Building a UHF Parabolic Reflector Like A CM4251?

[300ohm]

If someone wants a specific measurement of the CM4251, could try to get it.

Get all the measurements you can. Keep in mind, to model it, you would have to get all the measurements just like you would to build a clone of it. Thats actually quite a few measurements, heh.

I would be interested specifically in the bowtie measurements, their dimensions and feedpoint gaps and the distance between the two feedpoint gaps.

[tripelo]

300ohm - Get all the measurements ..., to model it, ...all the measurements just like you would to build a clone of it. ... quite a few measurements, heh.

Yes, it would be several measurements.

I would like to provide every pertinent measurement to specify this antenna, but it may not be practical unless the antenna is taken to the ground.

I will be working with minimal tools and do not anticipate removing the antenna and taking it to the ground. If something unexpected were to occur and it were feasible to take it down, then I could try to get all such measurements.

300ohm - I would be interested specifically in the bowtie measurements, their dimensions and feedpoint gaps and the distance between the two feedpoint gaps.

Yes, these are unique to Channel Master's design.

I realize that the following photos are not quantitative, they are sketches from the CM installation pamphlet. They may provide some insight that may not be apparent in the photo.

Channel Master 4251 Installation manual (page2):



Step 5 from the above image:



Step 6 from the above image:




I wonder if anyone reading this knows of 3D software that could derive accurate dimensional parameters from a set of photos (that included a measure standard in the image itself).

Probably, if there were good reason to do so, I could fasten a tape or steel ruler at a few specific certain locations on the antenna a take a set of photos from a distance.

[stampeder]

Originally Posted by tripelo

I could fasten a tape or steel ruler at a few specific certain locations on the antenna a take a set of photos from a distance

Actually a common-sized beer can would help to derive relative measurements, but above all be safe when you're up there.

[uhf]

Good Morning,
I have a wineguard 8275 amp and 4251CM antenna. Where can I mount the amp for the lowest loss. I thought about mounting it at the center junction point of the vertical mounting pole and the horizontal antenna pole that has the twin bowties mounted to them. I'm using RG-11. What is the lowest loss outdoor balun for CH 14-51 that I can use? I figured mounting the amp where I stated, would be an electrically "blind spot" since the square screen on the front of the bowties blocks oncoming out of phase signal to the bowties. So the signal is focused/reflected off the Parabola screen to the bowties. I read somwhere that FWD gain on the 8275 was about 22db and noise about 1 db. Is this true? Would a cm 7777 or 7775 be a better/stronger amp to use. I'm looking forward to some "RF" feedback on this my first post.

[stampeder]

Hi uhf, as you can see in this thread I've merged your post into, there are lots of fans of your big old antenna here at this site!

It sounds like you're trying to wring every last drop of gain out of yours, so I'd mount the preamp onto the backside of the reflector dish, right at the focal point. The preamp or any other obstructing piece must never be inside the CM4251's reflecting area.

As for a balun, see this thread for info on how to build your own coax-loop out of RG-59 that will give you the best performance:

Baluns (Brands, Designs, Losses, DIY Loops, etc.)

Use RG-59 for the straight run from the bowties aft of the reflector to where your coax-loop balun is attached, and then from it to the preamp. Don't worry about RG-59's loss rates because the differences in loss between coax grades are almost non-existent in such very short connector lengths. Keep the lengths of coax connections as short as possible, always allowing a bit of flex and length for drip loops but not much more. RG-59 is best for such jumpers because it is much more flexible. RG-11 is terrific for the run from the preamp output downwards from the antenna.

Can you please post your TVFool results here for us? (see Post #10 in the OTA FAQ for step-by-step instructions)

[300ohm]

I read somwhere that FWD gain on the 8275 was about 22db and noise about 1 db. Is this true?

Heres the Winegard specs on the 8275 :
http://www.winegarddirect.com/pdf/spec_AP.pdf

[M.Satterfield]

This might get you some dimensions from the collector located at the focus of the parabola:

http://www.summitsource.com/popup_im...b25a1c315a6f46

HTH,

Mike

[uhf]

Thanks for the replies stampeder and 300 ohm. I found my old wineguard catalog No. 110. On page 26 is the spec sheet for the PA8275. 28db gain and 1db noise on UHF, VHF was 26db gain and 2db noise for the PA-8275. Same figures for the PA-4975 which was UHF only amp. Are these figures correct for this older style wineguard amp and power supply (22VAC) that are both all aluminum and not encased in plastic like the newer models???

Also, I am using a wineguard model CS-775 VHF-UHF separator/combiner. Insertion loss spec on it is -0.3db on page 35. Can I trust these figures??? I know the modern spec sheet shows differently with 2.8 db of noise on UHF.

[uhf]

I noticed on Post 32 step 6 of the channelmaster wiring directions that it says you must dress/loop the 300 ohm upwards. Right now, my balun is underneath taped to the pole w/ the RG-11 attached. Is that ok? The twin lead on the end of the balun does not touch anything and the closest it is to metal is about 2 inches. Why do the say to dress it upward?? Is it refering to 300 ohm use/installation??

[300ohm]

. Right now, my balun is underneath taped to the pole w/ the RG-11 attached. Is that ok?

Is it in such a position that water has a hard time getting into the balun, while an easy time getting out ?
As far as position of the balun wires, you may want to experiment to find the best position. Sometimes that can make a noticible difference, at least on analog it did. With digital, with enough leeway, it may not be worth the effort.

[uhf]

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.

[300ohm]

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.

[uhf]

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

[300ohm]

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.

[tripelo]

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.

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

Previous preamp was a BlonderTongue BT-5124 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.