QRP Antenna Tuner with Audible SWR for Blind Hams
This little antenna tuner generates an audible tone that varies with SWR as you adjust the tuner controls. Based on a 4-States QRP Tuner kit designed by NM0S https://4sqrp.com/4stuner.php I added a VCO fed by the reflected power signal that in turn drives a small speaker. Two AAA batteries are required. It's quite sensitive and works reliably from 5 W down to 0.25 W and perhaps lower. In fact tuning by pitch is more accurate than tuning by the LED brightness.
Endfed Half-Wave (EFHW) Antenna Transformer
I built it to help Derek, KC1QXL, who wants to do some SOTA work with a radio that has no tuner. At this time I have no plans to build more but who knows.
After years of work on antenna design, I can't say I'm too much of a fan of end-feds due to transformer losses and a propensity for RF on the coax due to their extreme imbalance. But sometimes it's the only solution, and it's certainly easy to throw one up on any available support for portable operation. So here is my take on a fairly robust 100 W class transformer with a proper common-mode choke.
The requirements are: 100 W capability, high efficiency, a choke on the coax, and both BNC and SO239 connectors. This one also had to be usable by a blind ham so the binding posts had to be in unambiguous locations.
The 49:1 transformer is based on a Fair-Rite 2643251002 mix 43 core and it's wound as an autotransformer with 14 turns tapped at 2 using #16 magnet wire. The high-voltage end of it is insulated with PTFE tubing though this isn't absolutely necessary. A 100 pF silver mica capacitor at the input improves the match. A great deal of data was taken by MM0OPX proving that this is likely the most efficient possible implementation. His video report is here: https://www.youtube.com/watch?v=gNI-668wTJ4
Measured transformer loss is as follows. Running 100 W continuously will still overheat the core unless there is some ventilation, so the enclosure has a whole bunch of holes in it.
7.1 MHz 0.43 dB
14 MHz 0.49 dB
21 MHz 0.49 dB
28 MHz 0.57 dB
The common-mode choke is 15 turns of RG316 on a Fair-Rite 2631801202 mix 31 core. Impedance of this choke is mostly resistive and exceeds 5k over the bands of interest. Perfect.
SWR of the completed unit into an ideal 2450 ohm load was pleasingly low, below 1.4: 1 across all the bands though rising to 2:1 at 30 MHz.
Building Radio Projects (and Other Things...)
It's pretty obvious that I was born to design and build things. Whether it's wood, metal, electronic, software or whatever, I'm all over it. Here is a presentation that is popular with clubs and "makers" in general.