Homebrew ATU

Hi All,

More of my homebrew antics.

This time an ATU or Antenna Tuning Unit.

There always seems to be a point of contention regarding ATU's, misunderstandings, misconceptions, sometimes just general arguments. "It doesn't tune nuffin'", "They don't work", "What's the point?", etc. etc.

Well in my case I have a fan dipole array, all cut to resonance but due to the way they are zig-zagged in my attic space there are various impedances obviously affecting the SWR reading you get. All I'm doing is using a traditional Pi-Network style of ATU to match these impedances keeping my radio's output happy. This helps on receive as well as transmit.

I had a couple of 365pf air spaced variable capacitors kindly donated to me by a good Ham friend, (Hi Gary mate!)
Along with some scrap aluminium angle, a few bits of wood and a bit of workshop time I made an ATU that would not only work, but be nice to look at, (IMO), nice to use and generally do the job.

I had some knobs for the variable capacitors but did not like the look of them, so I played with some bits of wood on my wood lathe...



These work out around 2 inches diameter and along with the slope make for a nice 'grab' between thumb and fingers.

As these were 'coverings' for the original knobs it took some careful hollowing on the wood lathe...



There's nothing special about my wood lathe, it's 1980s vintage, spins wood, I point sharp bits of metal at it, otherwise known as chisels and with a careful bit of hand-eye coordination hopefully you get what you want

Now I have two knobs. For a change, I've made two of something by hand on the lathe that actually match in size!



They're gonna look pretty ugly with that dirty big tapped hole on the front of them! But take notice of the shallow hollowed depression, I'm gonna fill them with aluminium plates....



So with appropriately made jigs of the correct diameter I clamp some aluminium sheet inbetween. As this is aluminium it is 'soft' enough to cut with a HSS, (high speed steel), chisel by hand....



...and we land up with a perfect metal disc ready for the knob...



Clever eh?

Here you can see the finished knob I made and the rather ugly knob that it will cover....



You'll notice that I gave a nice rotary brush effect on the aluminium disc and a quick wipe of finishing varnish to bring out the colour of the wood.

Here we are checking out the knobs whilst building the rest of the ATU....



Starting to come together

Well after a bit more construction, hand brush finishing the scrap aluminium panels and finally adding some wooden buffer side panels we get this.....



None too shabby eh?


You'll notice that there are no 'nice numbers' on the variable capacitors or 'band settings' on the central inductor switch. I don't need them, it's not a commercial piece and as this is switching inductance and capacitance at my will, not needed. Also, as this is a Pi-Network tuner the amount of inductance required for each antenna tuning will vary according to my antennas, not by a 'band tuning'.

There are two little toggle switches added above the central inductor knob. The left one is to switch in extra capacitance to allow tuning down to 160m, and although my antenna is not cut that long it will at least give me some operation which is better than none at all

The toggle switch on the right was originally for a "RF Noise Bridge", (which I may still build), but at the moment found not to be needed. You go to the band of choice, twiddle the knobs until you get the loudest receive. A small tweak thereafter makes your SWR and impedance match perfectly. The idea behind the noise bridge though is to give you a RF noise source matched through a bridge transformer, (with one end tied to a 50 ohm resistor), and you adjust the ATU until the noise disappears - then you're matched to 50 ohms exactly!
Old technology, every experimental Ham should really have a RF noise bridge.....


The tuner is as nice to use as I envisaged. The wooden knobs are polished up with carnauba wax and feel gorgeous under your fingers with a silky smooth tuning to the ball bearing variable capacitors. The whole thing weighs a fair amount too, which means it doesn't slide around the shelf when your using it! (A bain of 'small' modern equipment requiring velcro hold downs or screwing down to the bench/shelf!)

Plus I now can achieve a 1:1 SWR @ 50 ohms for 160/80/40/20/17/15 & 10m....awesome



The cost? Not a ha'penny guv
Recycling at it's best and as I already have a lathe, some hand tools etc. I can't exactly count them in can I?


My apologies for the rather long, (me?), posting and the fact that it's picture rich. But I thought it might be worth it.


One last shot as I really can't help myself.....




This is not just an A.T.U.   This is a 'Victor' A.T.U.






All the best,
Victor

26CT3228
M7VIC

Like bit of furniture Victor , looks very well made mate I wish I was good at making things

Cheers Martin, really appreciate that mate!  

The wood lathe was a gift from my friend after I became disabled and took me a year or two getting to grips with it. Before that I'd made almost nothing from wood except the odd shelf! Now it's amongst the other hobbies I have, (I make bowls etc and even carve nowadays). Seeing as it was radio related it meant I could post it up

We can all 'make' things mate, I still love your garden radio shack!


All the best,
Victor

Only thing is Victor I didn't build the shack mate it was brought from a summer house company and they delivered it and put it up LOL,it was really just the inside that was done by one of sons ie: flooring and panelling the walls my help was getting in the way TBH lol , I  use to have patience when was younger but as time went I just sort of lost them ,perhaps I should try and get them back .

Absolutely fantastic Victor, what a talent.

You still had an input to it all though Martin and kit it out with your radios. 
Without your know-how it would just have been a shed, not a radio shack!

