Canadian TV, Computing and Home Theatre Forums banner
1 - 20 of 47 Posts

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #1 ·
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.
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #3 ·
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.
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #14 ·
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.
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #19 · (Edited)
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.
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #21 ·
In my case for this viewing area reducing the width of the reflectors on the bowtie reduces the need for a rotor. Rotorless OTA reception seems to be the most desirable benefit that my customers request. Hence my quest for the omni-directional antenna array development.

Reducing the reflector width on the 4221HD for some unknown reasons has actually improved the actual forward gain performance while also expanding the range of the side lobes. I agree with what the theory states, however the Erie signals improved significantly from 30% to more than 80% when the grid width was reduced.

In general terms, the 4221 is a UHF antenna. It could also be that the removal of those cosmetic plastic caps increased the 'V' element lengths to actually improve the VHF-Hi capability. On the flipside, lenthening of the 'V' elements may clip the upper UHF range, but then again, the UHF 50-69 spectrum won't be needed after the transition anyway. I could be wrong, but I don't believe that the cm4221 design was even considered to be compatable to the VHF spectrum.

With many years of experience working in various fields of product research and development, I learned to live by the '1st rule of achieving success'.​

" Theory is merely a wonderful guide to begin development.
Success is measured by the results "​


My earlier comment was derived from the words of a famous inventor :

" The proof is not always proven by theory, but it is always proven by the successful results. "
William Edwards Deming, [statistician, professor, author, lecturer, and consultant]​
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #27 ·
It may not show a significant signal strength # gain on your display, but you'll notice a gain in the form of stability. Narrowing the reflector did not reduce my forward gain toward Erie, it actually gave me a boost, and it did expand the width of the side lobes for a wider coverage area.

How much improvement depends on what you are trying to achieve and how you are aiming the antenna. Direct aiming for one broadcast tower will add some stability. If your using it to get signals from towers that are spread 45 degrees apart, then you'll notice the signal strength advantages of the modifications.
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #40 ·
Re: Post #34

I may be mistaken, I believe this was possibly answered in another thread, but here goes. :D Channel 29.1/.2 & 49.1/.2 are very directional for southern Ontario's reception. If you aim your antenna specifically for the best reception of channel 49.1/.2,... the other Buffalo channels should also fall into place. It appears that your aim may have skewed somewhat since the before and the after of your antenna modifications. Also it is important to make sure your antenna is aimed level with the horizon or even tilted slightly above the horizon. If your antenna is even slighty sagging toward the ground, you will pick up stray signal reflections from the earth, buildings or water surfaces and multipath becomes your enemy.

Overall it appears that the 4221 hack has worked in your favor, now you should just focus on eliminating line losses and make sure your cable terminations are secure. Possibly a pre-amp would help if you have a long downlead,.... or if you have a shorter downlead cable of less then 40ft, a distribution amp placed inline at the point of your downlead entry into the house may be all that you'll need.
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #44 ·
CM4221HD hack & VHF reception results

Try the first two steps of the 4221HD hack. Remove those pretty plastic caps from the 'V' element connection points to increase the surface exposure of the elements and then pull the balun out slightly from the mounting boom. The proximity of the balun's phasing bars to the mounting boom may be altering the 300 ohm feed between the balun's phasing bars. If your Hi-VHF is coming from a great distance [40miles+], then the CM4221may have simply been a poor choice.

I am 35 miles from Buffalo and I can get analog channels 2, 4, 7, but I can't get analog channels 5,6 or 9 from Toronto being that Toronto is 55miles away. However, if I remove those plastic caps, then I can get analog channel 9 crystal clear.
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #59 ·
gaddah

The 20" reflector has not affected my forward gain, but it has increased my beamwidth. The are some specific requirements that can be found within this forum for stacking identical antennas that are aiming in the same direction.

If the two antennas are placed close together it's an 8 bay antenna. If they are spaced apart, then they are simply two 4 bay antennas. Either way you do it, they have to operate in sync or you'll lose more than you'll gain.

Having the cable lengths the same won't mean much if the spacing of the antennas are wrong or if the antennas do not perform identically. If the antennas are mismatched in any way, you may have created multipath problems that can hamper your tuner's ability to make anything useful out of the signals that your antennas are receiving.
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #117 ·
I am only 20 miles away from the towers compared to 47 miles for zapperman.

I am experiencing multipath so maybe someone has an idea on how I can hack this 4221HD to make it more directional and cut down on that?
It would be unlikely that multipath is a problem with a single antenna. Being 20 miles from the broadcast tower, it is also unlikely that broadcast tower signal is blocking other channels.

Multipath would be caused by using dissimilar antennas that are receiving dual signals that are out of sync or by a mirrored signal bouncing back at your antenna from an object/building/etc. [thus the term multipath].
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #132 ·
Hi,

What about removing the CM balun, did you get better results?

And shortening the screen, will it give me a gain on ch 52 or 41 ?


Thanks,
Note: Your antenna is not the be-all and end-all of your reception capabilitys.

I'm not familiar with your location, however ch 52 or 41 may be too far away, the ERP may be too weak or there may be several other reasons why you can't get them such as co-channel, adjacent channel or multi-path problems relating to your antenna's location.
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #143 ·
Out of the box, the 4221 phase lines may be too close to the boom [bonded ground] which can result in signal reflections etc., and cause cancellation effects. On the older designs of the 4221, the instructions clearly stated to make sure that the phase line gap is maintained at least 1/2". The new antenna instructions omit stating anything at all. It's simply a 'cram-it-the-smaller-box' packaging syndrome to accomodate the manufacturer and to save shipping costs at the expense of the end user's antenna reception performance.

Cutting the reflector width will result in a wider beamwidth, but possibly at the expense of forward gain. For some areas, a wider beamwidth is more desirable than forward gain.
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #149 ·
Steve Smart,

Hmmm, I hope handy hasn't created a phasing headache for you. If you bought 2 antennas, I would assume that you plan on running the 2 antennas together as stacked or ganged.


If handy did it correct, it should look like this :

http://www.digitalhome.ca/forum/showpost.php?p=952715&postcount=95

The only problem with flipping the entire assembly like this is that the antenna's phasing has now been reversed. The main idea for this hack was to make sure the phase lines were not too close to the boom by simply pulling them outward and away from the boom.

Though I suppose as long as both antennas were done the exactly the same way, it should be OK to go ahead and gang or stack them together.
 

·
Registered
Joined
·
2,410 Posts
Discussion Starter · #163 ·
Razors,

On the older 4221's it is possible to repair the broken plastic brackets. I have repaired many that were damaged. If you have all the pieces, just use a thin coat of crazy glue to soften both broken surfaces and then work the surfaces together until the plastic gets noticably soft and then hold the pieces together firmly for about 10 minutes until the softened plastic cools and bonds together. It requires some patience and strong steady fingers.

Otherwise you could simply remove all of the wiskers and plastic mounts and use the post and reflector screen to make a small M4 style antenna using plastic bars or some plastic tubing or you can simply reuse the same CM wiskers and same spacing.

I have one old 4221 that I simply mounted a pair of GH elements onto and it seems to do OK, but the outer tips are slighty longer than the width of the reflector screen.
 
1 - 20 of 47 Posts
Top