YU7EF Duo 6 n 4

YU7EF Duo 6m + 4m Yagi

 A 5 Element dual band, single feed solution for 4m and 6m

In Dubus (2/2007) Pop, YU7EF, published a design he had been working on for a "space saving antenna" for the 4m and 6m bands. The brief - to design a real-world antenna based on a single feed point, with acceptable gain, pattern, side-lobe suppression (one of Pop's obsessions which pays big dividends) and all done on a 50 ohm feed needing no transformation. I am not entirely certain why Pop chose a single feed but I do know that he has an FT-847. Now the 847, which most people know can run on 70 MHz, has a shortcoming. The same antenna port is used for 50 MHz and 70 MHz. Such an arrangement means that working cross band with the correct antennas would only be possible with an arrangement of relays to switch antennas - not at all convenient. Also, with normal separate antennas, an antenna switch is necessary to drive 6m and 4m monobanders from the same antenna port. Only sacrificing the HF port to 6m would overcome this, eliminating HF from the equation. This single feed design is perfectly suited to the FT-847 and will allow seamless 50/70 cross band operation for 847 owners for the first time, along with a "no band switching antenna" for switching between bands.

No doubt the design process was a tall order but I waited with interest during the intervening period between his announcement that he was working on such a design to the moment it was published. Once the article had appeared in Dubus, Pop very kindly emailed over a .zip file containing the relevant data, polar plots, S.W.R. sweeps and of course the dimensions.

It is one thing to model antennas in cyberspace but to transpose these findings into a practical design can sometimes lead to frustration as real-world issues bring to bear minor changes due to materials not able to be included in the model. For any theoretical antenna designer, it is a trying time between the release of a model to the feedback of a built example which shows up the design for its glory - or otherwise...

As you will no doubt already know from this site, I have two YU7EF designs on the air already - the EF0407 - a seven element 4m Yagi on a 6.5m boom and an EF0606 (yes you've guessed it) a six element 6m Yagi on a 7m boom. Why then should I want to build one of these Duo's? Well as a builder of a great many years, I confess to a not insubstantial degree of curiosity. To justify the build I needed an application. With some discussion with the XYL we decided that we would undertake some /P operation.

This idea brought with it some extra challenges. Not only must I build the Duo to exacting tolerances but make it in such a way so that it could be repeatedly assembled, disassembled and transported in my small family car. Armed with the design, I looked to my pile of skeletons in the cupboard (or rather - behind the shack!) to see what metal lent itself to the project. Well, as it turned out there was not much. So for the first time in a few years I had to resort to sourcing in the metal. Living in a rural area and having my personal mobility problems somewhat restricted my options to online buying. I soon found that courier restrictions meant that there was a serious issue regarding the lengths of tube that could be supplied in the quantities I needed it - unless I was intending to mass produce...

Moreover, considering we started the process of metrification in 1969 in the UK, I was frustrated by the fact that all the UK based suppliers I found, offered imperial diameter tubing! The Duo's design calls for 12mm and 10mm tubing which I could not easily locate. To this end I approached Pop who kindly revised his design to compensate for 1/2" and 3/8" tube. In the end, the solution to both my problems were staring me in the face; buy in the element tubing on 1/2 element sizing and splice them in the middle. Of course this presents its own challenge but hey - when have I ever shied away from one of those?

Below are the S.W.R. plots supplied by Pop for his Duo model. Click on the chart to see an enlarged and more legible version. Both bands show an excellent response, with 6m having the edge. As the principal use of a horizontal Yagi is not normally FM ragchewing, the 4m response has been centred around the SSB/CW calling frequency, which most current allocations are based around. Even at the band edges on 4m the specification S.W.R. is still within an acceptable range.

Now on to the build. I make no apologies for not including a full step-by-step. The mechanical aspects of Yagi-building are well documented and not served by repetition here. Also there is a piece about my build of the YU7EF antennas on another page on this site, again I feel repetition is not warranted.

What should be noted however is the precision. Yes, Yagi building does require a fair degree of precision - the need for which increases commensurately with increases in frequency and error can be easily multiplied. The Duo's design requires intrinsic accuracy. By this I mean that every component length and position should be to less than 1mm tolerance. If you do not believe you can observe this, you may well need a network analyser to rectify it later. It is not the most tolerant design either; it willingly decouples to any nearby metalwork. When I was building it, once the elements were mounted I sited the antenna into my normal "initial test" position on the patio. Other yagis I have built have tolerated this temporary affair forgivingly, however this beast likes to play tricks!

I connected the network analyser and took the initial sweeps. . . I was bitterly disappointed by the results, both 4m and 6m were much further away from the specification than I had hoped. Right about then my wife moved a piece of metal which I was confident was not in sufficient proximity to adversely affect the readings - how wrong I was. As the tube was transported from its position to a new one I watched the continuous sweep of the network analyser swing to and fro. . . Indeed, just turning the antenna through 180 degrees, taking the reflector away from my aluminium windows behind, saw the sweeps evolve into readings much more kindred to the original specification. There was no alternative - a new testing ground was needed. (Yes that is an aluminium ladder you can see on the right of the shot...)

