[RickRae]

Hi, folks.

I'm a new member and this is my first post, but I've been following the GH and modeling/analysis discussions here for some time... so let me say "Thank you!" to everyone for being so willing to share your knowledge and efforts. And special thanks to nikiml; your site has been of tremendous help.

Here's my situation: I have available an assortment of digital and low-power analog stations with noise margins ranging from 10.5 to 46.6 dB (10' open-air elevation -- but the antenna's in an attic). All are UHF except for one on channel 12 at 42.6 dB.

As a first test, I cobbled together a crude GH0n3 (because it was simple and quick to construct) using 1/2" copper tape on styrofoam insulation board. It was able to snag all hoped-for stations, though 12 didn't have a lot of signal to spare. Since I want to allow for atmospheric variances, snow on the roof, etc., my actual build will be a GH6n3 (which has the highest channel 12 gain of the versions I studied).

Additionally, I've been tinkering with the design in an attempt to maximize channel 12 gain without unduly compromising the UHF band. At this point I seem to have a model that boosts 12 by about 2 dB over a GH6n3 with less than a half decibel damage to the UHF band, worst case. At this point it's a bit unwieldy -- the center reflector's over two meters wide for example, and the top-hat NAROD spacing looks disturbingly tight -- but I'm hopeful such things can be tweaked.

Before I spend any more time on it though, I'd really like someone with a practiced eye to take a look. I'm new to 4nec2 you see, plus it's been a LONG time since I earned my First Class license and most of my antenna knowledge is rusty from lack of use. The output seems reasonable, but I could easily have overlooked something or misused the program in some way. So, I'd love a sanity check just to make sure I'm on the right track.

Here's the nec file (this is with copper versus T6 aluminum, and #4 wire for the driven element and top hats, by the way):

CM CH12 OPTIMIZED COPPER #4 DRIVEN AND NARODS
CM Based on GH6n3 optimized by nikiml
CE
SY radius=0.003175
SY narod_surf_spacing=7.884e-3
SY narod_spacing=6.76e-3
SY feed=0.05417	'0.02, 0.07
SY b=0.1247179	'0.075, 0.25
SY a1=0.1518191	'0.07, 0.22
SY _G3=0.05551477	'0.006, 0.1
SY a2=a1+feed-_G3	'this is to avoid self intersection since _G3 is to be > radius+2.5mm
SY a3=0.123689	'0.07, 0.22
SY x=-0.08082828	'-0.06, -0.1
SY g1=0.01437272	'0.002, 0.045
SY g2=0.045
SY g3=0.04499394
SY l1=0.3186459	'0.1, 0.6
SY l2=0.3773999	'0.1, 0.65
SY l3=0.6086331	'0.1, 0.65
SY n_h=0.103995	'0.05, 0.14
SY n_f_dx=0
SY n_top_dy=-0.03689449	'-0.06, 0.04
SY n_b_taper=0
SY n_top=_G3+a3+n_top_dy- n_b_taper/2	'hat head size
SY n_f=x + n_f_dx	'so the hat does not tilt behind the reflectors
SY narod_top = a1+a2+a3+narod_spacing+n_h	'want to put 3 reflectors between z=very_small and z=narod_top +/- very_small,  1 reflectors around narod_top and 1 above it
SY dz3=0.1106235	'0.017, 0.15
SY above_below=-2
SY sign = abs(above_below)/above_below	'may fail at 0
SY z3 = sign*dz3+narod_top
SY dz = z3/5
SY dz1=1.116735	'0.2, 2
SY z1 = (1+(dz1-1))*dz
SY dz2=1.020749	'0.2, 2
SY z2 = (3+(dz2-1))*dz
SY ay1=feed+a1
SY ay2=feed+a1-a2
SY ay3=feed+a1-a2+a3
SY ay4=feed+a1-a2+a3+b
SY az2=a1+a2
SY az3=a1+a2+a3
SY n_b_z=az3+narod_spacing
SY n_b_y1=n_top+n_b_taper
SY n_b_y2=n_b_y1+b
SY n_z_coef=0.792873	'0.5, 1.4
SY n_zsz=0.361459	'0.3, 0.55
SY n_0sz=1.046914	'0.3, 0.6
SY n_x_diff=0.256792	'0.15, 0.4
SY n_x = x-n_x_diff
SY n_z = narod_top*n_z_coef
GW	1	11	0	feed	0	0	ay1	a1	#4	'#00f
GW	2	11	0	ay1	a1	0	ay2	az2	#4	'#00f
GW	3	9	0	ay2	az2	0	ay3	az3	#4	'#00f
GW	4	7	0	ay3	az3	0	ay4	az3	#4	'#00f
GW	5	17	x	g1	z1	x	l1+g1	z1	radius	'#0f0
GW	6	20	x	g2	z2	x	l2+g2	z2	radius	'#0f0
GW	7	32	x	g3	z3	x	l3+g3	z3	radius	'#0f0
GW	11	8	n_f	n_top	n_b_z+n_h	0	n_b_y1	n_b_z	#4	'#000
GW	12	7	0	n_b_y1	n_b_z	0	n_b_y2	n_b_z	#4	'#000
GX	20	010
GW	13	15	n_f	-n_top	n_b_z+n_h	n_f	n_top	n_b_z+n_h	#4	'#000
GW	15	43	n_x	-n_zsz	n_z	n_x	n_zsz	n_z	radius	'#D00
GX	40	001
GW	16	56	n_x	-n_0sz	0	n_x	n_0sz	0	radius	'#D00
GW	100	9	0	-feed	0	0	feed	0	radius*.8675	'#f40
GE	0
LD	5	0	0	0	58000000
GN	-1
EK
EX	0	100	5	0	1	0	0
FR	0	0	0	0	800	0
EN

