Projects

After serving me for many years, my good old keyer, microham mkII stopped keying last night.
Of course this happens at the least convenient time, during a contest.
I got an idea of where the fault lies, but if any of you guys has a schematic if the MKII lying around, I would VY much like a copy.
73 de LA6TPA
Geir

Failed Microham MKII

Got a whisper from a friend about his cheap single-board computer.

Have now two of these running with RTL-SDR receivers, decoding ads-b.
Works very good, much cheaper than original pi’s

 

armbian linux

$10 Orange pi zero.

opdate 08.07.2017

There have been some reports of some instability regarding the wifi on these devices, but I have been running mine 42 days without any incident.

I have now replaced the single 120mm 6000 rpm fan with twin 80mm 13000 rpm fans.
You cannot see them in this video clip, but you can certainly hear them.
The twin fan setup is a lot more efficient then the single fan. Full load temperature dropped somewhere around 7 deg. C, and that’s a lot. Also; it may be a bit hard to see on my messy work bench, but I have now modified the OP-amp circuits on the LED Audio VU meters, and they now shows forward and reflected power.(they are on the desk just in front of the pa-deck)
By the way, here finishing a QSO via the moon with DK4RC.

Here is a Saturday morning update report regarding my rf switch project.
Back to the good old drawing board, I guess.
Switch are switching ok, and loses are acceptable, this is fine.
However, it is picking up ambient noise. I suspect its being a shielding problem and an isolation problem between RF parts and DC parts of the relays.
To give you an idea of the rx degradation with this thing is in line compared to the narda 023.
Pointing the 144 array to a relatively cold spot, the wsjt noise floor rises about 4dB, and this is simply not acceptable. This thing is useless for weak signal work until these problems are rectified.
I have now installed a narda 023 again.
Hoping to do some EME contacts again this weekend. See you on the moon.

Morning report: Saturday 25.03.2017, still messing around with my quest for low cost high power rf-switch. I have cut and soldered a lot-lot-lot-lot-lot of coaxes since last post and….. Well, now it is looking really good, I have exchanged all the small diameter coaxes with larger ones, and there are now no heat loss I can feel with my fingers!
Operator is a bit too eager going on with the testing here, not checking all connection beforehand, resulting in the SSPA safety system trigging several times before it get going. (shame on me)
Here are a some pix & unabridged videos:

Been quite busy experimenting with this rf-switch the last couple of days.

There has arrived a lot on and off-list comments regarding rf-switches and I have not had the time to answer all, but thanks.

I am now up in what I call revision 4 switch.

What’s new:
Revised schematics for better RX/bypass-port isolation.
New 10 amps relays.
Have done a lot of λ/4 coax tuning.
TX loss is slightly better.
RX loss is slightly worse.
RX isolation is much better.

What needs to be done:
Switch works but the 75 ohm rg-179 stubs are getting way to hot when passing the Kw level.
Need to find something better.

Revised schematics for better RX/bypass-port isolation.

I have got some questions about the home brew rf-switch or relay in my previous post.
To clarify, I got this idea from Leif, sm5bsz web site.
He has done this with PIN-diodes. I don’t have suitable diodes on stock here so I substituted those with small relays. Problem is that lo-Z current at the relays are way to large, and I got to do something with that. Upgrading to larger relays would only make the Q higher and move the problem over to the coax subs I guess. I think. What is needed, is to lower the Q, maybe by adding a small resistor at the relays, say for example 0.1 ohm. (at the point of writing this, I must emphasize, I have not done the math yet.) But I have a feeling it my work. We will see………  🙂 

UPDATE: I have now done the math, and installing a series resistor at the lo-Z point will not lower the rf-current. It will only increase insertion loss. RF-current is about 4,2 amps, and my small signal relays are rated 2 amps @ 30VDC, Guess I got to find some larger ones.

Simplified schematic