Horn drift

[OldskoolPirate]

Been having problems with my rp70 horn drifting off lately. It's running at 7.9v. The transmitter site is only about half a mile away so it shouldn't have any problems. Anyone know why ? Also, no dish on the lnb.

There is also these thick metal plates in between the gunn and the flange. What will happen if I take them out ? Just looks like they limit power.

Thanks

[Albert H]

Can you get a photo of your "thick metal plates"? There are frequently tuning or polariser pieces of metal in the Gunn Diode aperture which will be crucial to getting the right frequency - don't try to modify the device until you're sure about what they're doing!

Incidentally, there's a great design in a late 70's ARRL Microwave Handbook for a transmit PLL for 10 GHz (actually an FLL - frequency-locked loop - as I remember). It uses a mixer to get the output frequency down to a few hundred MHz, where ordinary pre-scalers and logic can be used in the normal way. I'll see if I've got a copy here.

I used to use a "tracking" microwave receiver that would "follow" a drifting transmit end. It used a kind of AFC, deriving a tuning voltage from the IF. The systems would always stay locked together over any range of temperature I ever encountered.

[shuffy]

Since you mention the distance, it sounds like you're not absolutely sure it's the frequency drifting. As you say, with an rp70 you should be absolutely fine over half a mile with a bare LNB and from what I remember the larger C+K's are pretty stable around that voltage, although I don't remember ever using one at any more than 7V. (The smaller ones are a different story, they like 6V max and drift like crazy if you give them more). At short distances with no dish on the LNB, it's not quite so easy to get the alignment perfect (I know that sounds counter intuitive), and if you haven't done, then any slight drift will be more noticeable. If your receiver is free running (i.e. bog standard satcan with just a pot controlling the frequency) then ensure the link frequency aligns with the upper middle of the tuning voltage range (to try and get it on a linear part), re-check your link alignment, maybe drop down the Gunn supply voltage very slightly? That's all I can think of really.

[OldskoolPirate]

Iv tried dropping the voltage down to 7.2v as a lot of people recommend this voltage as the best sound quality. But if I go down past around 7.8 then it sounds all tinny and the high end clips (even when turned right down). When I run it at 8.3v it sounds really nice, but drifts a lot. I can't really win can I ??? I'll take a pic of the plates which I have both removed.

[OldskoolPirate]

[OldskoolPirate]

Also I removed the flange for a higher gain one from a agd horn. All that's left between the diode and the flange now is 1x silver plate (In pic) a block about 1cm thickness and another thin plate from the agd.

[DJ Mikey]

Remember having this 2 years ago when my horn was outside when ever sunlight hit it, it drifted out of link, i put it down to a faulty horn where i reckon it got wet once and bought a new 1, it done my nut in for ages, but 100% horn related.

[teckniqs]

Can we see a picture of the gunn? The small C&K is designed for 5v so at 7v+ they drift like mad if that's the type you have?
Try lowering your voltage to around 5 to 5v5.

[Tec1 87.5fm London]

It's the heat sun light on it put it in a box don't get hot and sand bag on it

[Albert H]

My outdoor Gunn Horns were always fitted into a white plastic box, with a plastic visor to keep the sun out of the aperture. They had camera mounts on the undersides so that they could be attached to a pole, a tripod, or any other camera-compatible mounting.

[thewisepranker]

The third plate from the left (the one next to the horn) is the iris. It is used to form the cavity, thus making it the correct size, and helps to stabilise the oscillator, as well as increasing stability where near field effects would otherwise make it unstable or change the frequency, such as a strategically placed roof tile or even a bin bag.

I think the two other bits are used before the iris allow a certain amount of tuning by changing the physical cavity depth. They may also be polarising filters.