I hope you're keeping well and enjoying your radio.
More of my DIY shenanigans shared in the hope of providing information.
(Would love to see others DIY antics too.
As some of you are aware I've been plagued with QRM woes from neighbourly VDSL and judging by the mess coming up my mains Earth a bit of PLT too!
Luckily I've been able to reduce a lot of it by having my 66 foot (20m to some) Doublet antenna
'bent' in opposite directions reducing the hash from S9+ to a bit shy of S5 on the lower bands and all but gone on the higher. Great stuff!
However I've never been all that happy with my Pi_Match ATU
and didn't get on well with my Homebrew Z-Match
either so decided to give the former a bit of well overdue modification.....
Yep, altered the simple 6 way rotary switching I had previously to a 7 switch system to individually select inductances required.
Some of you may recognise this configuration from 'automatic' antenna matching systems but here we have no need for a microcontroller to hopefully do the work but manually selected at will.
With inductances ranging from 0.2uH on the left in this picture to 12.8uH at the right I can select 127 (7 bit binary) inductance values from 0.2uH to 25.4uH in 0.2uH steps. It might look like a pain but infinitely cheaper than the prices people want for corroded old 'roller-coaster' inductors that's for sure!
Now a possible combination of 127 inductance values might seem like a laborious process to switch but there are only seven switches and in all reality I'm only using this on eight bands from 80m to 10m including all the WARC bands. The work in finding the values required for my antenna was made infinitely easier with the use of a NanoVNA (thanks again Alan
) and jotted down for future reference.
Having the ability to switch in 0.2uH steps plus the use of the variable capacitors means I can achieve a 1:1 SWR on all the bands of interest with the original 6 way switch now used to add the extra capacitance on the front end variable capacitor for the lower bands.
Not as difficult to use as it first seems, almost infinitely adjustable which means I can still enjoy the odd SWL session on the broadcast bands. Nicely by manually switching I don't have to "whistle-it-up" like automatic units on the Ham bands (annoying or what!) nor illegally transmit on the SW broadcast bands.
You may have seen similar circuitry used elsewhere by other operators but what confounded me was why some individuals used DPDT (Double Pole Double Throw) switches when all you have to do is short out each inductor individually using a much cheaper SPDT (Single Pole Double Throw)?
Maybe that was what they had to hand, who knows.
The big inductor?
That's a simple 1:1 current balun to help the ATU drive the doublet and out of every experiment I tried with 'balanced' tuners and such this was the simplest solution to the problem. Works well and endorsed by many.
Not very 'prettily' mounted though is it? However it does work at keeping it away from the aluminium cabinet sides. (That's a bit of pipe I found and an empty solder reel!)
Here's another picture and obviously taken from a planned angle so that my failing 'bear paw' hands and mole/bat like ageing eyesight doesn't show how wonky all the mounting holes actually are!
Always fun to build a bit of simple radio gear and waaaay cheaper than commercial offerings even if it is less 'eye-candy' looking.
Especially when you dig out bits of lawnmower lead from skips, capacitors from radios and whatever other junk you have laying around!
Oh, does it work?
Well I can hear more than I could, can hear further than I can transmit on digi-modes (my QRP operations assure that) but I've managed to bag another couple of countries on it so far and if I get the QRZ confirmations hopefully another QRZ award.....
More of my mumblings and musings but I do have fun and I hope it might help others too.
All the best,