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Discussion Starter #1
I've been playing with this "shorted" biquad design in nec. If I'm doing this right, vswr is less than 1.6 and below across the new UHF band with a fairly flat gain of about 10dbi. It has a roughly 70deg horz beam width (3db points). Would anyone care to sanity check my model and see if I did this right?

Code:
CM Shorted biquad by ocdman.  
CE
SY d=5.75	'higth width
SY g=.5	'feed gap
SY b=.5	'bisector higth (scaler between corners and apex)
SY r=.05	'radius
SY fr=.05	'feed radius
SY sw=d-d*(b)	'bisector width
SY sh=d+d*b	'bisector higth (also top/bot reflector spacing)
SY rd=-5.0	'reflector distance
SY rw=6.5	'reflector width (half)
SY rr=.3	'reflector radius
GW	1	10	0	-g/2	0	0	-d	d	r
GW	2	10	0	g/2	0	0	d	d	r
GW	3	10	0	-g/2	0	0	-d	-d	r
GW	4	10	0	g/2	0	0	d	-d	r
GW	5	10	0	-d	d	0	-sw	sh	r
GW	6	10	0	-sw	sh	0	0	2*d	r
GW	7	10	0	d	d	0	sw	sh	r
GW	8	10	0	sw	sh	0	0	2*d	r
GW	9	10	0	-sw	sh	0	sw	sh	r	'top bisector
GW	10	10	0	-d	-d	0	-sw	-sh	r
GW	11	10	0	-sw	-sh	0	0	-2*d	r
GW	12	10	0	d	-d	0	sw	-sh	r
GW	13	10	0	sw	-sh	0	0	-2*d	r
GW	14	10	0	-sw	-sh	0	sw	-sh	r	'bottom bisector
GW	15	1	0	-g/2	0	0	g/2	0	fr	'feeder
GW	16	10	rd	-rw	sh	rd	rw	sh	rr	'reflector
GW	17	10	rd	-rw	0	rd	rw	0	rr	'reflector
GW	18	10	rd	-rw	-sh	rd	rw	-sh	rr	'reflector
GS	0	0	0.0254
GE	0
GN	-1
EK
EX	0	15	1	0	1			
FR	0	0	0	0	585	0
RP	0	73	73	1510	-90.	90.	5.	5.	10000.	
EN
Here is the antenna fool results for where I would use it:

http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id=8d1747e702b4ba

I have some 1/16 brass rod for the active elements. Thought of using either conduit or copper pipe for the reflectors.

Any advice appreciated. Thanks in advance.
 

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Would anyone care to sanity check my model and see if I did this right?
Yeah, it looks good, but you should adjust the AGT to 1.0 (0db). Taking the correction into account, your numbers seem to be about right on.
I have some 1/16 brass rod for the active elements.
If you look at F1, Help > Reference data > Wire conductivity, brass is pretty far down the list. But on a cost benefit ratio, its better to use what you have, heh.



Here are some useful info and notes by Autofils, added onto by me :


For 4nec2, there are tests that you can perform, to ensure a reliable model:
----------------------------------------------------------------------------
1. From the Geometry (F3) window, Validate tab :
a) Run geometry check 'Q' and make corrections if needed.
b) Run segment checks '9' and make corrections if needed.

2. Run the Average gain Test (AGT) and adjust the wire diameter and/or the number of segments of the Voltage source tag to achieve an AGT result as close to AGT=1.0 (-0dB) as possible, at the middle and ends of the desired frequency range. For the new UHF range, the middle is 584mhz, ends are 470 and 698 mhz. Its also useful to have a printout of wire radius sizes in inches(millimeters) handy.

3. Run a convergence test, where the gain or F/B ratio has a relatively small change when you increase the number of segments in your model.

To find out more on AGT and convergence, type "AGT" or "Convergence" in the 4nec2 General Help index window.(F1)

Basic and "safe" segmentation rules for nec2 modeling:
------------------------------------------------------
Each element of the antenna that you model, is sub-divided into smaller sections called segments. The "safe" rules for how you determine the segments are:

1. Use at least 10 segments per half wavelength of wire at the highest operating frequency.
(Ten segments per full wavelength is sometimes used to specify the longest allowable segment length -- 0.1 -- but doubling the segment count yields more accurate results for a larger variety of geometric wire assemblies.)

2. Use a segment-length-to-diameter ratio of at least 4:1.
(Although the absolute limit is sometimes given as a segment-length-to-radius ratio of 1:1, the much larger recommended ratio tends to prevent problems in complex geometries with angular junctions of wires.)

3. If an element is composed of more than one wire, the length of segments on each wire in the assembly should be as equal as possible.

