CM4221HD Hardware Hacks

[holl_ands]

Horizontally-Stacked CM4221's with RF Combiner:

I initially checked balm's 4nec2 file (post #313) and found No Errors, but many segment Warnings.
AGT was slightly off in the Original: UHF AGT=0.99 (-0.1 dB), Hi-VHF AGT=0.93 (-0.3 dB).

Inspecting the file, I uncovered what I think is an unintended Wire Radius for the
Inner Feedline Radius (GW36-47 & GW152-163), which I changed from 0.1875-in
to 0.0595, matching what was used for the Outer Feedline Radius (GW20-35 & GW136-151).
All of the Segment Warnings went away...and still NO Errors.

After inserting my usual FR/RP Card Set, I added the variable SYmbol Rsrc to adjust AGT=1.0.
There was no significant change to either UHF Raw Gain or SWR compared to original:
UHF Raw Gain=14.1 dBi (470 MHz), 17.7 dBi (698 MHz) & 18.3 dBi (806 MHz), SWR<2.8.

To investigate whether the very short 0.5-in "Simulated Balun" wires connecting the SOURCE
wire to the antenna were causing any problems, I lengthened them to 2.0-in. No significant
effect on Raw Gain, but SWR at 470 MHz increased a bit to 2.9.

I then eliminated the "Simulated Balun" entirely, connecting the SOURCE directly between the
Feedlines (as is the usual practice with Multi-Bay antennas). No significant change wrt Original.

Finally, since the Original appears to have used Autoseg=6 (a bit small), I did Autoseg=11.
Raw Gain increased a bit: UHF Raw Gain=14.1 (470 MHz), 17.8 (698 MHz) & 18.4 dBi (806 MHz)
and SWR remained < 2.8. This is the final 4nec2 file I ran to obtain results posted here:
http://imageevent.com/holl_ands/mult...zstackcm4221hd

The 4nec2 File and Hi-VHF results were also uploaded. Hi-VHF was about as problematic
as the CM4228HD, with very irregular Gain across most channels (degrades DTV) and
Excessive SWR on all but Ch11 & Ch12.

Since the 4nec2 file tolerated these several different variations, I BELIEVE that these are
TRUE results and not some sort of calculation engine fluke.

The challenge now is to find an Interconnection Harness that doesn't lose as much as
the 0.5 to 2.0+ dB loss in a typical RF Combiner, see www.antennahacks.com

It would also be of interest to sweep the bowties forward (probably 1 to 2-in) and
try wider separation between the Bays and/or additional Reflector Rods to see if either
the UHF Raw Gain, Forward Sidelobes or Hi-VHF Performance can be improved.....

[balm]

holl_ands, thanks for fixing that file ...looks like I was overestimating low UHF, and underestimating mid-high UHF...

still, pretty decent results

as for the wire configurations and wire radii (not at the balun), I believe I used the same data as in you model of the 4221HD (unchanged radii)

I just took 2 of the 4221HDs as they were, and made the screens touch

it looks like your using separated reflector screens, similar to what I did in post #315



another idea might be to reduce tine separation to 2"



BTW, I CANNOT run your new 4211HD combined model...I get parser, constant, token errors, then I have to crash the program to get out...I thought it had to do with the quote/spacer markers, but after removing all comments, I have the same problem

EDIT: OK, tags #90 to #95 (reflector rods) got mangled out of position, and the radii got truncated to zero, in the opening of the file (I guess) - anyway I fixed it in my file

thanks

[holl_ands]

Although the uploaded file LOOKS good...and still LOOKS good when downloaded,
I have the same problem with GW90-95...maybe there are some hidden control char's???
Whatever it is, the NEW NEC EDITOR immediately recognizes that something is messed up.

I manually replaced all TABs with SPACEs and uploaded again....now it's fixed....I hope...

Looking back on my CM4221HD 4nec2 model, I see what you mean about the Inner
Feedlines being a much fatter 3/8-in, cuz it's simulating the FLAT BANDS going to the Balun.
Guess it doesn't really make much difference.....I'll fix the file tomorrow & repost results.

[balm]

I'm not sure if the EXACT detail of the "bumped out" Rabbit73 balun modification was modeled, or more importantly if it made any difference in the model results as compared to JUST the balun flip mod ? just curious.

Thanks

[balm]

Is there a reliable way of simply extending the CM4221HD whiskers to 9" long. I put an ALMOST perfectly fitting 3/16" aluminum tubing over the ends but I have not yet figured a way to fasten it solidly to the whiskers (its not quite tight enough to hold on its own)

So maybe some sort of glue that withstands outdoors ???, but then I assumed it would change the surface area properties and affect the antenna performance ...???

[Bass2Four]

Well, I've been reading this channel for quite some time now and decide to do the reflector hack. I cut back 2 inches from each side of the antenna, that proved a little difficult - spent a couple of hours as I didn't want to bend anything out of whack - yeah right.

So I managed to cut all the bars, reset the plastic supports - I think I may have bent the antenna a bit. Any how, I am currently using the antenna indoors (waiting for my roof to be redone before putting it up there) and the results are mixed.

The channels I have managed to retain are solid and actually appear much richer, indicated low signal strength seems to be holding better than before, but I lost a couple of channels...

Can anyone provide the technical measurements of the antenna, i.e. how far the whiskers should be from the reflector, how wide the spread should be for the whiskers - that sort of thing. I want to make sure my hack job didn't put the antenna to far out of spec.

I haven't tried the other hacks, balun or plastic cover removal, with this mod yet, although I did do those hacks initially (before the reflector mod) and saw no significant improvements...

[Bass2Four]

OK, completed the other mods, pulled-out the balun and removed the plastic caps - these mods do not appear to have effected the results over the reflector resizing...

NOTE:

I am currently using the antenna indoors, when I have a chance I will do some preliminary testing on-the-roof...

[stampeder]

Yep, testing a CM4221HD indoors is like boxing with your head in a canvas sack. These modifications show benefits in most outdoor scenarios.

[Bass2Four]

As mentioned in a previous post, I tried to be gentle with my reflector mod, but I may have bent the antenna here and there - does anyone have the spec measurement for the 4221HD antenna, i.e. whisker spacing, distance from the reflector, etc...

[BOUVAL]

spec measurement for the 4221HD antenna, i.e. whisker spacing, distance from the reflector, etc...

Bass2Four,

It's not that critical, take the average of the spacings and distances of what you've got now and adjust to that average and then tighten everything.

The reflector has to be parallel with the whiskers, around 4.25 inches in between, that's all.
.

[Bass2Four]

Darn, while resetting the alignment, one of the whisker mounting points has broken free from the boom - the rivet popped out. guess I'll need a screw or something now...

[stampeder]

Be sure to use the same metal type.

[stent00]

i already pulled the balun out about an inch. both feed lines are not exactly the same shape now. does this matter? Is there any easy way to straighten them out? I might just flip it tonight. Its a pain having the cable beside the feed lines and to close to the whiskers.