[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.
Well, the author used the term "speculation" because you cant actually see the signal bouncing all over the place, and hence cant prove it in every situation. But yeah, the gist of the article was about signals bouncing everywhere with multipath, fading characteristics rapidly varying in all 3-dimensions, and how the stack performs better/more consistantly under typical conditions than a single antenna of equal dbi.
So this would suggest that a vertical stack is generally better than a horizontal stack above your average home?
The vertical stack has the advantage of reducing overhead airplane induced flutter better than a horizontal stack. And I would think vertical stack is better for fading in general (the ground heating up during the day). But of course, there can be situations where the signals cancel each other out, so its location, location, location, determined by experimentation over time.
The horizontal stack should be better for a many types of multipath.
I am wondering how these hacks affect the upper UHF channels as well. Looking to get 58 and am wondering how the 2" off the screen would effect the reception on that channel.
In Post #106 hdCanuck reports an increase in gain on channel 53 after the hardware hack, so that's a good sign that 52 would be equal or better before and after the hack.
mdelleman, check out zapperman's many posts from Chiliwack to see how CBC does on 58.1 as you go further up the Fraser Valley.
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].
I just wanted to let everyone know Iv'e sucessfully hacked and stacked two of the 4221hd's. I started with the standard plastic cover mod, then removed both of the baluns. Both antennas are about 8" apart on the pole, facing the same direction. I made a phasing harness out of 12ga galvanized that is 38.5" which starts at the original 3" spacing and varies out to 4" at 4.75" down the harness. I utilized this length as a I calculated it out to 1.5 wavelengths at 620 Mhz. Since the original design was a theoretical 300 ohm impedance, putting the two in parallel would make a 150 Ohm impedance. I made a 2:1 Coax Balun Centered at 620 Mhz for this purpose to get a net 75 ohm impedance. This allowed me to remove the 2 Baluns and Combiner (Splitter in reverse.) and recoup most of the loss. I will post stats tomorrow as to the improvments made, but I know that the overall SNR ratio has shot up tremendously on a large variety of channels (from 23dB to 29db!)
OK, here are the channels that I was able to capture.. Being the most problematic out of the bunch, this is what I sampled. I know, I know, next time I'll take a full list. Have some pity, I'm just getting started in experimenting with antennas.
Keep in mind the before numbers here are after doing the cap mod and flipping baluns, antennas pointed in the same direction. The afters are with the new feed harness, 2:1 balun and antennas pointed in the same direction (I used reference marks on my mast.)
Code:
Ch Freq SNR before SNR after Diff Distance from TX
20 506mhz 22.9dB 28.3 dB 5.4dB 35.5mi (LOS)
33 584mhz 24.6dB 30 dB (max) 5.4dB 46.0mi (2edge)
38 614mhz 18.5dB 28.1 dB 9.6dB (WOW) 47.7mi (2edge)
39 620mhz 21.6dB 29.5 dB 7.9dB 46.4mi (2edge)
I realized from my calculations that the center freq was 600 on the money for the phasing harness and balun. The top end of the UHF band still seems to be ok as I'm still pulling in strong signals from the backside of the rig.
mogwai... great results... any chance you can post some pictures of the coax ballun and phasing harness installation. I have been considering ditching the stock baluns on my 4221HDs in favour of something more efficient and it would be interesting to see if I can duplicate your results.
Here are my photos, see above for the specs of each. I tried the cm4221hd's balun on the harness today, and lost about 3db across the board. So i'm sticking with my homebrew
those wires are about 80 feet away and about 100 ft in the air.. just for the safety conscious...
I'm going to put a bolt/nut combo with washers at the feedpoint to make sure it stays solid. The next harness I may flatten the wire at the feedpoint and see if I can drill a hole through so the nut/bolt combo can go right through. I'll let everyone know how I make out.
This mod is totally reversible and I'm curious to see if anyone with a better tuner than I have can post some decent signal numbers.
You have a basic problem of a copper to aluminum connection. That will have fairly quick galvanic corrosion over time. Youll have to redo it next year, I would say. Tinning the copper with solder will lessen the effect a lot.
I know about the corrosion factor and will solidify it once I'm all set with the results and mounting. The thought also crossed my mind just to solder it to the harness and be done. 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.
The CM balun is actually pretty good, the only reason I took them out was because i was stacking and didn't want the loss from both of them plus a combiner. You still need a balun! Mine is a simple coax balun, but it works..
From what I've read and modeled, reducing the width of the reflector causes a slightly higher gain on the lower spectrum of channels (14-30 or so) while stealing a bit from the high. It will give you a wider path, so if you have a station that is outside of the front 40 degrees you'll have better luck picking that up. Personally, I'd add more length to them to get more front gain.
From what I've read and modeled, reducing the width of the reflector causes a slightly higher gain on the lower spectrum of channels (14-30 or so) while stealing a bit from the high. It will give you a wider path, so if you have a station that is outside of the front 40 degrees you'll have better luck picking that up. Personally, I'd add more length to them to get more front gain.
'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.
The CM4221HD hack is intended to mimic the original 4221 dimensions and performance. IMO, the original CM4221 perform the best as they stand [un-hacked].
I'm thinking of modifying my reflectorless 4221hd by putting a 4 rod reflector, to boost my forward facing stations without completely removing my strong rear-facing stations.
I read somewhere that 34" or more reflectors would help pick up VHF-Hi which could be useful. Does anyone know if this would negatively effect the UHF stations?
I haven't gotten a chance to model this ( having 2 small kids kinda gets in the way of that )
Can anyone quickly model to see how it would respond?
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