4. To the degree possible, for parallel wires, let the segment junctions align as closely as possible.
(This rule is absolutely essential when wires are closely spaced, as in a folded dipole, and thus makes good sense as a general practice in all modeling.)

5. The Voltage source tag wire (the wire referred to in the "EX" card) should have a minimum of 3 segments. Using 1 segment can produce unreliable results.

More notes:
------------
1. Model just array-only first and explore it's gain characteristic for the freq range you are designing.
2. Try some variations in the array itself, looking for what variations provide the most raw gain.
Keep an eye on swr, as well as max raw gain. You should consider a swr of 4 as worst case for your design and have an objective of less than 2, and as close to 1 as you can get.
3. Once you have a good array design , now add the reflectors and run optimization runs to get the overall best design.
4. Set Auto-Segmentation to (per this forums consensus) of around 21 to avoid misreadings of gain when Auto-Segmentation is set to 13 - 15.

It's really that simple!!
=================================================================================

Since its not built directly into the program, Ill explain how to calculate Net Gain with 4Nec2.

Here is a method to calculate Net Gain:

NetGain = RawGain+10*log(Feed-pointGain)
where Feed-pointGain = 4*Zr*Zo/((Zr+Zo)^2+Zi^2)

The [10*log(base10)]" converts to decibels, and Feed-point Gain is less than 1, since there is a loss.

Zo = characteristic Impedance for the transmission line connected to the antenna (For use with 300 ohm twin lead or a 4:1 balun, Zo = 300)
Zr = real part of antenna's complex impedance at a specific frequency
Zi = imaginary part of antenna's complex impedance at a specific frequency
RawGain = Gain output given by 4nec2 at the specific frequency


The Raw Gain, Real Impedance, and Imaginary Impedance data can be obtained when a Frequency Sweep is calculated. The 4nec2 program then outputs a Imp./SWR/Gain (F5) Line Chart. Under the "Plot" option is the option to plot Forw-gain, R-in (real), X-in (imag). It will give an error message, ie wGnuPlot.exe not found. Ignore it and it will output/display a text file with the needed data. Cut and paste the relevant data into a spreadsheet of your choice for your Net Gain calculations.

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 
6.0     3.08
 

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Missing Step:
Prior to running the first AGT Check, Enable Autosegment (typ 21, but might need 7, or 15....or 33).
After running the first AGT check, press F8 to display the NEC Output file. Copy these segmentation
values into the 4nec2 file and DISABLE Autosegmentation. [The original file wasn't even close.....]
Now vary the SOURCE wire radius, etc until AGT=1.0. To save time, EDIT, SAVE, CLOSE & then OPEN
file again rather than simply rerunning it....thereby avoiding the need to reenter parameters for AGT.

BTW: Contrary to most antennas, increasing SOURCE radius REDUCED AGT.
PS: I didn't take any time investigating the cause of the various WARNINGS...they were only off by
a very small amount, fixable by manually tweaking segmentation.....and after all, they aren't ERRORs.....

In this antenna, there is a tradeoff of F/B and F/R Ratios vs SWR.
In the original file, SWR was excellent, but F/B and F/R Ratios were only good for low freqs.

In the revised 4nec2 file below, I rescaled all dimensions (except wire size) by a Factor of 0.95.
This small rescale keeps the SWR under 2.0 while improving mid to high frequency F/B and F/R Ratios.
Raw Gain is a flat 9.7 dBi across the New UHF Band, falling to 8.2 dBi at 806 MHz.

Rescaling to an even smaller size provided more improvement, but SWR on low freqs becomes excessive.
Perhaps "something" can be done to this antenna design to better align SWR with F/B and F/R Ratio curves.....

