[Techluvr]

I modded my CM4228HD as per Ken Nist's excellent article. I removed the hex screws holding the rods with the plastic balun box. I added a separate balun to each of the two antenna sections and a combiner. Measured with a laptop at the antenna using WinTV's Digital Signal Quality Meter I saw an increase in SNR from 27 to 30. I receive 4 digital stations, all broadcast from Camp Fortune at about 30Km north. The analog VHF Hi stations are also very clear; the VHF lo are a bit fuzzy. I'm about 5Km from Herbert's Corners, the other local broadcast tower; it's to the south. The UHF analogue stations and Ch 11 are watchable, but a little fuzzy. They are being received from the back of the antenna! The whole rig is in the garage rafters on the north side of the house.

I then took a CM4221HD and removed the bowtie assembly. I mounted that to the back of the CM4228HD; I'll call it a CM4229HD. I added another combiner that merges this new antenna section with the output of the first combiner of the modded CM4228HD. I saw no loss in the SNR meter and the picture quality of the VHF-Hi & UHF analog stations in the north direction were all still excellent. But now the UHF and Ch11 from the southerly Herbert's Corners are also very good. There is a slight multi-path ghosting on some of them, but still watchable.

Next, I disconnected the laptop from the antenna and replaced it with the 20 metre cable that leads to the VHF/UHF input of an Eagle Aspen S-4180-GX+ multiswitch. It mixes the VHF/UHF signal with the input from the satellite LNAs and distributes it to TVs and radios around the house. The loss for VHF/UHF through the multi-switch is 5dB. The signal at the TV outlets was pretty degraded. I couldn't get 2 of the digital stations from the north; most of the analog stations were very fuzzy.

Next, I put a CM7777 preamp at the antenna. This seemed to overwhelm the input of the tuner. I added a cheapo splitter between the preamp power supply and the multiswitch as a 4dB attenuator. That seemed to clear things up pretty well.

So, I'll probably leave everything this way until Canada's digital switchover and decide if it's worth the effort of putting this rig outside.

 

[cadsulfide]

Nice work!
Using the splitter as an attenuator is a good idea, I've had to use simular methods for my mini-headend to tone down modulator outputs.

 

[stampeder]

What is the URL to Ken Nist's article that you used?

 

[Techluvr]

What is the URL to Ken Nist's article that you used?

http://www.hdtvprimer.com/ANTENNAS/TemporaryPage.html

Given the name "TemporaryPage" I imagine it will be moved at some time.

 

[stampeder]

Nice hardware hack! This is just what the CM4228HD needed!

 

[tijaune]

Thanks, I will definitively try this. Lately, I just tight wrap the balun inch back and lost uhf 43. It was no fun to examine the balun closer in the attic.

Techluvr,

I guess you have remove the 2 original horizontal rods where the square balun is connected to? I read somewhere that it is impossible to pop open the balun.

 

[Techluvr]

Techluvr,

I guess you have remove the 2 original horizontal rods where the square balun is connected to? I read somewhere that it is impossible to pop open the balun.

Yes, it's very easy to do. It can be removed with just a hex-nut driver. I was careful not to bend the rods in case I had to put it back together. As it turns out I won't need to do that because I think the modified antenna works better. Of course that's just my subjective observation. I'm going to try to back that up with some real numbers this week-end. I'll post my results here.

 

[Techluvr]

Nice work!
Using the splitter as an attenuator is a good idea, I've had to use simular methods for my mini-headend to tone down modulator outputs.

That was just a temporary fix. I bought 2 6dB attenuators from The Source; they're on special for $1.98 ea. This worked even better than the splitter.

 

[300ohm]

I bought 2 6dB attenuators

I prefer the term Signal Discarding Devices, heh heh.

 

[mlord]

Reset Electronics in Ottawa also sells attenuators, which might be handy once Bell finishes their takeover of TSbCC.

cheers

 

[Techluvr]

Modified 4228HD... Some real world numbers

My 'real world' is a CM4228HD mounted in the rafters of the garage. The mast is a 1" copper pipe that I can rotate 360 degrees. I am located between two broadcast towers almost 180 degrees apart. One is north ~30KM; one is south ~ 5KM.

I took two sets of measurements from every UHF channel from both towers; one set with the original CM4228HD and one set with the centre balun replaced by 2 individual baluns (-2.5dB ea) and a combiner (-.5dB). The <10M cable losses and any other incidental connector losses are the same for both setups, so I ignore them. I took an additional set of measurements with a CM4221HD for only the south facing stations.

All numbers are dBm rounded to the nearest whole number.

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

Not fantastic; but for the most part, the modification is an improvement over the original. The 4221HD did just as well if not slightly better. Maybe if I have time, I'll swing the 4221HD north and see how it does on those stations.

 

[Muus]

Techluvr, what are you using to obtain those dBm values?

 

[Techluvr]

Techluvr, what are you using to obtain those dBm values?

A spectrum analyzer that I borrowed from work for the week-end.

 

[300ohm]

You might as well post the make and model number too, heh.

 

[Techluvr]

You might as well post the make and model number too, heh.

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

The procedure ( I'm sure that's the next question )...

Set the frequency to capture a contiguous group of stations from the target broadcast tower. Set scan rate of 500mS with trace mode set to 'erase/write'. Place marker on a center channel. Go up ladder and slowly rotate antenna while reluctant helper (wife) reads out dBm @ marker. Stop when highest figure obtained. Dismiss helper. Set freq span to entire UHF band; trace mode to 'max hold'. Wait for trace to stabilize at max values. Move marker to each peak and check if the frequency is a channel from the target broadcast tower. Record ( paper & pencil ) the dBm reading. Repeat from start with other broadcast tower.

Note: For each direction there was significant signal detected from the stations in the opposite direction. I'm not sure if this is off the back of the antenna or from multipath reflections. I do see ghost images with a large enough displacement that makes me believe it's reflections from distant objects.

 

[tvlurker]

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

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!