It's not really a radio shack without someone to operate the radio either. 

Yeah, in my eyes you made it mate

Hiya Paddy,

Cheers mate, really appreciate it.

It was just a coming together of appropriately scrounged and shaped parts. Not perfect, but it works and I'm happy with it


All the best,
Victor

Beautiful piece of equipment. You should be proud. As has been said, "wish I could do something like that"

Hi Ernest,

Thanks, really kind of you my friend.

I think our American cousins appreciate hand-made items much more than anywhere else in the world

Got to admit, I do feel a sense of pride as it sits at my station and smile every time I tune it. Those wooden dials really do feel nice under your fingers.


All the best to you,
Victor

Wow Vic, it looks amazing!!! 

Cheers Gary mate, really appreciate that

hi anyone made similer for 4 meters 70mz stay safe do radio paul m0kbd 73s

Hi Paul,

Here's an example of a pi-network somebody has made for 4m :-

http://ei4dib.blogspot.com/2015/09/70-mhz-antenna-tuning-unit-for-4-meter.html


You may have problems getting hold of such variable capacitors though.


If you're using relatively low power output, (<50W), you could modify a typical CB type such as the Zetagi MM27. Open one of those up and they'll be two polyvaricon type variable capacitors, (small square plastic variables like those found in old transistor radios), and a coil. Remake the coil with less windings, I'm guessing 4 or 5 turns around the same diameter and it should cover the 4m band.

I've ran a 'longwire' with a similar setup on 2m before - shouldn't work you'd think but certainly does! The ATU at least protects your output transistors on the rig

HTH.

Victor

Nice work, Victor.

Cheers John. Much appreciated mate

You are welcome, Victor.

hi victor i have found the site i will look on ebay for the right caps its frezing in stockton on tees keep warm and enjoy radio thanks  paul m0kbd 73s

That is SWEET!  I really think you should build more items using the same casing and knobs. So you have a row of them! Really nicely done. Love seeing projects like that. Far better than stuffed in an old tin can.
Well done!

Cheers Jeff, really appreciate it. 

As it was made up from scrap parts there's only one unfortunately. I thought I'd worked miracles making two knobs the same by hand let alone a row of them!

Still, I needed an ATU, enjoyed building it, love using it and it is as you imply, really nice on the eye.


All the best to you mate,
Victor

What can I say but I am impressed, that is a really nice bit of kit you have put together there it never ceases to amaze me on here the ingenuity and creative skill people have I am really digging those aesthetics too, the wood and aluminium ( I can't quite see if it is brushed by it appears that way) give a really individual and classic I feel, and it is unlike anything else I have seen.

Some labels for the switches would set it off very well I feel but it is certainly individual and stylish in addition to being functional I am sure, using a loft based wire antenna system myself I can absolutely relate to the need for a little additional tuning as no matter how well cut it may be its slightly compromised situation affects the SWR and a good tuner makes all the difference.

It was only quite recently I noticed tuning in the antenna on 80M made a great difference in the strength of my received signals and I can't say that is something I had picked up on previously, I have another antenna project ready to go I had all the parts out and realised I didn't have any little black tacks(nails) to faster my insulators and I couldn't pop ut to the hardware store for more it has to be eBay so the job was put off for another day...which was rather frustrating at the time!

Good to see your project doing so well and I hope to see it once you do the labels so we can marvel at the finished product!

Alan

Hi Alan,

Cheers mate, much appreciated

Yep, 'brush' finished, by hand....working through the grits 240, (the Ali was quite rough), 400, 800, then a final 1500 brush over. All whilst keeping the lines straight! (Made a little jig.)
Makes a huge difference with the loft dipoles which I've since changed to have ladder feed through a current balun then the tuner. Nice to have a HF antenna setup that can let me work the digital end of 80m one second and a quick twiddle for SSB at the other, then shoot off to 40, 20 or 10 with a quick flick of those knobs! You already appreciate what can be achieved with some wire slung in the loft.

Still no 'labels' as of yet, don't want to stick some tacky affair on it. Spent ages trying to find 'waxed etching paper' you used to stick through a typewriter, (yes, I still have a typewriter!), and electro-etch some nice typewriter font text direct to the ali. You'd have thought I was born in the 1850's asking for such stuff from the responses I got!

It's not an issue though, 80m is 9 o'clock, switch number 6, 3 o'clock, but surprisingly after a while you almost automatically tune it when switching bands. (A SWR meter helps on the final tweak but reception noise does get you almost bang on.)


Good luck with your antenna projects Alan, it's always fun to work on such things. Just a shame you had to stop due to a lack of tacks, but there's always something that stops us like that. Mind your back in that loft space too mate!

Always love a bit of homebrew and great to hear of or see others work.


All the best,
Victor

That looks amazing. Very stylish.  It makes me feel like I need to get myself building again.

Hi Nick,

Much appreciated. If my shenanigans enthuse anyone enough to get them building, then this posting is very much worth it. I always look forward to hearing others experiences, whether they sling up some wire for a home-brew antenna or go the full hog and build something a little more complex. (Even if it only just simply looks nice )

All the best,
Victor