[stampeder]

Several recent posts are now in the Antenna Chart thread: http://www.digitalhome.ca/forum/showthread.php?t=97121

 

[ChannelSeeker]

CM 4/8 bay antenna whisker screws

For another small and easy "mod" to perform right out of the box, check out my post #415 in this thread.

 

[Jorgek]

I guess this means a trip to the roof come spring and check all the bolts on the 4221 HD.

 

[stampeder]

Take along a tube of Loc-Tite too when you go up there for keeping them from getting loose again...

 

[Jorgek]

Agreed. I find contact cement works just as well loc tite.

 

[ChannelSeeker]

An important observation when combining with CM4228:

Just an observation from a guy who installed two combined CM4221 + CM4228 combos with stock baluns: for best results, DO NOT "FLIP" the balun on the CM4221! This will make the antennas out of phase and, even though they are pointed at different towers in my case, will result in some evident signal cancellation (observed in both cases).

Rather than flipping the balun, unscrew the metal bracket holding it from the whiskers, bend the aluminum bracket the exact opposite way from factory and reassemble it back onto the antenna. Now the configuration (phase) is still factory, except that the balun faces out the front of the antenna (instead of inwards) and has the required clearance, as per the mod requirement.

I saw a clear difference before and after doing said correction of my initial mod of just "flipping" the balun on the 4221's. In one of the locations where signal from the US tower was weak (the 4228 was my designated US antenna in both cases), signal cancellation was so strong that the signal on almost all US channels would flicker a few times a second and never stabilize. Putting the 4221's balun back in factory phase cured the problem instantly, even though the 4221 wasn't the one pointed at the US (it still gets a fair amount of the US signal though from the side... and can either add it or cancel it, depending on the phasing of the antenna, it seemed).

All that to say that both the 4221 and 4228 appear to have the same factory phase out of the box, so keep that in mind if you're connecting both of them with a reverse splitter/combiner. I tested two identical setups (2 x 4221 and 2 x 4228) in different locations and got the same result: if you combine them, keeping the 4221's balun connections stock is a must.

 

[AL B BACH]

CM4221HD Balun leads corrosion

While checking out my 4221HD before winter I discovered slight rusty corrosion on the balun leads.

I did the cap removal, balun flip and 20" reflector in 2009 at a different location. I've had it up here for a year. I had recently seen the balun spacer hack but didn't have all the parts, so I flattened the phase bars then used needle nose pliers to bend a 1/2" U to get the connection centered. Close but no.

ANT____ _______ANT
\_/__
BALUN

Should I try to open the balun box to inspect? Should I just replace the balun? I have zinc bolts/nuts or aluminum pop rivets to do a spacer now.

 

[Jorgek]

The best thing you can do for the 4221HD is to replace the factory balun. It attenuates VHF reception.

The other mistake is cutting the reflector. This mod also greatly reduces VHF reception.

I replaced the factory balun with a cheap $2.00 one as pictured. Not only did the VHF reception improve but so did the UHF performance!

 

[holl_ands]

For OUTDOOR use, you should be using an OUTDOOR Balun (Channel Master, Philips are lowest Loss), which is intentionally designed to withstand the ELEMENTS....and also has DC Continuity from the Active Antenna Elements thru to the Coax Cable, so that Static Electricity Buildup can be safely discharged via the Ground Block. BTW: This also allows Electrical Charge on the Elements to be near same Electrical Potential as the (hopefully grounded) Mast and Reflector Assembly, so the Active Elements are less likely to ATTRACT rare Lightning Hits.

BTW: Static Electricity Buildup is due to WIND blowing across the Elements (same as your feet shuffling across a Carpet)....hence more of problem in high wind conditions.

 

[AL B BACH]

Jorgek,

Another hack/mod I'm late on. Amazon has a CM 49999 High Performance Balum but it's out of stock till the end of the month. Guess I'll do some research on baluns.

