[ota_canuck]

My secret little CM4221HD hack.

[note: this hack has been tried and true over a dozen times]

Just thought I should share something with those of you who are stuck on the performance benefits of that old reliable CM4221 antenna design.

Here’s a little hack that I have been doing for a while now in order to beef up the new CM4221HD antenna design.

It appears that whoever it was at PCT in China that was responsible for the re-design of the new CM4221HD was more concerned with the cosmetic values of this newly designed antenna than it’s overall performance values.

There has been some concern among many about the lacking performance of the new CM4221HD antennas as compared to the old CM4221 antenna design. There are a few simple revisions that can be made to the CM4221HD that will restore those overall performance values and restore the familiar characteristics that the old CM4221 antenna had.

‘V’ ELEMENT OBSTRUCTIONS:
There are some cute little 1” plastic covers that hide a portion of the elements at the connection points of each ‘V’ element on each bay. Remove these 8 cosmetic plastic caps and just toss them away, as they actually shorten the overall calculated length of each ‘V’ element by almost 2”.

RESIZE THE REFLECTOR GRID WIDTH:
The next thing that will need some adjustment is the width of the 24” wide reflector grid. The new CM4221HD reflector grid is 24” wide and the old CM4221 is only 20” wide. To do this, just carefully remove the plastic side retainers from all of those grid rods. Then cut exactly 2” off each of the aluminium grid rods and then reinstall the plastic side retainers.

BALUN TO BOOM CLEARANCE:
The flat bars that the balun box is connected to are too close to the boom, so simply pull the balun assembly outwards and make sure that those flat bars are at least ½” away from the boom. I believe this may have just been an oversight that should have been mentioned within the antenna assembly instruction sheet.
[I believe this inward positioning of the balun was intended to accommodate the slim packaging dimensions and to reduce the risk of shipping damages]

You will now have the same basic dimensional and performance characteristics of the original CM4221 antenna design.

 

[300ohm]

RESIZE THE REFLECTOR GRID WIDTH:
The next thing that will need some adjustment is the width of the 24” wide reflector grid. The new CM4221HD reflector grid is 24” wide and the old CM4221 is only 20” wide. To do this, just carefully remove the plastic side retainers from all of those grid rods. Then cut exactly 2” off each of the aluminium grid rods and then reinstall the plastic side retainers.

That one I wouldnt do. Instead I would increase its width to 32 to 44 or so inches to pick up vhf-hi.

 

[ota_canuck]

more directional or more mult-directional?

Does a wider reflector actually make the CM4221 design more directional or more mult-directional? Either way that you choose to go with the reflector width, I have found that the forward gain on the center lobe remains about equal distance, whereas the forward lobes and outer lobes are much broader when using the narrower reflector width. The old 4221 design was more multi-directional for those who desired to receive from various broadcast tower locations without the use of a rotor.

When using the old CM4221 desin, I could get Erie PA WQLN and WSEE at 80-100% without making any antenna aiming adjustments. With the new CM4221, I could only receive either WSEE or WQLN by rotating the antenna back and forth by about 10 degrees, and the best reception I could get on either channel was 60% signal strength. When I modify the new CM4221 antenna as stated, I get the same results as I got with the old CM4221 design.

As far as VHF-Hi, I have found that the old CM4221 design was already pretty good at picking up analog channels VHF-5 through UHF-66. I suppose the new wider reflector design was intended to improve the VHF-Hi at the expense of losing some of the multi-directional qualities and the higher freqs above UHF-50.

 

[300ohm]

The old 4221 design was more multi-directional for those who desired to receive from various broadcast tower locations without the use of a rotor.

I think thats going to depend on which channels.

As far as VHF-Hi, I have found that the old CM4221 design was already pretty good at picking up analog channels VHF-5 through UHF-66. I suppose the new wider reflector design was intended to improve the VHF-Hi at the expense of losing some of the multi-directional qualities and the higher freqs above UHF-50.

Channel 5 ?? You must have very strong vhf signals. Post your TVFool. The 4 bay bowtie doesnt really kick in with really good vhf-hi gain until the reflector width is above 28-32 inches or so.
The difference in vhf hi between the old 20 inch width and the new 24 inch width is pretty small.

 

[ManBOOYA]

‘V’ ELEMENT OBSTRUCTIONS:
There are some cute little 1” plastic covers that hide a portion of the elements at the connection points of each ‘V’ element on each bay. Remove these 8 cosmetic plastic caps and just toss them away, as they actually shorten the overall calculated length of each ‘V’ element by almost 2”.

Being plastic, what affect do these caps have on the V element? (I'm going to try it when I get home)

BALUN TO BOOM CLEARANCE:
The flat bars that the balun box is connected to are too close to the boom, so simply pull the balun assembly outwards and make sure that those flat bars are at least ½” away from the boom. I believe this may have just been an oversight that should have been mentioned within the antenna assembly instruction sheet.
[I believe this inward positioning of the balun was intended to accommodate the slim packaging dimensions and to reduce the risk of shipping damages]

I'll give this one a try too. How much of an Improvement do you see? If I remember correctly the bars go in towards the boom. I wonder what would happen if you flipped the bars over (or replaced them with something else) so the balun would move away from the boom?

