[Bass2Four]

Reflector Mod, completed...

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]

Rough Cuts...

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.

 

[DXING.Bruce]

CM4221hd hacked specs.

I've seen the polar pattern for the old CM4221 but has anyone seen or plotted the polar pattern of the new CM4221hd hacked & un-hacked.
One hack is to shorten the reflector to 20" from 24" I'm interested in how wide the beamwidth would be for the 20" version.
I'm still trying to perfect my antenna setup in the Woodbine & Cosburn area in East York. I want to mount a new CM4221hd in a hacked 20" screen version, other hacks done already. I'm having trouble with the 24" un-hacked version with picking up all the Buffalo channels without using a rotator. I want to get away from using a rotator. I hope the 20" hacked version will have a wide enough pickup pattern to receive all stations. My antenna will be about 40' on top of my tower with a rca preamp with separate vhf & uhf inputs. I have the Antennacraft 5 element high vhf to pick up CHCH on channel 11. Does anyone know if the 20" screened 4221hd would be best for my situation or should I do a vertical stack of two 4221hd's 24" screens & aim each one for the different Buffalo channels then combine with a reverse splitter into the preamp. Any help will be appreciated guys.

Thanks.

 

[holl_ands]

I modeled the (Hacked) CM4221HD here:
http://imageevent.com/holl_ands/multibay/4bayrefl/uhfnewcm4221hd

Since it would be difficult for NEC2 to model the "proximity" degradations in the unhacked CM4221HD, I did not post a model for that configuration, knowing full well that it would be inaccurate.

I post 4nec2 files for every antenna I analyze, most of them using variable SYmbols that make it easy to change dimensions (but 4nec2's Geometry Edit will NOT save them). If you want to investigate some "what-if" questions, I would recommend you download and install the FREE (and easy to use) 4nec2 program and then download and unzip NEC2/MP into the c:/4nec2 folder:
http://www.qsl.net/4nec2
http://users.otenet.gr/~jmsp

Since you found that the higher Gain CM4228HD worked best, it's unlikely that widening the Beamwidth is going to help, since it ALSO reduces the Forward Gain. A Vertical Stack of two CM4221HD's doesn't change the Beamwidth, but can increase (theoretical) Gain by up to 2.5 dB....plus even more due to resistance to Multipath Nulls.

 

[byebye_cable]

Has the CM4221HD hack (flip balun & removing plastic caps) progressed to the point where it should just be done BEFORE even trying the antenna out-of-the-box?

Earlier in this thread it seemed the hack was to be resorted to only after height & location change avenues had been exhausted.

I'm setting up a friend, am a little nervous on the ladder so I don't see making multiple trips.

We won't be trimming the reflector initially.

 

[Archerotor]

RE: CM 4221HD spacers

On the most currently produced CM-4221HD's are the flat metal bars that run to the PCB balun box still too close to the metal boom?
I'd like to know- if they are- if there's a simple way or plastic block-like spacer material that I could wedge in between the flat metal bars and the square boom to maintain a precise and uniform 1/2" spacing where the bar is too close to the antenna boom. And I think, from reading the posts, that most people see an improvement in removing the decorative, black plastic caps where the whiskers attach, by producing a longer exposed whisker length.

I heard back from Channel Master, regarding the spacing issue of the flat metal feed lines and the boom and surprisingly, and somewhat condescendingly, this is what they had to say:

Aug 11 08:04 AM
Hello,
I have not heard of this being a problem with any of our customers. As for mounting the antenna, the best way is to install it vertically on a pole and use the U-bolts to fasten it to the pole.
Channel Master
Technical Support

 

[Jorgek]

The balun is attached with screws. If you really think this will make a meaningful difference then undo the screws and flip the balun over. I thought that plastic was transparent to RF so removing the tiny plastic caps show not make any difference.

 

[Archerotor]

I thought there were pop rivets holding the flat feed lines going to the balun box that some people here talked about having to drill out, before flipping over. I would have thought that thinner plastic would allow RF signals through, as many indoor antennas have bowties or UHF loops encased in plastic. But people here have testified to performance increases for the CM-4221HD after removing the black plastic caps.

