[antennaware] 40 M Yagi Problems

Eric Gustafson n7cl@mmsi.com
Sun, 30 May 1999 12:36:34 -0700

Hi Mike,

I think I can help you out.  As you have already discovered, your
element lengths are far too short for operation at 7.05 MHz.

I have been modeling 40 meter 3 element antennas for some time
now so that I might build one here soon.  N7DD actually did build
one to my model dimensions.  It played perfectly straight "out of
the box".  We didn't have to adjust any element lenghts by as
much as one inch!  So we are very confident of the stepped
diameter corrections and element clamp compensation that I am
using in my modeling.

Without information about your taper schedule and element clamps,
I can't give you dimensions to use to the inch.  But the
following dimensions should get you into the "working" ball park
regardless of these variables.  I doubt that you can be all that
far from what we did and still have a survivable structure.

These dimensions were developed for elements that taper from 2
inch to 0.5 inch diameter in 7 steps (not counting the boom to
element clamp compensation segment).  If your taper schedule is
different enough to significantly affect the array, you will most
likely degrade F/B significantly.  But the gain should remain near
8dBi despite the errors.  And it should be matchable and display
a reasonable bandwidth.

Build it.  Check the F/B.  If its terrible, send me your taper
schedule and detailed information about your clamps.  I'll see if
I can point you in the correct direction to tweak it into

I took the model of the known good working dimensions and
relocated the elements to your spacings.  Then I re-optimized the
element lengths for best gain consistent with >20 dB F/B ratio at
7.05 MHz.  This exercise resulted in the following dimensions for
the elements:

Half Length (inches)	Length (feet)		Length (meters)
-------------------	-------------		---------------

440.5			73.417			22.378

Driven Element:
Half Length (inches)	Length (feet)		Length (meters)
-------------------	-------------		---------------

419			69.833			21.285

Half Length (inches)	Length (feet)		Length (meters)
-------------------	-------------		---------------

408.5			68.083			20.752

I'm not a big fan of the gamma match.  So I model driven elements
for other matching schemes.  The driven element in this case was
sized to permit match by means of shunt inductor or "hairpin".
This resulted in an unmatched driven element impedance of about
26 -j25 ohms.  This is fairly close to the element size required
for a gamma match.  And, in fact, Larry didn't have to make any
dimensional changes to the DE to get his gamma matched just fine.
But some adjustment of the DE may be necessary to get the match
correct.  DO NOT adjust the parasitic element dimensions except
to modify the pattern characteristics!

Just for your information (and from my dim recollection), Larry's
gamma rod was sized and spaced similarly to yours.  But the
length was only about 65 inches.  And the series cap was only 140
to 150 pF.

If you wish to trade gain (slowly) for increased bandwidth (match
only - not pattern) you can push the DE farther out towards the
director.  But this will necessitate resizing the parasitic
elements a bit to maintain the pattern.  With your current
spacings, the element dimensions I have shown _should_ result in
a 2:1 SWR bandwidth of 200 KHz (and it _would_ with a hairpin
match).  You should get about 8.3 dBi gain and have more than 20
dB front to rear ratio (actually about 24 dB in the model).

I hope this is useful to you.  if you do try these dimensions,
please let me know how they worked.  I can always use another
data point.

73, Eric  N7CL

>From: W0YR@aol.com
>Date: Sat, 29 May 1999 09:58:53 EDT
>We at HG5C are stumped.  Building 3 el 40m yagi from WA7RAI
>design and it resonates at about 7.4 mHz!  We lengthened the
>elements in hopes of bringing it down to the design freq of 7.050
>but now it is extremely hard to get the gamma match properly
>adjusted and the bandwidth is ridiculously narrow (under 2:1),
>about 50 kc!!!  We built other yagis from this design and they
>played right off the bat, but this is a mess.  K4VX software
>shows very little difference in el lengths, etc.  We can't be
>THAT far off!
>Here is the dope ---
>40 Meter Yagi - HG5C    Budapest, Hungary
>Dimension		Meters				
>Boom Length		15.59		51.14 feet
>DE to DIR		6.69		21.94 feet
>DE to REF		8.95		29.36 feet
>Dir Length		19..65		64.46 feet
>DE Length		20.58		67.53 feet
>REF Length		20.94		68.71 feet
>Gamma Rod spcng	0.32		12.7  inches
>gamma diameter		0.02		 .63   inches
>gamma length		3.41		11.17 feet
>computed Gamma c	255.3 pf
>Boom Diameter		0.05		 2.17 inches
>Design Freq		7.050 mHz
>Height above ground    66 feet
>Please give us any suggestions.
>(Note: We believe there is no interaction from the guys and
>trusses since "Philllystran"-type line is used for element guy's,
>etc.  No difference in VSWR when turning ant.)

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