Here are the gain curves I get, plotted against a GH6n3 as a reference:

Here are the SWR graphs, again plotted against a reference:

As you can see, it's surprisingly good (well, to me as a newbie modeler anyway), which is why I assume I've done something wrong or overlooked something.

Here's a render as a visual aid; you'll see what I mean about the wide center reflector and tight top hat spacing:

I pretty much have no idea what I'm doing (grin), so I'm open to any and all input.

Thanks,
Rick

 

[nikiml]

-- the center reflector's over two meters wide for example, and the top-hat NAROD spacing looks disturbingly tight --

Besides the problems you already mentioned I didn't see anything else.
But it seems that if you distance the narods to the 'safer feeling' distance of 11mm you won't loose much gain. I guess if you are willing to accept the 2m rod you can experiment with the narod distance at test time...

 

[300ohm]

so I'm open to any and all input.

Great first model, youre well on your way.
Well, before doing frequency sweeps, AGT should be adjusted so that AGT=1.0 (0db). Hit F1, help for more on AGT.

AGT adjustment tips :

To Run AutoGainTest (AGT): [SOURCE Wire is the one in the EX statement.]

1. Select Calculate/NECOutput-Data/FarFieldPattern (or simply hit F7 key)
2. Select FULL, 1001 (top to bottom) and Freq (e.g. 198 or 584 MHz, but isn't critical).
3. Generate
4. If the AGT is larger than 1.00, reduce the SOURCE wire Radius, if smaller then increase.
5. Save revised file.
6. OPEN the revised file and repeat from step #1....and repeat....and repeat....

TRICK: If you simply Save and repeat from step #1 (without reOPENing),
you'll have to reenter all of the parameters again....wasting a lot of time....

If you do both UHF and Hi-VHF runs, the SOURCE Wire Radius will probably be different.

Other things that affect AGT:
a. Whether SOURCE Wire has 1, 3 or more segments.
b. Length of SOURCE Wire.
c. Segmentation

I run AutoSegmentation while doing AGT runs and after AGT=1.0, I press F8 for NEC Output
and copy AutoSegmentation results into the file. Then turn off AutoSeg and retweak AGT.
AutoSegmentation parameter may be small for "simple" antennas and larger for
more complex jobs....and some are a real pain (like the Corner Reflector).
It is possible to have TOO many segments, resulting in additional warnings...
(revised 10/07/09)

I am surprised that making the middle NAROD reflector almost 1.5 wavelengths does so much for the individual channel gain. It shows the GH design still has surprises left.

Looking at the TVFool for Quinton, VA, (post your exact TVFool) you really dont need 12 dBi to get a 45 NM channel 12. Thats way overkill, even in an attic, heh. Something else, like multipath, may be going on with your channel 12. 6 dBi should be plenty and you could make the antenna less unwieldy.
Are you just planning on having the antenna in the attic or are you thinking of mounting one outside for the stations at 156 degrees magnetic. In your situation, I would have two antennas with 2 coax downleads into an A-B switch at the TV. The stations at 156 degrees are 2 edge stations, but theres maybe enough NM there to make them consistently receivable with some effort outdoors.

4 gauge copper takes a lot more muscle power than 6 gauge to straighten, and in the attic its overkill and unnecessary and pricier.

As a first test, I cobbled together a crude GH0n3 (because it was simple and quick to construct) using 1/2" copper tape on styrofoam insulation board. It was able to snag all hoped-for stations, though 12 didn't have a lot of signal to spare.

Yeah, the GH0n3 even for vhf-hi is more bidirectional than a GH6n3. So if you have multipath problems on that channel, the GH6n3 could help. Another thing that could help with multipath in the attic is massive metal or chicken wire behind the antenna about a yard back.

And the 1/2" flat copper tape definitely doesnt make the best NAROD material since the NAROD depends on surface areas to work properly, so I think that may be a big source of your channel 12 problems on the GH0n3.

 

[RickRae]

Thanks for the input guys, and apologies for the long delay in responding. I had almost zero free time last week.

... before doing frequency sweeps, AGT should be adjusted so that AGT=1.0 (0db).

I gave that up to save time as the number of variations multiplied, figuring I'd resume once I'd homed in on a few candidates to fine-tune. (As I recall the AGT error wasn't bad on the model I posted.) But thanks for the reminder.

