[TowerTalk] Adjusting the reflector

[terry burge]

Hi John and the rest of the guys and gals,


My little bit of experience with both quads and yagi's has taught me a reflector should be about 3-5% longer than the driven element. Using the coils complicates the issue but sounds like you may have better tools than I did years back. I used to be fpretty good using NEC2 designing quads and still plan another big array.  As far as weather to make the parasitic element a reflector or a director I remember when building a 2 element 20 meter beam out of the parts from my old 'Gotham Quad' way back early in my ham radio time. A director has a little more gain than a reflector for a 2 element. Think that must have come from Bill Orr's beam antenna handbook that little tid-bit. That was way back in 1977 or so and Bill Orr's quad and beam handbooks were my 'bibles' for antennas.. I never was able to get the Gotham Quad to get lower than about 3:1 SWR. Worked anyway but being new to ham radio I never tried a tuner that would have cured my issues I think. Had a lot of fun building things from that cheap aluminum tubing. The 2 element beam worked pretty good using my home brew gamma match using gas line for dielectric insulation.. 


But the basic point is Yagi's and quads usually have 3-5% length difference one way or the other with closer supposedly giving higher gain. Add more elements and perhaps all bets are off judging by some of the detentions I've seen on various monoband Yagi's.


Terry

KI7M


> On April 8, 2018 at 8:29 PM John Keating wrote:
> 
> 
>     I'm getting ready to make another, hopefully the last, adjustment on the
>     reflector length of my 2 element 40M coil loaded short boom yagi. Looking
>     for recommendations or calculation methodology to optimize F/B and pattern.
>     Here's what I know:
> 
> 
>         * I operate mostly around 7.150 - 7.160. Driven element is tuned there, and
> 
>     its length is 408.25" tip to tip.
> 
> 
>         * Reflector length was originally 462.25" which resulted in maximum F/B at
> 
>     6.700, with a basic dipole radiation pattern at 7.150.
> 
> 
>         * Most recent adjustment shortened the reflector to 420.25", resulting in
> 
>     maximum F/B and good gain at 7.060; modest gain and less pronounced pattern
>     at 7.150. At this point, the reflector is tuned about 1.5% lower than the
>     driven element, and its physical length is almost 3% longer.
> 
> 
>         * coils are nominally 13.5uH with Q ~ 100 at 7.150, resulting in about
> 
>     200kHz bandwidth. Coils are approximately in the center of each half
>     element. Feedpoint impedance is close to 50 ohms with no match. Boom length
>     is 15.6 feet. Antenna height is 50 feet.
> 
> 
> 
>     N6BT's Array of Light states that shortened yagis require tighter coupling,
>     and recommends that the reflector elements be tuned about 1% lower than the
>     driven element. In my case that would mean shooting for around 7.090. If
>     behavior followed the linear relationship implied by the first instance of
>     shortening (8.22 kHz/inch), reflector length should be 413 inches, or
>     shorter by 7.2 inches. But I don't know if the length vs resonance frequency
>     relationship is linear for a coil loaded element.
> 
>     Alternatively, my EZNEC model indicates best F/B, gain and pattern will be
>     obtained with the reflector only one inch longer than the driven element
>     (yes, one inch!) but the SWR will deteriorate substantially due to change in
>     feedpoint impedance.
> 
> 
> 
>     Any further shortening of the reflector elements will require cutting
>     aluminum while in the air, so I want to be careful what I do next.
> 
> 
> 
>     Looking for any advice or suggestions other than "remove a half inch at a
>     time and measure."
> 
> 
> 
>     73, John
> 
> 
> 
> 
> 
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