I've got the balun completely weatherproofed with a layer of vulcanizing tape, 3M electrical tape, and brushed-on scotchcoating. I dressed the 300 ohm portion of the balun down in a drip loop so to speak which seemed to make better sense to me than up since I had rg11 attached to the balun. I also painted scotchcoate on the spaded lug end connections of the twin lead to the antenna to seal them. I guess I'll just have to experiment a little with positioning and observe the effect on the gain/signal bar-graph on the display mode on the TV. Thanks for getting back to me 300 ohm. I'm amazed at all the expertise ya'll provide.

I've got the balun completely weatherproofed with a layer of vulcanizing tape, 3M electrical tape, and brushed-on scotchcoating.
OK, it looks like you have the water getting in part well covered, but how about the water getting out part ? Condensation occurs with changes in temperature.

OK, it looks like you have the water getting in part well covered, but how about the water getting out part ? Condensation occurs with changes in temperature.


Thanks 300 ohm,
I've got this thing really well sealed from the outer jacket of rg11 over the Male F-connector, over the Balun completely to the 6" twin lead. I then painted the twin lead all the way up to the antenna wing nut connections attached to with the lugs at the termination end of the twin lead. So this thing is weatherproofed from the rg-11 cablesheath to and through the 300ohm connection at the antenna as shown on Step 6 of post 32. Does it rally matter if the balun with the rg-11 attached is looped upward as shown in step 6 which is actually showing it with 300 ohm twin lead. Or the same type loop downward which is a drip loop "looping" down? Also, is painting a thin, even coating of scotchcoate over the 6" piece of foam type twin lead that comes out of the balun going to affect, attenuate, etc. the signal at all? I most delicately soldered the foam twin lead into the balun in place of the original thin ribbon type. I'm not sure, but I believe I have a Blonder- tongue low loss balun with a low loss figure. Something like .5db on uhf band. Thanks for any info. UHF

Or the same type loop downward which is a drip loop "looping" down? Also, is painting a thin, even coating of scotchcoate over the 6" piece of foam type twin lead that comes out of the balun going to affect, attenuate, etc. the signal at all?
Probably not, but feel free to experiment. Each balun is a little different. Like I said, sometimes moving the balun wires around did make a noticible difference for me on analog. With digital, with enough leeway to the threshold, it may not be worth the effort.


Ken Nist on Balun wire positioning :
Balun wire positioning - If your balun has distinct wires (pigtails) instead of ribbon cable for its 300-ohm connection then read on:



For twin-lead to be 300 ohms the spacing between the wires must be about 6 times the wire diameter (more if there is insulation). If this is not true for the 300-ohm balun’s wires, the effect is equivalent to a small “point capacitance”. If the spacing is less than 6 diameters then the point capacitance will be positive, otherwise it will be negative.



The consequence of the point capacitance is hard to predict. Most likely it will reduce the antenna’s net gain on some channels, but it could actually improve the antenna on other channels. The effect is probably a few tenths of a decibel for UHF, insignificant for VHF.



Here is a rational approach to this dilemma:

1. Judiciously make a list of your must-have UHF channels.

2. Use the receiver to determine the weakest station on the list.

3. Note the signal strength of this station three times:

a. With the balun wires spread wide apart

b. With the balun wires pinched together

c. With the balun wires in a middle position

4. Leave the wires in the best position.

I finished the antenna work, replacing the CM Quantum 1110 with stacked pair YA-1713 antennas (height AGL ~60-61 feet at top of stack).

Reason for replacing the Quantum was that its gain was suspect for Channel 13, the Quantum had three missing floating directors and the antenna/preamp combo was not providing adequate reception for channel 13. Can post some observations and details when I figure out what thread to post them in:)

Relevant Quantum post located here (http://www.digitalhome.ca/forum/showpost.php?p=968508&postcount=69).

Did upgrade the preamp and rework transmission line to the CM4251.

Previous preamp was a BlonderTongue BT-5124 (http://www.blondertongue.com/reception/Reception.pdf) with 20 dB gain and Noise Figure of 3.5 dB at UHF. Was replaced with a Channel Master CM 0264DSB with 23 dB gain and noise figure of 2.2 dB.

The CM4251/BT-5124 was already providing excellent reception for UHF stations located 75 miles away over 2Edge diffraction (TVFool indicated the weakest signal strength at -106.0 dBm), so no significant improvement was noted.

Sorry, Stampeder and 300ohm, but I did not make any measurements on the CM 4251. It may be a possibility on a later trip.

:)Anybody got a gain graph/diagram for the Channel master CM4251 Parabola?
How would this antenna stack up against a gray-hoverman for channels 14-51.

I came across one of these for sale recently (new in box, never used!). At the time, I attempted to locate specs, but couldn't find any.