UHF Shorted Bi-Quad - Revised 4nec2 File:
Code:
CM UHF Shorted Bi-Quad, 3 Reflector Rods, 4nec2 by ocdman, 23Mar2011.
CM holl_ands mods: Rescale=0.95, Autoseg(21), Adjusted Rsrc for AGT=1.0.
CM Warn.: too sharp angle or too short/thick segment(s) for Wires 11 (tag 11) and 14 (tag 14)
CM Warn.: too sharp angle or too short/thick segment(s) for Wires 13 (tag 13) and 14 (tag 14)
CM Warn.: Wire 16 (tag 16), radius (8.e-3) above wlen/100 (6.e-3)
CM Warn.: Wire 16 (tag 16), seg-len (0.015) below 2 * radius (0.015)
CM Warn.: Wire 17 (tag 17), radius (8.e-3) above wlen/100 (6.e-3)
CM Warn.: Wire 17 (tag 17), seg-len (0.015) below 2 * radius (0.015)
CM Warn.: Wire 18 (tag 18), radius (8.e-3) above wlen/100 (6.e-3)
CM Warn.: Wire 18 (tag 18), seg-len (0.015) below 2 * radius (0.015)
CE
SY F=0.95	'Rescale Factor
SY Rsrc=0.22 	'SOURCE Wire Radius, Adjust for AGT=1.0: UHF=0.22
SY d=5.75*F	'heigth width
SY g=0.5*F	'feed gap
SY b=0.5*F	'bisector heigth (scaler between corners and apex)
SY r=0.05	'radius
SY fr=0.05	'feed radius
SY sw=d-d*(b)	'bisector width
SY sh=d+d*b	'bisector heigth (also top/bot reflector spacing)
SY rd=-5.0*F	'reflector distance
SY rw=6.5*F	'reflector width (half)
SY rr=0.3	'reflector radius
GW	1	17	0	-g/2	0	0	-d	d	r
GW	2	17	0	g/2	0	0	d	d	r
GW	3	17	0	-g/2	0	0	-d	-d	r
GW	4	17	0	g/2	0	0	d	-d	r
GW	5	 9	0	-d	d	0	-sw	sh	r
GW	6	 9	0	-sw	sh	0	0	2*d	r
GW	7	 9	0	d	d	0	sw	sh	r
GW	8	 9	0	sw	sh	0	0	2*d	r
GW	9	13	0	-sw	sh	0	sw	sh	r	'top bisector
GW	10	 9	0	-d	-d	0	-sw	-sh	r
GW	11	 9	0	-sw	-sh	0	0	-2*d	r
GW	12	 9	0	d	-d	0	sw	-sh	r
GW	13	 9	0	sw	-sh	0	0	-2*d	r
GW	14	13	0	-sw	-sh	0	sw	-sh	Rsrc	'bottom bisector
GW	15	 1	0	-g/2	0	0	g/2	0	fr	'feeder
GW	16	27	rd	-rw	sh	rd	rw	sh	rr	'reflector
GW	17	27	rd	-rw	0	rd	rw	0	rr	'reflector
GW	18	27	rd	-rw	-sh	rd	rw	-sh	rr	'reflector
GS	0	0	0.0254	' All of above in inches.
GE	0
GN	-1
EK
EX	0	15	1	0	1			
' FR	0	0	0	0	585	0				' fm ocdman
' RP	0	73	73	1510	-90.	90.	5.	5.	10000.	' fm ocdman
' FR Freq Sweep choices in order of increasing calculation time (fm holl_ands):
' FR 0 0 0 0 198 0		' Fixed Freq
' FR 0 0 0 0 584 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 48 0 0 430 10		' Freq Sweep 430-900 every 10 MHz - SPECIAL WB SWEEP
' 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
' 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 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 21 0 0 88 1		' Frequency Sweep every 1 MHz for FM: 88-108 MHz
' FR 0 28 0 0 54 6		' Wide Freq Sweep every 6 MHz for Ch2-13
' FR 0 37 0 0 96 6		' Wide Freq Sweep every 6 MHz for FM-13 & up to 300 MHz
' 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 (Fixed Freq)
' RP 0 285 73 1510 90 0 5 5 0 0	' 3D Full Coverage obscures antenna (Fixed Freq)
EN
 

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i just made a few ajustments to feed piont gap and reflector size and placement , If you want to see something cool heres a swr trick or treat
orginal reflectors lenght was 12.35 . After testing for some time i found the weak link to this setup and its in the reflector size if you go to 12.25 swr went up trick now for my treat i found that 12.975 puts swr <2

