CM4228HD Hardware Hacks

[tvlurker]

    Ch        Orig    Mod   4221HD
    14        -33     -34     -29
    20        -58     -58
    22        -52     -56
    24        -44     -42
    25        -50     -48
    30        -47     -45
    34        -51     -53
    40        -57     -58
    43        -50     -48     -44
    60        -60     -54     -53
    65        -57     -48     -48

techluvr, you don't get CITS channel 32 from Herberts Corners? I know it's the weakest station, so maybe it's buried in the noise on your analyzer.

How do your measured results compare with TVFool Longley-Rice predictions (after accounting for the antenna gain and cable loss?

[300ohm]

It's an Agilent E4402B. It's an 'economy' model; that's why they let me take it.

That would be what I would be looking for, heh.

[Techluvr]

Originally Posted by tvlurker

techluvr, you don't get CITS channel 32 from Herberts Corners? I know it's the weakest station, so maybe it's buried in the noise on your analyzer.

It was more of a procedural error. Ch 32 is the only HC channel between ch 20 & 40. I didn't realize I skipped over it until I had already modded the antenna. I didn't want to go back and un-mod just for one more reading. On the 4221HD it was -50dBm.

How do your measured results compare with TVFool Ongley-Rice predictions (after accounting for the antenna gain and cable loss?

The exercise was to objectively measure the difference between the CM4228HD out-of-the-box and the modified version as suggested by Ken Nist's article, so I haven't done any more detailed analysis of the numbers. I gave up on TV-Fool because the chart doesn't show all the actual channel assignments. Also TV-Fool assumes an antenna in open air, mine is indoors.

[300ohm]

Go up ladder and slowly rotate antenna while reluctant helper (wife) reads out dBm @ marker. Stop when highest figure obtained. Dismiss helper.

You forgot the standard disclaimer : Do not play with antenna when drunk and dont throw antenna at spouse.

[stampeder]

Oh and another piece of advice, typically from grandma: "It's all a lot of fun until someone loses an eye!"

Techluvr, thanks for the real world A-B test - that kind of info is gold!

[tijaune]

Techluvr,

I tried the mod this week-end and while the result are stronger for most channels, I lost my weak fox on uhf 43. I tried with 2 short 3' RG-6 cables, 2 differents splitters (as combiner at the antenna side) and also removed my attenuator (2 way splitter near the LG), can't catch it.

I don't have any tool to make my cable shorter but I know having good result with those 2 cables.

From your posted measures, can we conclude that the upper uhf range got worse?

[ppauper]

so the mod is basically to convert the 4228HD into a pair of 4221HDs ?
Did anyone try this with the original 4228 ?

[Techluvr]

Originally Posted by tijaune

Techluvr,

I tried the mod this week-end and while the result are stronger for most channels, I lost my weak fox on uhf 43. I tried with 2 short 3' RG-6 cables, 2 differents splitters (as combiner at the antenna side) and also removed my attenuator (2 way splitter near the LG), can't catch it.

I don't have any tool to make my cable shorter but I know having good result with those 2 cables.

From your posted measures, can we conclude that the upper uhf range got worse?

Just the opposite. Differences that are +- 1 dB are not significant. With the exception of a 4dB dip at ch 22 and slight dip at ch 34 the numbers for the modified antenna are higher or the same. At ch 60 I measured a 6dB increase and at ch 65 a 9dB increase with the modified antenna.

I use two 6" RG6 cables from the baluns to the combiner. I routed them behind the mast.

I don't know what else to tell you. Antenna design is science, but installing them has an element of magic involved. Every installation will be different. I used this test instrument to optimally aim the antenna and to measure the effect of the modification as it exists in my particular unique installation. Results may vary. Since we don't watch TV on a spectrum analyzer, the final test is whether or not your reception is as good or better with the modified antenna. If there's no difference in the other channels, but you've lost one, then the modification is not right for you.

Also with the CM7777 I found that if I switch to individual UHF/VHF inputs, everything goes haywire. The balancing of the signal with attenuators that I did with the combined input had to be completely re-done when I added the VHF antenna and used the split inputs.

[300ohm]

Antenna design is science,

Nah, nah, nah. There is still some black magic involved, heh.

[stampeder]

Originally Posted by ppauper

so the mod is basically to convert the 4228HD into a pair of 4221HDs ?

Nope, the result might seem like that but the purpose is to replace the existing feed problems. A CM4221HD still has a different feed system than what is created in this CM4228HD mod.

[sunnydays]

Hi Techluvr and all,

Good day to you people,

I am interested in modifying the new CM 4228HD but I have to admit I am not exactly a technically savvy person. I read the instructions posted by Ken but I still have some problems digesting what is written on his page.