The reflector mod actually helped at the first location but here I regret it. The towers I want are only spread 27*. My VHF are RF 11 + 13. Now they are reliable since I added 4 tube reflectors behind the bays. I'm trying out different lengths 29-34 inches. 29 and 29 1/4" gave me the best results so far but I recently read they should be 2-3" longer than the 1/2 wave length. IDK ?

My diagram didn't show correctly. I'd rather not bend the phase bars any more. Does the shape, length or position matter as long as it's at least 1/2" from the boom?

Thanks

 

[AL B BACH]

holl_ands,

Didn't see any outdoor Phillips on Amazon so I'll wait on the CM. Grounded, yep. The coax runs to a ground block just below the J-mount which is wired to the block then down to a 4' copper grounding rod.

Thanks

 

[Jorgek]

Channel Master did a good job when they designed the 4221HD. It is not an easy task to make an antenna with perfect gain across the entire UHF TV band.
Too many hackers think they can do better than CM. My experience with the 4221HD and the hacks that many profess are worthwhile is a myth.
Removing the black plastic covers was NOT a WOW moment.
Flipping the balun assembly again was NOT a WOW moment.
Cutting the reflector was a disaster! At least for me.

I don't care what the computer modeling projects. Its a lab environment which is perfect. Like comparing to an isotropic standard. Certainly not reality.

I did extensive real world testing up on my roof. That is the acid test.

I suspect the proponents of these hacks are mis interpreting their results. I certainly would not want to admit that I messed up my antenna!

The only real WOW moment was replacing the OEM CM balun. I immediately saw results!

The other important criteria for real world antenna testing is not to do it in the spring, summer, or fall. Atmospheric conditions are up and down like a yoyo and will actually skew results by the minute.

And just to leave you on another positive note, grounding your mast and antenna is actually making it into a lightening rod! However it is necessary and a building code requirement to protect you equipment. But, a direct strike will probably destroy part of your house regardless of all the grounding efforts.

Did I just open up a can of worms?

 

[HRE]

CM4221HD Modifications Measured

I was curious how the new CM4221HD compared to the original CM4221, but didn't have the urge to make any tests until ota_canuck started this thread. I bought a 4221HD and made some measurements with my Sadelco DisplayMax 5000 signal level meter (SLM).

TEST CONDITIONS:
OTA signals are constantly changing in strength, so I knew that I wouldn't get reliable readings if I made measurements before and after modifications. And, I don't have a constant level output signal generator to make antenna range tests like mclapp, so I decided to make rapid A/B comparisons between my 4221 as a standard antenna and the 4221HD as the test antenna. I found two suitable open areas for field testing, one was a large shopping center parking lot with the tree line about 200 yards away, the other was a clear LOS shot across water to the transmitters. I tried both, and the path across water had more stable signals (except when a boat went by!). What would be ideal is to set up on a flat roof of a tall building with LOS, but I don't have access to that here.

I mounted both antennas on the roof rack of my car; see the attachment.

The tests were as follows:
1. Compared the stock CM4221HD to the stock CM4221
2. Compared the CM4221HD with caps removed to the CM4221
3. Compared the CM4221HD with the phasing line strips flipped to the CM4221, without removing the balun box, which moved the strips away from the boom and put the balun box on the rear side of the strips

After looking at the numbers for test No. 3, I saw that the high channels weren't doing very well. I examined how the balun box was attached to the strips and noticed that although the rivets that attach the balun to the strips were centered between the two inner bays (point A), the effective electrical connection was not centered (point B). This meant that the two inner bays were not being fed in phase, and therefore all four bays were out of phase, which would reduce the gain.

