ATU CB master EM27

Does anyone has an manual for this one?  I recently bought one and on the Albrecht site the only instruction for use is something like "fiddle with the knobs". There is a "tune" and a "load" knob on the front. On other similar models i've seen two knobs "capacitive" and "inductive". Would "load" translate into "inductive" and "tune" into "capacitve". or is this too simple thinking?

73
Karel

Hi Karel,

This unit is a simple 'Pi-Network' tuner. (ATU or Antenna Tuning Unit.) Both the controls are a variable capacitor, (polyvaricon, usually 365pf or 500pf), and internally there is a coil calculated for the 26-28MHz region.

They are simple to use and hence why they don't usually come with instructions. (The Zetagi MM27 is similar and has no instructions.)

The usual way to hook them up is - CB Transceiver to SWR meter to ATU to Antenna. 

You simply adjust the controls until the SWR reads the lowest. This will depend on your antenna SWR and frequency in use so you will have to basically 'twiddle'

You do get the hang of them quickly and learn to 'drive' it with practice. You will usually also hear a sharp increase in received noise when you achieve 'tuning' also. (Should really be called 'matching' as they actually match the 50 ohms of your rig to the mismatch impedance of the antenna in use.)

They can seem quite sharp or fiddly to use at first but you soon get the hang of it. With the limited frequency range of CB frequencies you will find that once you have found the sweet spot you will hardly have to adjust off from the found settings.

I use a similar, (but multi-coil and bigger capacitors), circuit in a homebrew ATU and find it invaluable.

Some will argue about their use but usually as they have 'learned' the 50 ohms coax SWR method. My antennas are all 'resonant' to the frequency in use but due to their construction present a difference in impedance which the ATU matches for my radio. That's why they're usually called AMU's or Antenna Matching Units.

Be careful in use, they can tune/match in even the most appalling antenna setup which at worst will arc and burn out the capacitors. At best you'll just have a terrible antenna setup but you'll get a low SWR....not the best transmission setup!

The capacitors in them usually can't handle a bad mismatch of 50W or so meaning the typical 100W manufacture claim is widely out.


Some people may suggest 'why bother?'

If you have an alternative antenna setup such as a homemade dipole the impedance will not be 50 ohms and therefore the SWR will read high. You 'tune' the dipole to resonance by getting the SWR as low as you can at the frequency in use and then place the ATU in line further matching the impedance keeping your radio happy to see 50 ohms.
It's also handy to give your antenna a wider range across the frequencies.

Their effectiveness has and always will be argued about but results speak louder than words


Long winded I know but hope that helps you out.


All the best,
Victor

Thanks Victor, as always clearly phrased and reassuring.

I'm still searching for the easiest/best antenna to build for DX on 11m. Now i'm considering a moxon, but i am spoiled by the widebandedness of DEWD antennas. Thought this ATU might just help.

No worries Karel, I thought a fuller answer would be better than simply 'twiddle it'

For last Summer's seasonal propogation I used a simple Inverted V Dipole and managed a few contacts spanning hundreds of miles on CB frequencies. I also tried out a 'Hentenna' and a simple Horizontal Dipole mounted at the highest point on my roof in the attic/loft each with some success.

I had planned to play with a Moxon reading up somewhat, but unfortunately I don't have any experience with the Dual Element Wideband Dipole.

Since then I took my Ham licence and therefore started playing with antennas for those bands instead. My homebrew ATU/AMU was invaluable as although my antennas are Inverted V's, they zig-zag somewhat to fit the loft space so despite being resonant they have a different impedance to the usual 50 ohms. I had originally matched the impedance through standard coax but have since converted to 600 ohm homemade ladder line with a 1:1 current choke and then RG213 to my matcher.

This now allows me to make contacts several hundred miles and even over a thousand miles on certain bands with just my Foundation licence 10W! 


Good luck with your antenna experimentations, I do hope you achieve success. Summer will be upon us soon offering some good 'skip' at those higher frequencies. Looking forward to it myself!


All the best,
Victor

To add to this :-

The actual long standing convention to tuning up your antenna, which goes right back to early ham days and pre-ham, is to first adjust for high band noise.

So, pick your intended frequency, adjust each side of the atu for peak noise on an empty frequency. Now you've peaked for Rx, if you test on Tx, your SWR should be within a usable range, 2:1 or less, more likely to be sitting around 1.5:1. This is assuming your basic antenna setup is sound.