 

[stampeder]

Is there a NEC file for the CM4221HD to see how these hardware hacks would model?

 

[300ohm]

There should be soon. Ken Nist has been modeling the new CM stuff, so Im sure he'll post them soon. I stopped doing mine because, #1 I dont have the antenna in front of me and trying to get exact measurements etc for the model over the internet can be trying, heh.

 

[ota_canuck]

stampeder: how these hardware hacks would model

These 4221 hacks have proven to work well for this area. Seeing a model of the modified antennas performance would assure us that this would work in other locations as well where a wider coverage area is desired.

 

[300ohm]

Seeing a model of the modified antennas performance would assure us that this would work in other locations as well where a wider coverage area is desired.

Yep. Since you have the antennas in front of you, simply download 4nec2 for free and get busy, heh.

 

[stampeder]

Exactly, modeling will help tell if even further tweaks to these hardware hacks are possible too.

 

[ota_canuck]

4nec2 modeling

I just downloaded the 4nec2 tools. Now,... if I can figure out how to use it

 

[stampeder]

This is the thread to go to for 4NEC2 modeling:

http://www.digitalhome.ca/forum/showthread.php?t=83772

 

[firechkn]

Any update on this? I wonder if there is any downside to doing these mods.

Since it was easy to do, I removed those plastic clips and raised the balun and it appears I got slightly better results. No instruments to test just my signal meter on the tv.

 

[ota_canuck]

Narrowing the reflector grid by 4" will give your antenna a broader [45 deg.+] coverage area if that is the performance improvement you need. These modifications simply mimic the dimensions of the older 4221 design.

No performance measurement instruments are needed.
Realtime results that suit your needs mean more than any modeling or measuring devices.

 

[firechkn]

I didn't like how distorted the flat bars looked once the balun was raised, the result was that the balun was sticking way out. Can this effect the signal any?

I decided to remove the bars, flatten them, mark and drill new holes. Now the balun is in level with the V elements. If nothing else, it looks better now.

Would narrowing the reflector grid decrease the forward gain any? I like the forward gain now, but would be nice to pickup some side gain without the sacrifice. I don't have an original 4221 to test.

 

[kooguy]

A little challenging for me to visualize the modification...How about a picture showing the final modification?
Thx.

 

[stampeder]

Photos Please?

How about a picture showing the final modification?

Yes, some "Before" and "After" photos of this CM4221HD hardware modification would be nice.

How To Put Photos in DHC posts:

http://www.digitalhome.ca/forum/showthread.php?t=92167​

 

[firechkn]

You asked for it

In this picture you can see the plastic clip on the right and on the left removed:

This shot shows the flat bar mounted with new holes. You can see the old holes in the shot:

In this last shot I managed to flatten the bar and straighten it out otherwise the balun would be sticking way out, whereas from factory the balun is tight against the boom:

These mods are real easy to do and revert just in case. I didn't touch the reflector yet.

Sorry I don't have any before shots, but you get a good idea of what's happening.

 

[ota_canuck]

Good pics firechkn !

You've got the balun situation under control. Good job...

The old 4221 antenna came with instructions to make sure that the feed lines are at least 1/2 inch apart, to ensure that there is no cross resonance between the phasing lines. Cross resonance [or electo-magnetic shorting] between these phase lines can seriously reduce the overall gain of the antenna. On the new 4221HD design these phase lines come out of the box with less then 1/2" spacing from the mounting boom which bridges electro-magnetic cross resonance between the feed lines. I have notified Channel Master about the oversight in their instruction pamphlet.

The removal of those cosmetic plastic caps simply expose the full 8" length of the 'V' elements. [a little forward lobe gain].


The shortening of the width of the reflector grid to 20" expands the field of the side lobes. [ side lobe coverage gain]

I use this modified antenna as a secondary antenna that is specifically aimed at Erie PA, which is about 55 miles away from Dunnville. Before the modifications, the Erie stations are at about 30-40% signal, after the modifications they increase to 80%+.

I also noticed that the added side gain with the narrower reflector grid assists the side reception of the main antenna by increasing the signals coming from Buffalo to 100% from 80%. The main antenna is actually pointed [35 degrees north of Buffalo] at the CN tower.

The proof is not always proven by theory, but it is always proven by the tweeks of practical results.


NOTE; the main antenna on my mast is a combined VHF 7 to UHF 69 HBU22 that is mounted 29" above the 4221HD.

 

[300ohm]

The proof is not always proven by theory, but it is always proven by the tweeks of practical results.

Keep in mind the bowtie was invented in 1898 (and was re-invented 3 or 4 times since then, heh) and youre observing a very very old theory at work. Which is, when you reduce the forward gain of an antenna (by making the reflector smaller, for example) you increase the beamwidth of the radiation pattern. Which in the DTV age, can make aiming the antenna less critical and gives wider coverage. With digital, unlike analog, once youre comfortably past a certain threshold level, more gain can become irrelevant.

On the flip side, by reducing the width of the reflectors on the bowtie instead of increasing them to around 32 inches or more, you also give up vhf-hi performance, which in a lot of cases will be needed.