So, in line with Yagi practices, I up-ended the Duo and sited it down on the lawn, with reflector on the floor and D3 pointing at the sky. "Wow" I thought, "This is it!" The readings were closer to specification. Being a perfectionist, I wasn't satisfied with an S.W.R. of 1.2:1 when the specification shows lower was possible, also the sweep wasn't flat across the band indicating some correction was needed. I set to with enthusiasm in an effort to find the elusive Yagi-nirvana. Armed with some rolled up pieces of aluminium pie-tray to make sliding inserts (this is my cheat for quickly tuning wayward tubular Yagis) I soon found that I had to extend the 6m monopoles by a massive 30mm - which yielded specification S.W.R. and impedance. Confident that I "had it" the changes were made permanent and the antenna was taken down from the temporary tuning position and set aside for mounting on the telescopic mast which was to be this chap's final resting place during /P operation. The following day couldn't come quickly enough!

The following day she went up on to the portable telescopic mast. . . . Oh crap. . . . The sweeps revealed an S.W.R. of well over 1.5:1 on 6m and an equally unfriendly readings on 4m. I was miffed to say the least! In my years with Yagis of manageable size I have always tuned them up for the final commission with the reflector near the ground and the boom vertical. Indeed only recently I wanted to make some minor changes to the resonant frequencies of the EF0407 and EF0606. For the sake of curiosity, as I brought them down to the ground on the tilt-over-mast, I watched the network analyser sweeps which revealed almost no changes from operating position to tuning position as described above. Once the alterations had been made, I watched the sweeps remain steady as the antennas were winched back up to operating position. For this reason I had no qualms about treating the Duo in the same way. What I did not legislate for was that this beastie is really quite temperamental!

Duo 4m SWR Plot

And now for the moral of this story. After some tinkering, the specification target S.W.R. and impedance were obtained. How? You may well ask that question but if I told you then I would have to kill you. . . . Sorry, joking aside, so you want to know how I corrected it?

I returned the antenna to its specification dimensions! Yes, that's it, nothing more and nothing less.

OK, I guess I deserved that didn't I? Come on, admit it, you are laughing at me really...

The following charts are sweeps taken with the network analyser after completion of the above shenanigans with the antenna mounted on the temporary mast at 5m as pictured on the right, above. They demonstrate most admirably that the antenna as modelled is perfectly accessible in the real world - and indeed the final results obtained my me are better than Pop had modelled or could have hoped for! For the record the antenna is not as close to the shed below as the picture might suggest.

All that remains is to answer the most logical question which arises from the above text. "How does it perform?" My basis for comparison are as follows:- a YU7EF EF0407 at 16.5m and an EF0606 at 14m. I also have a Create CLP5130 which is a 50-1300 MHz log periodic on a 2m boom and around 6dB of gain. This is mounted at 5m.

My experience with the Duo is without doubt a positive one. Thinking "inside the box" one might entertain the notion that the 6m elements are all "in the way" of the 4m elements and as they are not electrically resonant on 4m that they would offer only obstruction. Of course this is a foolish notion that should be dispelled in an instant.

Using the log periodic as the direct comparison on tropo signals like the various beacons, I found the Duo to be of a very much lower order of noise and thus the signal to noise ratio was enhanced. Of course the Duo offers around 3 or 4dB of gain compared to the LP however this should not detract from the reality that the Duo does have good ears. On both 4m and 6m I spent hours sifting though the noises, stations and beacons on both bands, looking for weaknesses. When beacons were marginal on the larger antennas higher up, they were still able to be copied on the Duo albeit extremely weakly. The Duo did indeed prove itself to be a fine receiving antenna with no noticeable bad behaviour.

On MS, I watched numerous QSO's between MS enthusiasts, sometimes watching the waterfall on the radio connected to the Duo whilst listening to the audio pings and bursts on the other radio connected to the larger antennas. Other times I reversed this arrangement and listened to the Duo while watching the waterfall belonging to the larger antennas. I don't think that the Duo missed a single ping that I could observe.

On transmit the antenna showed an equal willingness to radiate in a manner consistent with its specification and worked fine from an MS point of view. I did not attempt to compare the antennas with SSB QSO's on tropo because there could be no common frame of reference due to the significantly differing mounting heights; no useful data could be gleaned save radiation pattern.

Finally for those who are interested, I now field the sweeps that I took with the network analyser. Most if not all are self explanatory and I therefore will refrain from commenting on them - the results speak for themselves. I do wish to comment briefly on the law of unintended consequences.
What!? You may ask. . . Well this antenna was designed for a purpose; to provide access to two bands with only one antenna and feed. When taking the sweeps I went to 180 MHz to look for potential harmonic resonances which could potentially offer a good path for spurious emissions from the radio/amplifier. As you will see from the sweep at the end of this page that there is is curious drop in S.W.R. in the middle 140's MHz.

It turns out after careful measurement that there is quasi-resonance on 2m!! Yes the S.W.R. goes as low as 1.3:1 and is low enough over the entire 144-146 MHz range! Naturally this is unintentional and the radiation pattern will be complete nonsense, however that does not detract from the fact that it could be used as a substitute for a horizontal dipole! I also include Pop's model plot for the 2m band should anyone be curious!!

YU7EF has now completed a page at his website outlining the Duo Design

It can be found at http://www.yu7ef.com/efDUOBANDS.htm