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).
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:
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.