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MClapping the 4221HD

So far I've done all the hacks except for the balun & reflector mods. On Mclapps' recommendations, I replaced the whiskers. I didn't have enough coat hangers, so I used 16 GA galvanized wire. They are 9 1/2 inches long (the stock whiskers are 8 1/8 " long with a gap of 4 3/4 ") and swept about 2 inches forward. This wire is pretty thin so it's difficult to get every whisker the same. Some ends are slightly longer than the other and the gaps between the edges of the whiskers vary from 4 1/2 to 5 inches. Despite all this, I'm getting a SNR improvement of 2 on average on each channel. I'm even getting Global on it so no need for the rabbit ears any more. SNR is also slowly increasing as the evening approaches. Two channels hover around 18 while most are 24 and above.

Mclapp suggested decreasing the gap to increase lower UHF but that will have to wait as the PVR is set. Reflector mods coming as soon as I can get out to the hardware store.

Thanks Mclapp!!!
 

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I hacked my 4221HD (removed the caps, inversed the balun and spaced it). Right after the mods it was standing on the table on the beam side, nobody was touching it then I hear *crack*.

One of the 4 plastic supports just gave up and one the V shape wing felt. I did care about not over-tightening, even put loctite on the screw... but it was still too much. I was able to rebuild the section with the same configuration. Beware!
 

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Bending out the Balun Bars

Do you need to actually flip the bars supporting the balun box? Is it just as effective to pull the balun box outward an inch (bending the support bars - which are very bendable)?

Nic
 

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Discussion Starter · #287 ·
Yes! Clearance is the only issue with the balun mod. The origin of the balun mod was because the product is carelessly jammed into a narrow box by the manufacturer and the antenna sometimes comes out of the packaging with the phase lines squished too close to the boom. It is a matter of either pulling the phase harness out a bit or flipping the whole phase harness assemby.

This is found in my opening post#1 of this thread:
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]
 

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1. In rabbit73 post #221, it appears the balun is flipped and further bumped out from the boom with a spacer

2. In addition, in the first picture showing the electrical connection at point B, its mentioned this is not the centered point between inner bays. However in the second picture showing the balun flipped, bumped out, and flat bar mounting, the electrical connection (would be point B,from above) still does not APPEAR centered

Is my understanding correct ?
 

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I've done this mod. The rivets that hold the balun to the harness are centered, but that's not the only part of the balun that's making contact. When you drill out the rivets and use the spacers, only the nuts and bolts holding the balun will make contact with the harness, in the centre position.
 

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I believe rabbit said the effective electrical connection is point B in post #221, which is off center.

But you are saying when bumping out the balun, to place the bolt & spacer connection in the same hole where the rivet was (point A in post #221) :confused:


also, did you mod help

thank you
 

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Discussion Starter · #291 ·
Shifting the feed point centering up/down may also have a little impact on impedance, or possibly SWR. In other antenna design applications such as the closed square loop or the slotted antenna designs, shifting the feed point a bit toward the top of the antenna may slightly raise the RF gain trend into the upper end of the UHF spectrum and lowering the feed point may slightly send the RF gain trend slightly toward the lower end of the UHF spectrum. It could also send the vertical beam upwards and/or downward, so if anything should be considered, it would be best to have the feed point higher rather than lower, in order to aim slightly to the sky and avoid sending the vertical beam toward the grass on your neighbours yard. Otherwise, a mere +/-1" shift in the 4221's feed point centering probably would have little if any noticable impact in performance.

PS, anyone is welcome to correct me if I'm wrong:p but the point is that the +/- feed point centering effect
is probably insignificant on an antenna of that size.
 

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It could also send the vertical beam upwards and/or downward, so if anything should be considered, it would be best to have the feed point higher rather than lower,
Ill have to check later, but I think that may be backwards.
 

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Discussion Starter · #293 ·
Possibly? :confused:

With the SH antenna, a shortened upper end [ which relatively made the feed point center higher ] sent the beam aim higher vertically and somewhat out the top of the antenna and it bumped up the UHF freqs toward the high end of the 14-51 spectrum. On a smaller antenna like the SH,.. the feed point center adjustment would have a more noticable effect. but with the size of a 4221, I don't think it would be significant. If someone has a 4221 nec model handy, it could be verified whether or not the +/- feed point centering makes a significant difference.
 

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I believe rabbit said the effective electrical connection is point B in post #221, which is off center.

But you are sayingwhen bumping out the balun, to place the bolt & spacer connection in the same hole where the rivet was (point A in post #221)


also, did you mod help

thank you
Yes, the connection is at point A. I don't know if this part of the mod makes any difference, but I'm happy with the results. At the very least flip the balun, that's an easy mod.
 

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each mod he did added significant increase, especially moving the balun way out, not sure if its recentred though, that was my original point....

EDIT: OK I think I got it now. thanks
 

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Discussion Starter · #297 ·
Balun centering and adding those standoffs

In rabbit73's results it does show a freq shift [leveling effect] and a slight overall improvement, but is it due to the feed point centering or is it that the balun spacers moved the feed point further outward from the face of the wisker elements?
 

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Ill have to check later, but I think that may be backwards.
Yep, like I suspected.

If you move the balun connection point down, closer to the lower two bays, the main lobe pattern points UP and vice versa.

I took a reflectorless CM4221a (not the HD, but it will be the same) and moved the connection point down 2 inches from centered. At 470mhz, the main lobe pointed upwards 10 degrees (when centered it was 0 degrees). At 585mhz, the main lobe pointed upwards only 5 degrees. At 698mhz, it was back to 10 degrees upwards.
Moving the balun connection point down will also produce some minor lobes pointing downward about 30 to 35 degrees.

Hmm, oddly which I didnt suspect, moving the balun connection point down increased the gain at 698mhz from 9.68 RG, 1.83 SWR to 11.1 RG, 1.41 SWR. But thats off center gain.
 

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Heh, yes if the balun connection point is above dead center. But just because the bulk of the black box is one way or another doesnt mean thats the connection point.
 
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