here my version. with holl_ands rescale

Code:
CM UHF Shorted Bi-Quad, 3 Reflector Rods, 4nec2 by ocdman, 23Mar2011.
CM wildwillie mod: Using holl_ands rescale and ajusting the feed piont gap to 3 inches and make placement ajustment and size ajustments to the reflectors a AGT of 1 and SWR below 2 and no warns
CM holl_ands mods: Rescale=0.95, Autoseg(21), Adjusted Rsrc for AGT=1.0.
CM Warn.: too sharp angle or too short/thick segment(s) for Wires 11 (tag 11) and 14 (tag 14)
CM Warn.: too sharp angle or too short/thick segment(s) for Wires 13 (tag 13) and 14 (tag 14)
CM Warn.: Wire 16 (tag 16), radius (8.e-3) above wlen/100 (6.e-3)
CM Warn.: Wire 16 (tag 16), seg-len (0.015) below 2 * radius (0.015)
CM Warn.: Wire 17 (tag 17), radius (8.e-3) above wlen/100 (6.e-3)
CM Warn.: Wire 17 (tag 17), seg-len (0.015) below 2 * radius (0.015)
CM Warn.: Wire 18 (tag 18), radius (8.e-3) above wlen/100 (6.e-3)
CM Warn.: Wire 18 (tag 18), seg-len (0.015) below 2 * radius (0.015)
CE
GW	18	13	0	-1.5	0	0	-5.4625	5.4625	0.05
GW	19	13	0	1.5	0	0	5.4625	5.4625	0.05
GW	17	13	0	-1.5	0	0	-5.4625	-5.4625	0.05
GW	16	13	0	1.5	0	0	5.4625	-5.4625	0.05
GW	5	7	0	-5.4625	5.4625	0	-2.867813	8.0571875	0.05
GW	6	9	0	-2.867813	8.0571875	0	0	10.925	0.05
GW	7	7	0	5.4625	5.4625	0	2.8678125	8.0571875	0.05
GW	8	9	0	2.8678125	8.0571875	0	0	10.925	0.05
GW	10	7	0	-5.4625	-5.4625	0	-2.867813	-8.057188	0.05
GW	11	9	0	-2.867813	-8.057188	0	0	-10.925	0.05
GW	12	7	0	5.4625	-5.4625	0	2.8678125	-8.057188	0.05
GW	13	9	0	2.8678125	-8.057188	0	0	-10.925	0.05
GW	15	7	0	-1.5	0	0	1.5	0	0.0285341
GW	20	11	0	-2.867813	8.0571875	0	2.8678125	8.0571875	0.0285341
GW	21	11	0	-2.867813	-8.057188	0	2.8678125	-8.057188	0.0285341
GW	22	27	-5	-6.35	0	-5	6.625	0	0.0285341
GW	23	27	-5	-6	10.5	-5	6.975	10.5	0.0285341
GW	24	27	-5	-6.25	-10.5	-5	6.725	-10.5	0.0285341
GS	0	0	0.0254		' All in in.
GE	0
EK
EX	0	15	4	0	1	0
GN	-1
FR	0	1	0	0	470	0
RP 0 1 73 1510 90 0 1 5 0 0
 

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Discussion Starter #5
more fine tuning

I applied as much of the info i could. Increasing segmentation dropped raw gain to about 9.5db. To get rid of some of the errors i dropped the refector diameter from .6" to .25", vswr 1.3 at low end, 1.8 at high with a dip in the middle.

Code:
CM Shorted biquad R2 by ocdman.  
CE
SY d=5.75	'higth width
SY g=.5	'feed gap
SY b=.5	'bisector higth (scaler between corners and apex)
SY r=.05	'radius
SY fr=.057	'feed radius
SY sw=d-d*(b)	'bisector width
SY sh=d+d*b	'bisector higth (also top/bot reflector spacing)
SY rd=-4.75	'reflector distance
SY rw=6.5	'reflector width (half)
SY rr=.125	'reflector radius
GW	1	17	0	-g/2	0	0	-d	d	r
GW	2	17	0	g/2	0	0	d	d	r
GW	3	17	0	-g/2	0	0	-d	-d	r
GW	4	17	0	g/2	0	0	d	-d	r
GW	5	9	0	-d	d	0	-sw	sh	r
GW	6	9	0	-sw	sh	0	0	2*d	r
GW	7	9	0	d	d	0	sw	sh	r
GW	8	9	0	sw	sh	0	0	2*d	r
GW	9	13	0	-sw	sh	0	sw	sh	r	'top bisector
GW	10	9	0	-d	-d	0	-sw	-sh	r
GW	11	9	0	-sw	-sh	0	0	-2*d	r
GW	12	9	0	d	-d	0	sw	-sh	r
GW	13	9	0	sw	-sh	0	0	-2*d	r
GW	14	13	0	-sw	-sh	0	sw	-sh	r	'bottom bisector
GW	15	3	0	-g/2	0	0	g/2	0	fr	'feeder
GW	16	27	rd	-rw	sh	rd	rw	sh	rr	'reflector
GW	17	27	rd	-rw	0	rd	rw	0	rr	'reflector
GW	18	27	rd	-rw	-sh	rd	rw	-sh	rr	'reflector
GS	0	0	0.0254
GE	0
GN	-1
EK
EX	0	15	2	0	1			
FR	0	0	0	0	585	0
RP	0	73	73	1410	-90.	90.	5.	5.	1.56E+07	
EN
Trying to get down to real channel 43 on my tv fool. The weaker channels are out of Toledo (approx 120 deg) and Detroit (aprox 60 deg) while still getting the stronger channels from Lansing (aprox 335 and 355 deg) without needing a rotor. So, F/B F/R ratios need not be great, just good enough to reduce multipath.

Thought the brass would be ridged enough to be self supportive but I have some #10 copper i could use if its too much of a hit.

This is a UHF only antenna. My VHF targets i can get with just a dipole.

Thank you for all the input.
 
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