I am surprised that making the middle NAROD reflector almost 1.5 wavelengths does so much for the individual channel gain. It shows the GH design still has surprises left.

I was surprised too. Thinking I knew what was going on, I stepped the length of the middle NAROD reflector from almost nothing to six meters in 20cm increments (these are raw gains):

LEN      207 MHz      177 MHz
       Gain   SWR    Gain  SWR
0.2    10.70 1.47    5.82 2.11
0.4    10.61 1.49    5.90 2.18
0.6     9.66 1.86    6.25 2.23
0.8    10.78 1.56    5.79 1.37
1.0    10.86 1.48    4.92 1.96
1.2    10.89 1.45    5.13 2.03
1.4    10.92 1.43    5.23 2.06
1.6    10.97 1.41    5.31 2.07
1.8    11.08 1.38    5.39 2.09
[B]2.0    11.58 1.26    5.53 2.10[/B]
2.2    11.30 1.39    5.81 2.11
2.4    10.93 1.93    6.42 2.18
2.6    10.90 1.43    4.19 1.97
2.8    10.90 1.42    4.48 2.02
3.0    10.89 1.42    4.65 2.04
3.2    10.87 1.41    4.72 2.05
3.4    10.71 1.38    4.74 2.06
3.6     9.98 1.29    4.73 2.07 <-- Gain peaks at 198 MHz
3.8    10.68 1.39    4.67 2.08
4.0    10.78 1.40    4.52 2.10
4.2    10.82 1.40    5.29 2.05
4.4    10.84 1.40    5.30 2.03
4.6    10.86 1.40    5.20 2.04
4.8    10.94 1.39    5.17 2.05
5.0    11.39 1.30    5.16 2.05
5.2    11.00 1.38    5.18 2.06
5.4    10.92 1.39    5.23 2.06
5.6    10.90 1.40    5.35 2.06
5.8    10.89 1.40    5.46 2.09
6.0    10.88 1.40    4.67 2.09

The results certainly weren't what I anticipated.

... post your exact TVFool...

Here ya go. Since I was requesting modeling rather than reception help, I didn't bother with it before, figuring it wasn't germaine.

... you really dont need 12 dBi to get a 45 NM channel 12.

Generally speaking I'd agree with you, but there are other influences.

Thats way overkill, even in an attic, heh.

That's one of the "other influences." I have a hip roof, meaning when it rains or snows the antenna will be looking through a sheet (or wall!) of water. So I want every dB of gain I can get to improve the odds (notice the posted model has a little additional gain from channels 24 to 36 as well). Plus there's the intellectual lure of actually learning something!

Something else, like multipath, may be going on...

There's definitely something odd about this location. For example, I'm lucky to get one or two bars of cell phone reception (outside) at the house, but at the causeway a few hundred feet down the road (which is at a lower elevation), the signal is solid. It almost seems like I'm in a slight RF shadow here. So based on past experience, I'm presuming my TV Fool numbers are a bit on the optimistic side.

That said, I've seen the TV report low to mid 90s for channel 12 from the mock-up GH0n3.

Are you just planning on having the antenna in the attic or are you thinking of mounting one outside for the stations at 156 degrees magnetic.

I'm staying in the attic if I can make it work. With the mock-up aimed dead west I can actually nab (real) channel 16 (13.4 NM) off one of the rear lobes at night, which is encouraging (and surprising). Once I have a "real" antenna in the attic I'll point it southeast to see how it does. If the results are worthwhile I very well might build a second one. The western stations are my primary interest, though.

4 gauge copper takes a lot more muscle power than 6 gauge to straighten...

I'll see your muscle power and raise you two trees and a Come Along. Or one tree and a vehicle with a tow hitch.

... in the attic its overkill and unnecessary and pricier.

Agreed on the overkill, though not really pricier at the moment. There's a local place that has a pretty good price on 4 gauge, making it only slightly more than the 6 gauge elsewhere.

I modeled 1/4" copper because I have a roll of tubing available to use. I wasn't keen on trying to make the driven elements or top hats from tubing though. I went with #4 wire because it seemed the SWR climbed as the wire got smaller (which I suppose was either the effective feed gap width change or the delta versus the other elements).

But at this point it's probably moot as I'm leaning towards 1/4" throughout lately.

... the 1/2" flat copper tape definitely doesnt make the best NAROD material since the NAROD depends on surface areas to work properly...

Ack! I'm a victim of my own two-dimensional thinking! I used 1/2" tape because it was available and fast. I figured between skin effect and the larger area presented to the signal, it would work as well as, if not better than, 1/4" rod/tubing.

I even adjusted the top hat NAROD spacing appropriately. But I totally failed to consider that the driven elements and top hats would be "looking" at each other edge to edge! D'oh!

Ah well, if that's the worst goof I made I'm pretty happy about it all. Especially considering the mock-up performs quite well.

Time will be tight for another week or so but I'll pass along what data I have when it's available.

Thanks again for your help, guys!

Cheers,
Rick