Amateur Radio

Stretched WA5VJB Cheap Yagi for 70cm

Introduction

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.

Correspondence

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 <thepalmhq@gmail.com> wrote:

From: David Palm <thepalmhq@gmail.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 <wa5vjb@flash.net> wrote:

From: KENT BRITAIN <wa5vjb@flash.net>
Date: Jun 22,2010 9:27 AM
Subject: Re: Question about extending "cheap yagis"
To: David Palm <thepalmhq@gmail.com>

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 <thepalmhq@gmail.com> wrote:

Subject: 	Fwd: Question about extending "cheap yagis"
Date: 	Mon,12 Jul 2010 11:22:16 -0500
From: 	David Palm <thepalmhq@gmail.com>
To: 	Bruce H McIntosh <scotsman@afn.org>

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