Invisible airwaves crackle with life
Stretched WA5VJB Cheap Yagi for 70cm
With the acquisition of an elderly Icom IC-451A,a need now presented itself for a 70cm Yagi. Once again the brilliant and inexpensive designs of Kent Britain WA5VJB were brought out and twiddled with.
Design and construction
Design rationalization proceeded much the same as for the 2 meter 10 element Yagi. The boom in this case was a length of 1x1 trim,primed and painted to protect it from the weather. The driven element was cut and formed from 1/8" brass rod; the other elements were cut from aluminum fence wire.
Up the mast and on the air
The antenna had its first acid test during the January 2009 VHF Sweepstakes. I did nearly as well on 70cm as I did on 6 meters,and that's with just 10W on 70cm vs. 100W on 6 meters. I netted one guy clear down around Fort Pierce,right on about 100 miles away. I can't wait for the June and September contests,where VHF/UHF propagation is generally much better.
Some time after I initially posted these articles,I got into an email exchange with David Palm W9HQ about my twist on Kent Britain's antenna designs. Some of the exchange is included below:
--- On Tue,6/22/10,David Palm <firstname.lastname@example.org> wrote: From: David Palm <email@example.com> Subject: Question about extending "cheap yagis" To: "Kent Britain" <WA5VJB@flash.net> Date: Tuesday,June 22,2010,8:37 AM Hello Kent,I've just been corresponding a bit with WA4UF and he mentioned building longer versions of your "cheap yagi" designs. I'd love to do this as well,for base operation. He's done a 10 element design for 2 meters and a 15 element design for 432. I'd be interested in extended versions for all the bands you have plans for. His note below says that he just took the length of the 2nd director and the distance to the 3rd director and used that to add additional elements: "Buried somewhere in Kent's original article was the notion that the designs were "extendable". I got to comparing element lengths and spacings,and deduced that if the 2nd director were duplicated and its spacing to the 3rd director similarly duplicated,you could add several more elements before the antenna got completely unwieldy. So that's what I did,and it seems to work pretty well." Sounds simple and easy. Do you concur that this would end in a higher gain version of your "cheap yagi" designs? Thanks and 73,David W9HQ --- On Tue,6/22/10,Kent Britain <firstname.lastname@example.org> wrote: From: KENT BRITAIN <email@example.com> Date: Jun 22,2010 9:27 AM Subject: Re: Question about extending "cheap yagis" To: David Palm <firstname.lastname@example.org> The problem is that I have never published a design that I have not built and tested on the antenna range. To make a Yagi longer it does kind of grow in the middle of the director set,but there is no formula. My suggestion would be to download a copy of Yagimax 3.0 or higher. The antennas are optimized for Front-to-Back,not gain,and keep the driven element impedance in the 18-20 Ohm range. Yagimax will not predict the length of the Driven Element with any reliability,so you will need to trim it for best SWR on the prototype. I'm sure UF is getting more gain out of the Yagi,but it probably not well optimized. I'm sure your first Yagi will be over optimized getting that last .01 dB of gain. But wnen you build it performance will be horrible. The dimensions get so tight you really can't build it. And Yagimax has a habit of making the first director a bit long. Back off gain about 1/2 dB and go for a wide bandwidth. Now you can be loose with the dimensions and not change the antenna very much. Good luck with your project,Kent WA5VJB --- On Mon,7/12/10,David Palm <email@example.com> wrote: Subject: Fwd: Question about extending "cheap yagis" Date: Mon,12 Jul 2010 11:22:16 -0500 From: David Palm <firstname.lastname@example.org> To: Bruce H McIntosh <email@example.com> Hey Bruce,You may be interested in this. I e-mailed Kent Britain about the idea of extending the "cheap yagis". His reply is below. He suggested modeling the lengthening using Yagimax,which I just got around to trying. According to that modeling tool,anyway,your 10-element 2 meter beam does indeed perform better than the 6-element model. But the 15-element 432 design probably doesn't do better than the 11-element version WA5VJB published. Interesting. I'm going to play around with this a bit more,but you may wish to explore this a bit too. If you find some really snazzy longer boom/element designs,pass them on to me. I'll do the same. Thanks and 73,David W9HQ