Sat dishes vs snow (was "Re: [BC] KFI Tower replacement ?")

[Cowboy]

 All of this is based on "been there, done that."

On Wednesday 04 January 2006 11:01 pm, WFIFeng at aol.com wrote:

>I was thinking more along the lines of placing a small electric heater (the 
>forced-air ceramic type) inside the dish, and just run an extension cord to the 
>building. That is what I meant as being disruptive to the focus. I don't 
>think I want to try that, though.

 The signal footprint in most any dish.....

 Almost none beyond 75-80% of the dish radius.
 So, disrupting the almost nonexistant signal at the edge is
 unlikely to cause a problem unless the dish is already inadequate.

 Dead-center of the dish.
 Again, almost none. Blocked by the feed horn, and since the dish
 is nearly flat at this point, provides almost no gain at all.

 The type of heater you are suggesting, I'd put dead center of the
 dish, as low-profile as possible against the reflector surface, for
 minimal disruption to signal, and maximum distribution of heat.
 Been there, done that, works poorly, but does work.

On Thursday 05 January 2006 12:18 am, Fee Lee wrote:

>There is a heating tape made for the refrigeration industry that may be an
>answer for people who may want to "roll your own" dish deicing system.

 The big problem with heating ANY dish, is the surface area of the dish itself.

 Generally, unless the dish is spun aluminum, the heat transfer is very poor,
 so it takes very large contributions from the heating elements to have much
 affect, or to have the *entire* surface heated.
 This is primarily why heat tape type solutions don't work very well.
 They can work marginally, but the heat tape needs to be on the front surface
 of the dish, and disrupts a surprisingly large amount of signal when arranged
 in this fashion.
 You'll find that the heat tape keeps snow clear about two inches from the heat
 element itself, except in windy conditions, when it's worse, and sometimes
 barely keeps the tape itself clean.
 Heat tape works best on the back of a metallic ( high heat transfer ) dish.
 Unfortunately, metallic dishes get rid of the heat best, too.

 The larger the dish, regardless of the ability to conduct heat to the
 problem areas, the more efficiently it gets rid of any heat put to it, anywhere.
 As a result, to have any affect at all, the heater needs to impart a large number
 of BTU's to the surface, and do it faster than the area of the dish can dissipate
 that heat in a wind.
 Those jet type barn heaters can impart a large number of BTU's to a relatively
 small area, so they work well, but are expensive to run, dangerous, flamable,
 just have a host of associated problems.

 You're putting heat to one side of the dish, but the dish is dissipating off of
 both sides, so you need at *least* twice as much heat as you would think.

 The ideal dish would be fiberglass, radomed to the front, insulated on the back,
 and have integral deicer heat wires laminated into the reflector structure, much
 like the resistance defoggers common on automotive rear windows.
 It would still take an inordinately large amount of power to warm it to an effective
 surface temperature, but should work.
 To the best of my knowledge, no one makes such an animal.

 An almost effective alternative that doesn't need several kilowatts, are simple
 flood lamps, mounted at the focal point of the dish, and aimed at the surface.
 Again, performance is marginal, but there are advantages.
 The heat is radiant, so primarily there isn't any heat until the light is
 absorbed by the surface, whether that surface is the dish itself, or the snow
 and slush on the surface. 
 One might think "heat lamps" would be better, but in fact, they are not
 as good as broad-band, white, unfiltered floods.

 Another problem with any heating solution, is that just like deicers in 
 FM antennae, they don't really solve the problem, but merely lower the
 temperature at which the problem occurs, and depend on on/off management
 to simply move the problem temperature window outside of the current
 ambient range.

 Still, the *most* effective anti-snow device I've ever found ( and trust me, I *did*
 put considerable time into it, including custom engineered and manufactured
 "solutions" ) is still a properly tensioned cover/tarp.

 Cheap to buy, install, and "operate". Maintenance is near zero when tensioned correctly.
 Disposable, and effective in all but THE most severe conditions.
 ( and by that I mean snowing 5 inches / hour at 33 degrees ambient )

 Combined with a hot-air type arrangement, primarily to heat the air between
 the tarp and the dish surface where wind dissipation is minimized, and heat
 requirements are also minimized ( only need to heat the tarp, not everything )
 and augmented by the mechanical shake-off of a flapping waterproof tarp,
 it's unlikely you'll experience problems.

 Unfortunately, this type of solution has practical limits dictated primarily
 by the size of the dish.  
 Tarps work very well to about 4 meters.
 Beyond that, anything is a challenge.

 In Willie's case, it's my opinion that Robert M. has indeed made the best
 and most cost-effective suggestion possible.

-- 
Cowboy

http://cowboys.homeip.net

If I had a plantation in Georgia and a home in Hell, I'd sell the
plantation and go home.
		-- Eugene P. Gallagher