Date: Wed, 1 Jun 2016 08:30:05 +0000 (UTC)
From: Dan Maguire <djm2150@yahoo.com>
To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises
Jim VE7RF wrote:
>>> The F12 N style 40m eles are not full size. They range from 59 ft for the
>>> REF...down to aprx 52 ft for the DIR.
Well, now we're making some progress at reaching common ground. I created a
dipole model with 1.5" diameter aluminum wires and adjusted the length for
precise resonance at 7.15 MHz in free space. Then I changed the length to 85%
of the self-resonant length, added loading coils mid-way out each leg, and
adjusted the coil inductance to again get precise resonance at 7.15 MHz. And
finally I changed the length to 70% of the self-resonant length and again
adjusted the coil inductance to get precise resonance at 7.15 MHz. Coil Q was
kept constant at 200.
Then I switched from free space to average ground and varied the height of each
dipole from 10 ft to 100 ft, this time adjusting the *frequency* for precise
resonance at each height while keeping the length and coil inductance (if any)
constant.
http://s33.postimg.org/v51yi2n5b/resonate6.png
100% length (69.79 ft), no loading, +218 KHz change from 30 ft to 60 ft.
85% length (55.93 ft), 3.297 uH coils, +137 KHz change from 30 ft to 60 ft.
70% length (46.06 ft), 6.721 uH coils, +78 KHz change from 30 ft to 60 ft.
So it seems that more loading results in less change in resonant frequency as
the height above ground is varied.
Dan, AC6LA
## each f12 40m N style ele starts at 1.875 inch OD....and tapers down to
just .375 inch OD at the tips. The oem F12 linear loading uses 3/16 solid al
rods...and starts at 7 ft 9 inches out from the boom.
Both LL rods extend outwards towards the tips. ( as noted in a previous post,
I replaced the LL rods with .375 inch silver plated cu tubing coils).
## If u see a 78 / 137 khz change from 30 to 60 ft....then every rotary
dipole maker would be warning its end users to tweak tips vs installed
height....and they dont. And I sure didnt see any 218 khz change when
increasing the height of a full sized wire dipole on 40m. ( 40 ft to 76
ft). It barely shifted. So what gives ? Is the model wrong ? Something
is wrong somewhere. The software doesnt jive with real world results.
Jim VE7RF
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