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Antennas

[Note. Summer 2008 once again took its toll, this time the rotator was lost. Too expensive to continually re-manufacture and replace/repair I have now made a set of Nested Cubical Quads to replace these antennas]

My current VHF antennas are homebrew insomuch that they were constructed by me. The design of the antennas however is not my own. All my previous antennas have been either commercial or homebrew arrangements depending on circumstances and QTH. I moved away from commercial antennas in the 1990's because I felt that their design was lagging behind the emerging computer models. Indeed, I took one commercial antenna from a UK manufacturer known for their 'Value Antennas' and modelled it. I was shocked at what I discovered and started down a path of modelling my own.

My designs were quite good; they performed at least to expectation, if not beyond them, at one point I had a 13dBd 9 element up for 4m with a 10m boom. Boy was that an awesome antenna both to behold and to use! However there is more to an antenna than gain alone. With my 13dB monster, I sacrificed much F/B to squeeze the last drops of forward gain but in the process also engineered in some annoying side-lobes. In many locations these may not have been an issue but I now live in an incredibly noisy location, crammed with PC's, TV's and a host of other nasties containing SMPSU's, not to mention birdies from a host of digital controllers. These were being picked up in their droves and my weak signal work was in trouble...

The EF0407 Vertical

YU7EF has approached me about the materials I used in the construction of the antennas, with particular emphasis on the brackets I used to mount the elements. The interest has come about from questions sent to Pop arising from the photograph he has on his website of both my constructions. As a result I have prepared a small piece about the :-

Making of the YU7EF antennas.

The EF0407 Horizontal

Then YU7EF came to the rescue. He published his designs on the web. Pop's antennas are not just good - they are very good.

Pop has dedicated hundreds of hours of his spare time to designing and then optimising his designs. He openly admits to robbing 0.5 - 1.0dB of forward gain in order to kill the side-lobes and improve F/B. This decision has paid dividends and his antennas truly are Low Noise. Of course it is not possible to engineer away all noise with a simple yagi (or with any antenna) but these designs do make a positive contribution to "noise management".

Pop very kindly customised two of his designs for me to accommodate me using existing metalwork from previous builds and to enable me to reduce the weight slightly by tapering the elements. I built the EF0407 and EF0606.

The results were astounding! My noise floor was lower - in fact the initial twenty or thirty minutes I was deeply concerned that they might not be working properly. Then the moment of truth came; I heard GB3ANG on 4m. Nothing particularly unusual you might think? I had heard the beacon hundreds of times on Es, MS and AU, but NEVER had I heard it before on Tropo!

What makes this all the more interesting is that I am on the side of a large hill, which rises another 250m very close to my home. This hill presents a large obstacle on VHF from 350^o-90^o QTF and GB3ANG is well within this arena at 10^o.

The beacon was there for all to see. I was so impressed I recorded it! Now some considerable time has elapsed and I know that conditions are down when I don't hear it! So you see these antennas transformed my operating conditions substantially! Let us apply some common sense to this scenario. Is it logical to say that my 13dB antenna had poorer reception capability than Pop's 11dB design with a shorter boom? Of course not, the difference is not the signal - but the signal to noise ratio. My previous design was collecting noise from the unwanted side lobes increasing the ambient noise floor presented to the receiver. Now with the EF antennas the noise sources themselves appear more directional and present less of a nuisance except on a direct path.

One of the great features about all of YU7Ef's antennas is that they are naturally 50 ohm feed. This simplifies the feed arrangements, with only an RFC needed to get started. I opted for a 1:1 coax balun as I knew I would be QRO (on 6m at least).

Also the antenna's impedance makes it ideal for multiple antenna systems; lower impedance designs often are intolerant of other nearby yagis, the 'EF monoband designs have demonstrated that they are very tolerant.

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