I would like to introduce you to my current project, called Spota One.
The trx is made exclusively for activation and only works on one frequency - for example on the 20m band - with a telescopic antenna like the AX1. Here are the preliminary data.
Frequency 14,058kHz
Power out 5W
Integrated telescopic antenna with adjustable charging coil
Built-in Lipo battery 7.2V
RX 0.06A TX 1.0A
Direct conversation rx
CW filters
AGC
0.5W speaker
adjustable keyer with memory
73 Chris
Here’s a clip about it.
Technical background
More technical background in german
!!!Previous version, look below for the current one!!!
Hi Chris,I hope you success with your project I recently created something similar using the QCX mini 20m.Still waiting for QMX 20-10m, then it will be on 5 band.
Even though this is definitely not my area of expertise - indeed I find electronics to be mostly a black art, “dark magic” and mostly incomprehensible apart from the little I learned to pass the exam - I nonetheless find your videos fascinating and even somewhat comprehensible: and I liked them too!
I’d be interested in hearing more as you develop the idea. Oh, and “Schwingkreis” translates to tuned or resonant circuit, sometimes known as a tank circuit - but you knew that already, right?
Yes, that are right words here. It works as a resonance circuit like a preselector and a tank circuit to transform the 50 Ohms from the antenna to the 1.5k input resistance of the NE612. By the way it has 12dB gain for better signal noise ratio.
73 Chris
A few days ago I had my first qso at a summit with the Spota One to Lars, SA4BLM. He gave me S9 with my EFHW and S5 with the whip antenna. That was expectable.
But I didn’t expect the strong qrm of radio stations, especially with the broadband EFHW. It worked significantly better on the 1.2m long whip.
That’s why I examined the frontend selection more closely and improved it.
EDIT Here the frequency response before and after, the new circuit diagram and the clip only in German.
73 Chris UPDATE
Applaud your concept! Clever design to use the 74hc 240 as oscillator and to control TR. Also, using separate crystals in Rx and Tx eliminates the usual 600Hz Tx offset crystal pulling.
I build something similar I call it 2B2C, dual band but also one crystal on each of 40 and 20m. Works really well and the DC Rx is very sensitive. I’ve worked VK to Eu on 20m several times with a wire on a pole on a Sota outing. Great fun!
I look forward to seeing it boxed up and doing the same thing as a KH-1 (albeit with limitations)! I love carefully designed minimal QRP homebrew!
Together with Uwe, DK8OA, I tested the new circuit design (check the update above) today during a Pota activation. Namely on the 20m long EFHW. There was no QRM from the radio stations, but there was, among other things, a nice qso with Manuel, EA2DT.
Now I will build the circuit again on a breadboard board, as small as possible and then in a housing.
I will use a Amidon toroid T80-6 and a rotary switch for changing the windings.
I you look at the site T80-6 you can check the number of turns to calc uH.
In preliminary tests I measured an inductance of 8uH with the 1.2m whip and 4.5m counterpoise.
These are my planned values.
I asked myself the question of the most suitable antenna for the Spota One. Since I plan to make the case as small as possible, a whip antenna might become impractical.
That’s when I came up with the idea of using a 9m long EFHW, which you can lay on the ground if no other option is possible. Just like the AX1’s counterpoise.
I see your idea there, if you are using an AX-1 or KH-1style 1.2m telescopic whip you need the 4m trailing wire counterpoise. So, why not just dispense with the whip and lay a full EFHW on the ground? Any you’ve demonstrated that it is workable. On receive at least.
On one SOTA activation some years ago, I set up my 40 and 20m linked dipole on a pole and worked a dozen chasers. I went to pack up, dropped the pole so that dipole was lying loosely on the ground, and was called by Peter VK3YE, who was operating portable on the beach. We had a QSO and I got a decent signal report, maybe two to three s-points lower, but good copy both ways. With a dipole on the ground! I wouldnt rely on a ground antenna in all circumstances, but it can work. Of course, if you can hook the end on a tree branch, or lay it over a bush, all the better.
Your 10m EFHW will double as a quarter wave ground plane on 7MHz too, although you will need to lay out a counterpoise (7 to 10m). Just feed it directly, no balun or impedance transformer required, the feedpoint resistance will be lower than 50r but your KX3 tuner will match it.
I still like the short telescopic whip option, as it gives you several bands, 20 to 10m, and the option of 40m with more base loading (although it’s all a big compromise on 40, but probably good enough to make a few CW contacts).
Here comes the update of the circuit diagram with the Fuchs network and the associated SWR diagram for 3.2kOhm impedance
Finally, the spectral analyzer diagram shows the losses. With theoretically lossless transformation 1:8 from 50Ohm to 3.2kOhm it should be -18dB due to the measuring circuit.
The led network has a loss of -0.3db
73 Chris