There are some fan websites devoted to this beast. I'd try contacting these websites to see if this information can be found.

I came across this spreadsheet some time ago. Look under the tab "Specs UHF Only" for the CM4251. Quite the antenna!
http://hdtv.forsandiego.com/messages/4364/DTV_Antennas-2841.xls

Here is a link to a tribute site with more info:
http://www.rocketroberts.com/cm4251/cm4251.htm

Here are the dimensions of the pickup portion of the CM4251. I still have the bow tie part from my first 4251. I can't get up on the roof and measure the dish part on my second one (still in use). look at Rocketroberts page under Richard Gray for the setup as of present.
Bow tie spacing apart: one inch
Middle of rivet to other rivet: 1 1/2 inch
top to bottom spacing (rivets):8 1/16 inch
overall bow tie length: 5 1/2 inch
Spacing to rear screen: 5 1/4 inch
Bow tie at it's widest: 3 1/4 inches
Any other questions, please ask. I will try to answer.
And it does out perform anything I hade tried. It's just a bit of a worry on windy nights. My mount has withstood a 52 mile an hour night. Katrina, on the otherhand turned all the antennas into scrap aluminum.
Was real happy to find a used 4251 and a new VIP-307!

overall bow tie length: 5 1/2 inch
Thats per whisker section, correct ? That is a small bowtie. Basically a 9 1/2 inch whisker section is optimum for the new channel range, but with solid whiskers, it could be shorter, say 8 1/2".

Any other questions, please ask. I will try to answer.
Do you know the size of the bowtie reflector, and the grid size on it ?

the bow tie is a triangle 5 1/4 inch (both log sides) and 3 7/8 inches on the wide end.
The rivet end is rounded to the rivet head size, no real point. Same applies to the outer corners. Real slight rounding of corners. Guess it's to protect fingers.
The screen is 15 1/8 high, 14 inches wide.
The squares on the screen are 1 1/4 inch by5 3/4 in the outside two rows, 2 3/8 inches by 1 1/2 0n the middle row.
Next time I go up on the roof, I will try (can't promise I can get to it) to measure the distance of the reflector to the dish.
Let me know if this makes sense.

Let me know if this makes sense.
Yeah, that makes sense.

What is the gauge and distance apart of the phasing lines (center of wire to center of wire) ?

Also, Im guessing the bowtie is made from 20 gauge metal, correct ?

the phasing lines are 1 3/4 inches on the center of the wires. The wires are around 10 gauge but of aluminum instead of copper.
The phasing lines connect in the middle on half circles at 2 1/4 on rivet center.
The half circles bow out, not in like the connection point of the bow tie.
I would guess 20 gauge aluminum for the bow ties. The aluminum is very strong for it's thickness.
The rivets are about 3/16 inch body diameter.

The screen is 15 1/8 high, 14 inches wide.
The squares on the screen are 1 1/4 inch by5 3/4 in the outside two rows, 2 3/8 inches by 1 1/2 0n the middle row.
Whats the gauge of the screen ?

Next time I go up on the roof, I will try (can't promise I can get to it) to measure the distance of the reflector to the dish.
Also get the depth of the dish. And double check the diameter of the dish. I know they say 7 ft, but that could be +/- 4" in real life, heh.

The screen wire is actually about 3/16 inch thick. Channel master built these to take a good bit of abuse.
I should be getting another in the near future and that should allow me to get more measurements and pictures of the dish.

3/16" is about 5 gauge.

are going to model, or remodel this one for eventual building

Im just modeling it for the heck of it, heh.

The screen is 15 1/8 high, 14 inches wide.
The squares on the screen are 1 1/4 inch by5 3/4 in the outside two rows, 2 3/8 inches by 1 1/2 0n the middle row.

I think you mean 1 1/2 inch by 5 3/4 inch in the outside two rows. In the drawing of the CM4251, there are 11 horizontal wires (10 spaces) on the bowtie reflector.

3D pattern of the CM4251 bowtie section at 645 mhz, the center of the old channel 14 - 69 band, SWR = 1.58. A little better than I expected, especially in the SWR dept. Keep in mind though, a higher gain driven element on a parabola doesnt always translate into higher overall gain on a parabola, as Ive shown in Area 51 :

http://img340.imageshack.us/img340/3060/cm4251bowtie3dpattern.jpg

Raw Gain:
http://img248.imageshack.us/img248/9195/cm4251bowtierawgain.jpg

SWR:
http://img22.imageshack.us/img22/12/cm4251bowtieswr.jpg

NEC file : http://www.wuala.com/300ohm/Documents/CM4251Bowtie.nec

Would increasing the size of the bow ties help since there is no activity above channel 51 around here?