I added nylon spacers between the strips and the lugs on the balun box using the 8-32 hardware that I had, moving the balun to the front. To duplicate the mfg method, you could use metal spacers and long aluminum pop rivets. If you decide to use the spacers, flat strips would probably be OK, but I haven't made measurements. Inserting a thin insulating strip between the two wouldn't be sufficient, because it would act like a coupling capacitor at UHF:

4. Compared 4221HD with spacers to the CM4221

Here are the test results in table form. I have converted the measurements so that they all have the same value as the first test for the reference antenna while retaining the differences between the reference antenna and the test antenna to emphasize the gain (or loss) after each modification. This allows all tests to be combined, hence the notation "relative dBmV" in the chart. The channel numbers are the real RF channels, not virtual:

RF  Stock  Stock  dB Gain  No Caps Gain   Flip Bal Gain    Spacers Gain
CH  4221   4221HD  over    4221HD   dB     4221HD   dB     4221HD   dB
    dBmV    dBmV   4221     dBmV            dBmV            dBmV

16  14.3    15.1   +0.8     16.0   +1.7     17.1   +2.8     18.4   +4.1
29  17.0    17.2   +0.2     18.4   +1.4     18.9   +1.9     18.9   +1.9
31  17.2    16.3   -0.9     18.2   +1.0     19.5   +2.3     18.7   +1.5
33  17.4    16.9   -0.5     19.0   +1.6     19.3   +1.9     19.0   +1.6
40  15.9    16.0   +0.1     16.3   +0.4     16.4   +0.5     17.6   +1.7
46  11.7    12.0   +0.3     12.2   +0.5     13.2   +1.5     15.4   +3.7
50  17.6    18.7   +1.1     19.2   +1.6     17.6   +0.0     19.4   +1.8

And as a chart:

CONCLUSIONS:
My measurements confirm the claims by ota_canuck for the caps and the balun flip. I really can't explain the cap results, because they should be transparent to RF, unless the plastic contains a metallic compound as a UV stabilizer. The balun flip, of course, is obvious because it moves the strips away from the boom.

I didn't do anything to the reflector, because it would be hard to reverse that modification. As already mentioned, reducing its width will broaden the horizontal beamwidth which will allow the antenna to be aimed between two transmitter azimuths if you don't want to rotate your antenna. There is another possible reason that it might help certain channels. There are two types of reflectors, one is resonant (as in a yagi) and the other is non-resonant like a wire mesh screen. For certain channels the reflector rods of the CM4221HD can be resonant at 3/2 wavelength which would put the voltage null at the center vertical support, making the rod seem as if it were insulated from any support.

Because accurate antenna measurements are difficult to make even with a professional test range, I can't make any claim about a certain amount of gain for each improvement. I am, however, confident that CM made two modifications to their new design that compromised its performance: pushing the phasing line strips back so that it would fit in the box was a bad idea, as was attaching the balun box electrically off-center to make it easier to connect the coax.

Even though it is possible to make signal strength measurements to compare antennas, the piece of equipment that determines which is ultimately better is the tuner. Tuners vary in their response not only to signal strength but also signal quality, which is why I like to make measurements of sensitivity and margin-to-dropout at the "cliff" using an attenuator.

It looks like the spacers are at point "A" my question is on the elevated balun how far forward is the distance to "B" ?

 

[Jorgek]

I also flattened the balun rails and cut off the excess rail length. I then removed the CM OEM balun and replaced it with a pigtail type as pictured in previous post. I then mounted the new balun half way between the inner 2 sets of wiskers.
Immediate improvement was I could now receive CFTO rf channel 9 at that time of the side of the antenna. Its since moved to rf 8 and I had to install a home made folded dipole to receive rf 8 reliably. That was the only hack for the 4221HD that mabe a visible reception improvement.

 

[rabbit73]

It looks like the spacers are at point "A" my question is on the elevated balun how far forward is the distance to "B" ?

I never needed to measure that distance, and I no longer have that antenna. The metal tabs are fastened to the balun at B. The holes at the other end of the tabs connect to the phasing line strips at A.

I flipped the phasing lines and raised the balun on spacers at A.