Hey Techluvr, care to do me a favor? Can u post more pictures of your modified CM 4228HD and I would try to modify my new antenna according to what is posted on the pictures cos I am not sure what exactly is a phasing harness and how do I replace it with 2 baluns, a combiner plus 2 short cables.

I guess a pic tells a thousand words and I would appreciate if u can help me out with them.

"Fortunately fixing it is trivial: You just replace the phasing harness with two baluns and a combiner. Those plus two short cables will cost you another $25, but a nut-driver is the only tool you need. If the loss in the baluns and combiner is 0.5 dB then the performance will be 0.5 dB less than the B3 plot above"

I hope it is ok with you, thank you so much for that!

regards
sunnydays

[Techluvr]

Originally Posted by sunnydays

I am interested in modifying the new CM 4228HD but I have to admit I am not exactly a technically savvy person. I read the instructions posted by Ken but I still have some problems digesting what is written on his page.

The first thing I want to say is that you should understand what you're trying to achieve before you tackle any project like this. In Mr. Nist's article he predicted an increased gain for the higher number channels by modifying the antenna. In my case, I receive a ch 65 from a broadcast tower about 5Km away, but the reception for some reason was not that great. Modifying the antenna provided a better quality picture for me, and to eliminate any subjective bias on my part, I used the spectrum analyzer and measured a 9dB ( 8 times ) increase in gain at the antenna for that channel. If you have no issues with reception of high channel numbers, I don't know if this modification will do anything for you.

Hey Techluvr, care to do me a favor? Can u post more pictures of your modified CM 4228HD and I would try to modify my new antenna according to what is posted on the pictures cos I am not sure what exactly is a phasing harness and how do I replace it with 2 baluns, a combiner plus 2 short cables.

I guess a pic tells a thousand words and I would appreciate if u can help me out with them.

I did take a few more pictures during the installation...

First you have to remove this thing shown below. There are two plastic caps on each side; pry them off and loosen the hex screws. Remove the rods that lead to the plastic box. There is another plastic do-dad on each side that holds the longer rods in place, just gently pry the rods out of it.



Next, you put a balun on each side. Make sure there's good electrical connection to the vertical rods under the hex screws. The baluns must be hooked up the same way on each side so that they are "in phase". If there's no way to tell which lead is which, you may have to try reversing one if the picture actually gets worse.




Using a splitter in reverse as a combiner, attach 2 equal length short RG-6 cables from the two new baluns to the "outputs" of the splitter. They are really inputs now. Run these cables behind the horizontal rods. The labelled "input" of the splitter is now your output to the TV.

[300ohm]

I receive a ch 65 from a broadcast tower about 5Km away, but the reception for some reason was not that great. Modifying the antenna provided a better quality picture for me, and to eliminate any subjective bias on my part, I used the spectrum analyzer and measured a 9dB ( 8 times ) gain at the antenna for that channel.

Only 9db ? So my observation/theory about dead spots near strong broadcast towers is correct ?

[Techluvr]

Originally Posted by 300ohm

Only 9db ? So my observation/theory about dead spots near strong broadcast towers is correct ?

That's a 9dB difference between the original CM4228HD and the modified version. The actual numbers were -57dBm to -48dBm. The station on ch 65 broadcasts 500kW ERP at 215.4 HAAT.

Yes, it is strange that I don't get this station very well. In contrast, ch 14 broadcasts 595kW ERP at 202.3 HAAT, and I measured ~ -33dBm before and after the mod.

Like you say... black magic, heh

[tvlurker]

That's a 9dB difference between the original CM4228HD and the modified version. The actual numbers were -57dBm to -48dBm. The station on ch 65 broadcasts 500kW ERP at 215.4 HAAT.

Yes, it is strange that I don't get this station very well. In contrast, ch 14 broadcasts 595kW ERP at 202.3 HAAT, and I measured ~ -33dBm before and after the mod.

Like you say... black magic, heh

Perhaps not. It might be that the Mod corrects a SWR problem that occurs at the high end of the band that is not present at the low end of the band.

There is also a possiblity that the CITY-TV-3 ch. 65 antenna has a narrower vertical beam than the OMNI2 ch. 14 antenna. One way to find this out is to see if the original CRTC application documents contain the transmitter outputs, so you can figure out something about the gain of each antenna. It may have an omnidirectional horizontal pattern, but a difference in gains will tell you how sharp the vertical pattern is. (The FCC applications show all this data, so you could figure this out for American stations.)

I'm not sure if TVFool looks at the vertical patterns when calculating ERPs -- I think it migh,t, but of course not for American stations. Also I think the attena pattern info in the American database is only for the horizontal plane. The vertical plane info is used for calculating health and safety limits ( aka Code 6 in Cnaada) for the base of the tower.