can I ask something else? The thickness of your wire LPDA; does it vary? I ask because I don't yet understand why, on LPDAs with standard dipoles, they often make them thicker further in. Why is that, may I ask? Does it simply conduct the signal better, when you increase dipole thickness going back?
Do you mean thicker for the longer elements ? Thats done for structural support.
Or do you mean thicker (like the tubing inside another tubing) at the mounting point to the boom ? Also for structural purposes.
 

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Thicker in the longer elements. As long as it's structural, that gives me more freedom to experiment without having to concern myself with element diameter.

Thanks for the reply, 300ohm.
 

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For vhf, the most common tubing size used is 3/8" in North America. In Australia, they use 7/16" and 1/2" because of the parrot problem, heh.
 

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Parrot problem... parrot problem... parrot problem?

What parrot problem? ;)

Until today, I was thinking about using extensible dipoles on a UHF LPDA. Now I realise it makes more sense to use them on a yagi, so it can be adjusted to pick up problematic stations as needed, without having to build a new one within the same area of the spectrum. Funny how the obvious is not so obvious until you bump into it.
 

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I was thinking of a multi-part slider boom of some sort, akin to many slide rules, or Hot Wheel track. That concept, anyways. Or maybe like a curtain track sort of setup where the elements can be insulated, moved, and locked down. So, there's two choices; the track moves, or the elements do. Needless to say, the structure would best be used indoors, because of it's innate weaknesses. Or shielded where possible.

Heh, I just realised that I'm trying to invent a Transformers antenna.

I just thought; if the boom is kept at the longest length it would need for UHF, the dipoles could be attached to clamps, and moved and added as needed. Seems like less work.
 
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