Point A is the mid point between the two inner bays, so that is where the balun must be connected. But, the two metal tabs that connect point B on the balun to point A on the phasing lines are in direct contact with the phasing line strips, so the balun is actually connected to the strips at B in the stock antenna. Raising the balun on spacers assures that the balun is connected to the mid point.

It is not necessary to drill any new holes unless you flatten the strips.

 

[rabbit73]

I also flattened the balun rails and cut off the excess rail length. I then removed the CM OEM balun and replaced it with a pigtail type as pictured in previous post. I then mounted the new balun half way between the inner 2 sets of wiskers.
Immediate improvement was I could now receive CFTO rf channel 9 at that time of the side of the antenna. Its since moved to rf 8 and I had to install a home made folded dipole to receive rf 8 reliably. That was the only hack for the 4221HD that mabe a visible reception improvement.

Good idea to improve VHF-High reception with that antenna. The PCB balun for the CM4221HD passes UHF quite well, but passes VHF-High signals poorly. But, that is still a compromise solution because the CM4221HD is designed for UHF. It will work if your VHF-High signals are strong.

 

[HRE]

View attachment 12226

View attachment 12227

View attachment 12228

I never needed to measure that distance, and I no longer have that antenna. The metal tabs are fastened to the balun at B. The holes at the other end of the tabs connect to the phasing line strips at A.

I flipped the phasing lines and raised the balun on spacers at A.

Point A is the mid point between the two inner bays, so that is where the balun must be connected. But, the two metal tabs that connect point B on the balun to point A on the phasing lines are in direct contact with the phasing line strips, so the balun is actually connected to the strips at B in the stock antenna. Raising the balun on spacers assures that the balun is connected to the mid point.

It is not necessary to drill any new holes unless you flatten the strips.

Thank You ! Thank You ! Great Work ! My only fear with the raised Balun on Spacers it leaves it subject to moving in the violet winds here in NH. So I need to support it underneath. Any thoughts on that ? I planned on using Hypact UMVHD which is RF Transparent will I be creating a UHF Capacitor to Ground ?

 

[rabbit73]

Thank You ! Thank You ! Great Work ! My only fear with the raised Balun on Spacers it leaves it subject to moving in the violet winds here in NH. So I need to support it underneath. Any thoughts on that ? I planned on using Hypact UMVHD which is RF Transparent will I be creating a UHF Capacitor to Ground ?

I'm not exactly sure what it would look like, but if you feel that the mechanical support is necessary at your location, I think you should add it. It's difficult for me to predict how it might affect the performance of the antenna. Just make sure the connector on the balun faces down.

Keep in mind when you are making comparisons that OTA signals constantly change in strength. I used this type of switch to make a rapid comparison between the two antennas:

and this meter that gave an immediate reading at center channel:

I have been experimenting with SDRs to find a less expensive way to measure signal strength. An SDRplay RSP1A SDR ($125US) with SDRuno software can give a reading that is just as accurate as a signal level meter, is less expensive than a signal level meter, and allows you to see what the signal looks like:

Justification for bandwidth correction factor (verified with signal level meters):

Video (I use VLC Media Player)
Watch the calibrated scale dBm reading change as the signal strength changes.
Download and view now:
2021-09-19 15-28-28CH16ampSS.mp4

Download now, view later:
https://www.dropbox.com/s/ww24ykp44fbl1p8/2021-09-19 15-28-28CH16ampSS.mp4?dl=1

 

[Yurii Pylypenko]

I have been experimenting with SDRs to find a less expensive way to measure signal strength

less expensive way is using $60...$100 meter (GI TerrF, Amiko Multitracker 2 and many others)
here is my TerrF for $70

 

[coffeecastle]

I have another alternative to measure signal strength an old cable modem. In my case an Arris SB6183 just check 192.168.100.1:8080. I have found some additional info here Cable Modem Spectrum Analyzer – KC9UMR if interested.