Poor Man's Variable Inductor - Modifying my ATU

Hi All,

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.....

Definitely works.


More of my mumblings and musings but I do have fun and I hope it might help others too.


All the best,
Victor

Nice one Victor.
Wish i could get my head round making that sort of kit but i will stick to antennas balund ununs the easy stuff.
Sent one down to South Wales the guy is in a 2 up 2down miners cottage with a 12ft square yard and he is working top band now.

Cheers Alan.  

It's all bits of wire wrapped around stuff however you look at it.

You'll have to post up some of your projects mate. (I enjoyed your shack build.)

I know you said it's not prettily mounted Victor, but it looks great to me. The inside wiring is very tidy too and both photos clear to copy from if anyone wants to.

Do you use the coax output at all, or just the feed for the doublet now? Are you able to match enough to make 160m work? ..just must mulling options as pushing myself to try new-to-me bands and modes currently.

Glad you've managed to take the edge off your noise issues and not letting it spoil your fun.

Hi Neal,


The SO-239 output was historical back from when I used it for my dipole fan array but handily left in place should I play around with other resonant dipoles. (You can 'tweak' a dipole with an ATU rather than trimming it or even make it work on a completely different band. )

The 1:1 BAL-UN works well (better than a typical 4:1) and along with a homebrew Maxwell or W2DU coaxial choke from the radio to this matcher job's a good'un.

With the 66 foot (20m) doublet I can match from 80m to 10m and although I can get pretty good reception at 160m it really needs the high current matcher I specifically made for 160m to transmit. (It enhances reception no matter where you are in the shortwave bands though. )

L.B. Cebik did much work on doublets and found that anything as small as 3/8ths wavelengths works quite well.
I've squeezed 'performance' from less with my 160m antics and Continental Europe contact on such is not to be sniffed at.


Most concentrate on the "best-of-the-best-of-the-best" regards radio but we don't all have property sizes to string up long antennas or sometimes even shorter ones. I always say 'any antenna is better than no antenna' and in this instance it fits my needs squeezed in to the loft allowing me to work all the HF bands at my disposal.

With FT8 (and burgeoning CW aspirations) along with my QRP fun it all works rather well which is all I can ask for. (Worldwide contacts keeps me happy and there is always FT8 action on all of the bands. )


Now that my noise levels are alleviated somewhat, (still working on that) I can hear as much as many of the online SDR's which for a bit of lawnmower lead, homebrew and my forty odd year old Yaesu isn't bad going.


It's fun to build Neal but for QRP operations even a cheap commercial portable Z-Match unit should do the job (although I gave up there due to the 'twitchy' nature of tuning them) and are available as kits should you wish to sort-of 'build'. Most cheaper 'auto-tuners' will not have enough range for such antennas on all the bands and neither will 'built-in' tuners on modern radios. Even the more expensive affairs will still need to see you adding BAL-UN's and CMC Chokes appropriately mounted so it's not exactly straight forward.


I'm happy to discuss anything about it all on the forum with you Neal and hopefully others may benefit too.

(Although Rick DJ0IP has a wonderful site concerning much about antennas and matchers here :- https://www.dj0ip.de/ )


Have fun, enjoy yourself and all the very best,

Victor

I like the creativity. Never seen this before. Usually see a large coil with taps. This type of effort is what makes radio fun. 

Cheers Mike,

It was a lot of fun sticking it altogether and I'm having fun using it.


All the best to you there Stateside.