At the very least, you can leave it set as such comfortably. Now how often you need to re-peak it depends on the band spread in use and antenna type.

So a random wire (a non resonant short or long driven element)/long wire (many multiples or at least 2x band wavelength in use) and an earth gives you a bit of a tuning sensitivity that's eye of the needle for best peak and movement each way tuning wise before re-peaking can be incredibly narrow.

And remember as your wavelength changes (by harmonic and overtone multiples), so does the effective bandwidth between adjustment need and how sharply it is razor edged.

Height is important to ant setup, although some antennas like STL (mag loops) are better near ground but are more sensitive to height changes in that they can go from razor dip tuning with extreme off angle rejection to the same off angle rejection and super razor sharp dip tuning within a matter of say a ten foot offset. But you'll see very rapidly why an STL has more qualities than flaws and where in most cases Brits don't have antenna farm scope in your average garden or window box, STL's have qualities than way exceed their deficiency.

Now your typical EF wire can be as minimal as you need, or you could literally take a large reel worth and run it out to a pole or fixing point (attached with insulators preferably) and carried on until you've navigated the perimeter and used a fair chunk of wire. Having hit extremes, you could continue inwards in a spiral or any way of effectively squeezing more copper into an available space until you almost have a bit of a spiral web going on. So if you can somehow get a physically long, but electrically very long antenna without the atu in place, add atu and a switch and you've got yourself a very good SWL antenna to use between 11/10m and incredibly effective on harmonically bands relative to it's natural EL defined resonance in wide band receiver use. It's not bad, for Rx use, even on non harmonic related bands.

But returning to the ATU, it'll be good enough on 11 & 10m where it's likely to have been built for. But for not a lot more, or even the new asking price of such a crippled ATU (meant as a blunt truth, not in a 'its bad' way), you can get one with a range selector that's a multitap selectable adjust that's your super broad per band optimiser, using the two main rotaries as fine match and superfine respectively. You used to be able to pick up 30W rated items for peanuts at one time and after buying a LPF similarly overrated for CB rig level outputs power, and still buy lunch in the way of a few sandwiches and a cuppa or two at your average radio rally. OK, that sentimental journey was from twenty/thirty years back when I was a CB/SWL user (only I got multiple LPF's inside that pittance budget more by luck). But notably as in critical, if using a naturally non resonant wire or tape or whatever silly sized driven element material you use, the LPF becomes non-optional if you intend to avoid an ongoing set of finger pointing and hard time disputing interference claims. As I said elsewhere, a LPF has a lot of sanity saving value and a bloody good practice in station keeping in general.

Some better ATU's have a LPF fitted internally, but an external one added (so a cascaded pair effectively) won't do any notable harm to your output or Rx capability.

Just please avoid Rx preamps and bilateral amps (RX amp and Tx highly devoid of linearity amp pairing in many cases, or a Rx Amp and a fairly linear Tx amp best case) to compensate for poor performance on restricted antenna use - it's a bloody mucky way to avoid the real issue whilst adding more issues to the equation.

So what you'll find, returning to the peaking process is there's a slight offset between tuning/adjustment on antennas between best 'noise' (Rx peaked) and best SWR. Well if the SWR is inside of sane limits and you still get a pretty lively bright band noise, leave it Rx peaked. The few dB output potentially difference between that and best SWR isn't really enough to keep chasing each time you re-peak as tuning dictates.

Without getting into detail you could more easily digest from established sources, the reason there's a differential with tuning is essentially where Rx peaking only needs to be within a gnat's eyelash each way to be right on the money on a longish wire or tuning a dipole outside its natural resonance, the relationship of current/voltage distribution across what's the driven element Tx side isn't overly razor edged for efficiency RX wise, but Tx wise, being inside of resonance (synthetic transmatched or directly electrically) is the difference between maximum amount of energy departing the antenna going airborne vs what's reflected back at the finals of your transceiver's Tx PA. So what's low reflectively optimal and simply peak noise optimal can have a fair, but sometimes very razor edged difference and depending on focus, one becomes more important to you.

Personally, I can live with a 2:1 SWR worst case and have awesome Rx response/scope with limited site setups. After all, what's the point of being heard through a peaked Tx setup if you're making Rx of the distant contact you're chasing harder because of a Tx max Puff obsessive adjustment of your ATU/Transmatch/Matcher?

But ultimately, get yourself a decent earthing setup - many avoidable evils become non issues when your rig and kit are truly well grounded across the board.