DiSEqC switches, watch your back!

[jgbayless]

I use a box the came with a used primestar dish I was givin. It was hooked to side of the house, ground and all. Has about 8 points of entry for wires at the bottom and screw holes inside for at least 2 4-way switches

 

[DC061668]

Switch

I came home today to no signal.. (at all) Turns out my DiSEqC switch died. all 4 ports, dead to the world. The back of the case was slightly dented, letting moisture inside and corroding all the internal stuff.

The dent pulled the seal away slightly so the water could get in. The switch is in the dark, so it never got any sun to cook the water back out.

I think it is a poor design. It should be filled with potting. I was handling
mine when I grabbed it to tighten the cable the plate dented and I think it
shorted out. Either potting or the edge needs to be soldered....DC

 

[123tim]

I am going to "deback" one of the spares I ordered today and fill it with 2 part epoxy.

I don't think it'll affect it's signal handling capabilities much.

SatAdam,

Once again, I hope that I don't offend by stating that this might not be for the best (if you don't have extra switches). Don't get me wrong - I'm all for experimentation and for finding things out, but I think that there might be a good chance that you'll detune your brand new switch. However if you have a couple and don't mind loosing one, I'd say go for it. As for what I'm going to say, I checked with a forum member whom I greatly respect, just to make certain that what I said was on track.

At microwave frequencies (and probably others) just about everything acts as tuning...the distance of the cover, the proximity of the case, the thickness of the circuit board, etc. I've seen amplifier circuits that were actually tuned by using thin solder spikes on the RF traces. Even the angle of the spike would change the signal characteristics.

A much better explanation was given to me in these words:

"...The traces between the ports on the circuit board inside a DiSEqC switch are meant to have the same characteristic impedance as the coax cable they connect to (RG-6, RG59 = 75ohms). The traces are printed with a specific width which is related to the thickness (distance to ground plane on bottom) and type of material of the circuit board. An assumption is made is that the top of the board is exposed to air which has different characteristics (the most important being dielectric constant) than compounds like epoxy.

If you pot the circuit board with something like epoxy you will change the dielectric constant that the trace "sees" and then it won't be a 75 ohm trace anymore.

The net effect will be some overall signal loss as well as possible frequency specific losses due to impedance mismatches."

I was thinking that the circuit would be totally detuned, but that was probably a little extreme:
"....In reality the whole consumer level satellite/cable TV setup using cheap cable, cheap F connectors, and cheap parts is a real RF mess anyway, kind of like a leaky hose, and so you may not notice unless your signals are marginal to start with - possibly weak transponders will suffer. "

I'd like anyone who might be thinking of doing this to keep this in mind. Make certain that you have a backup just in case. I'd also be interested in hearing how the experiment goes. Maybe I'll be proven wrong.

Just one other thought. Condensation occurs from warm humid air coming in contact with cooler surroundings. My suggestion (for what it's worth) would be to keep the switch enclosed in a light colored (white?) box with adequate airflow to limit sudden temperature extremes.

Please forgive the length of this post.

 

[dish_in_the_sky]

Yikes!

Epoxy and microwaves don't go well together. Past 0.5-1 GHz, Teflon, Ceramic and Rodgers (corp.) microwave substrates are used because epoxy gets too lossy. It has a dielectric constant of ~4.5, compare to air at 1.0003; this means all capacitances increase by 4.5 X if you totally fill the box.

Potting the switch in epoxy will therefore increase mismatch and through losses, and reduce port-port isolation. How much I'm not sure, but I think it will be noticeable.

Fortunately you're only switching 2 GHz, not 12 GHz. If it was 12 GHz, you would probably lose a lot of signal!

I agree that the weak transponders are likely to suffer the most. A line amp on the LNB side of the switch might mitigate the effect, provided it doesn't saturate with a strong signal. Better still, solder it shut and pot it on the OUTSIDE of the box (of course leave the connectors exposed!)