 

[stampeder]

In 2009, when Channel Master was going through the process of shutting down its US operations and changing over to the current Chinese designed and manufactured products, the immediate impact on consumers was not pleasant. We saw that here first hand, and those of us who had a warm spot in our hearts for Channel Master's industry leading OTA gear of yore were left cold.

The info in this thread goes back a long time and is considered accurate, yet we have never had good contact from Channel Master techs, and no amount of info provided by us ever seemed to go anywhere with them. Instead, we did the right thing and directed our knowledge straight to consumers.

Because CM would not listen, we've done the engineering work ourselves and kept this great information here in this thread for everyone to get the best results from.

Beginning in 2009 I've specified on the Antenna Chart that the CM4221HD should be modded, and in the OTA FAQ I have likewise encouraged consumers to do these modifications since that year.

I don't think it is helpful to contact Channel Master at this point or to rehash the same topics that have already been discussed within this thread after all this time. You can trust what we are recommending.

Since we are an active research & development forum the offer stands that if someone comes up with a very new, WORKING improvement to what is already in this thread then by all means post it.

Thanks for your understanding.

 

[holl_ands]

Unfortunately the NEC2 Engine can't model Dielectric Materials EXCEPT as a thin insulating coating on wires....so without access to expensive modeling programs, we can't investigate the effect of plastic caps. HOWEVER, quite some time ago I DID model a G-H with and without PVC Insulation on the Element wires, finding very significant Gain Loss at higher frequencies and also degradation of the SWR:
http://imageevent.com/holl_ands/grayhoverman/dbghcombo

So we KNOW that Plastic/PVC can degrade UHF signals (VHF not so much), but we don't have a good way to assess HOW MUCH, other than difficult to reproduce OTA Tests.

BTW: I've also read that Plastic/PVC pieces on the TIPS of the Elements can be especially detrimental. So a good rule of thumb is to MINIMIZE the contact area between Elements and PVC Pipes and keep a bit of separation distance to surrounding enclosures.

 

[aquatarkus]

HOWEVER, quite some time ago I DID model a G-H with and without PVC Insulation on the Element wires, finding very significant Gain Loss at higher frequencies and also degradation of the SWR....

What I wonder is how accurate their model for PVC insulation is. A simulation is only as good as its model library.

So we KNOW that Plastic/PVC can degrade UHF signals (VHF not so much),....BTW: I've also read that Plastic/PVC pieces on the TIPS of the Elements can be especially detrimental.

If so, it would indicate that PVC is lossy as a dielectric at ultra high frequencies, which may well be true. Something I hadn't considered up until now. Looks like I may want to pull the caps off my CM4428 although I'm not so keen on the idea of the dipole connections becoming more exposed to the Canadian weather.

 

[Jorgek]

I pulled the caps off my 4221 hd 2 winters ago. Antenna still looks like new. So if the caps are dialectric then removing them will in effect increase the length of the bowties thereby making the ant more responsive to lower frequencies. Is that right?

 

[aquatarkus]

So if the caps are dialectric then removing them will in effect increase the length of the bowties thereby making the ant more responsive to lower frequencies. Is that right?

No, if the caps are a perfectly lossless dielectric (insulator), they would be transparent to the radio waves, so that there would be no signal attenuation. I only recently came across this thread, but there appears to be a consensus that the PVC material being used here is actually an imperfect dielectric that is somewhat lossy at UHF. So removing the caps gets rid of the attenuation they are apparently causing, but without changing the effective electrical length of the bow-tie elements. So this mod should not shift the antenna's frequency range.

 

[BOUVAL]

Aquatarkus,

Even if the caps are not insulators, they are still an obstruction to RF signals. So you gain a fraction of a dB removing them and shift the frequency a fraction of a Mhz downward.

I did install a CM4228HD at a friend's place back in october 2011, I did remove the caps and after 3 winters the antenna looks like new.

The only hack I'm against of in this thread is reducing the reflector's width of the CM4221HD.

And remember that the Canadian weather conditions are worst in the Montreal area than the GTA.

Conclusion: The caps are only aesthetic, they protect nothing and consequently they are useless.
.